Harrymague@aol.com wrote:
>
> First, what is castor and if you can't adjust it then why have
> a value?
Castor is the inclination of the steering axis (the imaginary line that
the vertical link/hub/wheel assy rotates around as you turn the steering
wheel) in the fore/aft direction. Measuring it on a car with no
adjustment provides an indication that all is well with the front
suspension, errors may indicate bent suspension components, bent frame
or even early signs of failure. I've heard this is a common failure
point on the TR6, so check your frame carefully. Having a pivot rip out
of the frame can be an exciting experience of the worst kind.
> 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. I have looked all through the manuals and books and
> all they say is it can't be adjusted. All the other specs for the front turn
> out great, and that is after 1/4 of the front was cut off and a new piece
> welded in place. The car drives straight and doesn't pull. I just put new
> michline redlines on and don't want them to get eaten. So what the story on
> castor.
Castor's primary function is to cause the steering to return to center,
it also helps stability and wander. Errors in castor don't normally
cause the car to pull, or the tires to wear unevenly. It can however,
lead to squirrelly handling in some circumstances. IMO, a 5 degree
error in castor is enough to be dangerous, you need to find the problem
and fix it.
I'm no TR6 expert, but ISTR there are shims between the inner lower
A-frame pivots and the frame. Moving a shim from the rear to the front
will increase the castor angle. Camber and toe-in will have to be
checked/set afterwards. BTW, my Haynes says only "If the shims are
mixed up, the castor and camber angles will have to be checked and
adjusted by your local garage."
FWIW, one of my TR3A's had been incorrectly repaired, with the result
that the castor angle (probably camber too) was badly wrong on one
wheel. The steering wheel didn't want to center after a right turn, and
stepping on the brakes in a moderately hard curve would cause gross
oversteer and a subsequent spin-out. Fortunately, it was only a
cornfield where I discovered this !
> Secondly, the camber in the rear end. The data for this says it is 1/2
> degree positive. What does positive mean? Does the top to the tire go in
> torward the center to be positive, like most TR6s I have seen or should it
> actually go away from the frame at the top.
Positive camber means the top of the tire leans away from the car. Most
TR6s that I've seen seem to exhibit negative camber, presumably from
sagging springs. However, my TR6 Haynes lists the rear camber as 1
degree negative, +/- 1/2 degree. Note that due to the design of the TR6
rear suspension, the camber angle changes as the suspension moves. This
means that the camber angle specification should also include how
heavily the car is loaded, and that load has to be supplied when the
angle is measured. Unfortunately, my Haynes doesn't seem to specify
this important point. (The same thing is true of the front suspension,
but since it mostly carries the weight of the engine, loading of the car
has less effect and is usually ignored.)
In general, a slight negative front camber improves handling, I presume
the same thing would apply to rear suspensions. The idea being that as
the body leans in a curve, you want to keep the wheel as perpendicular
as possible to the road. Negative camber means that the most important
wheels (those on the outside of the curve, carrying the most load) stay
more vertical as the body leans.
Randall
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