pic: One Tough Train

[Tristan Lall]

That looks interesting....

Originally Posted by sanddrag

A few notes:
-It might be better to mount the CIMs by the 10-32 threaded holes in their face rather than claming them on the can.
-Why use a coupling on the CIM shafts rather than just putting a gear?
-The transmission directly drives the wheel, I LOVE IT!
-The transmission has three plates, make sure there are never 3 bearings/bushings on a single shaft.
-For the sprockets, what size are they? I would recommend going the smalles you can (which would be 13T for #25 with 1/2" bore)
-Finally, have you considered a welded box tubing frame, they can come out to be very lightweight.

I mostly concur with sanddrag, except for that sprocket thing.

A couple of years ago, in 2003, Woburn used those same little sprockets, and promptly broke something like 13 #25 chains over the course of three events. Since there's no such thing as too much horsepower, the chain simply wasn't up to what we demanded of it. (It undergoes higher stresses as sprocket radius decreases.) You might consider looking at Tsubaki's General Catalog, Section 1, pages A-6 and A-7, where it gives power vs. speed vs. teeth-on-small-sprocket charts for #25 and #35 chains. (I'm always recommending this, it seems.)

Some quick calculations say that for your 5.5 in wheels at 6.5 ft/s, you need a wheel speed of 271 rpm. Plug that into the chart, and you get a rated power of 0.17 HP for #25. Two CIMs at 341 W each are good for about 0.91 HP, or 5.3 times your chain's rated capacity. (You're screwed ; that's even worse than Blizzard 4, in power-to-rating terms--it was an already-horrifying 5.1, even after accounting for its extra power!)

Of course, there's an answer: move up to #35, where the rated power for a 13 tooth sprocket (note that it's bigger than the #25 version, in all dimensions) is 0.94 HP, which is perfect.

I don't know how much of a safety factor chain manufacturers build into these ratings. I've got empirical evidence to demonstrate that it's not 5, however. Maybe 2, if you like to live dangerously--but gambling that the safety factor exceeds 5.3 really won't be pretty.