UKC

Hi,

I'm looking to alter the gear ratios on my road bike a bit, and although I think i've got a good grasp on what i need to buy/replace and what i can replace it with, I'm not all that confident in my thoughts so I thought I'd pose the question to the wonderful world of the UKC users...

I'm running a Shimano Tiagra 9 speed triple set up at the moment. Because i'm so unfit/plain lazy I'm looking to make the lowest gear lower. Currently I've got an 11-26 on the back, but having hired a bike in France last week that make riding up Mt Ventoux surprisingly easy I'm looking to change it to something like a 12-30.

Now, I'm starting to gather than this may not actually possible, depending on the rear mech I've got...I'm led to believe that i can't go any bigger than 28 if the rear mech is a short cage, whereas i should be fine up to about 32 if it's a long cage - does that sound right? Currently I'm not quite sure what i've got and because it's an older model of Tiagra the product codes don't match up any more. However, am I correct in saying that if i've got a triple on the front, that the rear mech must be a long cage to be able to take up the slack created from dropping into the small ring on the front?

In other words, with the current set up, will I be able to just switch the 11-26 casette out and put a 12-30 (or similar) in? Or have I just misunderstood entirely and I'm completely lost?

Any help much appreciated!

In reply to cymjt:

You need two figures.

1. Chainring capacity, this is the number of teeth on the largest chainring minus the number of teeth on the smallest chainring.

2. Cassette capacity, this is the number of teeth on the largest sprocket/cog minus the number of teeth on the smallest sprocket/cog.

There's a third figure which is the sum of these two which is drivetrain capacity.

So having done that for Shimano the following holds:

SS - Short Cage Road Double - Maximum Cassette Cog is 27 and Total Capacity is 29

GS - Medium Cage MTB/Road Triple - Maximum Cassette Cog is 34(MTB)/27(Road) and Total Capacity is 33(MTB)/37(Road)

SGS - Long MTB - Maximum Cassette Cog is 34 and Total Capacity is 45

Your proposed new cassette has a capacity of 18. A Tiagra triple chainset has 52, 39, 32 chainrings so has a capacity of 20. Thus you have a drivetrain capacity of 38

Now you can usually get away with being a bit over the stated capacity but it can be a bit tight. From all the above I'd say that you can use a medium cage rear mech.

In reply to cymjt:

I think the triples have a long cage rear mech usually. id just put a new cassette on and give it a go, I think sram are better than shimano and the chain too

In reply to ow arm:


Triples work with short cage. Mine is that way now, only because I ordered it by mistake as a replacement for previous long cage broken one. As ling as you don't use the silly large to large or small to small rings you'll be fine.

In reply to cymjt:

Okie dokie,

Cheers, looks liek it'll be worth a punt, after all, they're not overly expensive!

In reply to cymjt: I replaced the 11-25 rear cassette that came with my bike with as 12-30 without changing the derailleur. The chains pretty tight but as long as I don't use the two largest at the rear with the largest at the front its fine. Its a double though not a triple.

In reply to cymjt:
You should be fine with a 12-30. Even though it is 10 speed my missus' bike has a triple and 12-30 at the back. Long cage is standard with a triple. Can't see any reason why 9 speed would be any different.

Unlike with a double where the chain can be tight at the extremes, on a triple it will be fine. You may have the opposite issue where the chain is a little slack when on the small chainring and small sprocket. But that isn't really a very useful combo anyway and I imagine you would've changed to the middle chain ring before you got to that particular ratio.