Mountain Bike Cassestte on Road Bike

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So, after many emails and telephone calls, this is what I learned:

Specialized (the bike maker) does not have any information about conversions to wide range cassettes.

Shimano (the component maker) does not make any rear cassettes with more than 27 teeth in 10 speed, and does not recommend after market conversions.

IRD (maker of wide range cassettes) said the following: "As far as compatibility and shifting performance, the two biggest factors will be the derailleur cage itself and the person setting it up. With the 30t, a short cage will be fine. On the bigger ones, the mid-cage mountain or mid (longer) cage road will usually work, but we officially recommend a long cage mountain derailleur. Shimano lists limits and we have found them to be overly conservative, really most short cage derailleurs will perform the same as other short cage derailleurs.Second, your local mechanic will be your best resource for fit. See what your shop recommends and refer back to the general derailleur length guidelines above."

This was very vague to me - it did not specify which rear derailleur to buy as there are many of them. I did not want to buy all these parts and then have the bike not work right. The 2 LBS that I went to and asked about the conversion said they did not shift as well. They recommended that I should learn to stand up and pedal...... It seems that on every ride here in the Santa Cruz Mountains, there are a few places with really steep, long grades that are not fun and could use a lower gear.

Then I went for a club ride and one of the women had what appeared to be a bigger cassette on her 10 speed triple road bike with 105. Her bike was custom made, so I asked for the maker's email address. He gave me a simple answer to the question of which components to buy (which was the same one that Unrealcycles had given me in the first place). " Lilly Ann has an 32 tooth IRD cassette with a Shimano 105 long cage rear derailleur as I remember, I also remember that the shifting seemed fine to me... Any good bike shop should be able to help you with that... A double crank shouldn't be a problem... You should stick with a 105 10 speed long cage derailleur as most MTB stuff is still 9 speed... Hopefully someone can get you rolling, it shouldn't be a big deal..."

So, I ordered a 12-32 IRD cassette, 105 chain, and 105 long cage derailleur today. It seemed logical to me that the reason that some people have problems with IRD cassettes and some don't is that the components are not compatible. Why would IRD recommend a MTB rear derailleur that is 9 speed and not a long cage road 10 speed derailleur that would fit better? Perhaps it may have to do with the larger 11-34 cassettes, but I don't need that.

I'll let you know how it works when I get it installed.

[SadieKate]

I would recommend that you do a little research on chain wrap. Not all long cage dérailleurs have the same capacity. And it's not simply a function of the largest cog on the cassette. You also need to take into account the chain rings.

Here is an explanation with some Campy info, but the same concept holds true for all dérailleurs.

http://branfordbike.com/articles/rea...leurs-pg62.htm

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I did do a calculation of chain wrap for my bike before I ordered the parts. I went into the Shimano website and looked up the specs on the derailleurs for chain wrap capacity. The front is 50T-34T=16T, the new rear cassette will have 32T-12T=20T. 20T+16T=36T. The chain wrap capacity of the long cage 105 derailleur is 37T, so it should be ok. I could have gone for the 11-32, but didn't think I would need higher gears and wanted to play it safe. An 11-34 would not work.

What is ironic is that the original bike components are out of spec. With a 12-27 rear cassette, I get a chain wrap capacity of 31T, but the small cage rear 105 derailleur has a rating of 29T chain wrap on the website.

The problem with the MTB cassettes is that most seemed to have chain wrap numbers of 33T for the small and 45T for the large, and I don't know if they are attached to the bike differently. (I have learned all this in the last week.) Those seemed too small or too big. So, now I will cross my fingers and hope that it works or I am out $200+.

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So, I had my new components installed yesterday and went for my first big ride today (in between terrible rain storms - no power for 2 1/2 days, much debris in the road). The bike worked just great! It shifted as well as before. There is no rubbing and it is very smooth. I now have 2 new lower gears, and I could get up the big hills much better than before. Instead of 34/27, I have 34/32. Why is there so much lack of knowledge about how to do this from all the professionals? Why would they recommend a mountain bike cassette which is for a 9-speed bike, when a long cage 10 speed derailleur would obviously be more compatible? I think this mismatching of components is why other people have had problems with this conversion.

To summarize, this is what I learned. I installed an IRD 12-32 cassette, with a new chain and a long cage 105 10-speed rear derailleur on my Specialized Ruby Compact. To find out what size derailleur you need on your bike, you need to figure out the chain wrap capacity.

You take the number of teeth on the largest front ring and subtract the number of teeth on the smallest front chain ring. (ex. 50-34=16). It doesn't matter if it is a triple or a compact double. (In a compact double the small ring has the average number of teeth between the small and middle ring of a triple.)

Then you take the number of teeth on the largest ring in the rear cassette and subtract the number of teeth on the smallest ring in the rear cassette. (ex. 32-12=20).

You add these two numbers together to get the chain wrap capacity.
(ex. 16+20=36). The chain wrap capacity for my bike with the new cassette is 36. Then you look up on a website the chain wrap capacity for your derailleur in the bike specifications. My small cage rear derailleur that came with the bike had a capacity of 27 and would not work. The new long cage derailleur has a capacity of 37, so will work with the new larger cassette. An 11-34 rear cassette would probably not work or would need a (larger) mountain bike cassette, but would have the issues of 9 vs. 10 speed.

I hope this will help anyone else who needs to get lower gears on their road bikes. I am really happy with mine.