Friday, January 29, 2010

Rear Transmission Pt 2

I got some parts back from Commercial Machine Service and put them together to see if my design would work.  Well, the addage "Measure twice, cut once" came up to bite me.  Since one of the goals of this design was to make the chain sprocket center distance adjustable, the plates that mount the motor and clutches slides along the joint between the upper and lower parts of the transmission.  Well, upon trying to put it together, I found that I made an error somewhere and the chain was too tight; couldn't even put it together.    Luckily, the joint has plenty of material and I only need about 1/8th inch.  So, after MUCH grinding to lower the shelf that the Motor/Clutch plate slides on, here is the result:

Saturday, January 2, 2010

December Update - Rear Transmission

I've been working on my transmission issues for the past month and here is what I'm going to try first....


Snowmobile CVT - Basically, since I geared the car for 60+ MPH, I set the motor/wheel ratio to 4:1 (basically 4th gear), not good for driving up steep hills. However, since my initial concept was for one motor for each wheel, I went for direct drive meaning no way to change gears, hence my problem. Here is my current solution:

video


I'm in the process of modifying my direct drive set up with a snowmobile CVT between the small sprocket and the motors. The CVT will will vary from 2.4:1 to 1:1 depending on torque/motor RPM giving me an effective ratio of 9.6:1 (second gear). The installation requires that the center distance of the drive (motor) clutch and the driven (sprocket) clutch be very exact. In order to fix this distance, I designed a designed a plate that will mount the motor and two clutches at a fixed distance. I looked around for available software and found a program offered by Solidworks. It is very powerful and I ended up modeling my whole transmission.


And, since I'm modifying my transmission, I'm using the opportunity to fix the center distance issue. With all chain drives, a method to tension the chain is required. Most of the time, that involves a method to change the center distance between the sprockets but I didn't initially design in a way to do that. Instead, I tried to use skateboard wheels attached to swing arms and springs to keep a constant tension on the chains but I greatly underestimated the forces, which explains my noisy drive. As the chains rotate over the small sprocket, it flops around, making a lot of noise. By putting the new motor/CVT combination on a sliding plate, I'm able to adjust the center distance up to 0.5"; it isn't much but hopefully it will be enough. Commercial Machine Service is in the process of making up my pieces so I can give it a test.

On another front, I ordered a set of front struts from a BMW 328xi. It is the four wheel drive version on BMW's 3-series sedan, the descendant of the 2002. Next step there is to try to get the struts installed in my '02 and then move towards motors on the front wheels. More on that later.