July 28, 2012

Electric Scooter: Drivetrain Refinement

In the scooter's original build report, I said:
"Since the sprocket on the gearbox is not located at the pivot point of the rear swingarm, as the suspension is compressed, the effective chain length increases. To take up the slack in the chain, I added a chain tensioner"
As it turns, out, the problem was not as simple as having a chain tensioner to allow the chain to lengthen when the suspension is compressed. When riding the scooter over rough terrain, the chain constantly came off the drive sprocket. I added a chain guard to fix this initially, which worked great, until it broke. As far as I can tell, the problem stemmed from the fact that, when the suspension is compressed while the scooter's throttle is applied, the top of the chain is put under a ton of tension. My previous sprocket and chain guard arrangements could not deal with the extra tension, causing the chain to pop off either the drive or idler sprocket. A second contributing factor was that the original chain tensioner did not store nearly enough extra links of chain to allow for much suspension movement.  If all the extra chain stored in the chain tensioner was used, as it might be during large impacts, the chain would be put over even more tension than it would be while going over typical bumps.

To (hopefully) fix these problems, I completely redid the chain tensioner and idler sprocket arrangement. As far as I can tell, there is no way for the chain to come of either the idler or drive sprockets without either breaking the chain or bending the idler mounts. The idler is now fixed to the gearbox, so it does not move with the suspension, as it did in the previous configuration. Also, the pin below the idler prevents the chain from disengaging the teeth, even when there is slack in the top of the chain.

The new chain tensioner works similarly to a rear derailleur on a bicycle, and bends the chain around in an "s" shape, to store more chain. When the suspension bottoms out, the drive sprocket, wheel sprocket, and chain tensioner sprockets all line up so that there is a straight chain line directly between the sprocket on the wheel and the drive sprocket, so that none of the extra chain links are wasted.

I ran out of pre-made idlers, so I had to fashion a new one myself. The sprocket was originally meant to be fastened to a motor shaft, but I cut of the bit with the set screws. I drilled out the center to 1/2", and then filed the hole larger, until I could press some roller blade bearings in with a vice:

During the most recent test run I did, I managed to burn out two of the CIM motors while attempting to power up a long, steep hill at full throttle.  Fortunately, I had one spare, and a different Ben let me have an extra one he had lying around, so the motors were replaced quickly.  Also during that test, I managed to bend the load bearing part of the disk brake where it bolted to the frame fo the scooter.  I rebuilt the brake mount using a lot more metal and many more bolts in the spots that bent.  It's not very elegant, but hopefully it will last longer than the old one.  

I haven't actually tested any of these changes yet, so I have no idea if they've improved anything.  As the scooter was before, it was completely adequate for riding on roads.  But what's the point of building  such an oversized scooter, just to ride it on roads?  For now, at least, if it can't handle a little off-road thrashing, it isn't finished.