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.
I know that there have been studies & write-ups on this but I think it comes down to wear -vs- weight. So long as the timing rings are the same size, there is no mechanical advantage to having them bigger (or smaller).
Have used TA timing rings as small as 28T back in the '70s and those TA rings lasted 25,000 miles on our custom Assenmacher tandem.
Currently using 38T on our Zona Tandem and have 16,000 miles on them and we are on our second chain.
How long rings last depends on quality/material and keeping things clean/maintained. Advantages and replacement time will be affected, but not to a large degree.
That is our experience/opinion.
Pedal on TWOgether!
Rudy and Kay/zonatandem
Larger rings make the captain's crankset stiffer and more efficient. It's easy to understand this if you imagine the crankarm as one end of a lever, the bottom bracket spindle as the fulcrum, and the radius of the chainring as the other end of the lever. As chainring radius increases, you have less leverage because the crankarm end is proportionally smaller, that is, it is a lesser multiple of the chainring radius.
This matters because it isn't very hard to wind up some chain (which shows as slack in the bottom timing chain run that isn't there without pressure) and bow the bottom tube /frame toward the timing chain run. Effort spent bowing the frame is not power efficiently transmitted to the rear wheel. This is especially noticeable at high torque levels, which may be part of sprint or hillclimb efforts--or maybe you are just that strong all the time.
I use 50T timing rings. The slight disadvantage is that you may have to use a slightly longer bottom bracket spindle to get the larger ring to clear the chainstay--no problem at all in front.
I can tell the difference and I prefer a stiff captain crankset over a mushy one.
Incidentally, a large-large combination (on a track bike?) used to be thought to be more efficient because the chain has to curve less (therefore less friction from the link joints) around the larger radiuses. This has been the driving force behind the larger rear derailleur jockey pulleys in recent years.
Last edited by SDS; 08-06-07 at 04:57 PM.
It seems that the large rings would be better in the front,and worse in the back.
It seems the small rings would be worse in the front,but better in the back.
I would think that overall,large rings are better due to the reduced radius angle and load sharing among more teeth.But really I have no idea.
Clearly,there is no substitute for a stiff bottom end.
Yes, they do...
Originally Posted by lhbernhardt
However, what do a tandem's sync chain and timing rings have in common with a track, fixed-gear, or single-speed bike? And what is it that you can do with a direct drive or single speed bike's chain ring to double its service life? That's right... you just flip it over and you get a whole new set of perfectly good teeth.
Seriously... you can basically run a tandem's sync chain and chain rings until the teeth on the timing rings are pointy so long as you keep the same chain on the bike (and in the same link orientation if you are running even numbered chainrings) and wear it down to nothing along with the rings. In fact, once you've used up all of the eccentric's ability to take up the slack you can replace a full link in the timing chain with a 1/2 link or perhaps even pull out an entire link and keep on keepin-on. You'll eventually reach a point where a chain's rivet or roller will give out, or where the teeth on the rings wear down enough to start skipping, but you're looking at 10k (probably more like 15k) miles before that happens.
You can also flip-over a set of non-ramped timing rings or, for ramped and pinned ones, just move the front chain ring to the back and the back to the front to "rejuvinate" the rings for another 1/2 life with a new chain.
Finally, you can just be really anal about chain maintenance and make a point to replace your timing chains before they wear enough to start doing any serious damage to the chain rings, which will also extend their service life well beyond the number of miles that only 1 out of 100 tandems sold will ever see.
Last edited by TandemGeek; 08-06-07 at 07:50 PM.