Bump Steer Question

[Ralph Z.]

I'm having issues with my 1968 Alexis Mk14 FF diving left or right under heavy braking. I'm thinking that bump steer may be to blame.

So, I did a quick check on the fronts and found that the LF seems to be OK while the RF toes in a significant amount in a bump condition.

Is it common to have only one side toe in like this? Seems to me that both sides might behave similarly.

Any thoughts on what might be causing and ideas on solutions.

Many thanks for your insights.

[Daryl DeArman]

If you are confident your measurements are accurate then look at the lengths of the tie rods, lengths of the a-arms and angles of all of them at ride height.

It's simply the difference in arcs scribed by the tie rods and the a-arms. Something is different Left to Right.

Ensure that you were measuring toe change through the same range of travel relative to ride height. For example +/- 1" from static ride height, not 2" of bump on one side and 2" of droop on the other.

Make sure bearings are properly tightened and the wheel is true if measuring off the wheel.

[Ralph Z.]

Would it make sense to duplicate all the measurements from left to right as a starting point? And, then, align from there?

I notice that the right side is slightly wider than the left. Maybe I should get back to a baseline first.

[Steve Davis]

Would it make sense to duplicate all the measurements from left to right as a starting point? And, then, align from there?

I notice that the right side is slightly wider than the left. Maybe I should get back to a baseline first.

Absolutely - that is the first step. Unless you are running ovals, your left and right should match.

You should also check the shocks/springs to make sure that they match. I had a situation where the left front shock failed and caused the car to dive right under heavy braking - quite upsetting to the car at speed.

Steve, FV80

[kea]

Of course, it goes without saying, the caster is the same for both wheels.

[Ralph Z.]

Speaking of shocks and springs...We just dynoed the shocks and springs and found that the front springs are off by 40 lbs from left to right. This may be contributing to the problem. Making matters worse, the shocks are shot. Almost no rebound.

So, we probably have multiple issues here where bump steer may be just one contributing factor.

[Daryl DeArman]

Whether or not the shocks are good/bad, caster the same l/r, those items won't affect your bumpsteer measurements. Of course they would nduce handling issues and need to be corrected but they don't have anything to do with your bumpsteer measurements.

[Brian]

You should get some 1" square aluminium tube and some fishing line, and make you a string box to let you do a proper alignment. Almost the hardest part is finding the true center of your car - those old vintage cars could be pretty funky once you start trying to measure them to a 1/32 of an inch... Anyway, get the square box, set the car on blocks at ride height with the springs and shocks off the car, and do your baseline alignment. Measure everything to the string with a dial caliper. Do wheelbase, width first, then do toe front and rear, then do castor and camber front and rear, then start over. It usually takes a few iterations because changing one thing makes something you did before wrong. Don't just look at the front of the car. Darting under braking can easily be the rear is bump-steering into a toe-out condition. Usually setting the rear castor to zero makes the rear neutral bump steer but not always. After you have a perfect alignment at ride height with the springs off, then raise the car three or four inches on blocks and start to do the bump steer at all four corners. Make sure you center the steering rack, lock it centered so that it cannot move, and only then start to set front toe. For bump steer purposes I would set toe to zero.

Brian

[Ralph Z.]

Whether or not the shocks are good/bad, caster the same l/r, those items won't affect your bumpsteer measurements. Of course they would nduce handling issues and need to be corrected but they don't have anything to do with your bumpsteer measurements.

Yes, I understand. Thank you.

[stephen wilson]

Did this problem start recently? Is the car new to you? Too much rear brake bias?

[Ralph Z.]

Did this problem start recently? Is the car new to you? Too much rear brake bias?

The car is new to me. Due to really weak springs the brake bias is toward the front.

[Modo]

check for cracked welds on wishbone location points and mounts, a car that old, you could overlook something moving around .... less than 2 cents

[Ralph Z.]

I just took some measurements and found that the RF suspension is .25" wider than the LF.

Would this be enough to create a bump steer problem?

[Modo]

.25 maybe not too much, maybe too much with that year .......what really affects toe is steering rack location and steering link swing, ie for zero toe the steering link would have to be the same length as the wishbones and the links at the same angle as the wishbones .... wishbones go up or down and in towards chassis center on arc, steering links do as well ..... a lot cars have slightly shorter link geometry to insure slight toe in with bump....... anyway, check that your rack is centered, the links are the same even if the steering rack assembly is not center mounted ........ then the .25 could be it, dunno ...... wonder if you could align with rack off center to overcome difference, HMmmmmm

[DaveW]

I just took some measurements and found that the RF suspension is .25" wider than the LF.

Would this be enough to create a bump steer problem?

Bumpsteer is not directly related to suspension arm length. It is more related to the arm (upper a-arm, lower a-arm, toe link) angles. These are affected mostly by caster and other things that change that relationship. Adjustment is usually made by shimming to change the angle of the toe link, or similar means.

[racersonly]

I've tackled the alignment on a couple of Alexis FFs in the past, and no matter what I did, there was a bunch of bump steer in the front. It seems to me that Bruce Lindstrand came up with a fix for it on one of his customer's cars, with included a rod end on the outboard end of the steering (toe) link. Lindstrand Motorsports is cff96 here on Apexspeed.

[Daryl DeArman]

Bumpsteer is not directly related to suspension arm length. It is more related to the arm (upper a-arm, lower a-arm, toe link) angles.

Hesitant to disagree with the man, but I'm going to anyways

I'm not certain I would say one aspect is more or less related. When things aren't right both can have a huge influence on bumpsteer.

For example, I can have the links parallel to each other and have the toe link 1" long and the a-arm 12" long and things will get real ugly real fast.

I can also have all the arms 12" long but at all kinds of steep angles relative to each other and things will get just as ugly just as fast.