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Mark Ortiz Automotive is a chassis consulting service primarily serving oval track and road racers. This newsletter is a free service intended to benefit racers and enthusiasts by offering useful insights into chassis engineering and answers to questions. Readers may mail questions to: 155 Wankel Dr., Kannapolis, NC 28083-8200; submit questions by phone at 704-933-8876; or submit questions by e-mail to: email@example.com. Please note that this is a new e-mail address. Readers are invited to subscribe to this newsletter by e-mail. Just e-mail me and request to be added to the list.
CHASING A PUSH
This month we are departing a bit from the usual format of a single answer to a single question, and presenting something more closely resembling a series of messages between me and a client. Ordinarily, such exchanges occur in confidence, but this client consented in advance to publication. I have edited and added to the material slightly for publication. Client's words are in italics.
I work with a road-racing RX-8, and recently we have been experiencing high front tire wear. It was never previously an issue, but in preparation for a recent race, a softer front anti-roll bar was fitted to the car and a bit of rake was added. Our front tire wear was measurably poorer at that event. It is my suspicion that the softer front bar resulted in less diagonal weight transfer to the rear and consequently, more front tire wear. Do you have any thoughts on the subject?
Anti-roll bars do not transfer wheel load diagonally in the sense of transferring it from one wheel of a diagonal pair to the other, and they do not transfer it front to rear. We might say they transfer wheel load diagonally in the sense that they transfer it from one diagonal pair to the other diagonal pair. Putting it another way, they change the dynamic diagonal percentage, but not the diagonal front, rear, right, or left percentage.
The RX-8 has double wishbone suspension in front, unlike all RX-7ís, correct?
A couple of questions back to you: did you increase the rake by raising the rear, or by lowering the front? How do your static camber and toe settings compare, before and after the change? Where do the front tires wear, i.e. what part of the tread?
Ordinarily, a softer front bar, or a stiffer rear, helps front grip and front tire wear. Does the car in fact have less understeer?
The chassis that we are running indeed has a double wishbone front suspension. When we increased the rake, we added height in the rear; we try to run the front as low as possible at all times. We were sure to set the camber and toe to the same setting as before our changes. We are usually able to achieve very high setup accuracy. The tire wear was strikingly even across its full width. As far as the car's balance, we have been experiencing mid-corner understeer for the entire life of the car, and that did not go away with these changes. It didn't get any worse either though.
The fact that the understeer wasnít affected by the a/r bar change, together with the fact that it is most evident mid-turn, suggests that something in your front end is running out of travel at full roll. The coilover or shock could be bottoming, or it could be something else. Other things that can happen in lowered cars, particularly when youíve found a bit more bump travel at the shock, include ball joints (usually upper) running out of travel, and control arms (usually lower) hitting something, like the frame. Sometimes tie rods or a/r bars hit things.
Do you have travel indicators on your shocks? If so, and if they donít indicate that the shock is bottoming, take out the springs, disconnect one end of the a/r bar, and move each front suspension through its indicated travel with a jack, then a bit beyond that. See if anything binds or hits. See if the steering still works freely.
If everything checks out at full compression, try the same thing, for full droop. In lowered cars, it is less common for the inside wheel to top out than for something to bottom on the outside one, but this can happen in cases where a shorter shock has been used to get more bump travel.
There is one other possibility, particularly if you are satisfied that nothing is binding or bottoming.
In some cases, mid-turn push can be caused by the brakes failing to release as they should, and dragging for a time following trailbraking. This can even be driver-induced, when a left-foot braker unwittingly fails to release the pedal completely. If you can rule out the driver-induced situation, it can be a bit hard to tell if the brakes are releasing slowly. One quick test is to try hard cornering, not immediately preceded by braking. That is, either test on a skidpad, or drive into a turn fast enough to put the car at the limit going through the turn, but approach the turn at that speed so you donít have to brake. If the car still pushes, you can probably rule out the brakes. If the push does not appear when you don't brake immediately before, suspect dragging brakes.