November 16, 2009 · Print This Article
Magic is an interesting concept. If we look back through the ages at things which were not understood at the time, they were often referred to as magic. Pinion angle is a term I hear tossed about as some sort of magic incantation quite often. Let’s dispel the myth.
Pinion angle in and of itself does not affect the way a car launches. A change in pinion angle does change the angle of the four link or ladder bar brackets on the rear axle housing. So a change in pinion angle will change the angle of push and pull on your rear suspension components, but, and this is the important part, we do not want to use pinion angle as a rear suspension tuning aid.
What we do want to do with pinion angle is have the drive-line in the right relationship so the universal joints work correctly.
Universal joints; this is really cool. Picture a simple shaft, now add a single universal joint in the middle of it and bend it slightly. You can now rotate one end of the shaft, and the other end of the shaft can also rotate, even though it is, in effect, going around a corner. (that’s not the cool part) If you could rotate the input end of the shaft at a very accurate constant velocity, and if you could accurately measure the velocity of the output shaft as it rotates, you would find that the output shaft does not rotate at a constant velocity. In fact through one revolution it actually speeds up and slows down several times as each cross of the universal joint changes angle, even though it still completes one revolution in the same amount of time as the input shaft.
The amount of this acceleration and deceleration changes with the angle of the universal joint. (you can see where I am going with this already can’t you?) From there it is pretty simple to visualize what we might want from a driveshaft with two universal joints in it, we would want both joints to accelerate and decelerate the same amount, at the same time, if we do not, we end up with vibration. We will not always feel the vibration in a drag car that is accelerating, but it is tearing up universal joints as they fight each other. So, pretty simple, we want both universal joint angles to be the same. Mark Williams has a great drawing here to show what I mean.
Now comes the bit that seems to be magic, we want the universal joints to operate at the same angle. . . when the shaft is turning.
Generally if the car has universal joints, it has suspension, suspension allows movement, which will change the angle of the u-joints, so we need some sort of compromise. Logic says we would want our universal joints to be operating at the same angles when they are under the greatest load. Which in our case is when we launch the car. If you remember back to a previous installment, when we launch the car, the rotation of the pinion gear on the ring gear causes the rear axle housing to rotate around the axles in the opposite direction of the tires. Put simply the front of the axle housing wants to go up, the front where the pinion goes in. . . the pinion that is on the end of the driveshaft. . . after the universal joint. So we will want to take this movement into account when we are setting our pinion angle.
So now, when you are setting up your pinion angle on the floor of your shop, you want to add a little to the angle to compensate for the rotation of the rear housing when you accelerate, here it comes that number you have heard, two degrees down. Ok, two degrees is not always the “exactly” correct angle, but it is a good starting point. but this does not mean that your pinion is set to two degrees, what you want is two degrees more to compensate for rear axle housing rotation. Let’s refer to the Mark Williams drawing here. If Angle “B” is ten degrees, you would want angle “A”, at the pinion, to be pointed down a little to compensate for torque reaction during launch, (the rotation of the front of the axle housing upward), so angle “A” should be about eight degrees. Of course the number itself is dependent on where you measure the angle, but let’s put it simple, you want the pinion pointed down about two degrees more than the back of the transmission. Of course this is all at ride height with fuel load and driver weight in place.
I think one place where many go wrong with the whole pinion angle deal is they do not correct pinion angle after they adjust the rear suspension. Particularly with ladder bars, if you change the front mount location, you must correct your pinion angle.