[TR] [Spits] GT6 Bump Steer

[Jim Muller]

On 15 Aug 2007 at 10:32, Randall wrote:

> When the suspension moves, it moves through an arc...
> If memory serves, bump steer can also be caused by changes in
> camber as the suspension moves.

Indeed the question of camber change is essential to the subject.  
The upper and lower A-arms are (almost) never the same length.  The 
upper is generally shorter so that the top of the spindle carrier 
(i.e. the upper ball joint) moves in a circle with smaller radius 
than the lower (i.e the trunnion).  The inner ends are positioned so 
that the camber is at spec when the car is at normal ride height.  
But when the suspension moves either up or down the upper ball joint 
moves inward more than the lower, resulting in more negative camber.  
This compensates somewhat for the fact that the car body leans to the 
outside of a turn, keeping the outer wheel more vertical.  As the 
inside wheel moves down in the suspension it leans more to the 
outside of the turn, but since it carries less vertical load it 
doesn't reduce the cornering by much.

So the question of bump steer really depends on the vertical 
positioning vs. length of the tie rod.  Suppose the A-arm mounts were 
parallel and perfectly longitudinal.  For simplicity suppose that the 
inner tie rod end is in the same vertical plane as those two pivots, 
and the outer end is directly in front of a vertical line between the 
ball joint and trunnion.  If it is at the same height and is the same 
length as the lower A-arm, vertical movement of the suspension will 
make the tie rod swing in the same arc as the lower A-arm, so no bump 
steer will occur.  But what if it is higher?  A point on the spindle 
carrier midway between the top and bottom will travel in an arc with 
radius shorter than the bottom arc but larger than the top arc.  So 
the tie rod needs to be a length between them too.  Also if the A-
arms are (let us suppose) horizontal when the suspension is at rest 
but the tie rod points down or up because the inner and outer ends 
aren't at the same height, then the outer end cannot possibly track 
the same arc that any part of the steering axis will, so bump steer 
could be serious.  There there are the cars with upper and lower A-
arm pivots not parallel, so as to produce caster change too but...

Aw heck, it's too hard to explain in words.  Moving pictures would be 
some much more informative.

-- 
Jim Muller
jimmuller at rcn.com
'80 Spitfire, '70 GT6+



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