Derailleur compatibility lower gears

[awbikes]

My wife's bike seems to be perfect with one exception. It needs lower gearing. She only carries about 25 lbs of gear and generally speaking we are flat landers with the exception of some hills. Her current set-up is Tiagra front and rear dérailleurs and a Tiagra crank with 50/34 rings. Rear cassette is a 25/12. Although our style of riding does not require super low gearing, she has problems with hills and bridges and needs to go lower than where she is now. I believe the primary limiting factor is her Tiagra dérailleurs. What is the lowest front and rear rings/sprockets her current derailluers will handle.

[DaveB]

Her "compact" crank is already geared about as low as it will take.  A 110 mm BCD crank will accept a 33T chainring as the smallest and that's not enough improvement to bother with relative to the 34T it now has.

The rear cassette could be changed for a 12x27 which will give an 8% lower gear.  An XXXx30 or XXx32 cassette may (repeat may) work but it has to be tried since it exceeds Shimano's maximum cog spec.   

If your Tiagra group is 10-speed there is now a 12x30 10-speed cassette that should work with your existing rear derailleur and a 32T cog may also work with it .

To go significantly lower, you have two choices:

1.  Replace the current rear derailleur with a 9-speed era MTB rear derailleur (note the new 10-speed MTB Dyna-Sys rd's are not compatible with road shifters) which will let you use a cassette with a 32 or 34T large cog.

2.  Fit a triple crank.  You will need the crank and a triple front derailleur.  The 9-speed Tiagra brifters were doubl/triple compatible so no change will be needed there.   the 10-speed Tiagra brifters are specific to double or triple so new shifters will be needed if you have the 10-speed group.

[RussSeaton]

The information given is great.  My only addition is to definitely try your current rear derailleur with a 11-32 or 12-32 cassette.  I'm guessing it will work just fine.  My brother ran a 11-32 cassette with a very old short cage Shimano 600 road rear derailleur.  Worked just fine.  So the maximum cog size limitations given are not to be believed.  And as mentioned, cheap 9 speed Shimano long cage rear derailleurs for mountain bikes can be found for $20 and will clear a 32 rear cog just fine.

[awbikes]

Thanks for the info. I will try the 11/32. As I am just beginning to learn my way around the mechanics of bikes, I would like to try the swap myself. Any helpful hints would be appreciated.

[RussSeaton]

To change the rear cassette you will need a cassette lockring remover, a big Crescent wrench to turn it, and a chain whip.  You will also need a new longer chain if you go to a bigger cassette.  So a chain tool is needed to shorten a new chain to the right length.  You can use the chain tool to reconnect it with the special pins provided.  Or use a quick link/master link from SRAM or Wippermann that can be used to reconnect the chain without any tools.  If you get a new rear derailleur, mountain, you will need the chain tool to break the chain to thread it through the pulleys, remove current chain, and a 5mm Allen wrench to install the rear derailleur in the rear dropout.

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[DaveB]

Russ' info above is good.  I further recommend you look up Park Tool's web site (www.parktool.com) for their tutorials for installing the new cassette, rear derailleur and sizing the chain properly.   Also carefully read and follow their tutorial for properly adjusting the limit screws and shifting for the new derailleur.

[PeteJack]

a big Crescent wrench to turn it

My lock ring tool has a 1/2" socket. If you have a socket set with a breaker bar you can use that in the socket, it's less likely to slip than a Crescent wrench

[DaveB]

a big Crescent wrench to turn it

My lock ring tool has a 1/2" socket. If you have a socket set with a breaker bar you can use that in the socket, it's less likely to slip than a Crescent wrench

Some common lockring tools have a hex that will fit into a 1" (Park)  or 15/16" (Shimano)  1/2"- square drive socket so that works too.   A lockring tool with a center pin keeps it from popping out of the splines under torque.

[PeteJack]

When you reassemble remember the lockring is supposed to be very tight. Here's the specs from Park Tool

[Pat Lamb]

When you reassemble remember the lockring is supposed to be very tight.

True.  Never could figure out why it needed to be so tight when the thing feels so notched going on, but after it came loose two years in a row on the same fall century, I now use the "tight as I can get it with a 12" wrench" spec.  Haven't had any problems since.

[waynemyer]

Most cassette lockrings I have seen indicate a spec of 40nm, which is a good reefing with a normal wrench, but not really a lot of torque, and certainly nothing that calls for a breaker bar.

[RussSeaton]

A lockring tool with a center pin keeps it from popping out of the splines under torque.

Just put the quick release skewer through the axle and through the lockring remover tool and it will never come out of the splines.  I suppose you could still figure out a way to make the wrench flats come off the flats of the lockring tool.  You would have to work at it.

[DaveB]

A lockring tool with a center pin keeps it from popping out of the splines under torque.

Just put the quick release skewer through the axle and through the lockring remover tool and it will never come out of the splines.  I suppose you could still figure out a way to make the wrench flats come off the flats of the lockring tool.  You would have to work at it.

That works but the remover tool with the pin is faster and easier.  I invert the tool so the pin points up in my bench vise, drop the wheel, cassette side down, on it and wrap the chain whip around a convenient cog.  One pull and it's loose.  The pin also allows using a 1/2" square drive socket and a torque wrench on the tool to tighten the lockring with no problems with the tool rocking or popping out of the splines.  Even the nut side of a qr skewer won't clear the socket.