Sticky #4: Alignment and Bump Steer

I just reinstalled my rebuilt front a arms. I did the Pat- Johnny add caster 3/4 inch arm straighting. I like the way it steers but when I hit a bump I feel it a lot more in the steering wheel. When I think of "bumpsteer" I think of it in a corner. What is happening on the straight on bumps?

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When you change caster you are altering the steering "steering axis inclination" or the axis on which the spindle is turning. Adding caster brings this axis a little closer to parallel with the ground effectively making you feel it more in the steering when the suspension hits a bump. If this axis was completely perpendicular with the ground you would feel no steering movement at all on a bump but the car would be a unstable at speed. I would imagine this could be affected by scrub as well. Hopefully I explained this ok. This is also only relevant if ONE tire was hitting a bump independently of the other as they would cancel each other if say you went straight over a speed bump..
.....or to put it another way, when your car hits a bump with one wheel, as that wheel rises it pulls on its steering arm and changes toe-in, which you feel as an unstable momentary twitch. Bump steer corrections are completely different from adding caster for better high-speed stability (with both wheels on smooth pavement). You can adjust but not entirely eliminate bump steer by lowering the rack, or by raising the steering arms.
Am I right in assuiming that I have eliminated "some" bump steer by leaning back the top and thus raising up the steering arm, making it more on the same plane as the rack? Blane, You know I hit the bump at a bridge and felt nothing, I guess your right about the canceling effect... MMMMM... thanks Bill 1362
quote:
Originally posted by lastpushbutton:
I just reinstalled my rebuilt front a arms. I did the Pat- Johnny add caster 3/4 inch arm straighting. I like the way it steers but when I hit a bump I feel it a lot more in the steering wheel. When I think of "bumpsteer" I think of it in a corner. What is happening on the straight on bumps?


Here is detailed information on Pantera Place that explains bump-steering http://www.panteraplace.com/page135.htm that will help you understand what is happing and why adding caster made it happen.

Mike
Hi Bill,

Did you add the camber lock also? As you have a narrow car try setting the camber at 1/2-degree negative. The toe in at 1/16th" and the castor at 5-degrees positive. Do that and it will steer beautifully.
You have got to get rid of those dangerous brakes!

Johnny
Hi Jhonny, I recently bought a 1986 GT5-S front and rear A-arms with the brake systems to put on my 1971 #2012 project. Is it all the info needed to get the front aligment done with wider wheels? Have you the spec for the rear aligment too? I want to do a group 4 clone...
Serge (the ¨Québécois¨ who bought few nice body parts from you)
Hi Serge,

Your GT5S A arms are the same as the 70's cars. You will need to make the top arm wider to get the castor you need. On the wide cars I have found 6 degrees of castor works well. I set the toe in a 1/8th and the camber at 1/2 degree negative (in at the top). On the rear I set the toe at zero and the camber at 1/4 degree negative.
Follow Jacks advice for bump steer on Pantera place.

Johnny
WOW! I made new shims and did what Johnny said. 9mm in front of the upper ball joint and replaced the 3/4 spacer. It is amazing that such little change, makes such a big difference. The car drives much better and I don't get the sterring wheel bounce I had with the larger spacer. Any body need some nice 3/4 ball joint spacers? Bill 1362

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Bill,

How goes it!
I thought you could use this,
http://www.longacreracing.com/....asp?id=1422&catid=5
I bought one and it is absolutely bitchen, road racers use them for setting up in the Pits.
I set up my 67 SS Nova after a total tear down and it came in perfect, straight steering and flat wear, by the way setting the camber, caster & toe is not precise, degree's are like fractions compared to thousands of an inch or minute's or seconds of a degree.
you can use a tape measure or build your own I.D. mic's for the toe.

Anyways I like it cause know I don't have to use a shop.

Mark
What did you replace the 19mm spacer with? Or did you shave it down to suit the application?

Johnny, those alignment specs given for the wider car, would they suit my car even though it has standard control arms but bloody big wheels?
Robert,

In a word 'yes' as all the body styles have the same stock a-arms, the extra width in a wide body car is obtained purely by the wheel width and offset, which is one of the primary reasons the wide body cars drive so badly on a stock caster setup.

Julian
A couple of questions while we're on the subject:

Why is toe specified in inches (or millimeters), rather than degrees as the other settings? I would think that tire diameter would affect the measurement, whereas diameter would be irrelevant if you measured toe in degrees.

Also, is the measurement given as one tire with respect to the center line of the car, or both tires measured from one to the other?
I want to amend some of my earlier posts. I installed new front tires and noticed some odd wear. I did the old school string alignment and found too much toe in. I made several adjustments and drive tests and NOW most all of the bump steer is gone. The car tracks like it should and like Johnny said it is a pleasure to drive. Bill 1362

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