Corner weights hope to give you an indication of whether your frame and
suspension components are straight and equal in strength and length.
Consider a car that has it's center of gravity equidistant from front and rear,
and from left and right. This would imply that .25 of it's weight would be
measured at each corner. The problem comes if one corner's spring is
a little stronger, or a little longer, or it's tire is over-inflated, or the
upper
mounting point for the spring is slightly lower etc. Any of these factors
would cause that the corner to be carrying more weight than .25.
Of course the sum of weights at all corners is the same, and if you draw
the picture, you will see that the diagonally opposite corner will now also
carry equally more weight, and the other two corners lighten their load equally
to compensate overall. The mathematical trick that provides the info, is that
regardless of the static weight distribution of the car, a perfectly, neutrally
suspended car will have the sum of it's two diagonal corner weights equal!
The weight measured at the right rear plus the weight at the left front should
equal the sum of the left rear and right front. Any structural imbalance will
cause the cross-weights to differ.
Getting your corner weights right won't guarantee you *equal* cornering
ability in both directions, because a car whose weight is skewed to the
left will always have the potential to corner better to the left. What corner
weighting does do is get you as good as you can get for a car that must
turn both left and right as well as possible. The fact is that corner weight
imbalance will often *help* turning one way, but will hurt turning the other
way even more. Circle-track racers purposely run imbalances by running
a larger tire on the outside rear to get more weight there, and therefore
balancing the important outer front and rear corners in their big front-heavy
american sedan chassis. If they had to turn around and race the other way,
they'd be so loose at the rear it'd be comical/suicidal.
Other racers do play with corner weighting on road courses. Interestingly,
at Sears Point, which is a clockwise course, you would think you'd bias
toward right-handers, but experts I've talked to say that the two big
left-handers,
into turn 1, and the 'carrousel' are so long and fast, that one should tune for
them, and tolerate some resulting tail-happiness in the other corners.
Note how many factors can contribute to corner-weight imbalance. More
than one of these problems can exist at a given time, and the seductive
danger is that they can cancel each other out *at a single suspension position*.
For example, if one of your springs is a little stronger than the others, but
the
tire on that corner is under-inflated, you may measure perfect cross-weights,
but are you ready to race? Here's how to solve that: Change the suspension
position and measure again. You change the suspension position by adding or
removing some weight from the car for the second measurements. It needn't be
anywhere special, just load the car differently (still level on the scales) and
if the
car really is perfect, the new corner-weights (which will be different than
before, obviously) should still provide equal cross-weights. The usual way to do
this is to do your corner weights with you in the driver's seat as you would
race,
and then do the weights with you out of the car, or if you want to be pushy,
with
you sitting on the right rear fender!
If the car, suspension. bushing friction, compliance, anti-sway bars,
and tires etc are all perfect, then both sets of cross weights would have
matched.
In reality you usually get a numeric indication of how imperfect your car
is... In real life, anti-sway bars are hard to tighten at their ends so they
don't
impose some static torque one way or the other, and this muddies the
corner-weight measuring, so some folks disconnect one end before even
taking the measurements!
The point of taking two measurements is that some shops will try to patch
one number. In my example, let's say you checked your pressures, and
filled the soft tire and found the imbalance due to the stronger spring The shop
might lower the stronger spring's mounting perch until the corner weights were
equal in the one weighting condition you were measuring. Are you ready to race?
In fact the problem might not be that you had a stronger spring, but the spring
perch
was too high before, and now you're cured, but if it *was* a stronger spring
(such as if the other one was older, and owing to abuse or poor quality had
sagged), you're still in the same unoptimal state, and won't know it. Of course
you're perfect when you are cornering under these exact conditions, *with no
lateral load and weight transfer*, but as soon as you are really cornering, all
bets are off. You need the second measurement to know whether your hot new
aftermarket suspension installer has a puzzle they must solve before you leave
the shop.
Joe Weinstein
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