According to United Bicycle Institute:
- Determine Maximum Chainring Difference by subtracting the number of teeth in the smallest chainring from the number of teeth in the largest chainring
- Determine Maximum Cassette Cog Difference by subtracting the number of teeth on the smallest cassette cog from the number of teeth on the largest cassette cog
- Determine Total Drivetrain Capacity by adding Maximum Chainring Difference to the Maximum Cassette Cog Difference
- Record the Maximum Cassette Cog (the number of teeth on the largest Cassette Cog)
For Shimano:
SS - Short Cage Road Double -
Maximum Cassette Cog is 27 and
Total Capacity is 29
GS - Medium Cage MTB/Road Triple -
Maximum Cassette Cog is 34(MTB)/27(Road) and
Total Capacity is 33(MTB)/37(Road)
SGS - Long MTB -
Maximum Cassette Cog is 34 and
Total Capacity is 45
For SRAM:
Short -
Maximum Cassette Cog is 34(MTB)/28(Road) and
Total Capacity is 32(MTB)/31(Road)
Medium -
Maximum Cassette Cog is 34 and
Total Capacity is 37
Long -
Maximum Cassette Cog is 34 and
Total Capacity is 45
For Campagnolo:
Short -
Maximum Cassette Cog is 26 and
Total Capacity is 27
Medium -
Maximum Cassette Cog is 29 and
Total Capacity is 36
Long -
Maximum Cassette Cog is 29 and
Total Capacity is 39
NOTE - THIS INFORMATION IS SUBJECT TO CHANGE BY MANUFACTURER
so that is a good general rule but how far have you stretched it and gotten away with a larger cog?
Today I had a buddy over to convert our old 90s race bikes into something we could use daily on the bay area hills. I set him up with a 9speed deore XT dr with a 34t, and I happen to have this unicorn of a dura-ace gt that I set up with a 30t on my 8 speed. and it worked but its close.. may.. maybe could go to 32.....
