YAMAHA CHOPPER PROJECT
DESIGN | MAIN | ASSEMBLY

2004/12/01:
I needed a rear sprocket that met some odd criteria:
1. Bolts to Harley hub (00-04)
2. Has 42 teeth
3. Fits "number 530" chain size.
So I was looking for something that had the number of teeth for a Japanese engine, but the bolt pattern of a Harley. Harley sprockets were not an option because the smallest I could find was 47 teeth, and Japanese sprockets didn't have the metal in the middle to drill out to make it boltable to my hub.

I chose 42 teeth because the stock Yamaha is 34, but this rear tire is a significantly larger diameter (so I increased the number of teeth by the same factor), plus I wanted to trade some top-end speed for the low-end torque I think I'll need to move this beast around town. The Yamaha gearing seemed a bit too tall to begin with anyway.

From all the screwing around looking for sprockets and coming up blank it occured to me that it can't be too difficult to just get a custom sprocket made. I found a great site that shows how to CAD sprocket teeth:
Notes On Sprockets And Chains.
And I measured the Harley hub to figure out the bolt holes. So here's the spec for my sprocket:

42 Tooth sprocket for 530 Chain, mating to Harley Davidson 00-04 Rear Hub.


I even created directions for how to machine it from a raw piece of metal:

1. Hub: Cut a circle out with a diameter of 2.25"
2. Bolts: Cut a .433" hole 1 5/8" from origin. Copy this hole around the origin at 72 degree spacing for 5 total bolts.
3. Teeth: The roller's center is 4.1817" from the origin. To make each tooth:
3a. Cut a .400" circle for the roller.
3b. A .850" inch circle with the same center inscribes the outer edge of the neighbouring teeth.
3c. The tip of the tooth should be trimmed off flat.

But finally I got tipped off to a place that supplies industrial sprockets. I could get a blank sprocket of any number of teeth I wanted through IMW Industries Ltd (Thanks Rob). The only issue was they sell standardized ANSI sizes, not motorcycle sizes. What I should have noticed though on the Notes On Sprockets And Chains site was that Standard ANSI #50 is identical spec to the 530 chain I was after.

So I had access to a blank sprocket, I'd just have to cut out the center and bolt holes. But why risk error margins when I could get IMW to machine that for me before sending it out? I sent them my Step 1 and 2 data, and a few days later I had my sprocket. And cheaper than I thought I was looking at. 35 bucks for the blank plus 40 for machining the holes.

Dead perfect fit, too. Guess I got lucky on my hub measurement calculations.

The only other note is that since the sprocket is just a flat piece of metal (no dish, no spacer), and the Hub sticks out only barely past the edge of the tire, I needed to get the sprocket out from the hub so the chain wouldn't rub on the tire. I had ordered a few sprocket spacers from Dennis Kirk to stack and set the sprocket out the width of the chain overhang plus safety margin.


DESIGN | MAIN | ASSEMBLY
email: mikebike@rocketcar.net