Shimano MTB vs Road Cassettes

gsd

Hi all,

I've just stripped down my wife's bike and replaced the chain, chainrings and rear cassette (amongst other things) after about 5000 miles cycle-touring round New Zealand.

The bike has 105 components with "below the bar" shifters on MTB style handlebars. She'd had an 11-25 cassette which was a bit of a killer on the mountain passes and so I thought I'd get her a bigger range on the new cassette. After ordering the bits in the LBS and having to wait 3 weeks for all the bits(!) he'd forgotten this cassette and so grabbed a 12-32 9-speed MTB (LX I think) one off the shelf and said "oh, yeah, this will do." I even asked him if he thought there'd be enough clearance for the rear derailler which he said there would be.

Long and short is: there isn't enough clearance for the 32 or really the gear below it.

But... my question (thanks for your patience) is this: I absolutely could not tune the derailler to select all the gears. I could either get the smallest 4 cogs working or the top 3 or the middle 3. I couldn't get it to work across the range even forgetting about the 32. Is this because the derailler was being pulled too much? I thought that I'd still be able to tune the gears that I could get to and was really surprised that I couldn't.

Any advice would be much appreciated.

Thanks,
Scott

http://www.dupoy.com/cgi-bin/photo.cgi?b=7375

maddog 2

as you say, MTB cassettes aren't compatible with road rear mechs as (Shimano) road mechs (long and short cage) are only rated for sprockets up to 27t (officially) and 28/29t in practice.

To run anything more you need a MTB mech. This isn't simply because the mech cage is longer - it's the geometry of the mech itself.

Either, fit a mtb mech or run a 12-27 with your current road mech.

The shifter is compatible with both types. If you can't get the shifting sorted it's something else that is wrong (wrong speed?, cable routing...?)

gsd

Thanks Maddog2,

So, ~27 is the magic number - that's exactly what I needed to know.

In what way aren't the rear deraillers "compatible"? Would it explain the indexing problems I was having? I thought perhaps this was because of the over-extension of the shifter.

Cheers,
Scott

John.T

If you are on 9sp there should not be any indexing problem.
MTB mechs differ in 2 main ways. The first is they can take up more slack chain with the longer cage. The second is that the top jockey moves differently to enable it to clear larger sprockets.

maddog 2

what John said

on a 12-27 you could swap between a road and a MTB mech without indexing issues

for a 12-32 you need a MTB mech, but this is compatible with both road STI and MTB shifters.

gsd

Thanks guys. I've just gone and got an Ultegra 12-25 so fingers-crossed that should be fine.

japsy

gsd wrote:

Thanks guys. I've just gone and got an Ultegra 12-25 so fingers-crossed that should be fine.

I thought the original problem was mountain passes? If so, how is this new cassette going to help? It's still 25.

For your information, I recently replaced a tiagra rear mech with a xt one for the same reasons you described in your original post. It cost less than 30 quid and nothing else needed to be changed.

Anonymous

Shimano 105 mechs are for road only right?

My Carrera had a 32 on the back and also a long cage 105 and it worked perfectly.

John.T

You just got lucky for once Will. Anything larger than a 28 can give trouble. even if it works the B screw has to be backed off so far it compromises the change on the smaller cogs.

croggy

I run a Shimano HG50 11-32 9sp with a Veloce 10sp medium cage.
Works perfectly.

rickhotrod

I've never seen a 9 speed 12-32 cassette in anything but XTR. Can you confirm Scott, that you have an HG70 (LX, 105) cassette in this size? (and not an 11-32 which is available)

gsd

You've got me! It was an XTR 12-32. Sorry for the confusion. I was a bit mad yesterday and seeing everything through a red veil of crap LBS advice fury.

After your (much more knowledgeable) advice I changed back to a smaller cassette (Ultegra 11-25, can't remember the HG number I'm afraid), tuned the indexing and the stop screws and it rides like a dream.

Phew. We're off to Devon on the overnight ferry tonight (from Jersey)!

As for the mountain passes... My wife managed pretty much all of the passes in New Zealand (see below) on this configuration so she'll be fine. We're not camping this time so we've got loads less stuff and have even managed to squeeze in a board game for our friends! Cycle-touring rocks!

(NZ passes: Lewis, Arthur's, Porter's, Burke's, Milford Sound Hill (x2), Crowne Range Road, Haast, Takaka Hill (x2) and as far up Tongariro and Taranaki as you can cycle!)

gsd

Would it being an XTR cassette make any difference to the derailler being able to handle it? (Or were you just curious about my mysterious cassette rickhotrod :-) )? I would have assumed the size of the 32 would have been the limiting factor.

Oh, and the hanger got bent in the middle of nowhere once and might be just ever so slightly malaligned after I bent it back in place! I guess that could have a detrimental impact on the tuning when it's at its limit?

rickhotrod

A 9-speed 12-32 cassette is an interesting size as it doesn't have a 17-20 or 18-21 jump that most MTB cassettes have. However, it doesn't have a 21-24 jump so isn't ideal for road use. All big cassettes need an MTB derailleur, even XTR!