Fast Toys

We've had a number of questions about what the Spohn rear adjustable upper control arm does and how you adjust it. Here is a little write up.

The rear upper control arm is used to set the pinion angle and to keep the rear end from rotating under power. This iw what helps to keep the rear end planted and give you a good launch. Since there is only one on the Mustang, it MUST be able to articulate. For this reason, you should avoid any upper control arms that have only a solidpoly bushing as that is a problem waiting to happen.

Setting Pinion Angle

There are two angles to deal with:

1) Driveshaft angle
2) Pinion angle

You subtract pinion angle from driveshaft angle to get TRUE pinion angle

Here's how you do it:

Using an angle finder place it on the underside of the driveshaft and record the angle indicated.

Next, place the angle finder on the underside of the pinion yoke and record the angle indicated.

Subtract the pinion angle from the driveshaft angle. The result is "TRUE Pinion Angle". In order to apply preload you need negative TRUE pinion angle. Adjust the upper control arm so that the front of the pinion goes down; continue to check each angle until the pinion angle is more degrees down than the driveshaft angle.

We recommend -1 degrees on a mildly modified daily driven car. For high horsepower applications we have gotten the best results with -2 to -3 degrees. There is no reason to run more negative then that, it will actually hurt your performance because it will induce driveline bind. The goal here is 0 degrees pinion angle when under load.

Here's a tip. When adjusting for your TRUE pinion angle, count the number of flats (or the 1/6 of a turn) as you turn the adjuster, to know how many it takes to adjust 1 degree of negative TRUE pinion angle and in what direction (clockwise, or counter-clockwise). Once you know that, then adjusting the arm at the track or before a race will take almost no time, and no angle finder will be needed.

CrazyAl

Nice writeup, but a quick tip: while technically you are looking for a negative number, it may not work out that way depending on how your angle gage is calibrated...just be aware of that.

Also keep in mind that these directions are appropriate only for the OEM 2-pc driveshaft. If you use a 1-pc driveshaft then things are different.

Here is a writeup for the 2-pc shaft with pictures showing how you place the angle gage: https://mustangforums.com/m_2570375/tm.htm#2571016

And here is the technique for an aftermarket 1-pc shaft:
https://mustangforums.com/m_2570375/tm.htm#2571832

And finally I'd like to mention that an angle of -2 to -3 degrees is great for a mildly modded car, but the more mods you have the less negative angle you need. Becasue a fully modded suspension flexes less than a stock (or nearly stock) one, you don't need to dial in as much allowance for housing rotation.

Fast Toys

Awesome write up too. Is there a tech section where these can be bookmarked for future reference?