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!


  1. Please, please let us know your methods and pointers for measuring a production suspension for input, for example, in to Bill Mitchell's excellent program. Doing that properly is the key to making any other changes, and pointers for us newbies would be very helpful.


  2. Eric, the topic of measuring cars could easily fill an entire post. Ouch!

    For now, here are a few ideas that spring to mind, in no particular order.

    1-It's hard to measure a car with wheels on, but it's necessary to get the suspension to its loaded position with wheels off. Take some care in doing that.
    2-Finding the center of production ball joints is difficult, but really important.
    3-You'll probably have to make up a spreadsheet to do some conversions from local coordinates to global. Some things are easier to measure and translate than to measure globally.
    4-IIRC, fore-aft measurements aren't as critical as heights and widths, except for caster and trail.
    5-Do some sanity checks. Make sure the various components add up as they should. Things like upright geometry, link lengths, etc.
    6-You'll probably need to make up some special tools. I've had cones for ball joint axis, J-hooks to reach inaccessible points, sliding scales on the floor, etc.
    7-Work hard to find the true centerline of the car. I use midway between the inner pickups of the lower A-frames. Non-symmetric oval track cars present a special challenge in this respect. Sometimes you just have to set an arbitrary center for them.

    There's probably a lot more, but that's a quick shot at it.