ball joint and bump steer

[Ishmael]

I've been searching and lurking and some ideas are clearer while I'm more confused than ever in other areas. I'm trying to get a handle on bump steer. Got scared a few times before I tore the car apart and I want to make sure I'm good before i get it back on the road. Other than the G-Mod, from what I "understand" it would seem that one of the biggest/cheapest improvements I could make using my stock 68 firebird frame would be to get some aftermarket UCAs and taller baller joints with the taller ball joints like the ones at SC&C being the key. Am I right in this assumption?

[Apogee]

Scott, the taller upper ball joints essentially achieve the same effect as the G-mod, just at opposite ends of the upper control arm. Moving the inner mounting point down has almost the same effect as moving the outer pivot up...granted the G-mod also moves the mounts rearward which gives more static caster, something a ball joint can't do. The primary purpose of either modification/upgrade is to achieve greater negative camber gain under compression of the suspension. This means that during a turn, the top of the outside tire leans inward towards the center of the car when the car rolls left or right. End result is a larger tire contact patch throughout the turn, thus allowing higher lateral g's.

Bump steer is mostly a function of the length and angle of the tie-rod relative to the control arms. Variations in the swept arcs over the range of travel is what induces dreaded bump steer. Worn parts can feel like bump steer, so it's important to make sure you're dealing with tight pivots and steering components before you jump to the conclusion that you've got major bump steer issues. A lot of these cars were relatively bad from the factory...but wear and tear can and will make them downright scary.

If you're thinking about running a tall ball joint kit, I'd suggest giving Mark at SC&C (www.scandc.com) a call. While a great modification for the money and worth every penny, it's not going to address any bump steer issues you may have.

Tobin
KORE3

[JRouche]

Tobin nailed it. I cant add to what he said. Ok, maybe a lil :)

What you were asking about was bump steer. The control arms cant really be changed to limit BS, unless you are building a suspension from the ground up.

Bump steer issues happen when the constraints of packaging all the components in one tight space becomes a problem. ALL the engineers of the stock suspensions knew this when they designed the various suspensions. But they gave up one aspect of suspension control and packaging for another. Swapping out the bad for the better, in their world.

Bump steer happens when the steering arm doesnt travel on the same arc as the control arms, lets pick the lower control arm.

I like to picture all the suspension movements with a circle. All the pivot points describe a circle on the end of the arms. Whether its a control arm ball joint or steering arm tie rod.

So when the steering arms outer point ( the tie rod) makes a smaller circle than the control arms outer point (the ball joint) you will have bump steer. Ideally the two arms would occupy the same space. They would be on the same fore and aft space and the same up and down space. Ummm, not gonna happen. They would have to be the same bar. Thats ideally. Zero bump steer.

If the steering arms circle is smaller than the control arms circle you will have a faster circle for steering. So as the control arm goes through its travel (the circle) the steering arm (and the tie rod) is traveling around a tighter circle. So the steering arm is pulling faster than the control arm during suspension travel. The faster circle means its gonna pull the steering arm in and cause a steer. Bump steer. The bars are out of sync, control arms and steering arms.

Not to mention the offset of the arms front to back. If your arms are perpendicular to the frame then thats a good thing. But some arent. Some arms actually point forward or backward a lil, that adds to bump steer. It makes the steering arms (the tie rod ends) describe an even smaller circle during suspension travel..

So anyway. Some folks combat the small circle or the steering arm with a lower joint mounting. They lower the tie rod on the steering knuckle (spindle) to get the steering arm sweep to be closer to the control arms sweep. It works!!

Get the sweep of the steering arm as close to the sweep of the control arms and you will get rid of the bump steer. But dont forget the front to rear offset. You want to have the arm moving vertically, if its pointing backwards or forwards then that displacement is gonna cause some bump steer also.... JR

[John Wright]

Don't just assume your car has bump steer....check it and then you will be able to see what you may or maynot be up against.

I think it was David Pozzi who had a jig made up(bolted to the wheel studs) to jack the suspension through the range of travel and was able to measure the bump steer and plot it so he could see the curve.(best to remove the coil spring and reassemble to check this)

BTW, You can get outer tie rod ends that are adjustable to help adjust out most of the bump steer.

Another thing the tall ball joint helps with is the ball joint angles...it can help if the regular joint is binding or running out of room to pivot.

+2 on calling up Marcus.


edit: Here is a bump steer jig already made up if you don't feel comfortable building one....and also instructions on how to use it in the link under the picture.

http://www.bakerprecision.com/longacr17.htm

[John Wright]

LOL...thinking about bump steer, my mind wanders over to our drag strip where you can (see) all sorts of cars leave the lights hanging the front wheels in the air and then you can see with your eye(no precision measuring tools required)the terrible issues with long travel of mostly stock suspensions....front wheels pointing in extreme toe in-out directions as the suspension completely unloads and loads again.

[Apogee]

LOL...thinking about bump steer, my mind wanders over to our drag strip where you can all sorts of cars leave the lights hanging the front wheels in the air and then you can see with your eye(no precision measuring tools required)the terrible issues with long travel of mostly stock suspensions....frontwheels pointing in extreme toe in-out directions as the suspension completely unloads and loads again.

That's funny as I was thinking about the same thing when I originally read this thread. It's always amazed me that people put up with that severe of toe-change on launch...tends to lead to a lot more steering input required in the first 100 feet.

[Ishmael]

Thanks guys. I think I learned what I did from John and JRouche's earlier posts and Tobin how is it you only have 708 posts?
I changed most of the little suspension pieces with a kit not to long before I drove it last so I think I'll get the UCAs and taller ball joints in just for better handling then let everything settle over a month or so then see if what is going on is safe. If not I'll try to dial it out a bit with adjustable tie rod ends. I think a lot of guys get too complicated with street riven cars and end up spending money where it may not be necessary just to say they have it. As you can tell by my handle, I'm trying to avoid getting sucked in too far.
Thanks again,
Scott