Weight Balancing Explained

Here's the deal:
If you haev a 911 your car has torsion bars, but they are directly analogous to springs with adjustable perches.  A perch is what holds the spring to the shock absorber.  By raising or lowering a perch (they are mounted on screw threads on serious sports cars, see the 993 lightweight project)  At the factory the car is balanced, i.e. the weight distribution amongst these springs is evenly distributed.

The best way to think of why it's important is to see what happens when it is not set.  It's entirely possible to set your car up so that nearly all he weight of the car is supported by one diagonal.  This is just like what happens on a 4 legged chair with one short leg.  Most of the weight is supported by only two opposing legs and the chair rocks diagonally.  Now imagine these legs made of rubber, and you have the picture.

Here's the catch that really confuses people:  You cannot change where the weight is distributed in your car, only HOW it's distributed.  What?

Let's say your car weighs 3000 lbs, 1000 on the front tires, 2000 on the rear tires, and is evenly distributed right and left, plausible for a 911.  It is fairly obvious that the proper weight distribution is:
 
 
 
   
Front (33.3%)
   
 
500
1000
500
 
Left (50%)
1500
3000
1500
Right (50%)
 
1000
2000
1000
 
1500 RF+LR
 
Rear (66.7%)
 
1500 LF+RR

The bold numbers are four tire weights and total weight, the center is total vehicle weights, and the interstitial numbers are axle and side weights.  Please note that you cannot change total side, axle or vehicle weights or teh center of gravity without physically moving stuff in the car, so these are essentailly constant.  Note that the diagonals are equal.

But, it could be set to:
 
   
Front (33.3%)
   
 
0
1000
1000
 
Left (50%)
1500
3000
1500
Right (50%)
 
1500
2000
500
 
2500 RF+LR
 
Rear (66.7%)
 
500 LF+RR

And still meet the above criteria. Note that the left and right side totals are still the same, as is total vehicle weight, and so are the front annd back.  Note that the diagonals are now skewed. 83.3% is on RF + LR and 16.7% is on LF+RR.  In this case one tire is off the ground.  Note that it is not possible to lift a rear tire in any situation, this is the limit.

Also note that if you know one tire weight, and the total side and axle weights, everythign else is determined, i.e. there is only one indepenmdant variable.  For example:  Make the LF 600lbs.  Since the front axle is 1000lbs, the RF must be 400 lbs.  SInce each side is 1500 lbs, LR is 900 lbs and RR is 1100 lbs, and you will see that the rear axle then is 2000lbs, whcih checks out.  This means that if you don;t care abotu ride height, you only need one adjustment on any corner to acheive proper weight balance.

The rule is that you canít move total weight front or back or right or left, so this still meets the 1000/2000 FR and 1500/1500LR rule.  The car would handle horribly, with one front tire in the breeze (it could be completely off the ground) and I can tell you that in any real car the chassis would flex to the extent that you it may well be difficult to open the doors!  This is called "Cross-Jacked" because you have 83.3% of the weight on one diagonal, RF and LR. The rule is that the diagonals should each have 50% of the weight on them in all cases.

This can be adjusted on a 911 by screws in the front and eccentrics in the rear which determine both weight on each tire and ride height.  If you attempt to put more weight on a tire, that corner will also lift, and you can imagine if you try to put more weight on all four tires, the chassis will simply become further off the ground, or ride height will be raised.  On 944s (witht he exception of the 944turboS) there are only adjustments on the rear spring plates, the front cannot be adjusted, but as shown above, you only need one adjustment to set the weight balance, whether or not it is level is another matter.

How is this done?  With a set of scales connected to a central computer readout.  You adjust one corner at a time, keeping an eye on ride height and weight.  If you want to change weight distribution without effecting ride height you could raise one front adjuster and lower the other and the car will not move up or down much.

You can se a picture of this being done below.  It needs to be done before the alignment as changes in ride height effect alignment settings.  A set of scales is about $2000 so I tack $50 on to everyone who uses them to recover costs.  The actual adjustment, if the car is close in the first place is usually 15 minutes or so.

Whenever suspension components are changed or large changes in ride height are performed a weigh balance is necessary.

Why is this irrelevant for say, American cars, two reasons:  If springs are soft enough, and typical for an American car might be 100lb per inch, you have to be WAAAAY off to have it make a dramatic difference, and spring perches or leaf spring mounts or whatever are not adjustable. On the other hand on a race car, like the Tealamonstre, I run 450/650 springs front and rear, so a 1/4" deviation from optimal can make for  a large imbalance.

American cars with torsion bars, like the 1970 Dodge Dart, must have an adjustment mechanism.

Congratulations, you have just passed Vehicle Dynamics 101.
 
 
 

Here's the whole setup.  We have the car set up on the lifting tables, with the turn plates, then the scale pads on top of that.
We can pop off the alignment heads and go directly to eright balance.  This way the rear scales "float" on the turn plates.

Interestingly (and surprisingly) the settings were spot on to within the resolution of the machine (0.05 degrees camber, 0.01" toe)
The maximum resolition of the old optical equipment was about 1/32 in toe.