Tuesday, August 30, 2011

Trackside Tuning - Who Wins?

Here’s a common challenge when tuning a race car from trackside – you want a particular improvement, but you are limited to a specific choice of possible changes or adjustments. It’s not unusual for one change have several possible effects.

For example, let’s say the car is unstable on corner entry, due to some oversteer on initial turn-in. Let’s say you’ve decided to try to cure this by adding some anti-dive to the front suspension geometry.

In an ideal world, your car will have finely adjustable heights at both the fore and aft inboard pickup points of the upper and lower A-frames. You will whip out the laptop, make a quick run of your kinematics software, and away you go.

But, it’s the real world. Let’s say the only trackside adjustment available is the height of the inboard pickup at the rear of the upper A-frame.

See where this leads – adjusting that inner pickup point will not only change the anti-dive, it will also change the roll center height, the camber gain, and the caster gain. Will those changes work for you or against you?

In a fit of indecision, you decide to abandon the notion of adding anti-dive, and just make a shock adjustment to prop up the front end with a little low speed bump damping. Uh oh, trouble again. More low speed damping could indeed prop up the front, but then again, it could possibly make the entry oversteer even worse by sharpening the turn-in.

Things can get uglier. A stiffer front anti-roll bar, raising the front roll center, stiffer front springs, less front downforce are all changes with more than one possible outcome.

So, here’s the question: Who wins?

First, you’ve got to identify all the possible effects of the change you’re considering. Then, you’ve got to identify the magnitude of difference you are making on each of those effects. After that, you’ve got to decide how powerful each effect is on handling. Finally, and perhaps most importantly, you’ve got to assess whether the effects all work in the direction you want to go, or if some effects work against your desired result, or even in some wholly new direction.

If all the possible effects work in the same tuning direction, you have a winner! The probability of getting the desired result is pretty good, unless you commit the error of making too big a change and “going over center”.

The fun starts when some effects work against others. Using the anti-dive example, let’s say the only adjustment available to us is the height of the forward inner pickup point of the upper A-frame. If you raise it to increase anti-dive, you also lower the front roll center, possibly increasing front grip and working against your desired result. Who wins?

The danger…

As you stand there on pit lane, with a vacant look on your face, considering the possibilities, the driver and mechanics are growing increasingly impatient.

There are two ways to avoid this happening. One way is analysis. Work through the changes possible on your car ahead of time and consider your options. Hand calculations, vehicle dynamics analysis, and simulation runs can all be helpful. The other way is to scan various changes on your car and learn how it reacts, preferably doing this on an open test day. Both will help with a quick “gut call” on pit lane in the middle of a busy practice session on a race weekend.