Transmissions: December 2008 Archives

Mike asks: I hope I'm not too much trouble. I was just wondering what diameter of pulley you ended up using with the HXT. What a pain, trying to find a 12mm one! According to my calipers, it's actually like 11.8mm, even better.

I used a 1/2" imperial pulley on my 12mm nominal motor axle.

1/2" = 12.7 mm. So you can use the imperial if you make a small shim out of brass or aluminum stock. I found the sides of a soda can worked great.

I also machined a notch on the axle so that it would grab the little allen bolt better.


Mike asks: I was looking at your bike and realized: Your CVT might not be working properly. Actually, I don't know if it is or not. Here's what's on my mind:My concern is that the CVT on your bike (which is different from the Motobecane) requires TWO movable pulleys to function. As the engine pulley closes, increasing the effective diameter, the tension on the belt pulls the cheeks of the driven pulley (on the wheel) apart, reducing the effective diameter at that end.With only a single pulley able to change ratio under load, would the electric motor not be subjected to varying belt tension as the bike accelerates and decelerates? I could be totally wrong about this - but as far as I've wrapped my brain around it, I thought I ought to ask.

My response:

Continuously Variable Transmissions (CVTs) on scooters have two movable pulleys -- the rear one is a simple spring and the front one is connected to a centrifugal arm. The faster the front pulley goes, the centrifugal arm forces it to become smaller. This forces the rear pulley to expand.

When the bike is at standstill, the front pulley to rear pulley ratio might be 2:1 but when it is at full speed it might be 1:2. This allows for low-speed torque and high-speed acceleration.

What I did with my EV conversion was to replace the front centrifugal pulley with a fixed pulley. This is because the old centrifugal pulley wouldn't fit on my electric motor. The end result is that I have a fixed gear ratio on my scooter (disabling the CVT) but one side benefit is that the belt is always tension properly since the rear pulley is always trying to become as large as possible.

A fixed gear on a little scooter isn't a problem as my ideal max speed is 30mph and electric motors have a lot of low-end torque. If I wanted the scooter to go 60 mph, I might try to machine the original back on, but at this point it's not worth it.

Mike A. also asked where I got the 12mm flanged bearing that supports the right side of my outrunner motor. I got it from Carr-McMaster, part #1434K15:

LineMcMasterPart NumberDescription/Customer ID#YouOrderedWeShippedBalanceDueUnitPriceExtendedAmount

Pics and specs after jump.

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This page is a archive of entries in the Transmissions category from December 2008.

Transmissions: August 2008 is the previous archive.

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