bump steer

[doghousedevil]

i keep reading about bump steer. what exactly is it?

 

[Gastrick]

Bump steer is generally a condition caused by improper suspension geometry or poorly adjusted suspension componets. The term is fairly self descriptive. When you drive over bumps, the front wheels want to turn themselves causing you to have to fight the car to keep it pointed in the desired direction while traveling over bumps. Bump steer can range from very minor to totally intolerable depending on how badly the suspension geometry is off or how bad your adjustments are.

 

[35Mastr]

Its when the car hits a bump and does a lane change on its own.

 

[donsrods]

It is generally caused when your draglink from the steering box to the front wheel swings on a different arc than the radius rods. If you can imagine if you lift the drivers wheel up and if the draglink is mounted in a different plane than the radius rod, the action of lifting the wheel (or lowering the wheel) will move the steering arm slightly. It acts just as if you were turning the steering wheel.

That is why most people say to mount your pitman arm in an intersecting line with the frame end of the radius rod, or at least to have your draglink pass through the point where the two intersect. Here is a picture I just posted of my '39 Dodge steering setup. I got the pitman arm as close as possible to the spot where the radius rods mount. Sometimes is is impossible to do it exactly, but the closer the better.


Don

 

[doghousedevil]

thanks for the knowledge.

 

[rustybeercan]

Its when the car hits a bump and does a lane change on its own.

Oh, I thought that was just part of the fun of driving an old car.

Beercan

 

[LoneWolfStreetRods]

Oh, I thought that was just part of the fun of driving an old car.

Beercan

It is

 

[donsrods]

That is why the make two lane and three lane highways.

Don

 

[sc_sonoma]

This question is in regards to RonsRods response. My steering shaft is at a slight angle up (from wheel to pitman), rather than parallel w/ the ground. Could/is this the cause of some of the play in my steering??

These are the only pics I have on my work computer showing the steering setup clearly.

Thanks.

 

[donsrods]

Thanks for posting the pictures, they help a lot. Well, cowl steering is one of those gray areas when it comes to steering. My Son wanted to use cowl steering on his rod and we talked about how to make sure it didn't create any problems with proper geometry. In the end, he decided to not do it, but to go with conventional steering.

Here is what I see in your pictures. First of all, the draglink from the pitman arm to the steering arm on the wheel is extremely long.........that allows some flex and monkey motion. It is like taking short rod compared to a very long rod, you can bend and flex the long one much easier. Secondly, the draglink terminates at a different mounting location than the wishbone. If you were to lift up on the drivers side front wheel and watch the movement, the longer drag link would slightly "steer" the wheel. The two are not rotating in the same plane, so one will travel further on the arc than the other. The idea is to neutralize this difference in movement as much as possible.

I don't know what the fix is for your car. When we build a car that is a little unorthodox (and I don't mean unorthodox in a bad way.......my own T is unorthodox in it's lowness ) we have to accept that we may have some problems to overcome. Your car, by being so long, low, and having cowl steering could possibly have some geometry issues to correct. How to do that is what I am puzzled about.

I would play with the alignment, like toe in, caster, and maybe add a steering stabilizer between the tie rod and the frame or axle. Those are the mini shocks like VW's had on their front ends to damp out bump steer and death wobble. Different shops like SoCal sell them and lots of people swear by them. Nice rod by the way, and I hope you get your problem solved.

Don

 

[sc_sonoma]

Thanks for the input. I was afraid you're answer would be along those lines.
I'll check into the steering stabilizers.

 

[IAM-LDS-2]

Oh, I thought that was just part of the fun of driving an old car.

Beercan

A cars age has nothing to do with bump steer it happens because some part or parts in the front end are not doing what they are supposed to. If it ever happens to you FUN will not be one of the words used to describe it.