using xt 13-30 cassette on road bike?
I am thinking of using a deore xt 13-30 rear 8 speed cassette on a road bike with only a single 48t front chainring and a shortcage derailleur. I am wondering if there will be any problems with using the short cage with this wider range cassette which is typically reserved for mountain bikes. I am looking to make the bike sleek, but want a good range in the back to support any substantial climbs. Any thoughts would be appreciated.
me have long head tube
The derailler will probably have enough capacity to take up the slack - the range is only 27t. However, the issue is whether the derailler will move at an angle that can handle a 30t large cog. Shimano "road" rear deraillers are rated to 27t max cog, although in practice 28t always works, and often 30t will work too. However, a 30t large cog isn't always a sure bet with this setup.
I run a 13-28 7-speed freewheel on my commuting bike, with a single chainring. So, similar setup. But I've got an old Shimano Exage rear derailler, designed for mountain bikes (so, long-cage), with an official max-tooth capability of 32t, functional to 34t.
If you're using friction-shifting, this is probably irrelevant; your derailler will handle it. But if you're using indexed shifting, 30t large cog with a road rear mech is a "maybe."
... and I am using a friction downtube shifter.
The type of shifting is irrevant. The derailleur may or may not handle a 30T cog, no matter what type of shifting the OP has. The problem is will the jockey pulley clear the cog teeth, not will it move laterally enough.
Originally Posted by timcupery
cycles per second
It might work. I've run 13-30 6-speed and 11-30 8-speed with Shimano road short cage rear derailers. You can often get Shimano road derailers to run on a 30T. The issue is that the guide pulley may hit and run against the 30T cog when you shift to it. You might need to adjust the "B-tension" of the rear derailer, and if you have horizontal dropouts you may need to move the wheel forward or backward. If it's not quite working but is really really close, you could also insert a 1 or 2mm axle spacer to move the cassette further from the derailer (you usually don't need to redish for only 1mm but you might want to if it takes 2mm to make it work). This will result in the derailer angling down slightly further once it gets to the 30T cog. Alternatively, you could put a spacer between the hanger and the derailer and no redishing would be required.