Harry Mague asks:
> I am particularly interested in anybodys thought on the caster question.
A caster keeps your cold drink glass from making rings on your shellac-
finished wood table. Of course, if you drank your ale warm the way you're
supposed to, you wouldn't need one. :-)
> First, what is castor and if you can't adjust it then why have a value?
>... I had my car aligned and the machine said that my castor for the
> front wheel was -2.0 and -2.9, L&R. Obviously these values are outside the
> accepted data of +2.75.
Castor is the front-to-back inclination of the steering axis. The steering
axis is the axis about which the hub assembly rotates when you steer and is
determined by a line drawn from the upper ball joint to the lower ball joint
or trunnion. Its significance is complicated and subtle but not necesarily
critical the way camber is. To understand why you can't "adjust" it you
need to understand what gives it the value it has.
With the car sitting on a level floor and the suspension at rest, castor is
simply determined by whether the upper ball joint is in front of or behind
the lower ball joint, as seen directly from the side. If the upper joint is
directly above the lower, castor is zero. If your castor isn't within spec
then to a first approximation your frame was re-assembled out of spec. To
adjust it you have to move either ball joint forward or backward w.r.t. the
other, and to do this you have several possibilities, none simple. On
either upper or lower A-arm: 1. Remount the A-arm bracket slightly forward
or backward on the frame; 2. Re-bush the pivot bolt that holds the A-arm to
its bracket on the frame so that the A-arm sits further forward or
backwards; 3. Re-shape the A-arm by lengthening or shortening one of the
legs of the triangle so as to move the outer end forward or backwards; 4. Re-
position the trunnion or ball joint where it is held at the outer end of the
A-arm so as to move it forward or backwards.
The complication is that castor isn't necessarily constant as the suspension
moves up and down. If the A-arms are of equal length and their inner pivots
are parallel and the distance between them vertically is the same at both
inner and outer end, then as the suspension moves up and down they will move
identically and castor will be constant. But suppose the pivots aren't
parallel so that, say, the upper pivot is inclined "backwards", i.e, up at
the front, down at the back (or similarly, the lower pivot is inclined the
other way). Then as the suspension moves up, the outer end of the upper A-
arm will move backwards while the outer end of the lower A-arm will move
forward. The result is castor that increases with bounce, decreases with
rebound. If you have such a geometry and you've modifided your ride height
by changing or cutting the springs then this would have affected castor.
The point is, several thing could give you out-of-spec castor, either poor
re-assembly or worn pivots/bushings, distorted frame or A-arms, or
intentional "fixing" by a PO. (Usually you'd want intentional "fixing" to
make it go up, but if a PO had done something radical and compensated for
any expected castor increase, and then you undid it, it's possible you could
end up with lower castor than expected.)
Okay, so what does castor do and why might you change it? Primarily,
positive castor causes camber to change as you turn the steering wheel. The
outside wheel which takes most of the cornering load goes more negative, and
the inside wheel goes more positive, causing both to lean into the turn.
This helps keep the tread on the road, compensating for both tread tuck-
under and body roll. (This isn't the only camber effect, of course.)
Another effect of castor depends on your steering radius. Imagine the
steering axis projected downward toward the road surface, and then imagine a
line perpendicular from this axis to the contact patch center. This is the
steering radius and is the moment arm by which the steering manipulates the
tire/road contact. With positive castor, steering makes the ouside wheel
rise w.r.t. the car, and the inside wheel fall. This exacerbates body roll
slightly. It also transfers weight from the outer-front and inner-rear
tires to the opposite corners, which may or may not be good depending on
other factors. The larger the steering radius, the more pronounced this
effect is (and also the more steering kickback you get), so on most cars the
steering axis is tilted inward at the top to short the radius.
[Note, an interesting benefit of trunnions over lower ball joints is that by
threading them to the hub you can cause vertical movement to happen in the
"right" direction when the car is steered. Also note that steering radius
is affected by tire height and offset. More offset or lower tire height
increases the radius, thereby increasing steering effort and also the side-
pull you experience when you lose traction with one wheel while braking.]
So is slight negative castor bad? Not really. It simply means that you may
not be getting the best possible camber behavior while cornering. If the
body didn't roll and the tires didn't tuck under, then camber changes
wouldn't be necessary anyway. Low values have very little effect, and "big"
castor values would be upwards of 5 to 10 degrees. In any case, it won't
affect your braking and won't induce pulling. Its only impact on your tires
may be a slight increase in wear on the outside of your tread while
cornering hatd. If you dial in a bit more negative camber you can trade
inside for outside. So unless you are seriously into competition or you
have a seriously worn suspension mount, you needn't worry.
There is an interesting application for non-parallel A-arm pivots, one that
Jaguar tried, I believe. If the pivots are arranged as I described above,
with the upper pivot raised and/or the lower pivot dropped toward the front,
the result is castor that decreases with rebound. When the driver brakes,
the force of the caliper on the hub is to rotate the hub forward, thus
pushing castor down. This pushes the suspension down in its travel, thus
raising the nose of the car and providing a measure of anti-dive during
braking. Merely lowering the front of both pivots also provides some anti-
dive but this non-parallel technique could be described as "active". The
down-side is that squat during acceleration also lowers the castor.
> Secondly, the camber in the rear end...What does positive mean?...
> After extensive repair to the rear structure, I am have problems
> getting the toe straight...and the car has the TR6 squat.
Positive camber means tilted outward at the top, and is generally not good
in a performance-oriented car. The TR6 has a semi-trailing arm rear. (The
swing axis is at about 30 degrees to transverse, isn't it? That's the same
design that Datsun used on the 510, BTW.) Its camber goes negative with
bounce, positive with rebound, like a swing axle but much less. Toe-in is
at its minimum when the arm is directly horizontal, and goes more inward as
suspension travel goes either up or down from there. Of course, at rest
your car may not sit with horizontal arms, and you didn't say whether your
toe was in or out. Either way, it sounds like you should investigte how the
arm mounts are fixed to the frame.
Sorry about the length. Hope this is at least entertaining, if not
educational.
Jim Muller
jimmuller@pop.rcn.com
'80 Spitfire (Percy)
'70 GT6+ (Nigel)
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