> dooot dooot dooot, nerd alert....
> i have heard folks make such claims about bicycle sprockets, especially
when
> racers started using 11t cogs in the rear. it's definitely true that on a
> bicycle, an 11t cog wears dramatically faster than a 13 or 14. i know
> shimano started using 11t derailleur pulleys because the odd number of
teeth
> alternate hitting inside vs outside plates on the chain, so they
> (empirically) wear better than 10t derailleur pulleys (of course the extra
> tooth may also help).
>
On a derailleur cog the angle of articulation that the chain runs onto the
cog isn't as great as on a drive sprocket, and it is this angle that causes
the chordal speed variation. This angle is also lower on the driven
spockets on the rear wheel cassette due to the derailleur. Also, bicycle
chains do not run at the speed of motorcycle chains.
> reality check tho, clearly the stock 15t on the klr lasts a heck of a long
> time for most folks, so going to a 17 is probably in the realm of
tinkering
> that makes you feel better, but i doubt if it's really necessary (note, i
am
> trained in mathematics and finding sensible solutions, not in
engineering).
To quote THE book on the subject, (Mechanical Engineering Design, 5th
edition, Joseph Edward Shigley, pg 682), it says:
"For smooth operation at moderate and high speeds, it is consiered good
practice to use a driving sprocket with at least 17 teeth; 19 and 21 will,
of course, give a better life expectancy with less chain noise. Where space
limitations are severe or for very slow speeds, smaller tooth numbers may be
used by sacrificing the life expectancy of the chain."
All the ANSI tables for selecting chain sizes are based on a minimum 17
tooth sprocket, with a MINIMUM chain life expectancy of 15,000 hours of
operation. Lets see, 15000 hours times an average speed of oh say 25 miles
per hour is 375000 miles. However, the tables say to select extra chain
capacity for any of these conditions:
The small sprocket has fewer than 9 teeth for low-speed drives or fewer than
16 teeth for high-speed drives,
Shock loading occurs,
The lubrication is poor,
The chain must operate under dirty or dusty conditions.
Wouldn't it be nice to have a chain that lasted the life of the bike?
Jeff
PS So what if I am a nerd.