Porsche 944 turbo corner balancing
The question of corner weighting comes up frequently. A common misconception is that corner weighting balances the car left to right or front to rear. This is not true. What it does do is most effectively transfer weight to all four tires.
The center of gravity of a car is fixed and can only be adjusted by moving physical parts of the car that have weight. For example, you could move the battery of a car from the front to the rear or side to side to effect the total weight distribution.
Given all the parts of your car generally stay in the same place, you cannot change the center of gravity, and therefore you cannot change how much weight is on your front vs. rear axles, or on the left versus right sides of the car.
The goal, therefore, is to distribute the weight in the same manner on the front and rear axles, or on the left and right sides of the car. As it turns out, satisfying one condition automatically satisfies the other.
The goal of corner weighting the car is always the same:
(Notation lowercase side, UPPERCASE end)
l vs. r percentages are the same Front and Rear. i.e.: Fl/(Fl+Fr) = Rl/(Rl+Rr)
This must mean that:
Front vs Rear percentages are the same right and left. i.e. Fl/(Fl+Rl) = Fr/(Fr+Rr)
it can only be so. The first implies the second.
Here's why:
Assuming the car is kept fairly level, it is impossible to move the center of gravity of the car by adjusting the suspension. That is a physical property of how weight is distributed in the car. That being the case, the following totals will remain constant:
right side weight (Fr+Rr)
left side weight (Fl+Rl)
front axle weight (Fl+Fr)
rear axle weight (Rl+Rr)
Therefore, any car can be weight balanced by a single adjustment, as long as you
don't care how level the car is.
Spring or torsion bar adjustment moves weight from one diagonal
(Fr+Rl) to the other (Fl+Rr) and vice versa.
For example, moving a spring perch DOWN OR adjusting a spring plate UP
on the Rr moves weight off that diagonal (Fl + Rr) to the other diagonal (Fr+Rl). The reason for having adjustments front and rear, and left and right, is so a car can be leveled. Example 911: Begin FRONT 500 LEFT 700 REAR Here we have car with 900lbs on the front axle and 1500 on the rear,
for a total of 2500 lbs. It can be quickly calculated that 36.0% is in the front and 64.0%
in the rear, and that the car is 48.0/52% left-right overall. The car can be balanced by making sure: a) Each axle has a 48/52% split or b) Each side has a 36/64% split. (actually one cannot be true without the other) The answer is 432/468 on the front and 768/832 on each rear wheel FRONT 432 LEFT 768 REAR These are obtained by multiplying the total axle weight by the desired
percentages. By adding up you'll notice the same l/r and F/R ratios as originally
calculated. This can be accomplished either by increasing the weight (raising) on Ll and/or Fr or decreasing the springing (lowering) on Lr or Fl. Note that corner balancing should be done with the driver in place, and
generally with 1/2 tank of gas, i.e. in "as driven" configuration. Now all you need are a set of scales! Question: Can I corner balance by setting ride heights? Not usually. It is posible to set ride heights and corner weights
simultaneously. This is done by adding weight to corners that are too low, and subtracting from corners that are too high. Here's the problem: Let's say, for example, that your car has an effective spring rate of 200 lb/in (this is not too far off for a 911), and that you set the car
so all ender lips are at 25" (also pretty close).For illustrative purposes, we'll pretend you have a C2 with springs,
and the following measurements are taken from the lowest perch adjustment to current adjustment: FRONT 2" LEFT 2" REAR Note that if you take the same amount off two corners on the same side
or on the same axle, the weights will not change, the car will just lean or rock end to end. Again, you need to be concerned with
adjustments ot the diagonals. And we'll use the same weights: Begin FRONT 500 LEFT 700 REAR How is this possible? Well, cars generally aren't perfectly square. The chassis is a bit off.
Also, because of the heavier right side loading of this car, the perches on
the right side should be about 1/4" higher each (50 lbs/200 lbs/in = 0.25
in)What is the optimal answer without changing ride heights? You need to move 136 lbs from one diagonal to the other.
Here are the adjustments in inches and pounds: Begin FRONT -68 (-.34") LEFT +68(+.34") REAR The car should stay level. I picked a nice example with constant rate springs. In reality, you
look for a corner that's a tad low and raise it, or one that's a bit high and lower it. Since you have 4 corners to choose from it's not too bad! For example, if the Fl was too high, you could take an 1.36" out of that, the car
would pivot slightly on the Fr and Rl diag, and you should end up fairly close. It ends up being iterative, which is
what having 4 adjustments is very
handy. Dr. Steven J. Timmins PhD Mechanical Eng/Design (302)456-0630 H
400
RIGHT
900
468
RIGHT
832
2"
RIGHT
2"
400
RIGHT
900
+68(+.34")
RIGHT
-68(-.34")
http://me.udel.edu/~timmins 944turbo widebodylightweight (Someday)
373 West Chestnut Hill Rd.
Newark, DE 19713
(302)831-6675 D