Chainring ratios for a compact 9-sp XT drivetrain

ndimoff said:

I use a non-standard chanring set of 24/32/34 with a 9-sp XT compact
drivetrain. I replaced the original 22 granny with a 24 because the
chain was rubbing the bottom of the rear triangle of my 2001 Schwinn
Rocket 88 - a known design problem. Now the middle ring needs to be
replaced. Will the FD handle a 24/34/42 ratio? Or should I stick with
24/32/42 or should go with the original ratio of 22/32/42 and take one
or two chain link which is the other cure for the rubbing (but I am
taking a chance as if I go by mistake biggest on biggest then the
chain would be too short and my der. hanger - history)? Any experience
or theorethical agrument will be appreciated.

I use a non-standard chanring set of 24/32/34 with a 9-sp XT compact
drivetrain. I replaced the original 22 granny with a 24 because the
chain was rubbing the bottom of the rear triangle of my 2001 Schwinn
Rocket 88 - a known design problem. Now the middle ring needs to be
replaced. Will the FD handle a 24/34/42 ratio? Or should I stick with
24/32/42 or should go with the original ratio of 22/32/42 and take one
or two chain link which is the other cure for the rubbing (but I am
taking a chance as if I go by mistake biggest on biggest then the
chain would be too short and my der. hanger - history)? Any experience
or theorethical agrument will be appreciated.

The fder should handle the 18T difference, especially since it was originally equipped with a 20T difference. Shifting will be improved between the middle and big with the 34 vs. the 32, only slightly degraded from the middle to the small on the 34 vs. 32.

By bottom of triangle I suspect you mean the top of the right rear chainstay.
Is increasing the size of the larger sprocket(s) (my preference) another option on clearance besides the 24 sub for the original 22?

Since it’s only 2T diff on the granny, if fixing via sprocket isn’t practical, I’d prefer the taller gear over straining the rder if you err in shifting. However, my Shimano RSX derailleur has been pleasantly forgiving with my 2T chainlength error.

Another option (I’d give it a lower preference to the other 3), would be to put a recumbent idler positioned so it only contacts the chain when in the offending gear.