Is it wrong to run a compact chainset (22,32,42) with a seven speed block?
This will be for my 12 year old son.
The reason i may do this is i am having probs finding 165/170mm cranks in 110 bcd.
You may need a compact front mech which, in turn, may not work too well with 7spd sti but will be perfectly ok with thumbies.
There'll be no issue with chain rub but you can use a 9spd chain - they work perfectly well in a 7spd system.
If you look at this compact mech - even with a 46t, its not quite right as it curves more than the chainring, a standard mech cage will be the opposite
Heres my Eagle with a 42t and 'standard' front mech - notice how the mech sticks out a bit? Its also lower down the frame which may introduce tyre/ mech interaction too