Bump Steer

[Jay Mann]

I have raised the car an inch to allow for the road conditions and parking lot entrances in the Edmonton area. At elevated speeds on a less than smooth road the handling of the car was spooky in the bumpy areas. Since I changed the height of the car from the design spec, I suspected bumpsteer. Tim Whittaker at Kootenai Valley Customs sent me an early version of his bumpsteer kit. I installed the kit, I had to shorten the tie rods slightly to get proper adjustment, and did some initial testing to see how much effect the kit has. I removed the front wheels, disconnected the shocks & springs and locked the steering. I attached a laser to the vertical fender mount and shone it on the wall, approximately 4 ft away. As I moved the suspension up and down I marked where the laser pointed on a paper. The result is shown below. Raising the tie rod end definitely reduces the bumpsteer output at the new ride height. I have do more testing in the spring to get the optimum shim height. The next step will be to compare the path of the laser when the steering is disconnected and the spindle is locked.

Tim's kit was very easy to install, in addition to adjusting for bumpsteer, it makes setting the toe in much easier and looks great.

2478924790

[skullandbones]

Jay,

I know it's early but it looks like you have reduced the bump steer except for the very extreme is that what this is showing (am I interpreting the dots correctly)?

[Jay Mann]

Jay,

I know it's early but it looks like you have reduced the bump steer except for the very extreme is that what this is showing (am I interpreting the dots correctly)?

The left plot is with no spacer, the plot to the far right is the longer spacer. I think the bumpsteer can be tuned out for a certain amount of stroke up and down, but as the stroke gets farther from the neutral point some bump steer is inevitable. I will not be able to work on the car until April.

[skullandbones]

I thought that was what I was seeing. So the prelim looks good. I think it was CraigS who was doing something similar to check his bumpsteer with a laser pointer. Looking forward to seeing your results. Thanks, WEK.

[Jim Schenck]

Jay,

Your bumpsteer measurement is being thrown off by the scrub of the suspension as it moves up and down. The upper and lower control arms travel in an arc causing the wheel to travel in and out relative to the chassis and that extra distance is altering your measurement. If you set your bumpsteer to zero out this affect you will actually have quite a bit of angle change as the suspension moves up and down. The easiest way to measure the steering change is to check toe at every ride height increment with a set of toe plates or very straight pieces of tubing, and when you do I would leave just a shade of toe out as the car compresses as it help with high speed stability on turn in. You can also check it much more accurately against a flat surface that cannot move and a dial caliper on each end of the wheel but that requires more setup time and some equiptment not everyone has access to.

[Jay Mann]

Jim,

Thanks for the comments. Your input is very helpful. The initial plot was to see how much the bumpsteer changes with spacers. I understand the scrub of the suspension movement. I was planning to plot the scrub by stroking the suspension with the steering arm disconnected and the spindle secured from moving. This would give me the 'base scrub'. I would then attach the steering arm and set the toe. Next, stroke the suspension to see how much the motion varied from the plot and change the spacers to match the plot as close as possible. This should be very accurate depending on the distance of the laser to the wall. The final check will be to use a laser on each fender mount, ( rather than tubes), put the front wheels on pivots ( tiles with grease between them) and lift the car with a bottle jack through the suspension stroke to see how the toe changes. Then repeat the process for fine tuning if needed. Any comments would be greatly appreciated.


Jay,

Your bumpsteer measurement is being thrown off by the scrub of the suspension as it moves up and down. The upper and lower control arms travel in an arc causing the wheel to travel in and out relative to the chassis and that extra distance is altering your measurement. If you set your bumpsteer to zero out this affect you will actually have quite a bit of angle change as the suspension moves up and down. The easiest way to measure the steering change is to check toe at every ride height increment with a set of toe plates or very straight pieces of tubing, and when you do I would leave just a shade of toe out as the car compresses as it help with high speed stability on turn in. You can also check it much more accurately against a flat surface that cannot move and a dial caliper on each end of the wheel but that requires more setup time and some equiptment not everyone has access to.

[Jay Mann]

With the front shocks disconnected, I lifted the car up and down to simulate bumps and holes. I set car on wooden spacers to ensure equal height on both sides. After adjusting the Tim Whittaker bump steer kit, I was able to get the wheel movement down to +/- 1/16 of an inch at 41 inches in front of the car when moving up and down 1 1/2" each way. The car is handling the bumps very well.

[stack]

Jay Mann can you take a pic of what spacers you used?

mark