Compact Chainset Question

fizz

I've got the tour of wessex coming up. Last year I struggled ( read that as walked ) up the climb to King Alfreds Tower. Now they've gone and cancelled the premier race, they've put this climb back in and I dont wanna walk this year. So...

My current smallest gear is a 39 x 25. So I'm thinking of changing my standard double for a compact to give me some lower bailout gears.

Drive train is currently

105 standard double ( 39, 52 )
105 10 speed rear block ( 12 - 25 )
105 rear mech
105 front mech

Its done a fair few miles and some of the teeth are starting to look a bit hooked so its time to think about replacement anyway so I was gonig to change the rear block to I was thinking of 12 - 27 to be honest.

My questions :

1. Will my rear mech be OK.
2. Will I need a new chain, I.e a shorter one ?

Which is the 105 equivalent front chain set

This

http://www.pedalon.co.uk/acatalog/shimano_4550_compact.html

This

http://www.pedalon.co.uk/acatalog/shimano_r600_compact.html

Or this

http://www.pedalon.co.uk/acatalog/shimano_r700_compact_chainset_170.html

So is this all going to work ?

Edit - I should add I dont really want to go down the triple route unless I have to.

Thanks

Monty Dog

If you want to run a 27 tooth rear sprocket with a compact chainset you'll need to fit a longer arm rear mech as your existing mech won't be able to take up sufficient slack in the chain. If you change your chainset and cassette then definitely fit a new chain.

redddraggon

Monty Dog wrote:

If you want to run a 27 tooth rear sprocket with a compact chainset you'll need to fit a longer arm rear mech as your existing mech won't be able to take up sufficient slack in the chain. If you change your chainset and cassette then definitely fit a new chain.

It should be fine with a short cage mech. I used to run a 12-27 with a 50-36 and it was fine.

Pirahna

I'll go with redddraggon but will add be very careful not to run on the big ring and big sprocket at the same time.

In the past I've run a mountain bike triple (46, 36, 26) with a 12-27 cassette and short cage Ultegra mech with no problems at all.

robbarker

Monty is absolutely right according to the manufacturer's instructions, but in practice a 50/34 and 12/25 work fine together with a shimano short cage mech. I ran exactly that over the winter. I'd never recommend a bodge though, so do it at your own risk..

Personally, i've just taken my compact off as I've post patience with the big gap between ratios at the front and the fact that me chainline always seemed to be pants.

I'm going for 53/39 annd 12-27 this season, with a (thoroughly legal!) short cage mech. There really isn't that much difference in the lowest gear and the useability is much better.

For really low gears there's no substitute for a triple.

fizz

Thanks all, most useful

Umh..

robbarker wrote:

I'm going for 53/39 annd 12-27 this season, with a (thoroughly legal!) short cage mech. There really isn't that much difference in the lowest gear and the useability is much better.

II had thought of dropping the inner to a 38 and getting a 12 - 27, but I wasnt sure how much benefit I'd get with that either...

I do like the closeness I get with the setup I have now. My winter hack has some big jumps on the gears and I find it breaks my rhythm and I dont really want that on my good bike.

I dont really want to go down the triple route. I really dont like them and its totally irrational but I hate the way they look. Also I dont think I'd use the triple except on the TOW. I can get up every hill I live nearby on my regular rides on the 39 x 25 Its only the steeper climbs on the TOW route I'm worried about.

robbarker

In that case fit a 53/39 and keep a compact to stick on for really hilly rides. I'll probably be keeping my compact chainset for exactly this reason. It's a 5 min job to swap them over.

fizz

Do I need a special tool to swap them over ? Or is it just a case of undoing left hand pedal arm and then sliding the rest of it out. I'm guessing I wont need to undo the bearing cups if I am just changing the chainset ?

robbarker

I use an allen key, a torque wrench and hex bit, a rubber mallett and the tool to do up the plastic preloading cap (there's one on the back of the Park bottom bracket wrench). Anti-seize compound is also needed.

You can leave the bearing cups in place, assuming they're compatible (as are hollowtech2 105 and a shimano hollowtech 2 compact, such as the R700)

If push comes to shove you can get away with just the hex wrench amd improvise the rest, but I wouldn't recommend it.

fizz

I'll get the park tool I think, If I'm going to do it I want the proper tools. Lessons learnt the hardway in the past...

Thanks for advice and help.

John.T

A 12/27 gives a more usefull spread of gears at the lower end of the range. The gaps between the 21, 23 and 25 are too small. A 50/34 compact will effectively give you one lower gear than the 39. A 38 is only a little lower (1").
You can run a 50/34 with12/27 on a standard short cage mech with no problem. Keep the chain as long as possible while maintaining tension on small/small. i am running this on 3 bikes and can use all gears on all of them. I try to avoid the extreme cross overs though.
This set up works better with a 10sp as I really miss the 16 cog on the 9sp.
Your cheapest option is a new 12/27 cassette and chain followed by more training.

fizz

John.T wrote:

followed by more training.

and less pies !!!

Monty Dog

I should have qualified my statement - if you want to run a 50/34 compact chainset with a 12-27, then you'll need to look at using a long arm rear mech. You can get away with the short arm, but be careful what gear ratios you run - small sprockets on the small chainring could find yourself grinding to a halt. Make sure you test all gear combos thoroughly on a bike stand to make sure chain length is OK.

fizz

So I've got two options if I want to avoid trouble

52 - 39 with 12 - 27 rear

or

50 - 34 with 12 - 25 rear

So that gives a ratio of ( dividing teeth )

39 x 27 = 1.4
34 x 25 = 1.36

Not much to choose between them. I dont really want the extra expense of swapping my rear mech so I can run worry free 50 - 34 with 12 - 27...

ScottieP

Hi Fizz

I have full 9 speed 105 on my Specialized Allez and am getting an R700 Crankset fitted tomorrow as well as a 12-27 cassette, so I'll hopefully be out on the weekend to try it out. I have been struggling with hills (not fit enough) and am hoping this will help!

Cheers
ScottieP

leonlikestrees

Just to add another option, you could go with 50/36 and a 12/26 or 11/26 cassette.

Nice compromise, works fine with a short cage mech. The gap between 50/36 is a lot more managable than 50/34.

Ideally, if I ever wear out the big ring, I'll replace it with a 48 or a 49. A 48/11 top gear is almost as big as a 52/12, which is plenty big enough for me (unless I suddenly start time trialling again)

John.T

Fizz. You DO NOT need to change the rear mech to run 50/34 and 12/27. Just to set it up right. I have 3 bikes running just that.
As I said earlier I think the 12/25 cassette is a waste as the 21,23,25 are too close together to get much benefit. The 21,24,27 are much better and with the 34 ring will give a considerably lower bottom gear.

musto_skiff

I am really pleased having switched to the Compact; I now spend most of my time on the big ring only going to the small one on hills and it's nice to have plenty of gears left if it gets steep. No more worring about getting up hills as I can aways get in the 34/25 & spin ...

dennisn

redddraggon wrote:

Monty Dog wrote:

If you want to run a 27 tooth rear sprocket with a compact chainset you'll need to fit a longer arm rear mech as your existing mech won't be able to take up sufficient slack in the chain. If you change your chainset and cassette then definitely fit a new chain.

It should be fine with a short cage mech. I used to run a 12-27 with a 50-36 and it was fine.

I have used a 50-33 and 12-27 with Dura Ace "short" cage without problems.
Although chain length is critical if this setup is to work through all gears.

Dennis Noward

robbie the roadie

robbarker wrote:

I'm going for 53/39 annd 12-27 this season, with a (thoroughly legal!) short cage mech. There really isn't that much difference in the lowest gear and the useability is much better. .

This is what I've done as I've always thought the jump in chainring sizes on a compact was a bit much. Never ridden one mind

More practically 39-27 is approx 39 gear inches

34-25 is approx 37 gear inches

so in reality there's not much in it and a new cassette is far cheaper then a new chainset.

markos1963

Just to join in, my Orbea has a 50/34 Shimano 105 Hollowtech compact fitted and its great. But I find on the relatively flat roads around hear that the 34 ring is hardly used so is a bigger ring available? I was thinking of a 36 tooth. What part no should I be looking for? And any other changes needed? The cassette is a 12/25

aracer

I reckon all those who have an issue with the jump between 50 and 34 must be riding Shimano (or lower to mid Campag). It struck me today that when changing down from the 50 I invariably change up at least two gears on the back even when relatively bogged down - would be a pain with STIs!

Not intended as a troll, but a serious comment - I'm glad I went with high end Campag for my first foray with a compact.

John.T

I am on Shimano. I often change from the 19 to the 16 or even the 15 at the same time as going from the 50 to the 34. By the time the chain is on the 34 the rear change is finished. No trouble at all. I would not recommend doing this half way up a 15% grade though but I don't think Campag would fair any better (or a 'standard' set up). As I have said elsewhere if you think you will struggle on the 50/24 then get on the 34 early. I do at least use the big ring with a 50 whereas I only used the 53 when racing.

mercsport

My tuppence worth on this recurring debate is that I run a Campag CT setup : 50 x 34 and a 11 x 27 rear with a normal short cage . It works fine .

leadshoes

I'm thinking that maybe it might be best to invest in a 39-52 compact for the usual training rides and the swap for a cheapo 34-50 one for various lumpy sportives and the marmotte which I'm hoping to do this year. Question is, how compatible are the chainsets. For instance, if I get the ultegra double groupset and a FSA Gossamer MegaExo Compact Chainset, are they compatible?