[BeniBoy]

Kind of reminded me of the traditional wisdom that you should select chains and sprocket to avoid patterned wear. I don't know if there is an effect (especially with bikes and their low torque),but I always think about it when changing my chain. I suppose this only applies to single sprocket bike.

[jstanley]

This is called "hunting tooth". It applies to any system of gears or timing belts.

In gears, if you make sure the number of teeth of mating gears is coprime then you wear the teeth of one gear evenly against the other gear. If there are low factors, then each tooth of one gear only engages with a small number of teeth on the other gear, exacerbating wear.

With belts, if the number of teeth on the belt shares low factors with the number of teeth on either of the pulleys then you get the same effect: any given tooth on the belt only ever meshes with a small number of teeth on the pulley.

[dredmorbius]

You've just made me realise that several of the more popular freewheel / sprocket combinations are in fact prime: 11, 13, 17, 23.

Usually contrasted against the decidedly non-prime 52 (2 * 2 * 13) and 42 (2 * 3 * 7) chainrings. 38 if you're old-school triple (2 * 2 * 7).

My chain lengths would vary and I never counted them specifically, though I'd typically remove a few links for fit.

[fyhn]

Isn't this only relevant for fixed gear bikes, where you can skid the rear wheel to brake, and you want maximize the number of possible "skid patches", or orientations of the rear wheel while the pedals are in a fixed position?

[analog31]

It could also apply to the wear on the chainring and cog if their teeth are in a perfect proportion to one another, but without thinking too hard about it, I'm guessing that's actually rare. For instance a 2:1 ratio would be awkward for most cyclists riding on pavement (too low) and 3:1 too high, if they had to choose exactly one ratio.

I ride 46:19, because those are the parts that were in my bin when I built the bike, and it's a pretty good all-round ratio for city riding unless you're a lot more athletic than I am.

Also, those components wear out soon enough anyway -- a chain lasts 2 to 3 thousand miles, and a cog lasts a few chains.

[namdnay]

"I suppose this only applies to single sprocket bike."

[jstanley]

It applies to any bike. Ideally the number of links in the chain would be coprime to the number of teeth on every sprocket. The easiest way to achieve this is to make the number of links in the chain prime.

It'll still work just fine if you ignore this idea, but it might wear out more quickly. If you're a hobbyist just trying to make something work, you can safely ignore it and do whatever is most convenient. If you're a bicycle engineer trying to make things reliable and long-lasting, then there's no downside to making the number of links prime if you can arrange it.

I don't know whether bike companies actually do choose prime-numbered chains, maybe they have other constraints that are more important.

[dmurray]

> It applies to any bike

On a bike with derailleur gears, every time you change gears the derailleur will add some slippage so you won't get this effect.

> The easiest way to achieve this is to make the number of links in the chain prime

The chainring is fixed, but you might need to add or remove a link in the chain. In practice it seems more common to make the chainring have a prime number of teeth (53 or 47).

[jstanley]

Good point about the derailleur, I hadn't thought of that!

[1-more]

> then there's no downside to making the number of links prime if you can arrange it.

Bike chains always have to have an even number of links because they come in inner and outer pairs. But this has an effect on chainrings as well. When the tooth on a chainring or sprocket is in between two inner plates it's in a narrow gap. When the tooth is in between outer plates that's a wide gap. If you have a chainring with an even number of teeth then you can have the teeth match the narrow and wide profiles (called a narrow-wide chainring) which is supposed to make it less likely that you drop your chain off the chainrings. I'm not sure if it works, tbh. It seems to only be a thing in mountain or gravel bikes with a single chainring. I can't find any track chainrings that have the profile but I only looked for a second.

For chains themselves there's usually a pretty narrow number of links that work on a road drivetrain. I think I can live with one fewer or more pair of links on mine.

https://www.firstcomponents.com/narrow-wide-chainring/

[et-al]

Just to add on:

Track chainrings don't need a narrow-wide profile because there's very little slack in the system for the chain to come off. Saint Sheldon warns of the possibility of losing a finger also for this reason.

[jstanley]

> Bike chains always have to have an even number of links

D'oh! Of course, you're right.

[et-al]

Yeah, chains and cassettes are considered wear items, so I doubt manufacturers put much thought into patterned wear. Avoiding chain drop and crisp shifting are higher priorities.