Toe-in and bump steer

[worzel]

Hi All

Is it possible to have too much toe-in just on one wheel?

Thinking about this I reasoned that if one wheel toed-in more than the other what would happen is that in the dead ahead position the steering wheel would simply be slightly misaligned to compensate and that when the car was moving both wheels would simply "shift" to produce the same toe-in on each side.

Or putting it another way if one side was set with no toe-in but the other had say 2mm the result overall would be 1mm each side but the steering wheel would not then be in the dead ahead position.

Is my reasoning correct?

Regards

John

[Uboat]

I jacked my car up last week and found out that the setting of the right front wheel differed 10 mm from the left wheel. This has probably been the case since the car came back from a well known Lotus specialist, who fixed a broken front suspension, and I expect this to be the source of the heavy toe in of my car.

I will take a test ride asap to find out if this is also the explanation of why the steering wheel is not totally centered...
/Ulf

[paddy]

Is my reasoning correct?

Yes. The "straight ahead" position will just be whatever position has equal toe-in on both sides. You would just potentially find that there is more steering travel in one direction than the other.

The only way to know which side is "wrong" is to centralise the rack (ie find the centre position where it takes an equal number of turns to go to full lock in both directions), and check/set the toe-in on both sides. Then set the inner steering column to centralise the indicator cancelling mechanism; then set the steering wheel to point straight ahead.

Paddy

[davidj]

Rob,

Just looking at you notes for checking toe in. Very interesting, but should the slots be 25cm to 40 cm or 25 to 40 mm deep? 40cm deep and you have nearly chopped the boards in half, unless I have misunderstood your instructions!

Cheers,

D

[billwill]

I made a giant wooden caliper from a length of 4x4 and two thick right-angled plywood triangles.

I'll try post a photo of it soon.

===

I'm fascinated by the actual measurements for bump-steer correction, I didn't know it could actually be done systematically. I thought it had to be done by trial & error. The problem being I had never seen a satisfactory description of what causes bump-steer problems. I had worked out that the track rod ends have to be horizontal at normal ride height, but now I'm not sure if that is right?

I like the idea of using those cheap laser levels to check the varying toe in-out. I've got ONE of those, which I took apart to recalibrate as it was way out of line, but is OK now.

Time to buy another for the other wheel hub.

[paddy]

The problem being I had never seen a satisfactory description of what causes bump-steer problems. I had worked out that the track rod ends have to be horizontal at normal ride height, but now I'm not sure if that is right?

No, its not that simple.

If both the top and bottom wishbone were level at normal ride height, then it would also be correct for the track rod to be level to eliminate bump steer. You can imagine that with small movements of the suspension, the top ball joint and the bottom trunnion would each move strictly in a vertical direction, and so the upright itself would be moving strictly in a vertical direction. If the track rod was level, the steering arm would also move strictly in a vertical direction, and therefore the wheel would not be turned in either direction with the bump or droop of the suspension.

However, the wishbones are not level. At normal ride height, the lower one is more or less horizontal, but the upper one is at an angle, inclined so that the ball joint is higher than the inboard pivot. So when the suspension bumps (ie the wheel moves up), the top of the upright moves closer to the centre of the car, and moves further away on droop. This also means that, for the wheel to still point iin the straight-ahead position, the end of the track rod must also move slightly towards the centre of the car on bump, and slightly away on droop.

Therefore, bump-steer is eliminated when the tie-rod is inclined slightly, with the outboard end slightly higher than the steering rack. You can imagine that, very approximately, the inclination of the tie-rod needs to be "half" of the inclination of the upper wishbone.

Paddy

[prezoom]

Hi David,

I seem to have trouble converting from furlongs per fortnight to metric measurements. The slots in the boards/plates should be just enough to slip the measuring tape into. i.e., so you can "hook" the tape on the opposite side, while checking the measured distance.

As far as aligning the steering wheel in the process, what I have done, is to make a mark in the center of the chassis, between the inner lower control arm bolts for reference. Both front and rear. Using a plumb bob, from the center mark make a spot on the floor. Tape a string down, front to rear, stretched over each mark. Now you have a reference to the center line of the car and with the steering shaft centered, then you can adjust the steering wheel to be correctly orientated. Measure one wheel/tire combination, with one of the toe boards, square with this line, by adjusting the tie rod end, and you now have a good starting point to set toe.

Using a prick point punch to mark the center of the chassis, will provide a lasting mark for future alingment issues.

Rob Walker
26-4998



Rob Walker
26-4889