Who can tell me EXACTLY what it is, what causes it, and why TTB trucks are so susceptible to it?

(For the record, I know the answers. I'm just curious as to what the perception of the condition is).

 

Bump steer
From Wikipedia, the free encyclopedia

Bump steer is the term for the tendency of a wheel to steer as it moves upwards into jounce. It is typically measured in degrees per meter or degrees per foot.

On modern cars the front of the tire moves outwards, as the suspension is raised, a process known as the front wheels "toeing out". This gives roll under-steer. The rear suspension is usually set up to minimize bump steer, where possible.

A typical value is two degrees per meter, or perhaps more, for the front wheels.

Excessive bump steer increases tire wear and makes the vehicle more difficult to handle on rough roads.

Solid axles generally have zero bump steer, but still have roll steer, in most cases. That is, if the wheels move upwards by the same amount, they tend not to steer.

 

A clinical definition and as with most wiki answers grossly incomplete. (What I would call a "wiki" answer). Compression of the front suspension is not the only cause for starters. Think I'll let this stew a while and check it in a few days.

 

I would define bump steer as turning, tilting and sideways movement of the tires when the suspension cycles. (can be rear suspension) This is with the steering wheel held in place and tight steering linkage. This problem is a geometry issue.

Some people say that they have bump steer because of loose balljoints, tierods or even wheelbearings. I dont agree. Thats just sloppy steering.

Solid axles do have bump steer. When the axle moves up, the track bar arc moves the axle to the right. If the steering draglink is not paralel and the same length as the tracbar, it will add to the problem. If you have leaf springs (no tracbar), then the draglink really turns the wheels. (when hitting bumps)

The best setup would be leaf springs with full hydro steering. The TTB is one of the worse setups because of the wheels not moving up and down parelal and the steering linkage not matching the axle pivots.

 

I'm giving you an "A" .........it's all about the dynamic steering geometry ! As with most any suspension, there are good points and bad points......what can you live with is the question ? I love the individual articulation cabability of the TTB ......and in stock form think it is superior off road....... especially on off camber trails.....plus it is very dependable.....at least mine is....keeping it feeling "tight" is another matter...........

 

Resurrecting this because I kinda left it hanging...

First, thank you Helirich for the most accurate explanation. I will disagree that the TTB setup is the worst for this condition. I will AGREE that the STEERING setup that Ford used on trucks equipped with TTB axles is the single WORST case scenario for creating bump-steer conditions.

No steering/suspension setup is ideal simply because no matter how you set it up, the body still moves independently from the suspension...(if it didn't, it would pretty much defeat the whole purpose of "suspension" now wouldn't it?) The use of a solid axle with a solid tie-rod between the knuckles and adding a single link between it and the pitman arm would come closest to "ideal". (If there is no way for the front wheels to move independently from each other, bump-steer is near fully eliminated.) The major drawback to this is the simple fact that it significantly hinders handling and stability over setups that allow independent wheel movement.

The most reliable "compromise" would be the standard steering setup found under most automobiles using "A" frame knuckle mountings with pivot points very near the frame rails and a steering linkage that employs a center (drag) link between two shorter tie-rods. While the driver can still experience bump-steer under "hard-over" maneuvers if the suspension articulates suddenly, the pivot points for the A-frames and the tie-rods at the drag link remain much closer in phase than they ridiculous "inverted Y" setup found under the TTB-equipped trucks. Further, I would argue that a setup that would put a drag link into the steering linkage of a TTB-equipped truck with tie-rods that are equal-length to the TTB axles halves (like cars that have tie-rods equal in length to the A-frames) would greatly improve this common dilemma with these trucks. I don't even like the Superunner setup from Superlift all that much because, while it employs the drag link, the tie-rods are still too short to keep the steering in phase with the axle pivot points. It IS a step in the right direction. There is room for improvement though.

Just a bit of editorializing really folks. Take if for what its worth. Thanks again to helirich for the spot-on explanation.