Chain length query. Advice please!.

[onegoodbike]

Hi, i'm hoping to attract some knowledgable advice!.
I'm in the process of changing a few bits on my bike and have just taken off a long cage rear mech and replaced it with a short cage example. My query is, do i need to shorten the chain length due to the reduction in cage length?, the front chainrings (53/39) and rear block (13-25 i think) remain the same.
Any help will be gratefully received. Thanks Matt

 

[nammonk]

chain length

Almost certainly, yes. One or two links should bring it back to the same chain tension. The principal difference should be the difference in the distance between the two jockey wheels. You could always give it a go without shortening first and see if the chain is too slack in your smallest/smallest riding gear (not 39-13, of course, which you shouldn't be using...).

Depending on the mech you've put on, you might find that it doesn't have the necessary range for 53/39 and 13-25. Possibly not if it is really retro and Italian.

Nick

 

[cromoman]

There are two ways to determine the correct chain length.

1. without running through the derailleur, wrap the chain around the biggest chainring and the biggest sprocket, find the shortest possible length it could be and add 1 link.

2. with the chain routed through the derailleur, select the small chainring and the smallest sprocket. Shorten the chain just enough so that it doesn't sag - i.e. it's under tension.

Method 1 will result in the shortest possible chain for your setup, method 2 will result in the longest possible chain for your setup.

Provided your derailleur has the capacity for your selection of ratios, then either will do, and both may well result in the same length anyway.

If as Nick points out, it doesn't have the capacity then you have a choice. Method 1 and never use the small small combo. If you do, the chain will sag and might get caught. Or, method 2 and never use the big big combo. If you do, you'll bust the derailleur off the frame. Personally, I prefer method 1 on an old bike.

 

[onegoodbike]

Hi guys, thanks for the prompt response and advice . The mech and block are not particularly retro i'm afraid, dura ace 7700 vintage!.

 

[west green]

Chain length #2

Timely post, I'm having an issue - as follows - took my bike to LBS to set up 8sp chainset 53-39 front and short cage rear mech, with 7 speed cassette 12-28. Collected and found I couldn't use the last 2 large cogs with large ring. Small ring and all 7 cogs were fine. Took it back and was told that large ring meant tension would have been to loose and I need a smaller cassette ie 13-23. Not happy, but is that the solution or am I being fobbed off? I took it to the LBS because I wasn't confident about doing it myself and now I'm 2 gearings short. Is another solution to change large chainring to a 52T 0r even 50T. In hindsight maybe I boughtthe wrong chainset, but I already have a compact on my other bike so wanted to try another set up. As above , any advice appreciated

 

[Simon Daw]

First, you shouldn't be using those two large sprockets when using the big ring - the chain line is horrible (look at the chain angle from above with the bike in those gears - that misalignment you can see causes vastly increased wear). You also shouldn't use the small ring with the smallest sprocket or two either, though they're slightly less ghastly.

However, the gears should be CAPABLE of being shifted into those gears, and if they're jamming up when you try then the shop has stuffed things up (the chain is too short). You don't say what mech you're using, and that's important, but pretty much all modern mechs will handle the setup you've described. The Shimano 105, for instance, gives a max sprocket of 28 and overall range of 33T (see http://techdocs.shimano.com/media/techd ... 702204.PDF), so you should have capacity in hand (albeit not much). Many changers will give you more.

See the advice on that document about chain lengths. It's worth noting that Shimano recommend the big / big + two links method outlined in an earlier post for 28T sprockets and short caged mechs. The vertical jockey wheels method (which I prefer for closer ratio cassettes - as does your shop, I'd guess) results in a chain that's too short.

 

[west green]

Chain length #2

Thanks for the reply. Ok, I hadn't clearly understood the jockey wheel method, but not best pleased that I gave them a new chain and they've cut it short. I had read that it's not good to go big too big with the chain but I can do it with my other bikes - this one it's impossibly tight. So - next questions - which is the easiest way for me to solve the problem?

1. change the cassette to 13-23
2. change large chainring on the front to a smaller tooth ie 52T 51T 50T
3. extra links on the chain
4. new chain

 

[west green]

Chain length #2

Sorry, meant to say it;s a short cage Shimano tricolor, so 8 speed mech with 8 speed Chorus 53-39 chainring - that shouldnt cause any issues should it?

 

[Simon Daw]

Well, actually the (old) Ultegra mechs don't have quite the same capacity. I used to use a 6400 years ago, but only ever used it with a 23 sprocket maximum.

First, check which you have - RD-6400 or RD6401 (the number will be on the back of the mech). The latter has slightly greater capacity, and certainly shouldn't be jamming up on the 24T sprocket, though it might struggle with the 28. The 6400 would be stretched even with the 24 according to the specs. See these for more:

http://velobase.com/ViewComponent.aspx? ... 9535bd531f and

http://www.velobase.com/ViewComponent.a ... 236e2fd096

I'm assuming these are correct - my memory of such things doesn't stretch back twenty years I'm afraid!

The best solution depends to some extent on your requirements. Dropping to a 50T ring may not completely solve your problem by the sound of things, though it might make enough difference to be useable. The 13-23 cassette should fix it, if the substantially higher bottom gear is adequate for your needs. A longer chain won't help if the mech is unable to handle the overall capacity, in which case you'd also need to change the mech. Don't simply whack extra links in without taking advice - Shimano chains need special rivets which can't be reused for example (though many other brands are okay).

 

[hamster]

I think your setup should work - I've used an old 105 on a MTB. I think the bike shop has simply (as you suspect) chopped the chain too short.

I've even got a Campag Chorus short cage (and Campag cages are shorter than Shimano) to work with a 12-23 and a 30/39/48 triple.

The capacity of the mech is around 29T, so you are only using 26T.