Timing Ring sizes
 

Hello all,

We have an old Trek tandem that has finally worn out all the drivetrain. It is from the early '90s and we picked it up 10 years ago with about 500 miles on it at that point. Since then we have run the wheels off it.

My question is simple most tandems that have chains for timing (not belts) seem to use 38 T or larger rings, ours included. 36 T rings are much cheaper as are chainguards that we want to add.

Any reason not to use smaller rings other than wear out time? Bigger rings would be smoother in theory anything else?

thanks
Ruben in So Cal

36 should be fine.

Off-road tandem teams have been known to run even smaller ones for better ground clearance, etc.

Regardless what size you choose, you can periodically swap the front and rear rings to get more life out of them.

Here is another thread with people discussing timing ring size: https://www.bikeforums.net/tandem-cy...oth-count.html

Smaller timing rings = shorter chain = less weight 😀

36 tooth timing chain should work fine. Though theroretically you could go even smaller, it's not a practical way of saving weight, or possibly even cost of components. As the chainrings get smaller, chain tension goes up. So, parts do wear a little faster, including bottom brackets. This isn't a major factor at 36t, but it might be worth adding it to your cost calculations. Of course, I wouldn't know how to!

As far as I know chain tension is set by measuring chain sag, which is indepedent of chain ring size.

36T works perfectly for us. Since we have not too much clearance, I sometimes like the little extra space of a smaller chainring
(when rolling over some "sharp bumps").
When building our bike I experimented also with 38T and I really doubt whether one can sense a difference in torque between 36 and 38...

...and if the 36T chainring is only 10% cheaper than the 38T, the (theoretically) bigger wear is definitely overcompensated ;-)

I was referring to the tension and load on the chain and chainrings while pedaling. Because of the mechanical advantage of the crank, the load on the components in a normal setup of say 42 teeth, is about twice as much as the pedaling force.

As the chainrings get smaller, the mechanical advantage of the cranks increases. You really can't get a chainring smaller than, I think 32t, so the mechanical advantage and the load on components is probably not going to exceed 2.5.

However, in an extreme and ridiculous example, someone with access to a machine shop and a vivid imagination might get the idea of machining a set of timing chain cranks to accept a really tiny chainring or even a modified rear cog, the thinking being that this could be a way of saving a few grams. At the same time this could cause chain tension go up 3 or 4 times pedaling force.

We can agree that this is a crazy idea and that no one, not even the most extreme 'weight weanie' would try to do this. I mention this only to say that, while pedaling, as chainrings get smaller, chain tension goes up, and so does wear on components.

If you are powerful and have problems with frame flex from pedaling loads, you can reduce this by going to a larger timing ring.