rotarynemesis

Every time i hit a bump my car likes to swerve to the right. What would cause this?

Cheers!

bump steer occurs when the suspension travels through it's vertical motion. THe tie rod connecting the steering rack and the knucle moves up and down with the motiion of the steering knucle and essentiall sweeps an arc. If the design of the suspension system is such that hte tie rods are very short the arc it sweeps is a lot. As the tie rod sweeps in an arc the tie rod's distance it is away from the wheel is shorted by the cosine/sine law. The actual distance is calculated by taking hte component of the angle between the tie rod at it's longest position and its shortest distance and taking the cosine of that angle multiplied by the actual length of the tie rod.

So when tie rod travels up or down with the suspension travel the distance is shofted. Same effect as if you were turning the steering wheel and moving the teeth of the rack over to one side. You are shortening the distance between the inner and outer pivot poitnes of the tie rods. So whenever the suspension moves up the steering changes.

Bump is a term used by vehicle dynamics engineers when we are talkinga bout compression of hte suspension (ie shortening the spring shock assembly when load is applied, and droop is the opposite).

Bump steer is not the effect of when your suspension bottoms out and your shocks hit the bump stops and it magically steers.

To answer your quesiton... check the conditions of your shock absorbers... if they are bad your car will be unstable when it hits bumps.

DamonB

Most likely a dead shock or very worn bushings or tie rod ends on that side. Does the car pull straight on smooth surfaces? Even under acceleration and braking?

Bump steer comes about because the inner and outer pivots of the tie rod are not geometrically in line with the inner and outer pivots of the suspension arm. This causes increased toe angle changes as the suspension goes up and down.

Bump steer is fundamental to the suspension and steering design. Every FD with a 3 rotor that has moved the steering rack without making modifications at the spindle as well has much more bump steer than it did before.

eyecandy

iTrader: (19)

Have you changed the your suspension or wheels? Anything that effects the ride height and suspension geometery will effect the orginal amount of bumpsteer (yes all cars from factory have bumpsteer, but kept to a minium).

rotarynemesis

The car is lowered but still has stock wheels.

The car still goes straight under acceleration but under hard breaking it likes to pull to the right.

So you think I need new shocks? And as far as the tierod ends and the bushings where would I get those from?

eyecandy

iTrader: (19)

With the car lowered there is you prolem, BUT you would only notice it under wheel direction change (ie turning).

If its under hard braking then I would check the calipers and pads. I have never really heard of a blown shock cause a car to pull hard unddr braking, but its always a possiblity.

If you need tie rod ends, you can get oem, or get aftermarket liek SuperNows, with a lowered car I would get aftermarket tie rod ends which include the bumpsteer kit to remove the bumpsteer with lowering a car.

rotarynemesis

who sells these? I dont think I have seen them at rx7store or anywhere else.

Thanks for the info

OC_

This is wrong! And i hear vipers dont have bumpsteer.

bump steer is a function of tierod length and placement VS control-arm length.
How would wheels and ride hight changes change those?

When you lower you car, you have to get it re-aligned since bumpsteer exists within the suspension geometry. lets say your car gains a degree of toe when the suspension is compressed. now if your car is lowered, you have to get it realigned since is going to have the 1 degree of toe all the time since the suspension is always compressed! Realigning sets it back to zero making things all good.

eyecandy

iTrader: (19)

There is NO car (street car that is) that has no bumpsteer, all cars have some degree off bumpsteer, mainly becasue manufactures do not find a need to have zero bumpsteer and the amount of bumpsteer in the car is so minimal that people do not notice it.

You just stated yourseld how ride height changes bumpsteer, yes you can remove most of the bumpsteer when egtting it alined but there will always be more than stock since you changed the geometry. Along with that depengin on how much of a change there is, you need to repoisition the tie rod ends to the hub to remove even more or remove it all.

OC_

I don’t want to get it a flame war or anything, but I think your definition of bump steer is not totally correct. And, you really can’t change the suspension geometry short of designing new parts or modifications involving the use of metal cutting equipment. For instance, if I relocated the lower control arm on the front of my car 2” forward, I would be changing the geometry of the front suspension; I have changed the anchor points of the suspension on the chassis, roll centers, how it moves thought the arc and everything has all been changed; you can see what it means to change the geometry. But if I get my car realigned because of a ride height change, none of the anchor points of the suspension have changed. What happened was an adjustment was made, but all the factory geometry is preserved. If you were to calculate the movement of everything, you would find that bump steer deflections would be the same. Changing your cars suspension geometry is not the same as getting your car aligned, changing your geometry is changing how your suspension ‘works’, its hard to explain. i recomend you read Competiton Car Suspension by Allan Staniforth for a more clear perspective.

And tell me, do old cars that have solid, non-independent front ends have bumpsteer?

Alex

OC, you're ignoring the fact that bumpsteer is not linear. Manufacturers design the suspensions to have minimal bumpsteer at stock ride height. But it will increase as the suspension is compressed. If you have 1 degree of bumpsteer in the first inch of travel from stock, you can lower the car 1in and correct the toe. But now through the next inch of travel, the bumpsteer could be 1.5 degrees. So the bumpsteer on the lowered car will be more noticeable, even if nothing else has changed.
And yes, solid axles can have bumpsteer too. This can be caused by compliance in the lateral locators of the axle. If the bump causes one side of the axle to move rearwards, you will get a bumpsteer effect.
Manufacturers will typically design in about .01-.1 degrees per meter of bumpsteer since if creates an understeer effect while cornering.

OC_

good point. I recall the early 90's MR2 had a problem with the bumpsteer on the rear suspension, and it was fixed in, i think 92. But it was said that the 90-91 cars really started to roll, the outside rear would suddenly go into a toe out condition making the car a handfull. this is obvious non-linear bumpsteer.
Is the steering effect of a soild suspension called bumpsteer or is it scrub, since the wheels dont actually steer independently. i guess this is getting beside the point.

rotarynemesis

so this problem is caused by having too much toe out?

RX744CSP

iTrader: (4)

Yes, changing toe (either way) can induce bump steer. All cars have some bump steer, as noted earlier, and it can be made worse by raising/lowering the car on the suspension. Think of it like this: all the outer pivot points of the suspension move in an arc, not a straight line. The upper and lower control arms and tie rod are all different lengths which make different arcs, and there's where bump steer comes from. Strut suspension has the same thing, since the strut is angled to give camber.

I see from the original post your problem is the car pulling to the right over bumps. Toe out can make the car 'chase' bumps. If your left wheel is pointing straight ahead and the right is toed out, yeah it's going to chase bumps to the right. If the toe is OK, I'd look very carefully for something bent. Make sure the wheelbase is the same within 1/4 inch on both sides, if it's off start looking on the side with the short wheelbase.

rotarynemesis

and to measure the wheelbase can I just go from wheel center cap in the front to center cap in the back?

thanks for the info

DamonB

Yes. That's what wheelbase is.

mad_7tist

i would look into the brakes like eyecandy said. put the front on jack stands check for tie-rod slack by holding the rod while someone wiggles the tire from side to side. then apply 1/2 inch of brake pedal and compare the resistance in trying to turn the front wheels. apply a little more and make sure they lock up at the same pedal pressure. a suspension issue would tend to appear under more than just braking situations. not saying that it is but the odds are good so it is a better place to start