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Old 05-01-18, 08:36 AM
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Originally Posted by 09box View Post
Upon looking at my bike, it is a Tiagra RD-4601 with MA stamped on top of it. Looking on Shimanos website, there is no 9 speed version of this RD and only a 10 speed. Am I missing something here?
Tiagra 4600-series components are 10-speed, but they use the same cable pull ratio as Shimano's 9-speed components and are thus interchangeable for the most part.

Originally Posted by 09box View Post
I looked at my bike and called Shimano and it looks like the RD is a medium cage. Looking on Shimanos site, it looks like the max is a 40T. Would I still need the Wolftooth Roadlink?
According to the specs on Shimano's web site HERE, the Tiagra RD-4600-GS (GS = medium cage) derailleur is rated for a maximum cog size of 30 teeth. Its maximum wrap capacity is 40 teeth, but that's a different limitation. Shimano's generally pretty conservative with their ratings, so it wouldn't be surprising that a derailleur rated for a 30 tooth cog could handle 32 teeth. 36 would be pushing it, though.

But about that 40-tooth wrap capacity... The spring-loaded cage on the rear derailleur has to take up chain slack not just when you shift to smaller cogs on the rear wheel, but when you shift to the smaller chainring up front as well. So to calculate wrap, you add your cassette's range and your crankset's chainring range:

(largest cassette cog - smallest cassette cog) + (largest chainring - smallest chainring) = wrap

So if you've got a compact double crankset (34/50) and an 11-36 cassette, you'd need a derailleur with a wrap capacity of at least 41. A medium cage Tiagra RD-4601 has a published wrap capacity of 40. It'll probably be OK, but you're right at the edge of the published spec. If you size the chain properly for the big/big gear combo, you'll probably find the chain to be fairly slack in the small/small combo.

If you want to go to an 11-36 cassette and your Tiagra derailleur doesn't work as-is, my preference would be to pick up a mountain bike derailleur like the Shimano Deore M591 instead of using a Wolf Tooth Road Link. It's about $15 more expensive, but it'll handle your desired cassette size and wrap, without an adapter.
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