Plainsman

A friend of mine has a Specialize Ruby road bike, with a compact crankset and 105 ten speed components. She is looking for a little more climbing oomph. I've noticed recently that several 10 speed bikes utilize a 12-30 cassette with a 50/34 crankset. In fact, my 2014 Scott CR1 has this setup and does quite well. My question is this - I understand that putting a 30 tooth cog exceeds the recommended number of teeth with a short cage derailleur, yet many manufacturers seem to make it work. The Ruby is a size 54 I think, with 415mm seat stays. Is there any reason such a setup would not work on her bike as well?

shelbyfv

There is a good chance it will work. I've used the 12-30 on three different bikes with three different rear derailleurs. All were out of spec and all worked well.

Ontarget

I was able to install a 12-30 on my Fuji Roubaix (Shimano 105 (5700) with short cage derailleur) without a single adjustment. I believe derailleur hanger geometry has a lot to do with it.

Also, there is a newer version 105 short cage that is designed to accomodate a 30t cog, though that isn't the one I have.

Davet

If your friend's Ruby has a 5700 or 6700 series rear derailleur, you will be able to use a 30t cassette successfully. You will have to adjust the B screw so the upper ferailleur pulley doesn't bump the largest cog.

FBinNY 

There are two considerations here.

1-the maximum size sprocket exceeds the RD limit. There's fudge room here, and that should be workable

2-the maximum chain take up capacity. The 18t spread of a 12-30 cassette combined with the 14t spread of a 39-53 crankset calls for 32t of chain take up capacity. This is well beyond that of most short cage RDs. It's workable if you accept limitations. Measure the chain long enough to run the big/big combination with enough slack to shift effectively (usually 1"). Because the RD will be lower than normal, I suggest making this measurement with the chain threaded through the RD.

This will leave the chain too long for the RD to take up the slack when using the small/small combination, and probably limit you from using the small with the smallest few rear sprockets, but this won't be a problem in practice, as those are overlapped ratios anyway.

Bill Kapaun

Looking at the 2013-2014 Shimano catalog-
All the SS "road" RDER's have a chain wrap capacity between 33-37T, depending on the model.

FBinNY 

I don't know every RD spec, but I said it was consideration to be managed, and still think that's true. Anyway, based on your post, I edited mine to say "beyond the take up capacity of most RDs".

Plainsman

My friend's bike is a 2010 Ruby Elite. I'm not sure, but didn't 5700 come out in 2011? If her drivetrain is 5600 105, can this still work? I notice that in 2011 they took the Ruby frame and outfitted it with a SRAM Apex 10 speed set-up using an 11-32 rear cassette and a 50/34 crankset. She has a 105 compact crankset (50/34), and currently runs a 12-27 cassette. We want to keep the 50/34, and bump up to a 12-30. That gives us a differential of 16 in the front and 18 in the back, for a total of 34. How do I determine the actual chain wrap capacity for the 5600 short cage rear derailleur?

FBinNY 

As I posted earlier, going beyond capacity isn't a major issue, as long as you measure the chain to loop the big/big +1" combination. If the RD lacks take up, you'll have a droopy chain on the small/smallest few combinations.

So, in your shoes I'd give it a shot and see what I see. Worst case, you won't be able to ride combinations you don't ride anyway.

However, if you want to know in advance. Shift to the small/small combination, and derail the chain off the bottom of the inner chainring. Back pedal slowly until the RD stops taking up slack. Then while holding the crank so the chain can't move backward, pull the lower loop forward along counting the links as they pass a fixed point. Pull until the lower loop is straight. The number of links you counted is what the RD can take up.

Plainsman

Great advice, thanks!

cale

You can get a great medium derailleur, Tiagra, for $38.00 delivered from big UK catalogs. I'm shifting a 12-30 on a compact 50/34 with this derailleur. No fuss.