Harry wrote:
>As I continue to struggle with my 74 TR6 suspension, a couple of questions
>have come up. 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. 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.
Take a look at a bicycle, Harry. Notice the way the axis around
which the forks/handlebars pivot (this is called steering axis) is
slanted toward the rear of the bike. This is a great illustration of
POSITIVE castor. If physics was simple and easy, the steering head
on a bicycle would be straight up and down, directly above the front
wheel's hub. (Actually, I think those early bikes with the giant
front wheel were set up this way) Negative castor would be the
steering head on our bicycle being out in FRONT of the front wheel's
hub with the steering angle inclined toward the front of the bike.
Castor is related to camber in the following manner: Keep in mind
that camber describes to what extent the wheel is 'leaning' in one
direction or another. Now on that bicycle, if you point the front
wheel straight ahead, the wheel has no camber. But watch what
happens as you turn the handle bars
the more you rotate the handle
bars, the more the front tire leans. This is due to the angle that
the steering axis is designed at. Let's take it to an extreme as an
absurd and totally impractical illustration
imagine if that steering
axis was laid down so that it was parallel to the ground. What would
happen? When you would rotate the handle bars, you would ONLY change
the camber angle. The wheel wouldn't steer the bike at all
at least
not very well. So now take a look at your everyday commuter car and
crank the steering wheel all the way to one side. Now get out and
look at the CAMBER of the front wheels. You'll see that there is a
good deal of camber in each of the front wheels now, while when
pointed straight ahead the camber was not noticeable. You may even
notice that on one side of the car the camber is positive and the
other side is negative. The steering angle for each of the front
wheels is arranged similarly to that of a bicycle in that the
steering angle is slanted toward the rear of the car. Castor
determines whether your car will have a tendency to wander at higher
speeds. The more positive castor, the more wandering and
'nervousness' at higher speeds. However, positive castor offers
another benefit
positive castor generates a self-centering force in
the steering. If you set up a car with negative castor, it would
travel in a straight line all day long and you wouldn't have to touch
the steering wheel one bit. However, if you could manage to steer
the thing (steering this car would be a real bitch) when you let go
of the steering wheel it would stay at that angle rather than
snapping back to center. With the right amount of positive castor,
steering is light and easy, the steering wheel will tend to snap back
to center on its own, and directional stability is still reasonably
good. Also, because castor introduces camber into the wheel as it is
rotated around its steering axis, it helps the car steer. If you
turn the wheel to the right, the right front wheel's camber will
become POSITIVE and the left's will become NEGATIVE. When you are
cornering hard, your tires become distorted where they touch the
road
in very hard cornering, the tires may distort so much that the
sidewall of the tire comes in contact with the road, while part of
the tire's tread has lost contact with the road. This is NOT a good
thing. But thanks to the castor angle, when the steering wheel is
turned, each front wheel/tire is leaned into the corner in such a way
as to minimize this effect.
You actually can adjust castor to some extent on a TR6
but leave
this up to the alignment shop. Take your car to a good alignment
shop and bring them a good supply of the proper alignment shims for
both front and rear.
>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. After extensive repair to the
>rear structure, I am have problems getting the toe straight. I have to put
>wedges in so as not the bend the rubber bushing to bad. Yes I know I should
>have the frame shop fix this, maybe next summer but right now I want to get
>the car driveable so the wedges are my best bet. While I am doing this I
>want the get the camber right with the different brackets, one, two or three
>notches. Right now I am sitting at -3.1 and -2.6 L&R, and the car has the
>TR6 squat. Well any help would be appreciated. Thanks in advance.
Positive camber is when the top of the tire leans AWAY from the car,
negative camber is when the top of the tire leans INTO the car. Are
you using the proper alignment shims under the trailing arm brackets?
Camber in the rear will be more a function of which brackets you use
and the height and stiffness of the coil spring. But toe will be
adjusted by adding/subtracting the alignment shims under the trailing
arm brackets.
Best of luck!
--
Pete Chadwell
1973 TR6
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