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I'm currently running a 2x10 shimano tiagra 10-speed drivetrain with a 50/34 compact crankset, a 12-30 rear cassette, and a Shimano Tiagra GS-4600 derailleur (that's medium cage aka road long cage) on a light touring / road bike.

I tend to ride a lot of hills and want a touch more range for tough climbs. Can I make this work with an 11-32 or 11-34 mountain cassette or do I need to get a long cage derailleur?

It looks like I might answer my own questions, becuase the specs say, they say I shouldn't really run more than a 30t max in the rear. However, I'm wondering if anyone else has tried this and had success going past spec with this derailleur or if I should resign myself to spending more cash on a new derailleur.

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Looking at a thread on it seems like a few others have reported success using a Shimano 105 GS derailleur. I may be able to get away with sneaking a 11-32 cassette on my bike. It's just over the spec. I don't think I'd try to push it with an 11-34 cassette. I might have to lengthen my chain slightly though. – Benzo Jun 26 '12 at 17:24
More evidence of experiences running 11-32 that were successful, even using short cage ultegra.… – Benzo Jun 26 '12 at 17:30
Interesting article about adapting Ultegra Di2 to use a 12-32 cassette.… – Benzo Jun 26 '12 at 18:01
FWIW, new shimano 105 RD-5700-GS derailleurs (11 speed) are spec'd to work with 12-32 cassettes. I've seen youtube evidence that it may handle an 11-36. – Benzo Jul 7 at 15:34
up vote 3 down vote accepted

With the b-screw tightened all the way on the medium cage GS-4600 tiagra derailleur I was able to install the 11-32 cassette. It was obvious from the appearance that I wouldn't be able to squeeze any larger of a cassette on the rear wheel without switching to a long cage mountain or touring derailleur.

There were no issues when shifting with the 34t front chainring in or out of the 32t rear cog.

However, using the 50t front chainring, shifting out of the large 32t cog was a little off. It worked, but it was lagging a bit and having a harder time jumping to the next cog, probably due to the chain being too short since I didn't adjust the chain length.

This should be solved by adding a link or two to the chain. However, I'm going to be swapping in a 46t large chainring, so that should address the lack of slack. I shouldn't even be in the big ring in the back and the big ring in the front anyway to avoid cross chaining.

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Swapping the chainring to 46t removed the need for additional chain links. No issues shifting out of the large 32t cog in the rear when on the 46t chainring in the front. – Benzo Jul 22 '12 at 1:02
After riding with this setup for a while, the only issue I seem to have encountered is that if I'm cross chaining a lot on my little rings in the front and back, I get a bit more chain slap on my chainstays, which isn't really a big deal since it's covered with lizardskins to protect it. However, It could be that I just need to shorten my chain a link or so. – Benzo Oct 10 '12 at 14:38

You are correct. You have answered your own question. A 30t cog is maximum for the Tiagra derailleur.

Using anything larger risks damage to derailleur, derailleur hanger, rear wheel and your frame, since shifting into a cross chain position can tear your derailleur off the bike. At that point it usually ends up in your wheel, and jammed against the chain stays on your frame.

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I just modified my Merida Race Lite 903 which has a similar off the store set up like yours a week ago, the hills here are tough (Sangklaburi, Kanchanaburi, Thailand). I changed the Cassete that was 11/28T to 11/34T and it worked without replacing the rear derailleur with a long cage (XT Deore). did this by turning all the way to the bottom the screw of the tiagra short cage RD to adjust the pull of the spring on the chain... noticed that the upper pulley chain guide space is just a few millimeters but not hitting the large cog as i change gears... I did not change the chain either, but i cannot use the large chaining on the largest cog... this combination not a good combination anyway...

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Thanks for the feedback. Glad to hear this worked out for you. I have no issues through my whole range with the 32t max cassette. I'm sure I could probably pop on one size larger (34t) and have a similar experience where you only have issues when cross chaining on both big rings. – Benzo Jan 26 '13 at 21:17

Just to add one more perspective: I've done this on a racing route full of hills. I swapped out my medium cage 105 for a mountain bike derailleur (Deore 9 speed). I was teased a bit, but after 85 miles I was asked a lot of questions as to how I did this and why it worked. Large cog was 34, and I had no problems shifting. So consider dishing out for a mountain bike derailleur if this is to be an everyday sort of thing. They look fine, have tons of adjustment and are much cheaper than road stuff.

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