I need some advice, I'm changing the ride height on my car, how do you re center the rear diff? What do you measure from and to. It has an adjustable rear arm but I don't know what to center up on. Thanks!

 

Well, technically you're supposed to lower the panhard bar so that it's level at ride height, and then to center it you just measure out from a common spot on the "frame rail" on each side. It really doesn't matter which spot you pick as long as its the same spot on each side and to the same place on the rim/tire.

 

It's better to measure to some common place on the mechanical parts of the car itself, such as the inside surface of the backing plate flange or something like that on the rear, to some fixed place on the car chassis itself like the LCA mount bracket.

I.e., things you would NOT want to use, are anything that can move or flex or bend or be taken off and re-installed slightly differently; not the tire, not the fender, not the inner fender well, etc. All of those things can vary quite a bit.

 

You would have no problems measuring to the tire.

 

Except of course, if one tire had more air in it than the other; or the tires were different brands or sizes or something; or the wheels were bent; or one axle was pushed in, and the other pulled out (in a 10-bolt); or the rear had one front and one rear wheel on it; or any of a dozen other potential sources of confounding error.

Go directly to the thing that you're setting, which is, the relationship of the rear to the car; remove all other possible things that can create confusion and unnecessary variation from your "measurement" path; and get as close to the core thing you're measuring as you can. That's probably the first thing you would learn in a "measurement 101" class, if there was such a thing. (Maybe there is....)

 

The body is not symmetrical. Trust me, after installing large tires on my car, it's very noticeable. Side to side and front to back is not identical. That means there is no real easy way to simply do a measurement from the body to find center.

You can get the car body up on 4 jack stands. Rear axle hanging or resting on a jack. Using a plumb bob from suspension measuring points, map out on the floor the alignment of the body. Points can be at the end of the front spindle (wheels and hubs off), suspension attachment points such as LCA mount, A-arm mount, etc. Drop the plumb bob down from the slip yoke on the transmission. How far off center is it? That's where the yoke on the diff should also be so it's directly behind the transmission.

You can also go get it put on an alignment rack and have it done electronically.

When I installed my 4-link suspension, I did all the mapping on the floor. After installation, I had my car on an alignment rack to see how far out I was. I couldn't believe my back end was perfectly square to the front and didn't need adjusting. My back tires are toe out slightly but that was because of how I narrowed my diff and it's not really enough to cause problems although I'd like to straighten the housing some day.

 

And it's very easy to adjust an adjustable panhard bar - you can do it anytime with a pair of wrenches, even with full weight on the car. You'd think it would be hard to turn, but it's quite easy - I was amazed.

 

Easiest way I know to do it, is to measure from one point on the rear end itself, on one side; to a known accurate point on the CHASSIS (not the BODY) on the other. Then do the same thing except swapping sides.

A good pair of points would be the center of LCA bolt on the left end of the rear end, to the center of the LCA bolt on the right side of the chassis; and vice-versa, right of rear end to left of chassis. Or, you could use the gauge holes punched into the "frame" as your chassis points.

 

I think I would find a way to directly ensure that the control arm is perfectly perpendicular to the bolts holding it in. Mechanically this seem like the single most important aspect in keeping the rear centered. Perhaps a rod of some type through the hole and some kind of fabricated tri-square.

 

I do know some cars have a slight offset on the rear diff, My car is a 9 bolt. I do have an adjustable rear panhard bar. So going from the inside of the frame rails to the center of the diff will work? Thanks for the repleys guys!

 

Just stick your fingers between the tire and the fender... when its within plus or minus a finger, it might be centered, almost.