moto367

Is there a standard on how a manufacturer determines the gear spacing on a cassette? (be it road or mountain). For example: my XT is a 12-34; the next cog after the 12 is a 14, then a 16. Somewhere in the middle it jumps up by 3 teeth then 4. Hope what I'm asking makes sense. Thanks for any help.

Lightingguy

Sheldon's site does gear charts, and one of the features (when you click on stock cassette) is a listing of the current Shimano cassette ranges, including some custom buildups from Harris.

https://www.sheldonbrown.com/gears/

Campy and SRAM have their own, but generally a less complete range as compared to Shimano.

And in answer, there is no standard, just what the manufacturers decide is appropriate and what they can engineer and manufacturer as cheap as possible.

Part of that lack of standard is that the spacing between the cogs varies between the 3 biggies - Shimano, Campy and SRAM, so that while in the 9 and 10 spd. systems, the spacing is very, very close, it's not exact, with the result that "successfully" mix and matching between manufacturers is sometimes problematic.

Steve B.

moto367

Thanks. That's what I was looking for.
Steve V.

HillRider

Are you asking about cog-to-cog spacing or how the gear tooth choices are made?

Gear tooth choices attempt to provide the smallest % difference in gearing between adjacent cogs, as best the number of cogs and total range will permit. Since we are limited to integer tooth counts (i.e. there is no 14.5 tooth cog) the best that we can do is one tooth differences or a "straight block" as much as possible. For example a 12x21 10-speed cassette can be "straight" over the entire range. As the range gets larger or the number of cogs smaller, there have to be 2 or 3 tooth differences.

Also, as the cogs get larger and 2 or 3 tooth differences are used to get the overall range, the % difference between adjacent cogs gets smaller so a 2 or 3 tooth difference isn't as much of a change as it would be for small cogs.