Changing gearing -- front vs rear

Kabara, Richard M. richard.kabara at pw.utc.com
Mon Jun 23 07:49:38 PDT 2008


Jim, I believe there are two different phenomenon at work here. One is
contact wear from specific teeth repeatedly touching, the other is chain
"hopping" due to low number of C/S sprocket teeth...

Even ratios are bad in gears, because they force the same teeth pairs to
touch over and over again, accelerating wear. If you had a strong enough
microscope, every gear or sprocket tooth is different, slightly different
surface finish, material flaws, etc

So if you had two gears 15/45 and gear1 tooth #1 had a microscopic flaw,
gear1 tooth #1 touches gear2 tooth #1, 16, 31, 1, 16, 31,etc over and over,
that accelerates the wear.

If you change it to 15/44, then gear1 tooth #1 touches gear2 tooth #1, 16,
31, 2, 17, 32, 3, 18, 33, etc, etc....that 'averages out' or distributes the
wear among all the teeth.

It's different on a chain drive, the sprockets don't touch each other, so
the chain pins have to 'transfer' the contact. Again, having divisible
numbers of sprocket teeth AND pins make things worse, would be if the
sprockets were 15/45 AND the chain had 90 links (or 135 links, etc)

So having 15/45 with 109, 110, 111, 112 chain links should not cause unusual
wear...


The other thing is chain 'hop' (not sure what the actual name is)
If you wrap a chain around a 14 tooth c/s sprocket, the chain does not form
a circle, but a 14 flat-sided shape. The 'hop' is the difference in radius
to a 'peak' point and 'between peaks' point. 

Imagine an extreme example of a 4 tooth sprocket, when a sprocket tooth is
at TDC (the four chain pins form a square) that's the chain low point.
Rotating 1/2 pitch further, now the chain pin is at TDC (chain pins are in a
diamond shape) chain high point. So the chain has to jump back and forth
between these two positions, causing wear.

I'll have to pull out the book I read that in, they had a curve of chain hop
versus teeth number, 14 teeth was right where the hop starts to become an
increase, causing wear.


So Jim, I believe your 8-9K mile wear has more to do with the chain hop due
to the 14 tooth c/s sprocket...sound believable? Any other opinions?

If you liked the 14/45 gearing, install 15/48 (this will shorten your
wheelbase, making it wheelie even easier!)...as Randy said, one tooth down
on c/s is equivalent to three up in back.

Rich

-----Original Message-----
From: Jim Stewart [mailto:js at strappe.com] 
Sent: Saturday, June 21, 2008 1:44 AM
To: Suzuki SV650 Mailing List
Subject: Re: Changing gearing -- front vs rear

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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