Improvement #1: Gearing

A gear box is useful on an ICE automobile because it gets the engine operating in is very limited RPM range. This comes at the cost of the weight of the gear box and the frictional drag of the gears.

An electric motor has a much broader useful RPM range. The drag penalties of the gear box would mean more batteries to go the same distance.

A hub motor wound to be efficient at the correct RPM offers many advantages. The disadvantages are unsprung weight, road shocks directly to the motor, hard to set adjust perfect RPM for each race track (re-wind each motor?).

Our educated guess is that a motor per wheel with a fixed replaceable gear driving a half shaft should be the best set up.

With the Electric Imp, we are keeping our two motors, each driving a pair of wheels through a differential.

We choose our original gearing based on what is now called an interactive simulation. We drove a racing simulation and changed the gearing. The stock gearing gave the fastest laps.

Retrospectively, we have found there were at least two flaws to this test.
First, we did not know that we would not have enough stored energy to complete our races at full power. This means our actual motor torque figures are lower (except for full power qualifying).
Secondly, we were ignoring the effect of gearing on efficiency. If we geared the motors to make them operate in the RPM/current range where they are more efficient, we will translate more of our stored power into speed.

This graph gives some idea of how idea of what motor/inverter efficiency might look like (Graph supplied by

motor/inverter efficiency  

By increasing our RPM by running a higher numeric gearing, we should gain a few percentage points in efficiency. We also should accelerate quicker and we do not expect any loss of top speed.

We were using the stock 3.9 ratio 152 mm ring gear differential. There is also 4.1 and 4.4 ratio 160 mm ring gear differentials used in other Subaru models that are readily available which only require swapping out the half shafts. They are slightly larger and heavier.

We would like to make a bigger change in ratio. The 1.6 liter Mazda Miata came with a 6 inch (152 mm) ring gear. Mazda Motorsports offers a 4.875 ring and pinion set (p/n 0662-27-950A).



This will allow us to keep the smallest and lightest Subaru differential case. It will require some machine work.


back of Ring

We need a pattern of the holes.


make a pattern  

The original bolt pattern


stock holes  

New holes are drilled to allow the ring gear to bolt in place.


bolted on using new holes  

The next step is to take the existing pinion shaft, cut off the pinion gear and weld on the 4.875 pinion gear. This requires very precise work and the bearing surfaces on the shaft need to match up.

The differential is assembled and backlash checked. Spacers are added and subtracted to get everything right. There is a lot of detail work. All the seals and bearings are replaced and adjustments are made based on how easily the input shaft can be turned with the torque wrench.

We debate using ceramic bearings. Research shows they can offer lower friction. They are expensive and we probably will not get a great deal of advantage from them if we do not use lighter oil. Without testing, we are loathed to risk too light an oil and destroy a bearing.

We also make some modifications to the front differential. It is the same differential as the rear but mounted upside down. We were running an oil pump to keep the bearings lubricated.


diff carrier modified to move oil  
We replace the pump by adding an oil gallery and raising the oil level a little. Slosh should keep the bearing lubricated. The slightly higher resistance of the extra oil should be offset by saving the weight and power used from the pump.


oil gallery

A clear tube helps us set oil level.  


Regenerative braking efficiency
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