Sunday, March 8, 2009

Rear Dual Sprocket Differential

One of the two main problems that early automakers had to solve was the rear differential. The rear wheels spin at different speeds when turning - the outside wheel is traversing a larger circle than the inside wheel, so it has to spin faster. If you don't do this, the tire "scrubs" and the vehicle resists the turn. It's very hard to find pictures of how Benz solved the problem - but it looks like his belt-drive motor turned a drum that had an interior freewheel ratchet driving the wheels, so that they could spin at different rates. I adopted that idea to the technology I had at hand. I designed up a mechanism using two freewheel ratchets, one for each secondary axle, driven by sprockets on the tertiary axle, itself driven from the motor:

The ratios on the sprockets shown here are 15:80, or 1:5.333 - so when the motor is spinning at 1500 RPM, the secondary axles will be spinning at 280-ish RPM. The discs with all the holes are the brake discs - again, one for each axle, since they spin independently. The tricky part: how to make sure the torque is transmitted to the axle? Bicycles solve this problem, of course - so I drew it as a bicycle hub with brake mount on one side and freewheel thread on the other:

Unfortunately, I could find no such item handy (I'll keep looking). In the meantime, I did find two mountain bike front hubs (with disc brake mounting points) and a spare rear flip-flop freewheel hub:

The trick is getting it all together... I cut all the hubs in half using a pipe cutter on two of them (the third hub had walls 1/4" thick and I had to use the cutoff wheel):

After suitable cleaning, I expanded three of the spoke holes to 3/16", bolted the mismatched halves together with 8-32 machine screws, and applied liberal quantities of JB Weld to the mating points to hold it all together. The screws will stay to provide some additional mechanical strength:

Here is a mockup of the sprocket and brake disc on one of the hubs:

Still to do on this part of the project: weld a 1/2" threaded coupling to a washer, tap & die two set screw holes in it, and JB weld the whole thing to the hub.


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