While the body is off it makes sense to do any frame mods that makes sense. So what about bump-steer. Many folks (web search results) claim that if the inner and outer pivots for control arms and steering are on the red lines, then there will be no bump-steer. The statement must be incomplete since you could make dramatic movements of the tie rod end (yellow) and get radical bump steer. Maybe the angle of the steering link needs to be the proportional to the upper and lower control arm angles?
In any event, moving the steering rack up does get the rack outer ball closer to the inner red line.
Both in the manual drawing and photo the steering rack inner pivot is slightly outside the red line and the tie rod pivot is also outside the it’s red line. The angle of the steering arm is roughly the same as the lower control arm.
This is on a low nose, 1970 S2 with 205/60 tires on 6” magnesium Minilites and a Spyder frame. Getting consistent measurements is a pain. This is what I did.
• Remove the sway bar.
• Find the center of the frame by measuring between suspension pivot bolts.
• Find the center of the rack by measuring between the outer ball joints (take off the protective boots and centralize the steering first, you may need to clean out the old oil / grease to feel this).
• Fabricate some sort of moveable platform for the front tires. I used two sheets of wood with about 5 sections of ½” copper pipe between. This allows the wheel to move freely while the frame is moved up and down.
• Remove the springs from the shocks and re-install. I left the bumpers in to set the location of the most compressed suspension.
• I chose to set the static toe-in (about 1/8”) for the frame height that results in a 0 degree angle (between suspension bolts) for the lower control arm (6”).
• Fabricate some sort of fixture to attach to the wheel (not tire) to allow toe in measurement. I used some aluminum scrap to make an inverted T that is attached to the wheel with springs. Three long screws set the location against the rim. Two slots are cut at the lowest corners of the T to secure a steel tape.
This is not the same as the bump typical steer gauge method that is usually used but it seems to work OK and was just an extension of what I was doing for setting toe-in. It also provides an easy way to relate bump-steer to chassis position.
The chart below shows the results. The horizontal scale is the distance between ground and the bottom of the center of the frame T. The static ride height is 6” with the lower link parallel to the ground.
I’ll freely admit to not having any specific issues regarding bump-steer (and probably not being able to recognize it if I had it). I did have some issues with twitchy / wandering steering, at highway speeds, but always attributed it to poorly set front and rear toe-in and a little bit of aerodynamic lifting.
The “stock average” line actually looks pretty good. A small amount of static toe-in with additional toe-in when on the brakes. My take away is that it is good enough for regular driving and autocross for fun. It does go to toe-out when lifted over an inch from static but that would be the un-weighted tire in a turn or when flying. Both situations are don’t matter and don’t care.
I tried a few alternate positions since the general drift, on this forum, indicates a slightly lifted steering rack will help. For my car, anything over .2” would be a disaster. A small lift of .17” does extend the range of “good” toe-in further into the unweighted / flying area but it is not clear if this would be noticed.
I also tried moving the tie rod end inward about .125” (spacers under the control arm). This did move the tie rod end pivot closer to the line between the trunnion pivot and ball joint but did nothing to improve bump steer. I’m not sure why this happened. About .125” in the maximum easily done with spacers. Moving closer to the “optimal” red line would be much more effort so I stopped working that line of thought.
What do you guys think?
It would be easy to fabricate a platform to lift the rack .17” but would I notice the change? Should I just leave where it is since it has served me well enough for over 80,000 miles? Several folks have implemented lifting the steering rack to “correct” bump steer. Did this result in improved feel / performance?
Thanx
Ron