Sram Rival

olmo

I got a boardman with 10sp sram rival.When I put it onto the small chainring 34T on the front the chain rubs off the front derailleur when Im in the smallest 2 sprockets at the back 12 and 13.Halfords say nothing they can do with it as its a fault with the design. There is a 'trim' on the front derailleur but only when in the big chainring. Anyone else have the same problem or are Halfords tellin me porkies! Thanks

Chips Lovell

It's designed like that. Pretty much all gear systems are, not just Sram Rival. You're not meant to cross chain, i.e. use the largest sprockets with the big ring or smallest sprockets with the small ring.

olmo

Fair enough El Tonto thanks for that. I thought it was just mine.

DirkVoodoo

Sheldon's gear calculator gives a pretty good idea of why you shouldn't ever need to cross chain. Gear systems tend to have overlap in terms of ratios. The following shows a compact gear setup with a 12-25 cassette. The values shown are the speeds in km/hr at 90 rpm.



You can see that without using the 23-25 rear sprockets while in the larger 50 tooth front chainring and not using the 12-13 rear sprockets while in the smaller front 34 tooth chainring you get the exact same spread of gears. A bit of cross chaining tends to happen in a compact as the spread of gears is larger than a standard double, but generally you should avoid it if possible for lengthy periods of time. Parts wear out, noises are made, etc.