John Taylor [The Banshee] writes:
> >straight. This is usually undesirable! My TR2 has ZERO CASTER, which means
> >that the only thing making the car want to go straight, is the silentblock
> >bushings in the steering gear ( GAAK ).
A general rule of thumb which I've heard and which seems to be true is
that there are two major things which make a car want to self-center:
caster, and weight on the front wheels. So if a TR-8 and an X1/9 have
the same caster, the TR-8 will have a strong tendency to self-center and
the X1/9 won't (X1/9's, with almost no weight on the front wheels, are
famous for wandering in highway driving).
The question is, does this hold only with cars with positive caster,
or does it hold for all cars? I seem to recall that many pickup trucks
have negative caster, but are still quite stable in a straight line,
perhaps because of the excess weight on the front wheels.
> OK, so the Moke has positive camber. Why do racing cars have negative camber?
> Does this give better steering, or does it stop the wheels from being pulled
> under the car (| --> \ --> _) during high-speed turns, or is there some other
Yes, negative camber can help the tires from tucking under (SCCA requires
that cars with swing axles, like Spitfires, have significant negative
camber to prevent the wheels from tucking under and perhaps coming off
the rim and causing the car to roll). But the main reason most race
cars have negative camber is to compensate for decambering under load.
What you really want is to have zero camber at all times. When the
camber is zero, the entire portion of the tire's tread is in contact
with the ground, and the more rubber you have on the ground, the more
traction you have. Sounds simple, right? But when you add a cornering
load, you get more positive camber on the outside wheels (imagine the
whole car tilting so that the inside wheels come off the ground ...
notice what's happening to the outside wheels now), and with most
suspensions (e.g. Mac Pherson struts) this effect is made even worse
because body roll affects the suspension geometry to make the outside
camber even worse (that's why race cars tend to be very stiff --
you want to minimize body roll to minimize the decambering effect).
If you set the car up with lots of static (i.e. with the car sitting
still, no cornering load) negative camber, then when you get into a
hard corner, the suspension decambers and voila!, you have zero camber
on the outside wheels, which means you maximize your cornering traction.
Only one problem here: when you're going straight, e.g. when you're
braking in a straight line, you have no lateral force on the car,
and since you've set up the car for so much static negative camber,
only the insides edges of the tires are touching the ground, so you
get poor braking performance. (This also applies to acceleration, if
you have a car with enough power that traction is a problem on
acceleration. I wouldn't know about that. :-) )
So when you set up a car, you have to make a trade-off between straight-
line traction and cornering traction. If you make the car super-stiff,
to reduce body roll, then you need relatively less negative camber so
you don't have to compromise as much, but then you have trouble if you
want traction on bumpy roads. Using swaybars, you can get more roll
protection with less stiffening, so you're better on bumpy roads, but
then you've compromised the independance of the suspension and you may
end up with too much wheelspin accelerating out of turns. You also
have to take into account the tires: tires with soft sidewalls tend
to roll under during cornering, so they need more negative camber
(bias-ply tires, like Hoosiers, with their stiff sidewalls, work with
far less camber than radials -- on the X1/9, Hoosiers want less than
1 degree static negative camber, whereas A008R radials want about 4
degrees! You can tell by looking at the wear patterns -- Hoosiers
will wear first on the inside edges, A008R's always wear out on the
outside first).
So setting up any race car is a trade-off and you have to decide what
factors are most important to you. My personal choice is to use radials
(I don't like the way Hoosiers lurch), as much camber as I can get
(currently about 3.5 degrees until I get the time to fabricate camber
plates), super-stiff springs (I don't mind that the car gets airborn
over bumps -- in fact, I find it fun :-) ) and no swaybars (I had major
wheelspin problems with a rear swaybar, and none at all without it.
Besides, swaybars add weight). I leave the caster alone (never even
measured it) and use front toe to adjust turn-in characteristics
(and rear toe to adjust oversteer/understeer balance).
--
...Akkana (akkana@apple.com)
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