The Stovebolt Site Forums The Shop Area Driveline Bump steer

Well I took my 53 Chevy truck for a test ride it drove good but I notice that it had some bump steer not a lot but how do I fix that ? I did lower the truck in the front and back not a extreme drop .Everything has been rebuilt in the front end and new tires . It has not had an alignment yet .

Alignment will certainly play a big part, small issues are multiplied if caster, camber and or toe are not within spec. Depending on the extent of your modifications you might have to play with adjustments until it handles well.

Ok Thxs . Yeah it has not gone in for a full alignment so it needs one for sure I hope it will help . I just took out a a few springs in the back and did the double spring in the front 3 inch drop in the front and 5 in the back .

I suspect it needs more caster, it probably had 2 degree shims in it when new, 4 degree should help. Just watch out for thick shims or short center pins on the leaf springs, the center pin needs to pass through the shim and still locate in the axle center hole. I have seen some shims with a center hole and pin combination so the length of the spring pin is not a concern.

If the steering feels loose, or easy to turn, then it needs more caster. More caster is like adding weight to the front. You can tell a vehicle has a lot of caster when the front end rises and drops when going from full stop to stop with the steering wheel. With no or very little caster, the vehicle will stay level with the stop to stop turn test.

Joe

Take a look at the drag link. It should be horizontally parallel with the ground. The toe-in is critical. There are ways you can check & adjust it yourself. Do a search. A lot has been posted on this. If you have a shim between the axle & spring, make sure the thick part is to the rear.
George

Unless you're willing to change tie vertical position of the steering gear to compensate for the height change in the front, you need to learn to live with the problem you've created. The drag link must be level, no matter how you go about it.
Jerry

I picked up an alignment tool some time ago. It gets my camber, caster and toe in/out close enough to make it drive able.

I looked at the drag link it is perfectly level with the leaf spring is that what you are talking about now I know my toe in and toe out is not set right .The shim is in the thicker side the rear of the vehicle.

The draglink is level with the Leaf spring

I believe that you will have a big challenge in getting your bump steer to go away as well as getting good handling quality with the significant difference in ride height between the front and rear axles. I would suspect that removing springs in the rear and adding springs in the front would not result in good handling either. GM engineers did know what they were doing.
Get it aligned by someone who knows what they are doing.
A good indicator that you have found a guy who knows what he is doing will be that he will tell you right away that an alignment probably won't help very much if at all.

Thxs

Sal,

I did a 4” dropped axle in the front and a 3” drop in the back. I have not had any issues. As others will state and argue both ways, I would be careful removing leaf springs. I tried to drive mine with only half the stack and almost wrecked. I put them back in and love the ride despite being a little tougher.

Chris

How will toe in and toe help the bump steer ? And fixing the canster help bump steer?

Bump steer is over-dramatized. After a few hundred miles you won't notice it anymore. I've driven trucks with so much bump steer that when I switched over to a car without bump steer it felt like something was wrong. Like a friend of mine says, "Don't fear bump steer."

The drag link needs to be level and parallel with the FLOOR.

Agree, skinny tires following cracks in the road is way more problematic.

Course YUMV, extreme modified frontends will behave much differently than OEM spec'd....

I have 650 16 bias truck tires on my 53 3100, dont really notice any rut finding either, course the entire suspension and steering is brand new original. Plus, I carry a bit more toe in than most, sure it will wear the tires in the longer run but it tends to keep things really settled down driving around at speed.

Ok I will give it a try .

I sure didn’t do a mono leaf in the front it has two leafs

Why is the toe in do critical? I know it’s part of the alignment but can that cause the bump steer?

By added more toe in does that keep the bump steer down ?

Sal, what are you talking about with 2 springs in the front? I'm confused...do you mean you took out all the spring leaves but 2?

Well I don’t know know if that’s what did but they built new spring s for the truck cause there’s only two those aren’t the original springs .

Here is picture from GM showing the difference in '36 and '37 truck steering. Pay attention to the compression and rebound walk. If you end up with the drag link not on center of its theoretical path, you can end up with bump steer. This why everyone says the drag link must be level.

To explain caster angle, picture a shopping cart, it will easily go any direction you point it because of negative caster. Now picture one of the old time dragsters with the front axle laid way back, they would go dead straight almost by them selves because of all the positive caster. You enough want positive caster to go straight with little input, yet still be able to steer.

Toe-in adjustment causes similar driving problems. The tires just don't know which way to go so they hunt around for a straight path to follow. In actual driving, the tires should be dead ahead straight. The toe-in as set for the front of the tires ( at spindle hight ) to be 1/8" closer together and rear of tires 1/8" farther apart. When you are driving, all the slop in the steering joints is compressed and the tires end up dead ahead.

Joe, the idea that people who don't have a basic understanding of steering and suspension geometry are modifying their vehicles simply to get a desired visual effect is downright terrifying. It's sort of like getting brain surgery at the corner drug store- - - - -from the computer jockey cashier, not the pharmacist!
Jerry

This may help explain steering,

http://www.valleyofhastings.com/WheelAlignment2.pdf

In a nut shell, as you travel over a bump in the road at a high rate of speed the up and down movement in the suspension causes the wheels to turn left and right making the vehicle change direction. This happens because the geometry changes even though you were holding the wheel straight! By removing all but two springs, the up and down travel of the axle is now way greater than the steering was designed to handle which in turn amplifies the bump steer.

Toe-In is what helps keep the truck running straight when you take your hands off the wheel. It also helps automatically return the wheels to straight after making a turn.

Mike B

Caster also helps the wheels to self-center. Because of the geometry of a properly-designed steering system, as the wheels turn away from straight ahead, the front of the vehicle is lifted. Gravity pushing down on the vehicle makes the wheels try to return to center. As the caster angle is increased, the tendency to self-center after a turn also increases. Vehicles with power steering usually have much more caster than those with manual steering because the increased steering effort is handled by the power assist. All this is designed into the car or truck when it's manufactured, and thoroughly tested before a steering system goes into production. Making radical (or sometimes minor) modifications without totally re-engineering the steering and suspension often results in not just annoying, but sometimes deadly changes in the handling characteristics.
Jerry

Mike, thats not entirely true, the motion of the front does cause the bump steer only if the drag link is not where it's suppose to be. The amount of spring travel or the number of leafs has nothing to do with the steering, assuming the steering and all adjustments are correct. On these trucks, the axle travel has pretty limited travel so it still comes back to either caster, toe-in or geometry of the steering links.

Not to be a pain, but it's caster that makes the wheels return from a corner and what keeps it going straight, toe-in also does, but the caster angle is the more important one of the two. Go back and look at the picture two posts up, the arc of the drag link needs to follow the arc of the springs as closely as possible. If you start out with the drag link at either end of the arc, you get bump steer, even with a dead on everything right front end, you still get some bump steer, it's just the way they work.

The same concept applies to independent suspension. Unless the length and position of the tie rods is approximately the same as the length of the lower control arm, as the suspension moves through jounce and rebound, bump steer will be induced. People who mismatch a rack & pinion steering gear and an independent suspension system can have the same, or worse problems with the vehicle self-steering as an altered ride height vehicle with a straight axle. The tie rod, steering knuckle, and lower control arm needs to form pretty much of a rectangle with the wheels pointed straight ahead, with the inner tie rod pivot and the lower control arm pivot in virtually the same position, side to side.
Jerry

So more caster shims

Well how do u get the drag link level with the spring ?

How do you get the drag link level the leaf spring ?

Thats the trick no one has figured out yet. You can hunt for a shorter or longer pitman arm, which raises or lowers the back of the drag link, or you can bend the steering arm at the axle end of the drag link. If yours is like mine, the drag link barely clears the top of the axle in stock location, so bending it down doesn't get you very much. On top of that, if you do change the pitman arm, you change the steering effort since the leverage has now changed.

Best bet is to add enough caster to hold it down, but not so much you can't parallel park it. My truck is lower in front, and the drag link is not perfect, but it drives as good as anything on the road. I can drive 65 - 70 mph all day without fear of bumps in the road. Good radial tires, are a must have. With the axle bump stops and frame location, I can't imagine the drag link being to far off, there just isn't enough room with a stock axle. Now if you have a dropped axle, then all bets are off.

Some people raise the steering gear on the frame to try to compensate for the lowering. The best advice is what the doctor gave to the woman who had an ugly baby- - - -"You grew it, so you need to learn to love it!"
LOL!
Jerry

Here is a picture of my 4” dropped axle from Sid. It is level. I have 3” drop blocks in the rear. The change required a 3 degree shim and minor adjustment to the draglink.

Over the past 45 years I've always owned at least one AD truck and during that time I've seen scads of them that others have owned and worked on some of those. It has always been interesting to me that I've never seen a single one with stock suspension and steering that had a drag link parallel to the ground. Not even one. Also I've noticed there are images right here on the Stovebolt forum showing 100% stock suspensions with their drag links also leaning that typical 15 to 18% downward angle (using the steering arm as the center point, parallel as one radius and the drag link as the other radius). If anyone has a bona fide image or picture of an AD truck with stock suspension and steering (and with good original type springs...not old wallowed out sagging springs or himmy-jimmy lowered springs) with a parallel drag link, I'd be very interested in seeing it. Likewise if anyone can point me to a passage in the AD shop manual with wording saying "make certain your drag link is parallel to the ground or shop floor" please do so. I've read the thing many times and evidently I missed that wording.

One could lengthen the pitman arm about 2.5 inches to take care of the problem, but I wouldn't. I saw this done in the late 1960s on a Ford in an attempt to correct the same situation (caused by a guy who wanted it to sit up higher). The drag link was then parallel but the leverage changed enough to make the vehicle dangerous. As you'd expect, the wheels turned quicker and it was harder to turn them...scary, actually. As I see it, the only safe way to adjust the drag link is by bending the steering arm or somehow raising the height of the place where the drag link sits, but I'd suggest doing this cold and bending it won't necessarily be easy. Jerry would be able to tell you how to do it using heat and I'm certain he has ideas/knowledge on this whole subject.

Thanks! That's exactly my point. You've lowered the axle 4 inches and you've mounted the drag link upside down (on the bottom of the steering arm) to get it level meaning you have it a net 2.5~2.75 inches or so higher than where it sat when the truck was in stock condition.

No matter how simple or complex a steering system happens to be, it's all just a series of right triangles. The one we're dealing with in reference to the drag link/Pitman arm/steering knuckle setup is no different. If the drag link is angled up or down, as the axle moves through its range of motion in jounce (spring compressed) and rebound (spring relaxed) the length of the long side of the triangle (hypotenuse) is going to change. If the drag link starts out level, the change is minimized through the full range of travel and the bump steer will be minimal. Once any measurement in the geometry of the triangle changes, something else must be done to compensate for the error. The simplest approach is to alter the height of the Pitman arm pivot by relocating the steering gear vertically, but it's also the most difficult to deal with because it will change the height of the steering column. Putting universal joints in the steering wheel shaft would be an obvious way to compensate for that if the resulting angles induced into the steering column aren't excessive.
Jerry

Jon, the factory manual won't make any reference to the drag link being level because the suspension system in its "as manufactured" condition has that relationship built in. If the drag link isn't level, something is wrong with the springs, shackles, steering knuckle, or other parts that affect ride height.
Jerry