Two years ago I installed a mountain bike (MTB) derailleur (Shimano XTR) and cassette on Anne’s road bike at the time.  See the prior post. At that time all MTB setups were 9 speed but her road bike was a 10 speed.  Although there was a 3rd party 10 speed cassette with a wide range, it was expensive and most people didn’t feel it worked very well.  So back then I used a device to allow you to use 10 speed shifters with a 9 speed MTB cassette.  The device, which had two concentric pulleys, changed the cable pull as needed.  It worked, but never worked very well and it seemed very hard to adjust things so it would shift smoothly.  It also meant that one of the index positions on the shifters would not work since there were only 9 cogs.

Anne got a new road bike last year and has been doing well with a triple chain-ring and a 12-27 cassette.   In the meantime Shimano has come out with 10 speed MTB gearing setups.  So for a relatively small price I bought a Shimano XT 10 speed cassette (Shimano XT M771-10) with a range of 11-34 and moved the XTR derailleur (RD-M971) from her old road bike to her new bike for those few times when she might need that extra low gears.

Type of Derailleur

Most road derailluers are limited to no more than 28 or 29 maximum teeth on the cassette. SRAM does sell a road derailleur as part of their APEX group that will accommodate a wider range, up to 32.  These are designed for use with a compact crank, although they may work with a triple also.

The other approach is to use a mountain bike oriented rear derailleur.  A word of caution is in order.  For many years Shimano kept the cable pull the same between their road and MTB shifters.  However with their 10 speed MTB bikes, they changed the cable pull.  Therefore, you can not buy a 10 speed rear MTB derailleur and have it work with 10 speed shifters on a road bike.  Instead, you need a 9 Speeed derailleur.  That might sound wrong since it will be used with 10 speed shifters but the XTR is not indexed and the indexing is all done with the shifters so it works just fine since it matches the cable pull on the 10 speed road shifters.  Another point is you need to use a “top normal” derailleur.  High normal or top normal rear derailleurs return the chain to the smallest sprocket on the cassette when no cable tension is applied, the same as with road bikes.  Many newer mountain bikes have a bottom normal or rapid rise shifter which is opposite, where the derailleur moves to the largest cog with no cable tension.  It would work on a road bike but everything would be backwards and should be avoided.  For Shimano the right model is RD-M971 for the XTR series and RD-M771 for the less expensive XT series.

Cage Length

Mountain bike derailleurs come in various cage lengths, which define the amount of varying chain length they can deal with.  For the Shimano RD-M971 the specs for long  (SGS) is 43T and for medium (GS) it is 33T

The easy capacity formula is to add your big ring & cog sizes, then subtract your small ring and cog sizes. It looks like this:

cap req’d (T) = (BIG ring – small ring) + (BIG cog – small cog)

…so for a 53-39-30 triple road crank & 11-34 cassette…

T = (53T – 30T) + (34T – 11T)
.. = (23T) + (23T)
.. = 46T

Using this simple formula, you would need a derailleur with a 46T rated capacity to absorb all the possible extra links and allow you to use every conceivable gear combination.  The Shimano long cage allows for 43T, and knowing that Shimano is conservative, then it should work fine.  If you have something other than a triple, use your own values in the formula above.  For example with a double crank (53-39) and a 11-32 cassette, it is 35 so a medium cage is sufficient.

Cassette

Although they sell cassettes with a capacity up to 36, most 9 speed derailleurs only accommodate up to 34.  This is what I installed, but you might consider using something like a 11-32.  That will still give you a very low gear and less of a jump between gears and you can buy a road cassette (SRAM with the range 11-32).

Chain

Another word of caution is about the chain.  Anne has a triple crankset so according to Shimano you should not use the newer chains (such as the CN-7901), which are only suppose to be used with a double (or compact) crankset.   That is true even if you have the latest Shimano Ultegra Triple.  They recommend instead the older model CN-7801 or CN-6600 chains.  However for the cassette I installed, they recommend the CN-HG94 chain, which is for the Dyna Sys mountain bike setup and is directional (like the new DuraAce chains)  so I would avoid that.  It is not as simple as when you would be a chain from Shimano and it said for either DuraAce or XTR.  Having on hand a new DuraAce CN-7801 chain I used that and it worked fine.  That is what one with a triple crank should be using, even if they have a regular road cassette.

It didn’t take long to install the XTR derailleur and new 11-34 10 speed cassette. I put on a new chain while I was at it.   Adjusting the shifting was the same as if it were a mountain bike.

Yesterday, we rode up Henry Coe, near Morgan Hill.  There is one short segment with the grade getting to about 20%.  Anne said the lowest gear she went into was a bit too low.  I think a 11-32 cassette would have been a better choice for her, especially considering she seemed to have no problem climbing it previously with her old 12-27 cassette.  Still it is nice to know there is a yet another gear in case you need it.

Categories: Cycling Equipment
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