The motor mount is one of the two big fabrication concerns when you’re building a bike- the other being the battery rack or box. It actually was a lot easier to figure out that I’d expected it would be. If you look at it from the gas motor standpoint, you’ve got to figure the stress points on the frame are going to be the same. The sprocket is going to want to be in the same place, too, so that the chain routes properly- not off-axis with the swingarm. All you need to do is take a good look at how the original motor bolted in.
Here’s my motor- note the upper and lower mounts, and the sprocket.
Now, here’s my final motor, all mounted in the frame. Essentially, I just duplicated the position of the motor mount holes, then moved the NEMA C-face standard bolt pattern from my motor into position so the sprocket would be as close as possible to the original position.
Now, I did most of it using mockups with MDF and cardboard- like this:
…and originally planned it as one welded assembly with buttresses, like this:
…and as you can see, at that point I was planning for a larger sepex motor. In my excitement to run the bike, I just bolted the one plate that I’d had cut with the tubing I’d used to mock up with, and it was a perfect fit. It was also incredibly strong, even without the welding. I’ve run it like that all summer with not even the slightest indication of stress. I will, of course, finish it up properly, but at this point I’m going to wait until next winter to tear it apart.
The point being, it’s a heck of a lot stronger, and suffers a lot less stress than I’d thought it would. I often see guys building motor mounts that look like they could withstand a Titan rocket. Having seen this one, and several others that experienced builders have made, I’d definitely go with the basic, lighter weight approach and watch it carefully for the first few runs. The mount I made here seems to be a very common approach, with various adaptations for different frames and motor diameters- but it’s seldom you see the need for the huge, boxed in array that some people put together.
If you really have no feel for it, ask for help. The first place I’d probably go would be my friendly neighborhood welder guy- those guys know what holds up, and what’s going to fail, often just by looking at it. Just about the last person I’d ask would be an engineer. They’ll just tell you all the reasons they think the whole thing won’t work. 🙂