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
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt! There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway Love your enemies and drive 'em nuts!
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.
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
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt! There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway Love your enemies and drive 'em nuts!
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
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt! There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway Love your enemies and drive 'em nuts!
