Dirt Setup Contradictions
From your article on chassis setup fixes ("The Evolution of Stock Car Setups," March '11), I quote, "Change rear spring rates. Softening the right rear spring, and/or stiffening the left rear spring will increase the rear roll angle and will tighten the car, as will softening both rear springs. The inverse is true, stiffening the RR spring and/or softening the LR spring will loosen the car."

For mid-corner tuning this seems to be the opposite to what popular chassis builders post on their websites. Shaw, GRT, and Warrior say that more RF and/or LR spring loosens the car entry, but not to tune with springs for the middle. Add stagger and/or raise the J/Panhard bar, and more.

Why is this so? It seems that if the car was good in the middle but tight on entry, for example, then changing springs in the direction they suggest to loosen entry would tighten the middle by your formula. Can you explain? As I would expect, looser entry would be looser middle. Cheers.

Trevor

Trevor,

If you look at what Warrior has to say, it's correct in that the changes for exit coincide with what we say to tighten and loosen the car—i.e., to tighten the car, decrease the RR spring rate. Yes, these changes will affect the middle, but most dirt tracks don't offer much in the way of a middle portion as we traditionally know it.

The car is either entering or exiting the turns. It's only on the longer and faster tracks where the car spends any significant time in the middle at what we refer to as steady state conditions.

The dynamics of entry are far different for dirt cars than what we see on asphalt, and in some ways similar. Hard entry on asphalt is never good and ruins the rest of the turn. Hard entry on dirt is common and many of our problems stem from rapid changes in camber in the front tires.

It's the control of camber change that concerns most manufacturers and top racers. If the car is tight on entry, it may be caused by the RF diving too quickly and the camber changing too quickly and giving up much of the contact patch. This is opposite to what we would do to correct a tight car, and we need to consider the effect of changes we make on other portions of the turns.

A stiffer spring in the RF will slow the travel down and cause less movement overall, reducing the camber change and allowing more contact patch loss. More and more dirt racers are utilizing antidive at the RF to slow the movement. Excessive antidive can serve to lock up the frontend and that's why most builders don't openly recommend that. In moderation, it helps.