Changing gearing -- front vs rear

Jim Stewart js at strappe.com
Fri Jun 20 22:44:03 PDT 2008


This won't matter at the track, but I have some observations re: 14 tooth 
countershaft sprocket on the street.

About 5 years ago I replaced the chain and both sprockets - 14 tooth c/s and 44 
tooth rear (44 is stock on my SV-S).

I had fun with the throttle - front end comes up very nicely with that ratio, 
thank you. (The ratio 44/14 is pi to four significant figures. Nature is 
mysterious. Shrug.)

But after about 10K miles the drive train developed a resonance - at 41 mph, 
plus or minus 1 mph, the chain would - ummm - "thrum". It was obvious, 
repeatable, and coincided with the beginning of rather rapid chain wear. I 
continued with this situation for another 2K miles.

At 12K or so I replaced the whole kit - c/s sprocket, rear sprocket, chain - 
once again with the 14/44 combination.

After 8 or 9 K the resonance began again.

Hmmmm.

I replaced the c/s with the original 15 tooth sprocket and all the strangeness 
vanished. I ran the 15/44 with the same chain and rear sprocket for another 10K 
or so.

----------

I think this is the same phenomenon that happens with gears, where a simple 
ratio between drive and driven gear will let the teeth wear unevenly. This is 
well understood by mechanical engineers, so I've been told, and the choice for 
mating gears is a ratio that's not evenly divisible. Even/even (14/44) is even 
worse.

Yes, the stock 15/45 is a nice round 1:3 ratio, but (now we're talking about 
sprockets) it's not a problem, it seems. I've run 15/45 for the last 4 years 
with no problems. I gather 1:3 would never be used in a gear pair.

--------------

I don't know what the mechanism is here, but (as a longtime (50 year) RF and 
audio guy) I can assure you I recognize resonance when I feel it, see it, or 
smell it. Or get a nasty burn from it. (RF is tricky.)


------------------


Thoughts, anyone?

-js

-- 
http://www.strappe.com





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