Measuring bump steer.

[TBART70]

Is there a crude home way to measure bump steer to get an idea of how bad it might be. Marcus did some calculations on my chassis after I made changes and he said it was so so. I know he is basing it off my measurements and it can only be checked accurately on the car. I was trying to see where I'm at. I will buy a gauge eventually but was hoping to see if it is way out, it does not look right to me at this point. I do not think I can move anything around to fix it, the bump steer kit does not have any room left on from what I can see this going. This thing just keeps getting on my nerves every time I work on a problem.(some I created myself). Time to go back to the stock chassis and take the loss.

thanks for any info.

[David Pozzi]

Easiest way is to put a straight piece of wood or steel against the side of each tire about 3" off the floor. put two tape measures across them under the car, from side to side. Space them about 25" apart, a normal tire height. The hard part is to raise the car with a jack in 1" increments and measure toe change, the jack get's in the way of the front tape. If you can raise the car and place it on blocks before you start, this get's easier to do. Or put a jack on each frame rail behind each wheel.

To lower the car for bump measurements, open the hood, put padding down then add weight evenly over the radiator support area. I use old cyl heads, flywheels, etc.

To measure height, put a piece of masking tape on the top center of the wheel well, make a mark next to a good visible whole number on the ruler.
David

[TBART70]

Thanks David, the car is still in pieces so it won't be hard to work around, the coilovers are easy to remove. I can jack and drop the suspension through 6+ inches of travel. I can visually see it move which worries me!

[David Pozzi]

If it's down that far, you can do one wheel separately, it's easier to deal with one side at a time. Raise the car on jack stands, remove spring/shock and anti roll bar. Put a jack under the lower control arm. Make two simple frames out of angle iron or even better, square tube 3/4" or 1" but even plywood will work for a cheapie. Make a base, a triangle works well. connect it to a vertical frame that is in a rectangle shape. Connect the two parts via a piano hinge. Place the piano hinge parallel to the car's center line. This is crucial to getting a good measurement. Place a bare wheel on the spindle, then a c clamp a flat piece of aluminum 1/4", or thicker masonite. The plate should be as wide as the tire tread plus at least 2". The vertical part of the frame should also have a width the same as the tire tread, -around 25"-26"

You place the triangle on the floor, add weights square it to the chassis line. tip the rectangle piece against the wheel with plate clamped to it. At this point you can either make measurements of the gap that develops as you run the suspension up or down, one side will always touch the plate. or you can put a metal threaded stud on one side, a dial indicator on the other and make easier readings. If you use the second method, make sure to start the dial indicator at half travel and make the screw the same length so your wheel is parallel to the hinge and the hinge is parallel to the car centerline.

Hope this is all clear, I'll look for a link to something similar.

If you plot out a few inches of movement on a graph, it can tell you which direction to move the rod end and if the steering link length is correct.
David
https://static1.pt-content.com/images/pt/2008/12/Bumpsteergauge2-1.jpg

[TBART70]

Thanks, I'll rig this up like Macgyver.

[David Pozzi]

The important part is squaring it up to the car centerline. If you use a 1" travel dial indicator on one side, set a threaded screw at the same length as half travel of the indicator, that is your setup point. You can square it against a tool box or wall to get it adjusted. I loaned my rig out once, and they had it all screwed up (screw way too short) and their readings were all off!

Once you plot it the plot will show you if your tie rod links are too long or short, or the outer or inner ends are too high or low.

I had a friend with a formula car, I could stand on the frame and bounce it up and down and see the wheels changing toe! He'd hit a bump, and change lanes on the race track!!! scary!
David