This problem - simply a longer BB needed? Or another solution?
Instead of whacking the chainstay, you could consider:
Shorter cranks;
MTB cranks;
A bottom bracket spacer on the drive side, unless both sides are clipping the stays, in which case;
Longer axle bottom bracket.
Instead of whacking the chainstay, you could consider:
Shorter cranks;
MTB cranks;
A bottom bracket spacer on the drive side, unless both sides are clipping the stays, in which case;
Longer axle bottom bracket.