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Cassette: How big can I go?

Old 08-02-15, 10:02 AM
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Cassette: How big can I go?

I currently have a 9 speed Shimano Tiagra RD 4400. It appears to be a long cage. The bike has a 26 tooth cog as it's biggest. The bike also has a triple 52,42,30. My question is what is the largest big cog will the RD handle. Just looking at the distance the Rd will expand that it will handle a lot bigger cog but I am curious about the chain wrap that it will handle.

If need be I have a Deore RD that will handle a bunch of chain wrap and large cogs. If I install the Deore RD will the bar end shifters still index properly?

I am a big sissy and need some lower gears.

Thanks for the help
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Old 08-02-15, 10:06 AM
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Change the 52-42-30 to a 52-42-24

My 9 Spd touring bike came with a 11-32 cassette.
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Old 08-02-15, 10:15 AM
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As I understand it, all of the "modern" Shimano derailleurs have a shift ratio of 1.7, at least up to 10s (not confirmed on 11s). And it is the same for road & MTB derailleurs.

https://en.wikibooks.org/wiki/Bicycl...ing_Dimensions

So, other long cage derailleurs should work.

As far as chain wrap, it is best to have a derailleur that can handle Big-->Big and Small-->Small gear combos.

If your current derailleur can handle the combos, then see how many links you can shorten and still have adequate chain wrap for the Big-Big combo. That will tell you how many teeth you can add on your rear cassette without problems.

The derailleur also should be adjusted not to rub on the cassette when in the bigger cogs. Adjust the B-Screw as needed for minimum clearance.

You should be able to find chain wrap and max sprocket size ratings for your derailleur. But, most can handle up to 30T or 32T or so. The official Shimano ratings may be a bit conservative.
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Old 08-02-15, 10:23 AM
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A Deore will index the same as the 4400.
Not sure about the very newest, but older ones will for sure.
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Old 08-02-15, 10:36 AM
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So that you know the process, not just the answer:

Google to find the RD4400 large cog and wrap capacity. Then refer to Sheldon's entry about (derailleur) Capacity: Sheldon Brown's Bicycle Glossary Ca - Ce. You can get lower gears with a larger rear cog, smaller chainring, or both. Wrap capacity comes into play when you decrease the chainring size.
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Old 08-02-15, 11:01 AM
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Shimano says 27 but virtually everybody else says that's conservative. It has to do with the distance of the upper pulley from the big cog. I suspect it has to do somewhat with where the derailleur mounting bolt sits relative to the dropout. At any rate you can experiment with the "B" adjustment screw to see how it goes. I'm guessing 30 for sure, maybe even 32.

Slack take up is the one everybody always talks about but I don't think that it matters much. Make sure that your chain is long enough to handle the big/big combination. I have NEVER used my granny chainring with anything but my largest 2 or 3 rear cogs so what happens in the little/little combination is of no concern.

A Deore RD will definitely work and will index properly up to a 34 rear cog. I've made that conversion on a lot of bikes.
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Old 08-02-15, 11:15 AM
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It depends on how you are with cross-chaining.

With the Downtube shifters, I don't think I ever do 100% cross-chained.

But, with brifters, it is easy to march up or down the cassette without paying attention to the exact gear that one is in.
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Old 08-02-15, 02:55 PM
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I don't cross chain either. Granny ring and 3 maybe 4 inside cogs but mostly just granny and granny. The center chainring usually goes the full range of cogs. The big chain ring I use the smallest 4 or 5 cogs. I also have 3 bikes with Deore RD's and I have one in my parts bin I think. I have one Italian frame with DT friction shifter and a 9 speed cassette. Actually I still like the friction shifters. I was not sure the Deore would index with the current 9speed index bar end shifters. I think I will try the 32 cassette just for s%** and giggles just to see if I can make it work. Push comes to shove I will put a Deore RD in the system. I live a quite hilly area and low gears are a must for this old fat man. I am always amazed at the bike knowledge on this forum. This new stuff is all new to me. I am still riding mostly freewheels and friction shifters. I have an old great condition Neuvo Record RD that needs a home. I can no longer ride the gears this was designed for.
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