Question 911 front strut steering knuckle, bump steer Options

[Krieger]

I know there are shops out there that raise the spindle on bilstein struts then heat and bend the steering knuckle down to help with bump steer. I plan on lowering the front suspension about 1.5" by using the adjuster. I have boge struts and want to know if anyone has done just the heating and bending of the steering knuckle, does it work to counteract bump steer, is it hard to do acurately?

[J P Stein]

FWIW....My thinking is you need to approximately split the difference in the arc that the A arm & tie rods form during vertical travel and use that as your static height if you wanna go low.

For instance, if you are using 2 inches of suspension travel in your shocks, you would want 1 inch below parallel (to the ground) at the ball joint for static height. This would then give 1 inch above parallel at full compression. Actually, shading that down would be better..maybe as much as a half inch or so...that way you continue to gain negative camber for most of the stroke.

IMO, the tie rod should never go above parallel to the ground throughout the stroke....OK, maybe just a bit.... .200/.300". The bump steer you feel in the steering wheel is actually toe change. The tie rod "pushes back" as it (effectively) lengthens during compression....the tire is changing line. If it goes above parallel it shortens (effectively) and pulls. If you are turning into the force, you can handle a push back as you are loaded up against it....a pull goes the same way and that can cause you & the car to get upset....you then pull back against the pull....the effect ceases then you push again. (IMG:style_emoticons/default/blink.gif) Bump steer isn't much of a problem going in a straight line.....unless you hit a curb or something.....that's a good way to break some fingers.

When measuring bump steer, you are actually checking for toe change caused by (in front at least) the tie rod moving thru an arc. Lengthening & shortening.
As Chris says, you want to minimize that effect and if you can't, do the least harm. The center of the arc is at the steering rack. When the tie rod is parallel to the horizontal center line of the rack is where/when the tie rod is longest. Andy's drawing.....

All this is of lesser importance in a street car. You're not operating out there on the edge like in a race car. Take your street car to the track or AX.....then it matters. That's pretty much true with lowering also. On a race car it's a matter of lowering the CG of the car and you do what you have to do to get there. On a street car it's a matter of looking like a race car. (IMG:style_emoticons/default/biggrin.gif)