Join Date: Mar 2006
Location: Vancouver, Canada
Bikes: Rodriguez Shiftless street fixie with S&S couplers, Kuwahara tandem, Trek carbon, Dolan track
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Size of Timing Rings
Are larger or smaller timing chainrings more efficient/effective? I've read some back posts on this subject and the accepted wisdom seems to be:
- small rings are lighter but wear out faster
- there should be no difference in leverage or mechanical advantage
- small rings are better on mountain bike tandems for getting over logs
In track racing, the common "old wives' tale" is that sprinters should use a small-small combination (chainring to sprocket) because it accelerates faster, while pursuiters should use a large-large combination because it is easier to keep spinning. This sounds silly initially, until you think about levers and the weight of a chain. If you think of the chainring as a lever, with the bottom bracket spindle as the fulcrum, then the smaller the chainring relative to the length of the crank, the more leverage you've got. As well, if you think about how fast the chain is going, if you are spinning a 46-tooth chainring at 120 rpms, that's a chain velocity of 92 links per second. With a 52-tooth chainring at the same revs, that's 104 links per second. Obviously, it's going to be easier to accelerate a chain to 92 links per second than to 104 lps. (Obviously, there will be leverage issues with the resulting smaller/larger rear cog, but that will be pretty negligible.)
So looking at a tandem's timing chain, it seems to me that leverage will be improved with smaller rings. But how about the question of wear? Wouldn't chains last a bit longer with smaller timing rings? If we contrast two timing setups, one using 40-tooth rings, the other using 34 teeth, and we connect the two with 100 links of chain (the chain connecting the two sizes will be exactly the same number of links regardless of chainring size, since all we're doing is running chain between the midpoints of both chainrings, or the distance between the two bottom brackets). So the 40-tooth setup would use 140 links (20 links touching the front of the front chainring, 20 links touching the rear of the rear ring, and 50 links each at top and bottom, and the 34-tooth setup would require 134 links of chain.
Now, spinning at 90 rpms, in one minute we'd run through 12,600 links on the 40-tooth ring, 12,060 on the 34, about a 4% difference. You should be able to get about 4% more life out of the chain using 34-tooth rings, then? But because the difference between 40 and 34 is 15%, are we spreading 15% more stress on each of the smaller rings?
Anyway, funny what occurs to you on long rides. - L.