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July 25, 2017

Driveline Geometry

I had cause recently to be pondering drive shaft U joint angles.  I vaguely knew that the geometry of a drive shaft was important, and that a lot of thought goes into it (or should go into it, at least) for highly modified cars, especially things like rock climbers or cars with radically raised bodies.

The drive line geometry is something I didn't give much thought to on the TR6 project, since I made no modifications in that area.  But in response to a question about TR6 frame sag and how it might affect the alignment of drive components, I decided to take a little deeper look.  My frame did have some rear end sag, and I had to address that to get decent panel gaps and get the rear bumper brackets to fit right.  (Some pictures here of how I did that.)  During that project, I had calculated that the sag at the rear suspension was probably on the order of 1/4" or so, and I decided not to pursue fixing that.  I didn't, however, consider what affect frame sag would have on drive line geometry.  Rear frame sag would have the effect of rotating the differential into a slightly more "nose up" orientation, and this could theoretically affect the drive shaft angles.

A little Internet research suggested that even small amounts of driveline misalignment could cause vibration and premature wear, so I sat down with a pad and pencil and worked out some of the math behind U joints.  Those with an unhealthy curiosity about that math can see it here, but for the rest, take my word for it that the main takeaway is that the axis of the gearbox output shaft and the axis of the differential pinion shaft need to be parallel for all drive shaft speed and angle distortions to fully cancel.  If frame sag tips the differential up, it could be a problem.

This is where my usually fairly latent OCD kicked in, and I decided I needed to determine the alignment of my drive line.

So, out came the exhaust system, the rear part of the tunnel, and the drive shaft.

Now, I can't think of a good way to measure gearbox output or pinion shaft angles directly, so I worked from the respective flange faces, which should be pretty close to perpendicular to their axes.

In the left picture, a machinists 1-2-3 block was clamped to the flange and leveled side-to-side.  Then the magnetic angle gauge was placed on top.  Though I did calibrate the gauge to true horizontal, it really wasn't necessary since the angles of these axes to horizontal is irrelevant.  We just want to know their relationship to each other.  In fact, the back of the car was jacked up, so these don't represent any angles we'd see in normal operation.  The more artsy picture on the right shows the gauge attached directly to the diff flange.

The upshot of this foolery was that with the car in this attitude, the gearbox flange was pointing up at about 0.5, and the pinion flange was also pointing up at about 1.3.  For the axes to be parallel, the diff flange should be pointing down at 0.5.

A little math told me that, based on the distance between the front and rear diff mounts, to rotate the diff by 1.8 degrees, I could shim down the front mounts by a little over 5/16", or shim up the rear mounts by the same amount.  Unfortunately it looked like neither mounting point would accommodate that much extra thickness, but I could split the shims.  For the front, I made up a couple of these two-piece shims from 3/16" stock.  The little wedge pieces are so I could just lower the diff slightly on the studs, and insert the larger part of the shim, then slide in the wedge to lock it in place.  The shims went on top of the rubber mounts.

For the rear, I just used an extra 1/8" thick washer between the diff cover and the rubber mount brackets.

On re-measuring the angles, I got a final angle mismatch of only about 0.3, so this approach got rid af over 80% of the angle error.  I decided that chasing the remaining 0.3 just wasn't worth it.  Things like frame flex and flexing of the rubber mounts will probably cause more variation than that.

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