November 18, 2002

Mounting the rear differential

The rear motor will power the rear wheels through a stock 3.9 ratio Subaru differential. The differential needs to be securely mounted.

There are two studs at one end of the differential that bolt onto the rear cross member. These hold one end of the differential in position.
rear diff held by cross member
The other end of the diff was originally designed to be bolted to parts of the car that have now been cut out. Also, we have raised the differential to keep our flat bottom. This minimizes the angle on the half shafts (between the differential and the wheels) and minimizes drag by reducing friction. We spend a fair amount of time measuring parts and trying them in place. In the end, it works out nicely. We can weld a plate to our strong battery box/motor housing to support this end of the differential.

The plate is cut from 3/8 inch mild steel. We clearance where the diff's drive shaft will sit and drill out some holes to lighten the piece. We weld it in place and then weld two supports made out of chrome-moly tubing to the bottom of the plate.

diff support plate
We would like to add two more supports from the engine plate to the top of the support plate, but these need to be removable. We fabricate some appropriate size bushings.
bushing for fabricating supports

We rough cut the angle onto chrome-moly pipes and then use a grinding tool to round the metal to fit the bushing.

grinding the pipe to fit the bushing
A tight fit minimizes the welding.
fitting the bushing

The welding is done with the piece in the car to make sure the fit is exact.

top support pipe welded

With both supports done, the plate that the diff will bolt to is well supported.

motor plate bolted to diff support plate

There is about 6 inches between the end of the rear differential and the motor. We need a short drive shaft to connect them. We decide to make it out of aluminum because it is easy to machine and light.

We purchase a chunk of solid round aluminum and cut off the length we need.

cutting the drive shaft length

The actual machine piece is not too complicated but needs to be exact. One end must match the flange on the differential with its four bolt hole pattern. The other end must match the six hole pattern of the CV joint. The two flanges must be square with each other and around the same center. The drive shaft should be hollowed out for lightness.

the finished drive shaft

Now we are ready to fit things together. The splined stub axle (see Electric Imp Work History, February 25, 2002) goes into the motor. A constant velocity (CV) joint fits over the part of stub axle that protrudes from the motor. The aluminum drive shaft bolts on to the CV joint. The other end of the drive shaft mates with the flange on the end of the differential.

rear motor connected to rear diff

We use a level gauge with a bubble to check the tilt of the motor, the drive shaft, and the differential. The motor and drive shaft are dead level. We shim the differential to make it level as well.

using shims to level the diff

If the whole structure forms a straight line in the horizontal plane, then the last bolts between the drive shaft and the differential flange should slide in freely. They do not. We check and determine that the differential center is slightly higher than the center of the motor.

rear diff is higher than the motor

We have a way to adjust this. We loosen the four bolts that hold the rear cross member to the car, one turn at a time. A black mark on each bolt head helps us make each turn exact.

lowering the diff by lowering the rear cross member
When the bolts connecting the differential flange to the drive shaft slide in and out without restriction no matter how we spin the connection, we know the connection between the motor and the diff is true. We drill the bolt holes to attach the differential's front end to the support plate. The bolts hold the diff and its shims in place.
Other end of rear diff bolted in place
We measure the gap created by lowering the cross member and make spacers to fit.
measure gap to make spacer
Finally, we slowly disassemble everything. We look at how hard this will be to do with the floor attached to the car. Can we remove the differential without removing the motor? Which way should the head of bolts face to make them easiest to remove and replace? We discover that lowering the cross member that little bit has made pulling out the differential much harder. We must grind a little off its case so it can slide out. A little work now will make the job at the track that much easier.


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