Hi Guys
I am in the process of buying some kit to upgrade my bike https://www.halfords.com/webapp/wcs/s...egoryId_165710

to Shimano Tiagra and have noticed there are a few different types of mech that can be bought such as small, medium and large rear mechs.

Can anybody advise what the difference is apart from size and what would be most suitable for me to buy. Planning on upgrading to a 9 speed cassette if that makes any difference.

Thanks

What gear range are you planning on using? What range of cogs? that will determine short or long.
to look up the ranges google on ""Shimano.com" go to proper country, under products select Tiagra, select the RD and it will show the sprocket range, FD range and total capacity.

You will need to decide since you didn't provide enough information.

Derailleur specs include chain take up capacity expressed as the number of teeth. A typical short cage would be 25-27t. To find out which you need add the differences between the largest and smallest sprockets front and rear.

For example if you had 39-53 with a 12-23 cassette you'd need 14+11 or 25t of take up capacity.

You'll get the best performance using the shortest cage RD with enough capacity. If you're at the borderline, you can usually push the capacity a tooth or two since the ratings are conservative. This might make sense if you were using 2 different cassettes one just over the line and one smaller. OTOH if you're buying new, you may go with the longer cage, to give yourself the option of a larger cassette in the future.

Cages can be removed and changed.

Short and medium cages are used with double and compact double chainrings, long cages are used with triple chainrings though a medium can be used.

You get quicker shifts with short cages than long but the difference is minimal, you need a laser stop light to be able to tell the difference!

The cage takes up the slack in the chain when you shift from the large 52 tooth chainring to the 30 tooth chainring. Siince there is less of a size difference between a 50 and a 34 or a 50 and a 36 than a 52 30, the cage does not need to be as long to take up the slack.

I suppose theoretically they can but where do you buy the replacement parts and at what cost? I expect two different complete derailleurs would be less expensive than one complete derailleur plus the second cage.

True. on line you can buy a new derailleur cheaper than just the cage probably but I'm sure you can buy cages online as you can buy them at your LBS. If you have an Ultegra derailleur, you can buy a used Tiagra or 105 for cheap and switch the cages. The cages don't wear out unless they are hitting the spokes, other than the jockey and tension wheels. I've switched cages but that was because I was given some by either I upgraded a friend's bike, he upgraded, the bike was trashed in an accident, I bought at a bike flea market.

Thanks for all your advice so far guys

I have been checking my current set up and it is 36/50 Front and 12/25 8 speed cassette. I am upgrading to a 9 speed cassette so will proberbly keep to a similar cassette spec but maybe have a 27t as an addition but I will have to see if I can find one or I might just go for 12/25.

What would be my best option for size RD gage and any other advice you can give me?

Road long is some what shorter, distance between pulleys,
than mountain long cage.

wrap is (big+big) minus (small+small), it is the wrap up of slack ,
that has the distance between the 2 pulleys needing to be greater.

not much significant issues in erring towards too long, .. having too short a cage,
but having sufficient chain length, means some slack in the chain
is not tensioned by the return spring that rotates the pulley cage...

when combining small chainring and the smaller cogs on the wheel.

So, why not go with the longer cage ?

But you are only adding 2 teeth.

since the new chain types have lower tolerance for just adding a link,
It's a good time for a new chain,
and make it a link longer when you install it.

if that won't work well .. then, buy a longer cage derailleur.