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.
-Preload

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. http://en.wikipedia.org/wiki/Damping. 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?”

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.

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.

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.

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.