"Crowned road"

Corner Balance
 

Below is a writeup I did for the local PCA as an instruction sheet to go along with the local club's corner balance scales. It is a little long but for anyone interested, it can be cut and pasted for future reference.

Use of the Corner Balance Scales by Jim Williams

INTRODUCTION

What is corner balancing and how does it work?

Visualize a sturdy chair with legs of four equal lengths. Compare the chair to a car chassis (with four wheels).

Into the chair put a heavy object such as say a cinder block. Rather than put it in the center of the chair, move it off center, 6" back from center and 2" off to the left of center. Compare the chair above to a car, to which we will now lump all the car components, (fuel tank, battery, engine, transmission, driver, etc.) For the 911 with its rear engine, assume the weight distribution of all these components produces the same sort of off-center weight balance as will the concrete block in the above chair.

For the chair, if we never change position of the concrete block, the weight balance (front to rear and side to side) of the chair will always stay the same. The same holds true for the car. If the physical components of the car stay in the same place, the car's front to rear and side-to-side weight balance will not change.

We can calculate the position of the chair's Center of Gravity by placing a small scale under each leg. From the 4 scales, we can calculate 5 sets of figures: the sum of the front legs, the sum of the rear legs, the sum of the right legs, the sum of the left legs, and the total of all four legs. The front to rear balance is the sum of the front legs divided by the sum of the rear legs. Similar logic applies to the side-to-side balance.

Now let's trim a small amount off the left front leg, enough that with our sturdy chair, this leg just no longer touches the floor. Now the total weight of the chair and concrete block is borne by three legs, with part of the weight shifted to the right front leg and part shifted to the left rear leg. The total weight is still the same, but each leg is not carrying its share of the load. Getting each leg of the chair, or each corner of the car, to carry its proper share of the load, is what the corner balance process is all about.

SCALE SET UP

Our scale ramps were designed to accommodate different wheelbases. The center portions of the ramp dovetail for this purpose. Measure your wheelbase to position the scales/ramps. The scales must be level side-to-side or front to rear in order not to cause a bias (diagonal "jacking") in the readings. To be the most accurate, they should all be level, but for the accuracy expected for the use of these scales, may not absolutely necessary. Slight shimming under individual scales may be necessary to achieve leveling. Slip plates should be lubed and placed on top of each scale. The car can be driven slowly up the ramp, but stopped short of the scales and pushed on to by hand. If driven onto the scales, and if significant momentum is built up and braking is applied directly on the scales, the slip plates can slide off the scales, with possible damage to scales/ramps. Best to stop the car short of the scales, and push the car onto the scales manually. Also, individual scales that are not level can cause the car to slide off the plates.

Zero scales prior to driving car on.

CALCULATION OF DESIRED READINGS USING SCALES

1. (Optional) Weigh car empty for reference purposes. Note year, make, model, fuel level in gas tank, any modifications affecting weight from stock configuration.

2. Weigh car (with driver). Since the car will only be expected to perform with a driver, and the driver's weight affects corner balance and handling, it should be obvious why the driver or his/her equivalent weight needs to be in the driver's position. (If only one person is available to perform the measurement, position the scale display so it is visible from the driver's seat.)

3. Note (record) weights at each corner. Since on a rear engine car the weight bias will be to the rear, you should expect to see more weight in the rear. A 40%/60% front/rear bias should be expected on a 911. Due to the fact that seldom (read that *never*) will the Center of Gravity (CG) be in the geometric center of the car, the weights will be different at all four corners.

4. Even after the corner balancing of the car, an off-center CG will still result in different corner weights at each on the 4 wheels. This is due to the fact that the CG will still be, after the final suspension adjustments, where it was before the adjustments.

5. Although the corner balance scale readout can calculate front to rear weight bias, a short method for calculating the ideal weight to be attained at each wheel after optimum adjustment is as follows:

Designate the weights you initially measure at Left front = A, Right front = B, Left rear = C, and Right rear = D, and record these. The weight goal you want to attain for:

Left front = (A + B)(A + C)/(total weight)
Right front = (A + B)(B + D)/(total weight)
Left rear = (A + C)(C + D)/(total weight)
Right rear = (C + D)(B + D)/(total weight)

After adjustment, if everything is perfect, you want the weight to be read on the scales at each wheel equal to Lf, Rf, Lr, and Rr.

6. The above includes the necessary measurements and calculation required to do the corner balance, but leaves out the discussion of theory, and the actual adjustment procedure. I have several Excel spread-sheets that perform the calculations of the above formulas, handy if there is a laptop near to the scales.

7. Note that for some later model stock Porsches, the 996 being one, that unless the car has had a suspension modification performed consisting of adjustable coil-over shocks, the corner weights are not adjustable, nor is the ride height.

I had the same problem when I was teching a students car for a DE. It was a 1990 C2. When I accelerated, the car would want to steer to the side. Turns out the owner recently purchased a used set of wheels with tires mounted. The seller must have just mounted some tires he had laying around...the aspect ratio was different from left to right on the rears. A new set of tires corrected the problem. The difference in dia made the car steer to the side.

My first thought was air pressure in the rear tires, but a broken belt could also cause the problem.

Jim W.,

Thanks very much for the detailed write-up on corner balance - copied and put into my Porsche files!

After getting the car up on the rack, torsion bar rubber bushings were inspected and appeared as new. No debonding of rubber was evident on either side. Some prying with a bar to deflect radially showed equal results side-to-side. The two control arm bushings were inspected next, with gentle prying in both the radial and lateral directions. Both appeared firm and still compliant.

Wheel bearings were next on the list. Cocking the tires and axial load inputs showed no slop in either bearing. With a stethescope against each bearing housing, wheels were rotated individually and no audible bearing distress was noted. Both rear wheels rotated freely showing that brake drag was nill.

I think I found the problem! The left-side stabilizer bar flange had a hairline crack all the way through. This was easily welded and after some grinding and painting, was almost an invisible repair. I really find it difficult to believe this was causing the problem I was experiencing, but it is now repaired and the bar is supported properly.

During the drive home, it was too windy to evaluate if the pull-to-the-right tendancy was eliminated so this will have to wait for calmer conditions. Nevertheless, a component failure was repaired and this makes me feel better.

Thanks to all who contributed suggestions - greatly appreciated!

Jimmy Clark had a related issue. Unfortunately, at race speed. Glad you discovered it.

Sherwood