Changing gearing -- front vs rear
randygrein at comcast.net
Sat Jun 21 09:56:27 PDT 2008
Resonance happens in many systems, and sounds like you found a good
We don't have any tracks up here that require such low gearing (except
perhaps Mission, BC) so I see taller gearing. Some are still using
15/45 and bumping against the rev limiter, but sneaking a peak at the
fast guys I saw 15/43. My short-lived superbike did quite well with
16/43, thank you very much with no trouble with 2nd gear power
wheelies. I might have ultimately changed to 16/44, but it was well
past 9500 (power peak) at the end of the straight, so getting fully
dialed in might have made that pointless.
Randy Grein, WMRRA #41
On Jun 20, 2008, at 10:44 PM, Jim Stewart wrote:
> 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
> 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.
> 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
> Thoughts, anyone?
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