I wish to use a long cage Shimano 105 RD with a 11-34T cassette. The front crank is a double
(48-34T) with obviously a double FD. This will give me a desired low gear of 1:1.

The total capacity of the 105 long cage on the Simano site, and my calculations with the above combination, match at 37.

However, I am concerned about the chain hitting the bottom of the FD cage (double) when in the small front ring, as I shift to the smaller sprockets on the rear cassette. Obviously at some point (usually third from the smallest), the cross-chaining will force me to shift.

BUT, does this total capacity assume that a front triple FD is being used? If this is the case, and because I have a double FD, will this cause the chain to hit the FD cage sooner on the upshifts?

Hope that makes sense.

Thanks!

Digger

The Shimano site shows a 16 tooth difference for 105 double FDs and you only have a 14 tooth difference so there should be no problems.

Oh right! D'oh!

I am running an 11-34 IRD 10-sp cassette with a 105 double FD and an XTR long cage RD and it works fine.

If you already have the 105 RD, then try it and it might work. Don't buy one, though, as it is not adequate for the job, specs-wise. I don't remember the particulars, but I did not go willingly to an mtb RD for a road bike, except no road components were supposed to work (road cassettes are not that big, as you know)

No I don't have the RD yet. But if the specs match my setup then it should be ok(?). Total capacity matches with the 37 on the Shimano site and the front ring difference (as CACycling mentioned) is within the spec.

I know there is no "road" cassette but I will be using a "MTB" cassette.

I'd go with an MTB rear derailleur. Two things to look out for: chain wrap capacity and cog capacity. The chain wrap capacity for a road RD would be OK but it might not be able to get the jockey wheel low enough to clear the big cog. And if you can get the jockey wheel to clear then it will ride too far away from the cassette in high gears. The MTB RDs have a steeper "slant" to them that will keep the jockey wheel the right distance when using a large cassette. Generally if you have 11/28 or wider it's better to go with a MTB RD.

Fair enough. I'm just thinking of my commutter bike with a 1998 Shimano 105 RD (8 speed) with a 11-32T cassette and it works fine. That's with a 48-36T rings up front.

But sure, a MTB derailler might be a better bet.