Pinion Angle.

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.

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Comments

2 Responses to “Pinion Angle.”

  1. Mike Kennedy on October 18th, 2011 11:26 am

    excellent explanation. I’ve worked on many car systems, however not really the suspension system except to have a basic understanding of the parts and the experience to be able change out parts needing replacement. I’m working on setting up an 85 Monte for street / strip and i’m getting around to the suspension system. I’ve read a bunch of articles about pinion angle and what should be the ideal setup, and how ride height and engine angle is the starting point of all thoughts related to this topic, however what was missing was the WHY and a description of the associated torque dymanics occuring necessitating a look at this magic. i never thought to think about the changing velocity of the output shaft as it is spinning with the driveshaft at different angles. As a result of reading your article i can now understand driveline vibration & U joint wear, and also the twisting of the axle shaft which then causes the pinion angle to change. I thought it was the torque from the driveshaft itself that caused the pinion to raise, but now i understand the raising pinion angle is actually from the axle shaft wanting to rotate backwards. Through your description of these forces i was able to follow the WHY. Knowing the WHY something happens is very important. Ok, so now that i think i understand what is going on when i launch, other articles i’ve read state that only leaf spring cars w/o traction aids on the springs to reduce winding are affected by pinion angle changes during launch and that similar traction control devices are unnecessary on your typical 4 link rear such as you would see on a coil spring car like my monte. I am considering an adjustable rear supension kit that i thought would be a traction aid by effectively allowing a change in the pinion angle. If the pinion angle seems to be correct with the stock upper and lower control arms, and assuming the bushings are in good shape is there any added benefit to changing them to these adjustable aftermarket “lift bar” kits and gaining additional traction aid?

  2. Jim on October 18th, 2011 8:59 pm

    Thanks for your kind words Mike.
    It is difficult for me to assess if there is a benefit to you changing to this “adjustable rear suspension kit” as I don’t know what problem you might be trying to remedy? I gather that your pinion angle is correct. Are you having traction problems?, and if so, can you describe what your traction problems are? Then we can work toward identifying what is causing the problem, once we have identified what is causing the problem, then we can look at ways to resolve it.

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