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Pinion Angle question

This is a discussion on Pinion Angle question within the Suspension and Handling forums, part of the General Help category; So I already know that the correct pinion angle for a stock height is -2 degrees. My question is that ...

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    Honor-Courage-Commitment SajoR's Avatar
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    Pinion Angle question

    So I already know that the correct pinion angle for a stock height is -2 degrees. My question is that I have my car lowered with Eibach Pro Springs and Edelbrock shocks. Is the pinion angle the same or different?

    Wanting to know for my adjustable torque arm

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    Member c5z28's Avatar
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    It will be off, how much I don't really know. Most adj torque arms will come with a tool to help you get it back where it needs to be.

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    Honor-Courage-Commitment SajoR's Avatar
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    Anyone have a clue? Lol or should I just keep tweaking it until it seems to be at optimal performance?

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    Moderator 35th-ANV-SS's Avatar
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    Read this...

    Both pinion angle and instant center are effected by the torque arm. Lowering the car will effect both.


    Negative pinion angle is when the nose of the carrier housing (the pinion area) is pointing down towards the ground instead of parallel with the tailshaft of the transmission.

    Positive pinion angle is when the carrier housing & pinion are pointing up from the parallel line.

    Pinion angle is measured at the pinion when the vehicle is sitting at rest and is the angle of the pinion, it is not necessarily the angle of the torque arm.

    If you do not change the outside diameter of the rear tires, and you lower the vehicle with springs then the rear axle is still at the same height from the ground, but the front torque arm mount is lower in relation to both the ground and the rear axle. Since the rear of the torque arm is solidly attached to the rear axle, If you lower the front of the torque arm that will twist the front of the axle down. This would cause negative pinion angle. To return the pinon angle to 0 or to just make the pinion angle adjustable you can use a torque arm with a turnbuckle or threaded rod-end at the rear axle end of the torque arm. This will allow for adjustment of the pinion angle in relation to the transmission tailshaft. You can also change pinion angle by moving the front mount of the torque arm up or down but that also changes the point known as the instant center.
    Ideally you want 0 pinion angle at all times.

    When using constant velocity U-joint driveshafts, both ends must be at the same angle (0 difference) to maintain constant velocity. Any angle difference, positive or negative will cause driveshaft U-joint binding with pulsing and surging. This will eat U-joints over time and the vibration, pulsing and surging will cause traction loss and the binding wll slow the car.

    Drag cars are set up with negative pinion angle when at rest (usually around -2) so that when the axle twists under extreme force from acceleration and the pinion moves up towards the floor board the pinion angle will be 0. The idea is to have 0 pinion angle while accelerating so there is no surging, pulsing or binding in the driveline. If the pinion has positive angle (angled up) when at rest and then when under acceleration it moves up more, that would throw the driveshaft and U-joints way out of line and cause a very large amount of bind and surge.

    Now for the fun stuff:

    Where the instant center is effects traction much more than pinion angle. When the rear axle twists under acceleration it applies a rotating force on the control arms and the torque arm. This force pushes up and back on the body of car. The point at which this force is concentrated is called the instant center Moving the instant center back will push up on the car body over the rear tires, forcing the tires down into the pavement. Think about doing pushups. Your arms lift your shoulders away from the ground while pushing your hands into the ground. Moving the instant center up in relation to the cars center of gravity will help transfer more of the front weight of the car onto the back tires.

    Finding the instant center is easy on a car with a four-link rear suspension. Draw a straight line through the center of the front and rear bushings of the upper rear control arms to the front of the car. Do the same thing for the lower rear control arm. The point where the lines cross is the instant center.
    Move the mounting location of either the front or rear mounting location of either the upper or lower control arms and you will move the instant center. The front and rear mounting points of both the upper and lower rear control arms of a four link are static. Their bushings pivot around bolts and have no forward or backward movement.
    The front and rear mounting location of the rear control arms on an F-body is also static. The rear mount of the torque arm is also static but the torque arms front mount is not. The front of the torque arm not only rotates in the bushing but it also slides forward and backward in the mount. This makes it a little more difficult to find the instant center in an F-body, but it is still the same basic relationship between the torque arm and the control arms. In simplest terms the torque arm more or less replaces the upper control arms of a four-link.

    On an F body, instant center is affected by the angle of the torque arm (front mount height in relation to the axle), the length of its front mounting location from the rear axle, and the angle (front and rear mounting locations) of the rear control arms.

    When Tory moved his front torque arm mount up he moved his instant center upwards, in relation to the center of gravity, helping to transfer more of the weight of the car onto the rear tires. Moving the front of the torque arm up twists the axle and would cause positive pinion angle. When you add the natural twist of the axle when under acceleration it would go even more positive causing bind and pulsing in the U-joints. The pinion angle would then need to be adjusted to compensate unless the car had too much negative pinion angle before moving the front torque arm mount up.

    If you lower an F-body you lower the front mount of the torque arm in relation to the rear mount of the torque arm, moving the instant center down. You also move the front mounting location of the rear control arm down in relation to its rear mount. Due to the short length of the rear control arms their angle changes drasticly. This moves the instant center way further forward and way, way down. This will seriously hurt traction. A lowered car needs to have the mount of the axle end of the rear control arm lowered also to correct the instant center geometry.

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    Honor-Courage-Commitment SajoR's Avatar
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    Yup that sufficiently explains things to me so pretty much for my style of driving I need to maintain a negative degree pinion angle.

    Thanks for the knowledge, saving this for later

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    Moderator 35th-ANV-SS's Avatar
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    No problem. Glad it cleared it up!

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