I've attached a picture showing the widths of the upper A-arm, lower A-arm and rack arm pivot points. My question is, what should I expect in the way of bump steer? Do I need to go to a shorter rack? The picture only shows the widths, I didn't measure the heights.
Alan, I think you will have big bump steer problems here. Imagine that you have the lower control arm in perfect alignment with the rack. Then compare the arc of the control arm with the arc of the steering link. The steering link is much shorter, thus the arc of the steering link will be less. When you hit a bump toe in will be increased at that wheel and create bump steer. Then again I am not a suspension engineer but the geometry you describe does not make sense to me.
Alan what you want to do is measure the distance between the lower control arm pivot points front and rear.Since they are on a angle you need to split the difference to find the true pivot center line.If I remember right for a 68-72 that should be around 22 inches.Now you need to order the rack shortened to 22 inch pivot length.RJ race cars sells shotened racks.Next you need to install the rack so the tie rods are level with the lower control arm.This should take care of your bump steer issues.your upper control arm pivot won't have anything to do with toe change but it will have something to do with camber change over your travel.I ended up moving my upper control arm up 1.5 inches to help correct my camber curve.My car is very low so this may not be needed with you car.If you have any ? feel free to call me I have dealt with this a few times 708-756-7747
Thanks for the info. This is a Pinto suspension and the lower arm isn't the same as an A-body, I think it moves perpendicular to the frame. o No: I went to RJ race cars website and the narrowed rack they show should be perfect. I did a quickie check of the bump steer and came up with the chart below. (Checked one side to the frame and subtracted the highest reading from all the measurements, then multiplied by 2 for total toe) If I get the time tommorrow I'll see if I can scoot the rack over a couple of inches and check everything again to see if it gets better.
When I did mine I ended up with 1/4 total bumpsteer over 5 inches of travel.My rack is mounted pretty far forward to clear blower pullies since the motor sits very low in the chassis.It took some ciphering to get it perfect.I feel if I could get the rack back farther I could have got close to zero.For about 2.5 inches of travel it has no toe change which in the long run for a high hp chassis car that alot of travel.
I moved the rack over 2.25" and checked one side again, and it reduced the bump steer quite a bit at the travel limits. Looks like I need to spend some money! ou:
