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!