Gearing my Boxer

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While I'm waiting for the ESC and servo tester (aka throttle) to arrive, I'm trying to figure out what gearing I'll need with my new motor.

My Piaggio Boxer wheels are 14" in radius, so if my high-school geometry is right, that's 2 x pi x r = 88" in circumference.

If go 5 mph, that's 316800 inches per hour, that's 3600 rotations per hour or 60 rotations per minute.

My mo-ped's rear hub has a built-in 12:1 gearing reduction. The current 1.5" front fixed pulley I'm using with the little 250 watt MY1016 motor gives me a 3.25:1 gearing reduction to the back wheel. That gives me a final 39:1 gearing reduction.

5 mph = 60 rpm * 39 = 2340 rpm.......
10 mph = 120 rpm * 39 = 4680 rpm......
20 mph = 240 rpm * 39 = 9360 rpm......

This sort of explains the 8mph max speed that I'm getting with the 250 watt Razor motor. By the way, I bought a Doc Wattson which tells me that this motor draws 20 amps at "stall" speed (since I have a centrifugal clutch, it never actually stalls):

When I'm running just down the street, it draws about 10 amps.


So when I put on the HXT outrunner, it's obvious that I'll have to go to a bigger front pulley. If I use a 2.5" front pulley, the v-belt gearing shifts to something closer to 1:1. Doing the same math but with a 12:1 reduction gives me:

5 mph = 60 rpm * 12 = 720 rpm.......
10 mph = 120 rpm * 12 = 1440 rpm......
20 mph = 240 rpm * 12 = 2880 rpm......
30 mph = 360 rpm * 12 = 4320 rpm......

Since I bought the 130 Kv motor and was planning on running it at 24 volts or 3120 rpm, it looks like my max speed would have been limited to 21 mph.

If I instead run the motor at 36 volts, this would give me a max rpm of 36 * 130 = 4680 rpm which would give me a max speed of 32 mph.


In addition to stepping up to 36v, I could tighten the v-belt so that the rear gearing shifts to something like 1:2, which would give me a 6:1 reduction and effectively double all of my speeds @ 36v:

5 mph = 60 rpm * 12 = 360 rpm.......
10 mph = 120 rpm * 12 = 720 rpm......
20 mph = 240 rpm * 12 = 1440 rpm......
30 mph = 360 rpm * 12 = 2160 rpm......

For a max speed of 65 mph at 4680 rpm............. which is a bit too fast.

What I might do is a 6:1 reduction @ 24 volts which would give me a max speed of 43 mph which is just perfect.

This is all theoretical, I'll have to do some playing around when the rest of the parts arrive. But for now, it looks like the pulleys and v-belts I currently have will be just fine.

p.s. If I had access to a machine shop + was actually skilled, I could machine the HXT spindle so that it would fit on the old CVT from the moped. It would take some filing to make it D-shaped, and then some tapping so that the nut could screw on. Then I could take full advantage of shiftless gearing from 0mph to 65 mph.

(above, the spindle on the old ICE with the CVT front pulley attached)

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About this Entry

This page contains a single entry by Karen Nakamura published on July 17, 2008 7:26 AM.

Super huge motor arrives.... controller / throttle to follow was the previous entry in this blog.

Boxer Test Run 1 (250 watt Razor MY1016; 30a controller; 24v 7 Ah SLA) is the next entry in this blog.

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