quote:Originally posted by PanteraDoug:
Your '73 should already have the shims installed from the factory. Certainly '74's do.
Measuring bump steer is a PITA. You need to move the suspension through it's entire travel to measure the changes in toe in/out.
Generally speaking this entails removing the shock and spring so that you have easy free access to do this.
I don't think it is possible to completely eliminate it. Just reduce it.
On the Pantera it is more significant with a bias ply tire like the original Arriva Goodyear where the walls don't flex much.
On a radial tire, the tire works by allowing the wall of the tire to flex. A controlled flex.
That flexing is going to be likely much much greater then the toe changes.
Also, by changing the spring rates and keeping the car away from maximum suspension travel, which is where the maximum toe change likely is, you also reduce the change.
Unless you are running a wide body car with 10 inch wide (or wider) front wheels, this is all insignificant.
If you are racing on a big track and entering turns at like 170 mph, then this becomes very significant because you are trying to hit a very specific line through the turn other wise you will spin the car.
Race cars use different tires, spring and shock rates which coordinate with this need to eliminate or reduce the toe change.
On a street car with radial tires you aren't even going to notice the toe change unless you hit a bad pot hole and the suspension suddenly drops 12" or so.
Measuring the toe change has nothing or very little to do with the machine...just the use of the platform maybe as a lift?
Most cars live with no problem with FRONT toe changes.
On an IRS car, toe change in the rear can be lethal. You need to maintain toe in there at the risk of life and death if you don't.
What will happen if you suddenly get rear toe out, the rear will steer and if you are lucky you will just spin the car and not hit anything or run off of the pavement into a soft shoulder and roll the car.
Irregardless, toe IN is always your friend.
I was able to achieve zero bump steer by modifying the width of the steering rack. It's not something many would do.
http://www.rc-tech.net/pantera1/bump/bump.htm
The bump steer shim basically moves the bump steer arc. It puts the arc in a position where bump see's far less change.
Instead of an immediate divergence it spends more time crossing the apex. The blue line represents the start point. It does make droop worse which is pretty much a non issue.
As far as Akerman, you're not going to change the rack enough for it to change much so that's a non issue. You can't really move the rack enough in a Pantera forward or back to even address Akerman.