Alignment or Corner Balance first?
02-18-2009, 02:16 PM
#81
The spring rates actually do come into play when adjusting the cross balance. You use them to figure out how far to raise or lower the spring seat (regardless of whether you're raising it by moving the seat on the body, or extending the body with the seat on it).
Say you have a 4000 pound car with 1:1 motion ratios on all four corners, and 500 pound/inch springs. Suppose the weight distribution is 50/50 front/rear and left/right. When the car is sitting on the ground, it should have 1000 pounds resting on each wheel if the cross-balance is correct. Each spring is compressed 2 inches.
Suppose something is tweaked, and the weights look like
Front
950 1050
1050 950
Rear
So either your frame is warped, or your suspension elements are bent, or you have a spring that is longer/shorter than the others, or you don't have the spring seats all set symmetrically. What do you need to do? You need to lower the seats on the right front and/or left rear, and/or raise the seats on the left front and/or right rear. But how much should you move them? Well, the right front is carrying 50 pounds too much weight, so you would lower it by (50 pounds / 500 pound/inch) = 0.1 inch. Then you would lower the left rear the same amount, and raise the other two corners the same amount. Or, you could just lower the right front 0.4 inches and have a tilted car.
Now suppose you did the same experiment with the shock shafts welded to the bodies so that they were rigid, like Mike suggested as a thought experiment. Supposing you have the same geometric error as above, your corner weights will look something like:
Front
2 1999
1999 0
Rear
As you lower the / diagonal and raise the \ diagonal, those readings won't change much until you get all the wheels in the same plane -- the table legs the same length. If you go a little bit past, then the numbers will flip and you'll get
Front
1999 2
0 1999
Granted, if you're not going to try to calculate how much to raise or lower the spring seats, then the spring rates won't matter much to you. You just keep turning the knobs until the cross weights come out equal, then you stop.
The spring rates will not affect where the spring seats are when the diagonals are balanced. However, they will affect how far off the diagonal weights are when the geometry is not correct.
Say you have a 4000 pound car with 1:1 motion ratios on all four corners, and 500 pound/inch springs. Suppose the weight distribution is 50/50 front/rear and left/right. When the car is sitting on the ground, it should have 1000 pounds resting on each wheel if the cross-balance is correct. Each spring is compressed 2 inches.
Suppose something is tweaked, and the weights look like
Front
950 1050
1050 950
Rear
So either your frame is warped, or your suspension elements are bent, or you have a spring that is longer/shorter than the others, or you don't have the spring seats all set symmetrically. What do you need to do? You need to lower the seats on the right front and/or left rear, and/or raise the seats on the left front and/or right rear. But how much should you move them? Well, the right front is carrying 50 pounds too much weight, so you would lower it by (50 pounds / 500 pound/inch) = 0.1 inch. Then you would lower the left rear the same amount, and raise the other two corners the same amount. Or, you could just lower the right front 0.4 inches and have a tilted car.
Now suppose you did the same experiment with the shock shafts welded to the bodies so that they were rigid, like Mike suggested as a thought experiment. Supposing you have the same geometric error as above, your corner weights will look something like:
Front
2 1999
1999 0
Rear
As you lower the / diagonal and raise the \ diagonal, those readings won't change much until you get all the wheels in the same plane -- the table legs the same length. If you go a little bit past, then the numbers will flip and you'll get
Front
1999 2
0 1999
Granted, if you're not going to try to calculate how much to raise or lower the spring seats, then the spring rates won't matter much to you. You just keep turning the knobs until the cross weights come out equal, then you stop.
The spring rates will not affect where the spring seats are when the diagonals are balanced. However, they will affect how far off the diagonal weights are when the geometry is not correct.
02-18-2009, 02:28 PM
#85
Originally Posted by mikegarrison,Feb 18 2009, 03:22 PM
Who does that? Nobody I know sits there and tries to calculate that. Instead they just move it a little bit and see how things change.
02-18-2009, 02:38 PM
#88
Former Moderator
Originally Posted by Orthonormal,Feb 18 2009, 04:22 PM
Preload and ride height.
Come to think of it, I have a JDM bike as well (Suzuki)
