The Stovebolt Site Forums The Shop Area Driveline Bump steer

I understand all that Jerry and I like your clever right triangle explanation, but the drag link on the AD trucks (in stock condition) wasn't level and didn't have that relationship built in to it. I've tried for decades to find one which sat parallel to the ground and I haven't found one yet. Take a look at NDKid's image there...only because the axle was lowered 4 inches (obviously moving the steering arm higher by that same amount) and because the drag link was installed upside down (losing about 1.5 inches of that raised height), does the link sit level. If his truck had the stock steering setup, the steering arm would by definition sit about 2.5 inches lower than the center point of the pitman arm.

I've saved images of other people's steering which have been posted here. Here are 2 posted some time back by Longbox55...hope he won't mind me using them for illustration. His truck is complete and sitting on tires and in this post he said it was 100% stock. If you want me to send images of my truck (which has new springs and all good steering components, I can do it, but it will show you exactly what you see in Longbox55's image...

If the drag link isn't level, there will be bump steer- - - -no way to avoid it. Even from a level position, once the wheels leave a straight ahead position, some bump steer is going to happen. The best of all possible situations is to minimize it. Most people just learn to live with it - - - - -the further away from level we start, the worse the problem will be. I don't know of any baseball player who would willingly step up to the plate with one or two strikes against him from the get-go. Maybe that kind of logic escapes the hotrodders who think they can violate the laws of physics without any consequences.
Jerry

Hi Jerry,
Yep, no argument here and a zillion years from now all the geometry and physics anyone can round up will not have changed one iota. So if all the stock AD trucks suffer this same condition, do you have any ideas on how to safely raise the height of the steering arm tip by some amount that will get them closer to being on a level plane?

First, let me say thanks to all for keeping this discussion civil. Second, let me assure you that I’m NOT a steering geometry expert. I just googled custom Pitman Arms and got a boatload of options. Another option might be to contact Nostalgia SIDS and see if he has any ideas. Good luck with your projects.🛠

Steering arms can be bent - - - -COLD- - - - -never use heat on steering linkage- - - -to correct minor out of level errors. Re-arching springs also comes to mind if the drag link is angled up toward the front. Decades of bouncing around on less-than smooth roads will make springs sag and have a definite effect on steering geometry. On vehicles that have been slightly lowered, not to the ridiculous front bumper-dragging point, raising the steering gear on the frame is about the only option. At some point, the vehicle becomes a trailer queen that gets carefully rolled out for a car show, with no safe way to be operated on the street, but the guys who do those kinds of mods know they're unsafe, regardless of their protests to the contrary.
Jerry

Sid is great, he was the expert that bent my axle and such.

I probably shouldn’t admit this, but I never realized it was upside down.... Ops! I just knew it had to be level, I guess that sometimes stupid errors work out in your favor. Actually, I did all the geometry things that Jerry was talking about..... (ok, that is BS!)

Chris

I still want to know how caster could fix the bump steer or the toe in sitting ?

Short answer- - - -it can't. More caster will make the return toward center quicker, but the problem will still exist. It's like giving a brain tumor patient a big dose of aspirin for his headache- - - -it might mask the symptoms a little, but it doesn't cure the problem.
Jerry

I went out and measured my '37, it's the same basic setup as the newer trucks. The rear bumper is 7" higher then the front, so the it has a nose down stance. The garage floor is pretty level, bed rails are 4 degrees down in the front, running boards are 3 down in the front, frame is 3 degrees down in front. The drag link is 4 degrees up in the front, so it's NOT level by quite a bit. If it was level, it would be 7 degree lower in the front then it is now. The only way to level it would be to raise the gear box.

Point I am making, is it doesn't have to be level to still drive decent. I have 1/8" toe-in with newer style sealed tie-rod ends, newer style sealed drag link ends, NOS gears and bearings is a perfectly adjusted steering box, and 4 degree caster wedges ( thick end at back of axle ). I had the exact same set up with 2 degree caster and found the steering way loose, no road feel, and would not track straight. Going from 2 to 4 degrees made a huge improvement, if I could find a 5 degree I would go with it. The only thing that is altered from stock is the lower front end which changed the caster. I have custom made leaf springs with a much softer spring rate to improve the ride so they are softer and set the truck down lower. Caster may mask the problem as Jerry points out, but it's the easy way to fix the problem.

You still need all other adjustments to be right or never will drive right.

So, I just read through this entire thread and would like to add something.

My '53 1/2 ton is my only vehicle, so it's driven on a daily basis. As I read through this thread I began to wonder what it was I had done through the years to eliminate bump steer on my truck. Even if I hit a bump at speed big enough my front and back tires leave the ground my truck stays straight (for the most part). No involuntary lane changes. I've decided that you never fully eliminate bump steer, but:

1) Relax, fighting bump steer makes it worse.

2) You can minimize the causes and effects of bump steer. Only in the most extreme cases do I experience bump steer because I've found that:

a) When your tires are too hard your suspension reacts to pot holes, etc. more violently. When your tires are too soft the truck reacts too sloppy and recovers slow.

b) Steering and suspension geometry are key.

c) Wheel bearings, king pins, tie rod ends, steering gear box, spring mounts, bushings, shackles, etc, etc., have to be tight/no slop.

d) REAR spring mounts, bushings, and shackles HAVE to be TIGHT or bump steer will bounce your front end one way and the rear end the other way. Impromptu slalom course anyone? The last time I started to notice a return in bump steer I found my rear shackles were toast. When I replaced them it eliminated bump steer in all but the worst cases.

In other words there's no magic bullet (other than maintenance).

Sounds like u have the same set like my truck new lowered springs up front . I put new tapered roller bearings up front with sealed tie rods and drag link new every king pins shocks except no sway bar. I still haven’t got a alignment yet . I just test drove hit a bump I notice it pulled to the left so I figured it’s bump steer. It has one the caster wedge that came out of it but missing the other one I didn’t put back in cuz it broke cuz of rust .my original question was the bump steer caused because I lowered the truck and do I have to raise my put it back to stock height to get rid of bump steer ?

This has been an interesting discussion, from the perspective of several different viewpoints. I'm accustomed to dealing with the "go fast- - - -turn left" crowd, where we put a bunch of vehicles so close together they could pass notes to each other out the windows, for dozens of laps, and we do it at speeds well over 100 MPH. ANY unintended steering input under those circumstances is too much, and it can wreck the whole crowd. With that in mind, we go to extreme lengths to make sure the steering and suspension does exactly what it's told.

With a couple of traffic lanes to play around in, at speeds half or less what the round trackers deal with, I can see where herding an unpredictable vehicle in the general direction we want it to go might be acceptable. I just look at things from a different point of reference.
Jerry

You would have saved a whole lot of time for us if you would have disclosed, right from the get go, that you are missing the very part which would is likely causing you all of your trouble.

"A castor shim.
It is missing.
I have bump steer.
Anyone have any idea why I have bump steer?"

When he puts that caster shim back in should he lubricate it with castor oil?

"Inquiring minds want to know!"
LOL!
Jerry

I didn’t think one small piece of shim would cause the bump steer .

I did put back the bigger shim but the other one isn’t that big besides most everyone is saying caster isn’t what causes bump steer

Look at some of the vender sites in the above links, or go to your local alignment shop or spring shop ( old dirty shop will be a better bet ) and get two new 3 or 4 degree shims. Replace what you have, set the toe-in to 1/8" at the front, then let us know how it drives. You do not need to raise the front back up.

You have to have caster and toe-in set before it will ever drive right, how much of each is different for each truck, but having only one or none is not the correct amount!

Joe

Stovebolt front axles are stiff and heavy enough that it's not practical to make side to side adjustments, so putting the same amount of shim at both sides is about the only option you have. Putting different caster shims at each side just bends the springs- - - -and short of getting the alignment checked by a professional shop, there's no telling what the actual angle will be. Ford axles on vehicles with a single transverse spring could be bent with the right fixtures and big bottle jacks to get an exact alignment. Not so with Chevy and other vehicles with two front springs and heavier-made axles.
Jerry

Sal, are you pulling our legs? You are missing a castor shim on one side and you can't figure out why the truck doesn't handle properly?
Try removing one of your front tires and test drive the truck without it. Maybe GM didn't know what they were doing when they suggest that we use both front tires.

Carl
December fools day victim

No your not understanding what I’m saying both sides have caster shims the ones that are missing are the ones that crumbled from being rusted that I didn’t put back in . They were thinner than the ones that are in there now so I didn’t bother putting those back in . I figured since I lowered it would need different ones anyway .

Ok thxs . What is a good tire pressure setting ?I have mine at 35 psi on radials .I was think maybe running 32 psi soft tire maybe handle the bumps better not sure if that would help

35 should be ok, Sal and 32 should also be ok. Too soft and you won't recover from turns, bumps, etc as quickly. Also the wider the tire you have is, the less it is going to match the design of your front end and the worse your handling will be.

Now here is something I found interesting in one of the shop manuals I have last night. This is for the 48-51 years and the book was created by and copyrighted by The Chevrolet Division of General Motors in 1950. When you look at this scanned drawing you may see at least 3 interesting things:
1. The angle the drag link should be sitting at is shown, but is ignored by the remainder of the illustration.
2. The drag link sits at an exaggerated angle leaning upward from the pitman arm to the steering arm.
3. The springs are shown as being shackled at the rear. This obviously isn't correct.

Granted this drawing is to serve as an example for caster angle, but what was this technical writer thinking (or drinking) when they put this in and why go ahead and draw the correct line for the drag link? The correct number of leaves in the spring are shown, and the spring is shown at the correct arc. Just a little puzzling in light of the last 7 pages of discussion...

OK- - - - -this discussion isn't going to change any "hearts and minds" like they used to say about Southeast Asia, but the way we evaluated modifications on the racers was to chain the frame down to the floor on adjustable screw jacks at the desired ride height, assemble the steering and suspension without springs, and cycle it through the full range of travel while actually measuring bump steer. With a leaf spring setup and a straight axle, that would require fabricating a swivel arm to position the axle in place of half a spring, using the rigid spring mount as a pivot point. Nobody who is participating in this discussion is likely to do that, so I just wasted a couple of billion electrons and a few minutes of my time!
Jerry