Originally Posted by
mawn
I want to replace my old -but still good- 52/42 Shimano 105 crank with a 50/34 Shimano Sora (cheap), because i need an easier gear.
There’s one issue you ‘might’ encounter going from a 42 down to a 34 with the small ring. The smaller circumference of the 34 changes the angle of the chain back to the gears and could cause it to snag on the large ring. I wanted to change from a 52/42 setup to a 53/38. Once re-assembled, everything ran OK in the middle gears. However, the angle of the chain from the new/smaller ring back to the smallest gear caused it to snag on the large 53 ring. So I changed the small ring to a 39 and that increased the circumference jus enough so that it doesn’t snag on the large ring. Given that you’re also going down to 50 on the large ring ‘might’ allow clearance. But it’s and issue you might run into.
Edited to add a simple drawing to help explain. Obviously not to scale, but just to show where I had the problem.
Dan
Last edited by _ForceD_; 05-31-23 at 08:18 AM.