Compact chainset compatibility

[RobbieA]

Might be a silly / overthinking moment, but is there any issue with using pre-compact size front derailleurs with a compact chainset?

It's a Shimano set-up and I note that there would be a reduction in BB spindle lengths (122.5 to 113mm) - I will have the appropriate BB for the chainset, but would the derailleur arm be in the wrong place relative to the chainset, or could it be adjusted to accommodate?

Also, would the radius of the derailleur be different? Does it matter?

Should I have less caffeine? Maybe...

Cheers, Rob

 

[Fourbikes]

I run an m560 front mech with a compact chainset, doesn't seem to cause any problems. The cage on it is a bit longer than, say an m565, but works fine for me.

 

[RobbieA]

Good to know. I don't have a 110bcd chainset (thought I did somewhere) as long as it all works, for you, then I hope it will for me!

 

[Fourbikes]

You may have to position the mech slightly higher on the seat tube to avoid it touching the chain stay. That's the only thing that might cause a problem.

 

[hamster]

I've never had a problem running M735 (say) made for 26/36/46 with 20/32/42.

 

[RobbieA]

Clearly I'm overthinking it all, as you are both demonstrating!

 

[bikeworkshop]

Clearly I'm overthinking it all, as you are both demonstrating!

Not sure you are.

In most cases its just not a problem.
As @Fourbikes and @RobbieA say

The bb length is irrelevant if you're using the right one for the chainset
- the chainline is the crucial detail.
Road chainline is closer than mtb, which got wider as tyres got bigger.

This sometimes creates a compatibility issue between road and mtb kit

Changing outer ring size can mismatch the curve of the derailleur cage, or catch the stay.
Changing inner ring size can catch the chain or struggle to shift.

But there's often a work around or compromise.