Wednesday, February 8, 2012

Motor Coupler Part 1 - Design

This post really shows off my engineering side.  All day long I help design parts and create drawings for them.  This project is an opportunity for me to do all that plus figure out how to actually make the part.  This sort of project always gives me a deeper appreciation for the people who bring form to things I look at all day on a computer screen.  As a warning to all of you, this could get boring.  Then again, if you're like me this could be pretty interesting...or maybe still boring.  Enough said.

The electric motor has to somehow transfer power to the transmission.  Some people choose to set up this connection clutchless.  In this case, the flywheel, pressure plate, and clutch disk are removed and a single coupler attaches to the motor shaft and the transmission shaft.  Benefits to this system include simplicity and low rotating mass (i.e. more apparent torque for acceleration).  The downfall here is that changing between gears could be slow.  To other option, of course, is to retain functionality of the clutch, in which case you need a coupler that attaches the motor shaft to the flywheel.  The benefit here are that you shift as quickly as you would have been able to with the gas engine.  The downside is that the coupler design and fabrication is much more challenging and you have higher rotating mass (i.e. some of the torque that the motor produces is used accelerating the mass of the rotating components and not as much is available to accelerate the car).  Based on my friend's experience, I decided to incorporate the clutch.  And here it is.

In this next picture you can see how it fits in with the motor and transmission.  The motor is on the left.  You can see the coupler attached to the shaft and the flywheel attached to the coupler.  You can also see the transmission (or in my car's case the midshaft) that is piloted to the coupler.  This shaft must be supported with a pilot bearing so that it does not flex when you disengage the clutch, which could cause all kinds of problems.

So we have a couple important features here.  First, the ID of the coupler must be a very close fit with the motor shaft or it will cause vibration.  To improve the fit, I've added a slot in the coupler and a bolt that will clamp it to the motor shaft for a slop free fit.  Second, the face that the flywheel mounts to must be perpendicular to the motor shaft.  If it isn't, the flywheel will wobble causing bad wear on the clutch parts.  Third, the OD of the coupler pilots to the flywheel.  This must be a very tight fit or the flywheel will be unbalanced.  And fourth, the pilot bushing must be concentric with the motor shaft so that the transmission shaft is concentric with motor shaft.  Still with me?  Here are all the dimensions I used.

You can pay someone to build this for you for $500-600, but part of the fun for me is to figure out how to do this yourself.  Luckily I have some very generous friends who have the equipment needed to make it and were willing to help me out.  I have to give a big thanks to Larry for supplying me with around 12 hours of time on his lathe and just as much time in advice.

So now that we've got a design, it's time to start cutting chips.  I went to the local metal store and picked up a 4 inch diameter bar of cold rolled (CR) steel.  I got 9 inches figuring that would be enough to have two tries at getting it right.  I probably spent at least and hour discussing this machining process with my friend to make sure everything turns out the way I want it.  Considering how long this is going to take, this long conversation was well worth it.  After all this discussion we cut it down to about 4.25 inches long.  The shorter the piece, the less problem you'll have with wobbling, vibrating, etc.  Plus, even at that length, the starting piece weighs over 15 lb.

Next up, how to make it...


  1. Hello Micah,
    I'm starting myself on the same project. 924S 1987.
    I've been learning a lot reading your blog and I really appreciate you share with us your experience. would you also have the drawing of the adapter plate that you build? I do see it mentioned anywhere but can see it on one of the picture.
    Thank you!

  2. The motor to bellhousing adapter? I don't have a drawing, just a solid 3D model. When I had it made I sent the model to the machine house along with the chunk of metal and they used the model to program their CNC machine. It's based on the size of material I stared with, but can probably be adjusted for anything else.

    The only issue you may have is the bellhousing on your car may be different than the one on mine because mine is an Audi engine and yours was a Porsche engine. You are welcome to the solid model if you want it though.

  3. Correct. I just notice you are using one of the early model. I guess I will need to figure that out once I remove the motor. The exhaust is making me crazy. I'll probably have more questions down the road. Great blog. thanks.