November 21, 2002

Mounting the front differential

The stock front differential is part of the same casing that contains the transmission. It also is designed to connect to a motor forward of it. Since we will not be using the transmission and wish to mount the motor behind the diff, we needed a replacement. The most obvious plan is to simply get another stock Subaru rear differential and mount it backwards.

Since our motors are designed so that they do not care which way they rotate, we could drive the new front differential in the opposite direction of the rear differential to get the wheels to go forward. The negative of this plan is that the gear cog's faces are designed to be loaded turning in one direction. Basically, the diff designers, quite reasonably, did not expect people to drive their car very hard in reverse.

An alternative solution is to turn the differential upside down. When this is done, the diff can be powered in the normal direction and still have the wheels move the car forward. The biggest negative is that mounted upside down, the oil might have problems lubricating. We are specifically worried about the bearing at the input end of the differential. Overfilling the diff would alleviate this problem, but might create excess drag with the gears having to slosh around the excess oil. A small electric pump with an outlet aimed at the highest point is another possibility.

We drain the oil and remove the front cover to look at how the differential should sit. A big consideration is the angle of the half shafts that drive the wheels. We jack a wheel to our planned ride height and try and set the diff axle holes to match that level.
fitting the front diff
The distance between the motor and the differential is smaller than how we have mounted the rear differential. We decide we can use a small spacer rather than a custom drive shaft between them. The order is almost the same as the rear motor/diff connection: motor; splinned stub axle; constant velocity joint (to compensate for any movement between the motor and the diff); spacer; differential flange.
We also will not need to extend the differential support plate as far from the motor support. We cut it out of 3/8 inch steel and weld it in place. At this time we make support bars that goes from the top of the motor mounting plate to the front side of the diff support plate, but we do not install them yet. This gives us more room to work.
front diff support plate

The front (silver colored) cover on the differential is symmetrical. By turning the cover upside down on the upside down differential, we end up with the cover plate right side up. This puts the studs above the front cross member and simplifies installation. We mount an aluminum plate (#1) on the studs and grind two steel tabs (#2) to weld to the cross member.

fitting tabs (#2) on the front cross member

Now we recheck the position of the differential. Is it even with the wheel center at planned ride height? Does the center of the diff input flange match the center of the motor? Is the diff leveled the same as the motor is? This question requires a spirit level and patience. When the answer to all these questions is "yes', we clamp the plate in position and drill.

drilling the front diff mounting tabs

We drill the two holes for the bolts that will hold the input side of the diff to the diff support plate (the wrench is touching one). We also mount the support bars that run from the top of the motor mounting plate to the front of the diff support plate.

bolting the input side of the diff to its support plate

We spend so time thinking about how we will remove the differential at the track and how to fill and service it.

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