Tooth count question for chainring swap
 

My brain swears I read something about this somewhere (Maybe "Art of Urban Cycling"? Returned my borrowed copy...) I'm not so certain, and I need help settling the question between my brain and I.:p

I've got a Bianchi Castro Valley that is set up (stock) as a 1x9 . Bianchi's site says the crankset is a Sugino XD500t with 42 teeth (which I gather to be a 74/110 ring on a tandem crank). Bianchi also says my cassette is a SRAM 11/26t, but Sheldon's gear calculator only has a SRAM 12/26 9-speed which made me look once, and I believe I found that to be correct, not the Bianchi stats (a typo?). [I'm at work & forgot to check this a.m. before I came in.]

Anyhow, the question is this: I wanna fiddle with the gearing a bit (slightly high for my liking), and I know that swapping the chainring is the easier and cheaper way to try this quickly, but do I have to swap chainrings with a specific increment in teeth different from what I have already? In other words, can I use a 39 tooth ring with the same cassette, or should I only change the chainring in 2 tooth increments or something?

My brain is insisting that I saw something about wearing out chains faster with odd combinations, or is that only if I've got no deraileur taking up slack?

I think that you worry too much.

The even/odd ratio thing is definitely a factor in auto differential gears, for example, but I'm not sure how a bicycle chain with an unknown number of teeth affects the issue. At any rate I wouldn't worry excessively about wear on a $20.00 part that's easy to replace.

If it was my bike and I thought that a 39 chainring would be perfect I'd slap on a 39 chainring and see how I like it.

Hey, this is no problem with a multi-speed bike.

With a single-speed or fixie, it's ideal to have the number of teeth on the cog and chainring be relatively prime (e.g. 39 and 15 are NOT relatively prime because they have a common factor of 3, but 39 and 16 ARE relatively prime). This is because if the two sprockets have a common factor in the number of teeth, they'll repeatedly rotate into the same position, which can cause accelerated chain/sprocket wear if you apply more stress to the pedals in certain position (this particularly applies to skidding with a fixie).

So again, this isn't a concern for a multi-speed bike where you'll move the chain from sprocket to sprocket and you won't have these synchronised rotation problems at all. The thing you'll want to worry about is getting a front derailer that indexes well with your new chainring combinations... but with a little adjustment you can problem make your current one work, regardless of what Shimano marketing says :rolleyes:

Thanks, but in this case it's a single front ring, so no front deraileur. No worries there. :) I guess I haven't seen too much about odd-numbered teeth on chainrings for a multi-speed and worried that I'd be harming my drivetrain somehow if I made the wrong kind of swap.

I'll have to get my hands on a 39t ring soon to give it a try.

Yup, choose whatever tooth count meets your requirements, and enjoy the ride.

The other context in which odd/even tooth counts make a (small) difference is in a tandem's synchronizing chainset, with odd tooth counts "evening out" chain and chainring wear.

Sounds good! Check Nashbar for good deals on cheap chainrings, cause the Shimano ones are just ridiculously expensive. Nashbar's selective is dodgy but when they have a part they have a good deal.

Also, the shifting aids that make the Shimano chainrings more expensive add no benefit on your single ring set up.

Any knowledge/thoughts on BMX Rocket Rings? I've done a quick search or two here and seen mixed reviews, and mostly just people saying they thought they might go that route, but (to my knowledge) no posts much later giving a full perspective on quality.

Good point. For what it's worth, the cheap Nashbar rings include such shifting aids as well.

If you want singlespeed-specific rings, Surly makes nice steel ones, or find some old pre-Hyperglide ones from the eighties, or look for track rings. But frankly I think there's no reason to do that, since I've never seen any evidence that multi-speed rings throw the chain more or anything like that.

Just make sure that the bolt-circle-diameter is right.