Wednesday, December 30, 2009

DOE in NASCAR Sprint Cup

Brandon Thomas is Chief Design and Development Engineer for Red Bull Racing in NASCAR. He has the dubious distinction of being the first victim to be drafted for a guest blog on The Race Engineer. Thanks a bunch, Brandon. Without further ado...

I feel honored to be invited to guest blog on The Race Engineer blog. I made the mistake of updating my LinkedIn profile with some obscure details of my work history and Buddy came calling. He and I have traded EMails over the last few weeks, and some of that content he felt was worthy of the blog. So, here goes.

DOE, design of experiment, was a topic briefly glossed over in my required statistics and probability class during engineering school. Had it been more of a focal point in the class, I probably would have paid much more attention. Seems the biggest problem educators have is making the course content relevant to the audience. I cannot explain the intricacies, or even the theory, behind the method in a simplified discussion. The point of DOE is to build a response model based on several user-decided factors. Like all models, it requires the user to be pretty informed ahead of time of what is expected in the results, but also to be somewhat open-minded when the results diverge from the expected. I’ll leave it up to you readers to do more research on the www on the method and its origins.

DOE was certainly revered in the pharmaceutical industry – probably still is. Types of experiments, the number of factors involved, and the complexity of the experiment are all parts of drug trials in this era. What does that have to do with making your racecar go faster around a given lap than the competition? Working smarter, not harder, eliminating the surrounding static, and concentrating on the most important setup parameters to achieve a result.

In this realm, I am alluding to a method of DOE involving simulation code. There are many basic advantages to DOE in the virtual world. The repeats can all be skipped, the number of factors can be rather large, and you can accomplish a full factorial design with today’s larger multi-core machines. There are other methods of DOE outside the virtual world, but I’ll take the academic copout that those are beyond the scope of this blog, and are sensitive information in a competitive environment.

Now let’s get to the good stuff. What does DOE methodology do for the race engineer?

One of the most useful tools to a race engineer when away from the computer is a Pareto chart based on the results of the DOE. Here’s an entirely fake example of one.

In this case we decided to look at roll angle and the effects each of these basic factors have on the vehicle roll angle at a given point on the racetrack that we have decided as being critical. Did you see the number of times I mentioned the end user being involved in choosing how to analyze this data? Pay attention to that sentence again. The effectiveness of a DOE approach is only as sharp as the person implementing it. There are numerous pitfalls here.

  • Where do you look at the data on the track?
  • What vehicle response(s) are critical to improving laptime?
  • What factors were deemed critical enough to vary in the design?
  • How many factors were varied?
  • Was the design setup to include interaction of factors?
  • In brutal honesty – how well does your simulation capture the response of your car?
  • Will your setup stay close enough to the baseline that the DOE remains relevant?

You can see that, in order to effectively use this approach, you have to already have a good deal of knowledge of what you expect the outcome to be. You also have to be willing to accept when results are counterintuitive. I mentioned above about interactions, as you use higher order designs you began to capture interactions between your factors. This typically is when the human brain starts lagging in understanding – how does the engineer at the window of the car during practice comprehend how an interaction between all 7 factors listed above would affect the performance? This is when you must be back in front of the computer with the response surface calculation tool. The Pareto chart just helps you change front bar rate or the RR spring when the driver says the car rolls too much or too little – and don’t worry about the LF spring it won’t help here no matter how much you want it to. As in all real life, the previous example has a big “but” in it. What if your sim method doesn’t handle spring preload, bar preload, or jacking changes properly? Or maybe LF spring does affect roll, but you’ve just misled yourself away from that. Losing sucks, and being wrong is the geek’s version of losing.

Where do you look at the data on a given lap? Great question. This has huge implications on the quality of the fits, the impact on the setup, and the dreaded compromises that arise in any setup choice. This is trial and error, and no sane person would help a fellow competitor through this stage. Ever wonder why there are very few good SAE papers on racecar topics?

What vehicle responses are the most critical to improving lap time? When it comes to big stock cars (the vast majority of my time has been spent here), simply reducing roll angle isn’t going to make your car faster than everyone else’s, so the above example is a little off target. Simply matching dynamic crossweight to some magical number in the driver's and crew chief’s heads isn’t going to get it done either. The responses calculated are only limited by your imagination as to what defines better performance.

  • What factors were deemed critical enough to vary in the design?
  • How many were varied?

As the number of factors increases, so does the number of trials needed to capture the interactions of these factors. Think exponentially. This is where the multimode machines are making serious headway, no difference from CFD. What is important to your vehicle setup? What data about your model are clear enough to capture subtle changes? We all wish that tire data could be discreet to the point of being able to gnash our teeth endlessly over a very small pressure change. Vary inner and outer tire pressures (for a total of 8 factors and only 2 settings of each) in a full factorial DOE, and you’ve just signed up for 256 runs of simulation. Add only two more factors (let’s say front springs at only 2 levels) and that total is now 1024 runs. How fast is your PC? How accurate is that tire data in quantifying some change in the car? That may be better served by trying it with the driver strapped in and let the lap times make the call, except every major sanctioning body is trimming practice time as the years go by. When setting up your factors and their levels of variance, you have to allow a wide enough range to be helpful but not so wide that the response from one setting to the next is so different that the regression looks like a total mess. Maybe two levels of each factor aren’t enough, certainly when you are considering a spring change. No race team running for the big foam check at the end of the day brings only four spare springs! Now, consider that you decide you need to vary each of your 10 factors in 5 discreet values. That seems pretty reasonable. Put on your big boy pants, because you have just created a design with 9,765,625 simulation runs. Google fractional DOE design.

How well does your simulation capture the response of your car? As you can tell from the above discussion, this isn’t a topic to apply to your first runs in simulation. Get the validation work done, shoot holes in the data, and convince yourself that you are getting close. The alternative is wasting your time, which in this business tends to lead to unemployment.

You spent all week setting up the design and writing the script file to launch the sims and the array of CPUs burned through night and day without any interrupts in power or crashes in solution. Yeah, right, wait till you see some of the setups a five level full factorial design generates to run – you’ll be lucky if it can statically solve enough of them. On the final setup day, a decision is made to change the RF suspension geometry significantly. You now have a nice memorial to a lot of wasted time and effort. Once the setup winds its way outside of major parameters, the DOE becomes irrelevant. It’s a fact of life in this arena, you won’t be the first, second, or even 100th person this has happened to. The driver and team manager really aren’t going to want to hear about this. So, don’t bother. These are things that only other engineers are going to be sympathetic to, when we gather at the back of the garage to enjoy a Red Bull and complain to each other without giving away current projects.

DOE isn’t anything new. In reality, most of us were just waiting on hardware and software to catch up to concepts so it could be used. Most of my experience working on this topic happened between 1998 and 2004. The cost of such toys usually limits this approach to the big budget series. The tidbits and sarcastic remarks I have made all relate to working on this approach strictly in NASCAR Sprint Cup Racing at a couple of top level teams. I didn’t invent any of this. Like everything in racing, all of us claim to have invented it. My particular experiences happened while I was a part of a great group of engineers at previous teams. I have lived on every side of the race engineer role for the last decade, sometimes as an engineer helping support the race engineer’s job, sometimes as the race engineer, and sometimes as a crew chief using the race engineer to get the best out of the car, as quickly as possible. Luckily, my day job has nothing to do with simulation code, DOE, or race engineering these days, because this doesn’t even scratch the surface. I can guarantee you that all of the unknown faces in the Cup garage engineering these cars understand 100% of what was discussed here, and have for some time. Plenty of books have been written on the topic. I’ll leave you to go find them.

Thanks for reading,

Brandon Thomas
Red Bull Racing (USA)
Chief Design and Development Engineer

Tuesday, December 29, 2009

Setup Sheets, Part 2

Let's review the setup sheet layout. There are two fundamental layout concepts: by topic and by car location.

Sheets laid out by topic group similar items together. All the ride heights are together, all the shock info together, all the aero info together, and so on. For example, here's a prototype sports car sheet. This layout, although well done, omits some detail on brakes, tires, suspension geometry. It is used with "setup wheels", machined aluminum fixtures that replace real wheels and tires on the setup pad. Ride heights are calculated from measured drops to a point on top of the chassis, rather than actual measurements up from setup pad to the floor. Blue items are user input.

PDF - Print or Free DownloadXLS - Purchase Full Download

Sheets laid out by car location group information into a birds-eye view of the car. For each corner, you have alignment, tire data, springs, and so on. Information that doesn't fit that layout is placed on the center of the sheet or in a separate section. In a slightly different twist, the sheet that I use has chassis-mounted items and measurements like AR bars, ride heights, and packer gaps in the center, reducing the amount of info listed at the individual wheel. Here's a Swift 008a Formula Atlantic sheet. Note that this sheet includes some engineering calcs. It also has non-printing separate worksheets for vehicle dynamics and for shock build specs.

PDF - Print or Free DownloadXLS - Purchase Full Download

So, which layout to use? Both are popular, and both can be effective, if they are done well.

The main advantage of the topic layout is in grouping similar items together. For example, all the corner weights are in one spot, just like on the scale display. With so many different types of data to show, it can be a little scattered, unless it is carefully organized. The example posted here is one of the better ones.

The strength of the car location layout is in its ease of use. If you want to know something about the right front corner of the car, look at that part of the sheet. Some items, like brakes, corner weights, rake, or cooling, don't fall into the layout that well.

I use a layout that mixes some elements of both approaches. Go back and look at the sports car sample included in the Part 1 post, it's mine. Stuff mounted or measured at the wheels is out on the corners. Stuff mounted or measured on the chassis is down the middle. Front aero is at the front, rear aero at the rear. Gears, brakes, and weights are clumped together at their approximate location on the car. The next incarnation might get a new section for configuration file names for the data system, ECU, ABS, paddle shift system, etc.

Now, let's look at a layout for the worksheet that accompanies the car to the setup pad. This worksheet is a hands-on working document for use at the setup pad. Most of the teams I work with lack either the time, money, or resources for this to be used as a networked document on an smart phone or touch-screen PC. So, it's filled out by hand, and may or may not be scanned, depending on who needs copies and when. I like the cheap HP all-in-one printer/scanner/copier units for the trailer.

On the front, there are fields for Setdown, where we document how the car was found as it rolled off the track and onto the setup pad after preceding on-track session. The center column is used to enter the changes to make. The changes are then made, on or off the pad, and the car rolled on for adjustment. And then, the righthand column documents how the car rolled off the setup pad. On the back side, there is a worksheet for actually making the adjustments.

Yeah, I know, there's some redundency here, and opportunity to introduce error. We'll talk about this again in a later post, but cutting to the chase, I've found that a complete setup sheet doesn't work too well for calling out between-session changes. So, we do a setdown, fill in the changes, and finish the setup.

The example below is a scan of both the front and back pages of a setup worksheet after use. The links immediately above the form will download it, as well as blank versions of the front and back pages.

PDF - Print or Free Download Blank Form
XLS - Purchase Full Download Blank Form
PDF - Print and Free Download Completed Form

Tools and organization

OK, how about computing tools? Your choices are basically spreadsheet or data base. PDF forms with fillable fields don't have enough function. Spreadsheets offer plenty of formatting and calculating power, and are the near-universal solution. But, I've always wanted to try a database. The initial setup would be lot more work, but your setups would be available for the full power of database searching and reporting. I suspect that the ever-evolving nature of much racing might be responsible for the relative rarity of databases, since last year's setup is often no longer relevent. Series where you take the same basic car back to the same tracks, year after year, probably stand to benefit the most.

One thing is for sure. You have got to be diligent and organized in file naming and directory structure, or you will soon have an unworkable jumble of setup sheets files. Here's the file naming convention that I use:

Setup Seb090307 A04 P1 Start.xls

  • Sebring is the track
  • March 7, 2009 is the race date (not the creation date of the setup sheet)
  • Chassis number A04
  • This sheet shows how the car started the first official practice session

I place all the sheets for an event into a directory exclusive to that event. Use real-time archival software pointed at the location of all the setup sheets. You don't want to lose a year's worth of setups when the notebook hard drive crashes at the track.

Remembering that a setup sheet is a vehicle for communication, the next post will get into the process of using it. Once that's done, we'll dig deeper into content.

Monday, December 28, 2009

Setup Sheets, Part 1

I'm kinda excited about this multi-part series, since it's a bit of a departure from recent posts. We're going to review setup sheets in fairly complete detail, so this isn't your usual short-attention-span blog post. You'll be able to download PDF samples and working Excel spreadsheets from Scribd. Here we go...

What is a setup sheet, anyway?

In simple terms, it is a document that details the configuration and adjustment of a race car.

And what is it used for?

  • Define all the setup adjustments, like alignment, ride height, etc.
  • Specify commonly swapped parts, like springs, gear ratios, anti-roll bars, etc.
  • Document the car setup, for later reference
  • Possibly, link to analysis or simulation software to provide vehicle dynamics details
Let's talk first about general content. Future posts will cover how to actually use a setup sheet, communication issues, options for layouts, computing tools and storage, some recommendations, and more.

The simplest setup sheets are handwritten onto a basic blank form. In this guise, it is mainly a working document for crew adjustments to the car on the setup pad. It probably has no more than the following content, and maybe less, depending on what items may be non-adjustable, non-changeable, or non-existent on a specific car:

  • Ride heights
  • Spring rates
  • Anti-roll bar sizes and adjustments
  • Shock adjustments and gas pressures
  • Camber, caster, and toe settings
  • Aero adjustments, such as angles and dimensions
  • Corner weights and percentages

Here's the catch. If the setup sheet is to be a complete and unambiguous definition of how the car is configured, there is inevitably more information required. Sometimes, lots more. The possible list is endless, but here are some common items, in no particular order:

  • Bump rubber spec and packer gaps
  • Third spring and damper components and adjustments
  • Optional aero components and how they are installed or adjusted
  • Cooling configuration, both components and blanking
  • Gear ratios and differential setup
  • Optional suspension geometry and components
  • Brake components, pad/rotor material, master cylinders, bias setup
  • Multiple ride heights - aero components vs. structural/suspension
  • Specific assembly instructions - part numbers, shims, etc.
  • Tire sizes, compounds, and constructions
  • Tire pressures for both the setup pad and the grid
  • Driver weight and fuel load for the setup pad, starting fuel load for the track
  • Shock build spec
  • Part numbers or serial numbers for specific components and assemblies
  • Spring and/or pushrod installed length. Rocker ratio and position.
  • Ballast weight, configuration, position

This can get out of hand. Still, we absolutely have to be able to completely and unambiguously define how the car is expected to be configured when it rolls onto the track. At some time in the future, we need to be able to completely and unambiguously recall that configuration by reviewing the setup sheet.

Taking things a step further, there are two additional types of information that sometimes show up on setup sheets. They are specific component serial numbers, for use in part lifing, and vehicle dynamics calculations, such as wheel rates. I personally choose not to include these on my setup sheets. They aren't essential to defining the car configuration and clutter up its use by the crew. If needed, I think they should be on a separate document, or an "engineer's version" that can be separately printed.

We have to remember that a race car is always changing, and we race or test it as a snapshot in time. Some of these evolving changes are permanent, some not. They rarely seem to fit the existing format for the setup sheet. We have to decide whether, when, and how to indicate these changes on the setup sheet. I tend to mention permanent modifications in the comments section at the time they first appear, then delete them on future sheets - a solution I'll admit to being imperfect.

Sharp readers will have noticed no mention so far of engine configuration and tuning, nor of configuration options and file names for ECUs, data acquisition, traction control, no-lift shift, ABS, or any other electronic systems. Engines tend to be assembled, tuned, and maintained by a separate group which may or may not be part of the team. The electronics are typically maintained and tuned by one or more specialists, a process that can be a bit undisciplined, if nonetheless superbly executed. In an oddity of how things have evolved, the setup sheet is typically the configuration for the rest of the car.

One possible solution to some of these concerns is a "build sheet", produced either as a separate document or as a different print option, similar to what we've discussed for components and engineering data. It can include all sorts of information, like serial numbers, part numbers, modifications, file names, and so on.

So, here's my recommendation:

Use a comprehensive setup sheet that defines everything adjustable or changeable on the race car. Permanent modifications are either excluded or get a mention in the notes section at the time they are done. If it's appropriate for your situation, create a separate build sheet, either to define the car more fully or for part serial numbers. Keep engineering calcs off the main sheet. A worksheet accompanies the car to the setup pad for note-taking during the setup process.

To give you something to think about until the next post, here's a recent sports car racing prototype setup sheet. This sheet is fairly comprehensive, yet this car lacks certain suspension geometry and aero options that I've had on other recent setups. Numbers and other fields, of course, are changed to disguise the real setup. Fields calculated internally by the setup sheet show in blue. If you purchase the full XLS, you'll see the non-printing calcs for ride heights and gears.

PDF - Print or Free Download XLS - Purchase Download

Friday, December 25, 2009

Using Scribd

Hi folks.

There's a major new series coming for the Race Engineer blog. We'll be covering setup sheets in quite a bit of detail.

And, we're going to offer content for download in the Setup Sheet series, as well as in future ones. I'll be using If you're not familiar with the site, it's sort of analogous to iTunes, only for documents instead of music and video.

Every document will be available free as a PDF, either to print or download. Not free beer, not free lunch, but the best I can do in a blog.

Many will also be available for purchase in the native format, which will typically be XLS. For forms with a lot of formatting and information, like setup sheets, this will save quite a bit of time compared to creating your own version from scratch. For real engineering tools, you get all the calcs behind the visible input and results.

I puzzled some over pricing. Some of this stuff is pretty simple, but some of it represents literally days, if not weeks, of work. Radiohead's pricing strategy of letting the user decide how much to pay was intriguing, but Scribd doesn't work that way. I do need to get something for my effort, but on the other hand I want to make this stuff widely available. In the end, I decided to make the forms $10 each, unless they are really simple. You're already paying that for an album download on iTunes or Amazon. And it's waaaaay cheap compared to the time you'd spend duplicating it. Real engineering tools will be priced according to their content.

The imbedded PDFs will display a frame from Scribd. Preceding each will be two links, one to the PDF, one to the native format.

Click the PDF link to print or get the free download.

Click the native format link to purchase. There will be a big yellow button "Buy Now" on the right side of the screen which takes you to the typical online purchase dialog to enter your credit card info.

The Scribd site asks you to register to get the free download or print. Wish they didn't do that, but at least it's free. Oh well...

As a sample, here's a setup sheet for an IRL car in 1998. Left or right click on the Scribd window to activate the various controls, zoom, and so on.

PDF - Print or Free Download
XLS - Purchase Download

Please drop me a comment if all this doesn't work correctly for you.

Thursday, October 29, 2009

Vehicle Dynamics, Vol. 1 - Thinking in 3 Dimensions

At the outset, I promised that this blog wouldn't be a vehicle dynamics tutorial. Still true. Get out your Milliken, Gillespie, and even Carroll Smith's timeless "Tune to Win".

But, here's an observation.

When many race engineers and chassis tuners think about handling, they focus heavily on roll moment and roll stiffness. That's a vital topic, but it's only part of the total handling equation.

The car operates in 3-dimensional space, with forces and moments acting on it along the X, Y, and Z axes (longitudinal, lateral, and vertical to regular mortals). At any point in time, its handling is dependent on how it reacts to ALL SIX of those forces and moments. Limiting your thinking to roll only addresses lateral forces and longitudinal moments.

For example:
1-Savvy oval track engineers have long been focused on the vertical forces from both banking and aerodynamics.
2-When a car is cornering, it is rotating around its Z (vertical) axis. The are a number of very significant sources of moments around that axis, some of which are under the control of the engineer.

There is much to be learned from data acquisition. Install a steering travel potentiometer calibrated in spindle degrees. Install a 3-axis accelerometer and a yaw rate sensor. Just in the Z-axis, you can calculate understeer gradient (see Gillespie), yaw gain (yaw rate divided by steering), and stability index (see Danny Nowlan's excellent recent series in Race Car Engineering magazine). Use your imagination. There's more.

Apologies for the lack of illustrating diagrams. I'm still too busy with real work to explore the tools that Blogspot offers. Eventually.

The proverbial word to the proverbial wise should be sufficient. Go for it!

Thursday, October 22, 2009

Modifying Production Suspension Geometry

Well, after literally weeks of not having the time to do a new blog post, I was looking forward to a new post on testing. I'd even spent two whole airline flights outlining it.

But then, today, I had a rush job to prepare some modifications to the geometry of a production-based race car. And, surprisingly, that project generated the theme for a new blog entry. So, here we go...

We'll conveniently assume that you have already measured the car (probably a whole blog post of its own) and that you are already up and running with kinematics software. Bill Mitchell is an old friend, and I've used his programs for years.

The starting point to the whole process is learning what changes the race series rules will allow.

If the rules allow modifying or replacing the spindle/upright, you are most of the way to a from-scratch design. The steering geometry will be yours to invent, as well as most of the instant center-related items like camber gain and static roll center location. The series rules on inner pickup points will mainly handicap the ability to design the desired anti-squat or anti-dive, and will likely effect roll center migration.

If you're stuck with using the original upright geometry (with or without a usually tight spec on allowed modifications), then the freedom to improve the goemetry is much more constrained.

So, here's the basic situation:
-You will never be able to get a geometry with optimized roll center location and movement, camber gain, anti- geometry, steering geometry, and spring/shock motion ratio. Give up on that notion.
-The series rules and the car's original geometry will limit your options.
-It's pretty common that a geometry modification that helps you reach one of your targets will hurt your ability to reach another.

So, it follows that the MOST important job is NOT necessarily the design itself. It is, instead, successfully figuring out what change is most important for your car. For example, is it more important to lower the roll center, increase the anti-dive, or reduce the camber gain? Having decided what change is most important for your car, then it's time to review whether the almost-unavoidable tag-along changes will hurt the car, and understand how and how much they will hurt it. Finally, consistent with the theme of many of my posts, you will now hear me say that experience and education will only improve your chances of identifying what the car needs, not guarantee it.

Be prepared to be surprised. Design more than one option and test them all. This also guards against two common foibles:
-Believing that you know more about suspension goemetry than the car's designer. Maybe so, maybe not...
-Believing that you know what the car needs. Surprises abound...

Keep your mind open. Spacers, offset bushings, parts swapping, machining, moving inner pickup points, allowed tolerances, and fabrication may be among the options for revising the production geometry. Study the rules carefully.

Keep in mind that your changes should ideally be either high-confidence or reversible. It's a big plus to be able to A-B-A test them, too. If the changes require major fabrication or machining, or are irreversible, then they must be high-confidence as well.

Consider a simplified or partial version of the expected direction. For example, maybe it's an easy job to move the roll center 0.50", but a hard one to move it 1.00". But you want to move it 1.00", don't you? So, move it 0.50" and test that. If the car and driver like it, now there' s ample justification to try the more difficult second step. If not, much work and money was likely saved.

As always, geometry is just part of a total package. Be prepared to tune bars, springs, and shocks to optimize the car for the new geometry.

Good luck!

Sunday, September 13, 2009

I haven't forgotten you

Yes, I know the last new post was July 29. No, I absolutely HAVE NOT given up on the Race Engineer blog.

I have simply been covered up, in a major way, since late July. In today's business climate, that means, for me, multiple small jobs instead of one or two big ones. And, there's overhead associated with that. And, for you, an eventual blog post or two about how to get work, the new-job startup process, and juggling multiple clients.

There are also a number of new posts that are jelling in concept. Shortly, we'll commence two series, one on testing and one on setup sheets.

Hang in there, folks. Thanks for your understanding.

Wednesday, July 29, 2009

Philosophy, Vol. 1 Revisited

I'll touch on this subject one more time and then let it rest.

Just to show that our preconceived notions of what the racecar needs are often wrong, and that slavish devotion to simplistic notions of vehicle dynamics are equally wrong, it happened again this weekend at Autobahn.

One car I engineered felt like it needed a crisper turn-in with more support. But, it was worse with a slightly stiffer front bar, not better - as is often the case. Turns out it needed more low-speed bump damping and less front bar.

Another car I engineered lost front grip with a lower nose on the bump damping curve, despite expectations that it would gain front grip from that change. After all, the rear had just gained grip from a similar change.

At the next event, I may have to abandon these seemingly solid conclusions, too, if they prove to have been track-specific.

On my soapbox for the last time on this subject, for at least the next week:
-Listen to the car and the driver. They are reality. Your preconceived notions are not.
-The solution to a question may go contrary to common theory or your experience.

Thursday, July 9, 2009

Springs, Part 2

Tuning with springs

If there were any question that springs are a powerful tuning tool, we'd only have to look at the extensive spring inventory in the typical race trailer and at the amount of time the typical race car spends on the setup pad getting springs changed. A complete treatment of spring tuning would probably be a whole chapter of a book. Today, we'll just offer some fundamentals to stimulate thought.

There are two major influences that springs always have:
-Dynamic ride height changes from longitudinal load transfer,banking loads, and aero loads.
-Roll stiffness and the resulting front-to-rear distribution of roll couple
Tackling these first...

Softer springs allow more ride height and pitch change from braking and acceleration, stiffer less. For example, allowing the nose to drop more on braking should increase front downforce percentage and maybe total downforce amount, lower the height of the mass at the front of the car, and lower the front roll center. IF these work, the result is a sharper turn-in response, which in turn can lead to less mid-turn understeer (even after the nose rises) by the simple virtue of getting the turning done earlier in the corner. But, too much drop can destabilize response, shut off the front aero, move the suspension into the bump stops, cause adverse camber changes, blah, blah. A similar but different set of concerns exists for stiffer springs. And, a similar but different set of concerns exists for the rear springs. As always, the key is finding how much is just enough and how much is too much.

In roll, the springs affect both the roll stiffness and the feeling of support perceived by the driver. With front AR bars creating a fairly significant portion of front roll stiffness on most rear-drive cars, the effect of springs on roll stiffness on the front of the car is relatively subtle compared to their effect at the rear, where the AR bar is relatively soft or even disconnected. But subtle may be all you need, if the front-to-rear bar balance (a whole 'nother blog) is already good.

Of course, one reason springs are so powerful for tuning is that you get both ride height and roll stiffness changes in one tuning change. If you choose to run the same static ride height with a stiffer spring, you get more roll stiffness, less vertical movement, and higher dynamic ride height under aero/banking loads. If you choose to run a lower static ride height with a stiffer spring, you still get more roll stiffness and less vertical movement, but the dynamic ride height will depend on where you set the static. You have to choose.

But wait, there's more. We haven't gotten the full $29.95 worth yet.

Springs have to be matched to the tire. I've seen a change in rubber compound and carcass construction require as much as a 20% change in spring rate to retune a car. With serious sim work and full tire test data, we might be able to reduce the "cut and try". Outside of F1 and NASCAR, the average race team simply doesn't have the money or information, though. The tire might have different vertical or lateral stiffness, different contact patch shape, different internal damping, a compound that requires "working" the tire to generate sufficient heat, etc. There's also the issue of the extent that tire performance falls off over the tire's life.

Springs have to be matched to the track. In broad generalities, lower-grip venues require softer springs, within reason. At higher-grip venues, the car may feel sloppy and unresponsive if not supported by some combination of stiffer springs, stiffer bump damping, and/or stiffer bars. Bumpier venues may require softer springs and higher static ride heights, if a solution doesn't exist in damping. Tracks that put a premium on corner-exit traction or tracks with a persistent reputation for understeer may require a different front-to-rear spring proportion.

Springs have to be matched to the driver. Some drivers like softer, some stiffer. Engineering doesn't completely win out. If the driver isn't happy, you aren't fast. I once saw a team make a nearly 100% stiffer spring change to accomodate a new driver. He won the same number of races as the preceding driver, and was equally or more competitive in general. Same car, same tire, same tracks...

The holy grail of 7-post rig testing, the RMS variation of contact patch loads, is generally better as the springs get softer. But, we can't chase this in isolation either, as should be clear by now.

So, how do we choose spring rates and then tune them?
-Thoughtful consideration of the driver's comments
-Detailed review of the the logged data
-Careful analysis of the available tire data
-In-depth review of all the vehicle dynamics and aero data for your car
-Testing, either stand-alone or in the context of a race event
-Assessment of the track's needs

I can't over-emphasize the need to break free from dogmatic thinking. On two different cars in the last 12 months, I've improved front grip by going stiffer on front springs. Both cars were simply moving too much. On one of these, I saw too much front roll in the logged data. Fairly easy call. But, on the other, it came to a "gut level" call. Nothing in the data or the driver's comments pointed the way. On that car, I had to buck the skepticism of the drivers and the car owner. Luckily, one of the drivers was open-minded enough to go "Hey, it worked even though I thought it wouldn't".

And one detail - street car thinking says don't have the same natural frequency front and rear. See Gillespie. While many factors may well lead you away from this on a race car, I personally haven't found it to be a sin when it did happen.

Have fun!

Monday, July 6, 2009

Travel to Watkins Glen

This is as close as I'll get to personal blather on The Race Engineer. I did warn you that I'd write about life as a race engineer, and that includes travel. So, here are a few random notes about this weekend's trip to the Finger Lakes region of NY for a race at Watkins Glen.

-The dining scene in Corning has lost the old standby, London Underground.
-Tony R's, Corning's excellent new chop house, more than makes up for losing LU. Our group enjoyed steaks, seafood, and some of their Italian specialties. We also had the largest baked Alaska known to man. Not cheap, but well worth the trip.
-Ithaca/Tompkins Regional Airport is a nice alternative to Elmira.
-I finally got to tour the Corning Museum of Glass. Well worth the time.
-If you connect through DTW, the "trippy tunnel" connecting Terminal A to B and C is, well, pretty trippy.
-There is nothing in racing quite like the view from the paddock down the valley to Seneca Lake.
-Does it always rain at Watkins Glen? I've never had a totally dry weekend there.

Back again soon with another "real" post. See you then.

Tuesday, June 23, 2009

Springs, Part 1

As promised, we’re getting technical now.

This series of posts on springs will focus on vehicle dynamics and tuning issues. We’ll only delve into design and preparation issues (motion ratio, coil bind, material choices, type of spring, etc.) to the extent that they relate to vehicle dynamics and tuning.

So, you ask a race engineer, “What’s your setup on that car?” Usually, one of the very first things you will be told will be the spring rates. There’s no question that there are quite a few other setup choices that have a major influence on handling and grip. But, for nearly every race car, the choice of springs is not only powerful in its own right, but also in the effect it has on a number of other setup choices.

Everyone has their own definition of what springs do. Here’s mine:
1. Allow the wheels to move relative to the chassis
Absorb disturbances from bumps, curbs, etc.
Avoid suspending the car exclusively by the stiff and underdamped tires

2. Allow certain beneficial movement of the chassis relative to the wheels
For vehicle dynamics or aerodynamic reasons
In transient and/or relatively constant situations

3. Conversely, control the movement of the chassis relative to the wheels
Keep response to driver control inputs acceptably quick
Maintain the chassis attitude desired for aerodynamic, geometry, or weight transfer

Here’s a (probably incomplete) list of what we may eventually cover:
-Interactions and relationships with anti-roll bars and shocks.
-Bump rubbers. Either in this, or their own, series. They are, after all, springs.
-Why run stiffer? Why run softer?
-Racing series rules for minimum ride height
-Relationship of spring rates to tire vertical spring rates
-Third spring/damper/bump rubber setups. Either in this, or their own series.

To get things started, the first meaty topic in the series:

“Using natural frequency to choose and compare spring rates”

The sprung mass natural frequency of one corner of the car is:

Frequency = (1 / (2 * Pi)) * Square root (Wheel rate / Sprung mass)

See Wikipedia for a brief overview. Without fail, read what Bill and Doug Milliken wrote in their book, and Thomas Gillespie in his.

The answer is in cycles per second, abbreviated as Hz (Hertz). Don’t forget to use consistent units, subtract the unsprung weight from the setup pad corner weight, apply the proper constants to convert corner "weight" to corner "mass", and convert the spring rate to wheel rate correctly, using the motion ratio. This idealized formula assumes linear (non-progressive) springs, no friction, and no damping. Lotsa caveats there, but it will do well for the kind of coarse comparisons we have to do.

The reason why sprung mass natural frequency is important is that it lets us compare spring rates between cars. Two cars which are similar in design/race series/performance, may have differing weights and spring motion ratios, but still need a similar natural frequency due to their overriding similarity. OK, to be fair, a heavy car at a given frequency is not the same as a light car at the same frequency. But, it’s a starting point.

Let’s touch on one thing first. Some series have minimum ride height rules for tech inspection. Unless the ride height in the rules is relatively low, cars in these series frequently find themselves on artificially soft springs to allow the car to assume a lower dynamic ride height under the influence of vertical loads from banking and/or aerodynamics, as well as possibly being “pulled” down by rebound damping. That’s for later…

For oval track and road race cars running on pavement, I see four broad categories of cars:

Softer than 2.0 Hz
-Street cars and "showroom stock" racing classes. Autocross, maybe?
-Too floppy for the race track, without big crutches from the bars and shocks

Soft, from 2.0 to 3.0 Hz
-Setup emphasizes mechanical grip
-Little or no vertical load from aerodynamics or banking
-Chassis attitude changes don’t hurt, and may actually help, aerodynamics or vehicle dynamics
-Tires or track surface may have limited grip, or conversely may be just fine

Medium, from 3.0 to 4.5 Hz
-Setup is a compromise of mechanical and aerodynamic grip
-Moderate vertical loads from aerodynamics or banking
-Aero loads may be moderately sensitive to height of underside of car relative to the track
-Suspension geometry may be less than optimal, causing bad behavior with travel
-Good grip from tires and track

Stiff, from 4.5 to 8.0 Hz
-Setup is primarily influenced by aerodynamics
-The car has a high-downforce configuration, or it makes lots of downforce from high speeds
-Aerodynamic loads are highly sensitive to height of the underside of the car relative to the track

Let’s say I’ve been engineering a Daytona Prototype with front and rear motion ratios of 0.90 and I’ve found spring rates that make the car and driver happy. The team then decides to change chassis manufacturers and the new car has front and rear motion ratios of 1.05. Ignoring, for now, the effect this has on the shocks, I’m likely to get a good starting point for the new car by selecting springs that match my old natural frequencies. That’s because the old and new cars are on the same tires, and should have similar downforce amount, power output, total weight, weight distribution, etc. Of course, differences may exist in aerodynamic sensitivity to ride height, suspension geometry, stiffness of the chassis and suspension components, and more.

Nothing is ever simple, though, for my sample Daytona Prototype or for any other car. The front and rear of the car have different tasks to achieve, under different conditions. The need to control the height of an aerodynamically height-sensitive wing or splitter may lead to stiffer front springs. The need for corner exit forward traction may lead to softer rear springs. Certain tracks may have poor grip or a reputation for understeer. Our tire manufacturer may change the rubber compound and/or carcass construction. A change in damping may allow us to run stiffer or softer springs than would previously work.

The list of factors that go into finding the best spring rates is large. No doubt, that’s why it’s likely to be the first thing you hear when you ask, “What’s your setup on that car?”

Wednesday, June 17, 2009

Philosophy, Vol. 1

Here’s the first of several posts. Lacking a better one-word description, I’m calling them “Philosophy”. This series will address non-technical concepts, such as:

-Approaches to problem solving
-How to make decisions
-Common foibles and failures
-Old wives’ tales, Murphy’s law, and the like

If this list sounds too general, be assured the focus will be firmly on racing, not on folksy sermons. My caution to the reader is this – you’ll read this and say “Sure, I already knew that”. I hear you, but I don’t believe you. These are mistakes I’ve made over and over, and that I see others continue to make every day.

I’ll share my thoughts, ideas, approaches, and experience (with the usual qualifier that I’ll leave some things to the reader to figure out, and keep some key advantages for myself). I’m not presenting these as “universal truths”, merely as my outlook. You are free to disagree, in fact I welcome it. Enough blather, here’s today’s topic…

“Just because you think you have a good idea”

Three examples:
1-In “The Unfair Advantage”, Mark Donohue reckoned that he was lucky if half of things he tried on a race car actually turned out good.

2-When I was club racing, my engine guru and I decided, from our readings and musings, that the exhaust system on my car needed to be shorter. Dutifully fabbed up, the new system was a joy to the eye and ear. Several months later, it was found NOT to be a joy to the dyno. Until then, I had firmly believed in my own cleverness, not only about the exhaust system but about everything I did to the car. What an eye opener!

3-An unnamed race car, introduced in the last several years, failed to meet a major performance target. Its design was based, in part, on a fundamental concept that failed on another car some years earlier.

So, you think having the front roll center 1.0” below ground is a good idea. Or, you think qualifying on scuffed tires is a good idea. Or, you think the best aero balance will be 45% front downforce.

Here’s what’s potentially wrong with every idea you have – no matter how broad and deep your know-how and experience, there will always be unexpected and/or unknown factors. Our knowledge of vehicle dynamics and racing will always be flawed and incomplete. Those of us who complete engineering school come away with a cause-and-effect, deterministic outlook on problem solving.

The solution:
-Test new stuff, don’t just throw it on the car and pronounce it an improvement. Testing may be physical, at the track or other (wind tunnel, 7-post, shock dyno, K&C). Testing may also be virtual (CFD, FEA, etc.)
-Sometimes you don’t know if your idea is bad until you try something different.
-Keep an open mind when trying new stuff. Be prepared to give up on it.
-Learn how to recognize which new ideas have a better chance of succeeding.

Techno-junkies, thanks for bearing with me. The next post will be about race cars.

Saturday, June 13, 2009

Recommended Reading

I was thinking about writing a new post on choosing spring rates, when I realized I’d have to explain natural frequency. And I don’t want to do that, because it’s already been done, oh say, a hundred seventy three times. The idea of mentioning a good vehicle dynamics reference quickly spiraled off into a larger list of recommended reading. I’ve read all the following, and they belong in every race engineer’s library. They aren’t all in print, but hey, you’re supposed to be resourceful if you’re going to be a race engineer.

These two books offer some insight into life as a race engineer:
“Inside Racing: A Season with the PacWest CART Indycar Team”, by Paul Haney
“The Unfair Advantage”, by Mark Donohue and Paul Van Valkenburg

Books on race car technology:
Everything that Carroll Smith wrote. All of it. Really.
“Inside Racing Technology”, by Paul Haney and Jeff Braun
"Competition Car Suspension", by Allen Staniforth

“The Racing and High-Performance Tire”, by Paul Haney
“Racecar Engineering and Mechanics”, by Paul Van Valkenburgh
“Data Power”, by Buddy Fey. Yup, that’s me.
“Race Car Aerodynamics”, by Joseph Katz
“Competition Car Aerodynamics”, by Simon McBeath

Returning to sprung mass natural frequency, here are the essential vehicle dynamics references:
“Fundamentals of Vehicle Dynamics”, by Thomas Gillespie
“Race Car Vehicle Dynamics”, by Bill and Doug Milliken. The definitive masterwork.

Keep subscriptions to Racecar Engineering, Racetech, and Bernoulli magazines, and follow the regular contributions by these folks:
Mark Ortiz
Danny Nowlan
Simon McBeath
The data acquisition column by Pi Research

With apologies to Bill Mitchell and Warren Rowley, I own but haven’t read Warren’s impressive book, “An Introduction to Race Car Engineering”.

And have a look at Mike Fuller’s comprehensive web site, Mulsanne’s Corner. It’s in the links on this page.

Back to where we started, I hope you’re up on sprung mass natural frequency pretty soon, so my post on springs will make sense.

Monday, June 8, 2009

Random Notes, Vol. 1

-The sequence for tackling topics in The Race Engineer could be fairly random. They'll probably happen as I think of them and find the time to research and write.

-Topics posted in multiple installments are unlikely to be finished in a single string of posts. For instance, there will be more to the current series on race strategy, but I've got to touch on a couple of other topics first - partly because they are on my mind and fresh - but also because I keep thinking of things that need saying about strategy, meaning it's hard to just do it and call it done.

-Like a radio station, I'm open to requests. As to publishing a post - no guarantees on which, whether, and when. Shoot me an EMail if you have it, or tweet @buddyfey or send a direct message on Twitter. I'll try to figure out how to add a "Contact Buddy" feature to the blog.

-While this rank amateur learns about blogging, please bear with me. I'm trying to keep it a simple and quick, but informative, read. I'll be adding photos, videos, PDFs, etc. as I get them made and learn how to best incorporate them.

-I hope to offer some posts from qualified guest writers. The unwilling victims have yet to be informed, so if I see folks avoiding me at the race track, I'll know they are likely candidates.

Wednesday, June 3, 2009

Race Strategy, Part 2

Today, let's focus on a simple approach to race strategy. Or, how not to totally embarass yourself if you are new at it.

You need to do these things before the start of the race, whether you're a seasoned pro or calling your first race:

1-Establish your expected fuel economy under green and yellow flag conditions.
2-Calculate how many laps/minutes you can go on a tank of fuel.
3-For a car with a dash or telemetry fuel readout, confirm that readout vs. physical pump outs.
4-For sports car racing, work out a plan for when to change drivers. Account for the rules.
5-For IMSA/ACO races with tire changes after fueling, work out a plan for when to change tires.
6-Assess expected tire life. Develop a tire change strategy - which set to use when.
7-Take a solid educated guess at the number, timing, and frequency of expected caution flags.
8-Throw all the above into a pot, stir it, and work out a pit stop plan.
9-Pre-race meeting with driver(s), team manager, crew chief, etc. to discuss and firm the plan.
10-Pre-race meeting with the pit crew to brief them on the plan and contingencies.

OK, now you've got a plan and the green flag has dropped. Here are some good rules of thumb for dealing with what comes up in the race. Before I even list them, I'll say this - it's way more complicated than this simple list, and rules are indeed made to be broken. But, you've got to start somewhere. This simple approach got me through my first season without any major embarassments.

1-When in doubt, pit when the leaders pit. They have likely worked it out from both analytics and experience. Note that this is not a subsitute for working out your own strategy, but it will serve in a pinch when you are presented with a situation where you don't have a contingency.
2-Generally, you'd prefer to stop as few times as possible. Less time or positions lost in the pits, less opportunities for pit stop problems.
3-Keep close track of your actual fuel economy and adjust expected pit windows to fit.
4-Green flag pit stops are more risky. Any problem costs you more track position.
5-If your series allows pace car waveby, learn how to recognize if you are eligible and don't forfeit the opportunity by pitting, unless there's a compelling reason.
6-Use the radio to communicate your intentions (what's going to happen with fuel, tires, and driver changes) with the crew. It's loud out there, and yelling and hand-waving just raises the tension level.
7-Keep everyone, drivers and crew, aware of when the next expected pit stop will be.
8-Have the exiting driver report to you immediately after he/she leaves the car. Get a quick debrief and radio the new driver with any information he/she needs. Timeliness is critical.
9-After each pit stop, exhale and then take a minute to rethink your strategy for the remainder of the race. If nothing changes, great. If there's a difference, communicate with the crew.
10-Keep your radio persona calm. Everyone is counting on you to be cool under pressure.

That's it for this week. More to come on strategy.

Friday, May 22, 2009

Race Strategy, Part 1

Racecar Engineering magazine has had several well-written articles over the last few years covering race strategy. They are intriguing, but they focus on the Formula 1 challenge of how to run a specified race distance in the minimum amount of time. There's validity in this, but the US series for Indy cars, NASCAR, and sports car racing under Grand Am and ALMS offer full-course yellow flags with pace cars, rules on when the pits are closed and open, rules on when drivers must be changed, rules and practical decisions on tire life and fuel load, even "waveby" rules on being allowed to pass the pace car. And, in the sports car series, there is more than one class in the race. Clearly, there is more to a successful strategy than the quickest time to distance.

I'll be upfront here - I'm not telling what my strategies are, or even the processes that led to them. But, I will offer a few comments to kick off the brain cells.

  • "Seat of the pants" calls made on pit lane, no matter what your experience level, are rarely as good as the calls made by someone who has thought through the possible scenarios and how to react to each possibility.
  • The best race strategies may seem counterintuitive at first review. Not that I'm advocating contrarian thinking just for the sake of being different. That rarely works. You have to know WHY you want to be different, and not get so caught up in the uniqueness of your new strategy that you can't predict its potential weaknesses.
  • The race will be unpredictable and your strategy will likely have to evolve. When a revised situation presents itself, do you already know how you will adapt to it? If not, you're back to seat of the pants.
  • It helps to develop tools and methodology. 'Nuff said about that.

Here is a pop quiz question:
If the race is 250 miles or 2 Hr 45 Min (whichever comes first), how does your race strategy change for a time-limited race?

Thursday, May 21, 2009

The Race Engineer's Blog begins

Hello to all. And at this point, probably hello to a very few folks. Welcome to my blog.

I'll be sharing all sorts of stuff, on what will probably be an irregular basis. Who knows what will eventually emerge, but at this point I visualize:
  • Technical discussions about race cars, most likely focused on anything other than the engine.
  • The race engineer's role in racing, testing, and at the shop
  • Day-to-day life as a race engineer. Travel, reviews of actual events, career, etc.
  • Personal views on racing topics
  • Developing trends of all sorts

Race engineers are notorious for not sharing what they know for fear of a competitor taking advantage of it. I'm also like that to some extent, but I'm comfortable with explaining more than most. I've left a trail of information and tools with former race teams. I won't mislead anyone by posting outdated or false info.

That's about it for now. Away we go!