chain length

trig1

hi wondering if yous can help , put a new kmc chain on my bike when in 50 ring on front and on 25 at back the rear mech seems to be quite far forward, i know ill not be in this gear very often but wanted to make sure its ok, if i have the chain any longer it ll slap the chain stay when im in 34x12 , 13 14
cheers trig.

scherrit

Hi,
There are a few manufacturer's guideline/rules but this one is common sense really, the chain needs to be long enough to go over the big-big combo and short enough for the derailleur to wrap all the slack when the smallest cog-smallest chainring combo is engaged. If this is impossible, you are exceeding the design limitations of the derailleur set up and you are on your own (selected individuals can choose to/have good reasons for doing this but you are on your own wrt safety, the shifting is no longer idiot proof etc etc)

For a short cage shimano set-up with a smallish cog (say 11, 12, 13) as the smallest cog on the cassette, the derailleur pulleys are approx vertical when the big chainring-small cog combo is engaged - this is an example of a rule of thumb/manf guideline and it has some limitations, as you can see...

Another rule of thumb is big-to-big chain length (ie tight over those) plus one full link (or 2 half links).

As an aside, the chain will only bounce off the chainstay if there is no pedalling tension in it and the deraileur tension is usually not enough to stop it bouncing even when it is being held far away (chain on big ring) but with no pedalling tension - that's why bikes tend to have chainstay protective stickers, or lots of scars in the paint!
Good luck!
S.
www.thebikewhisperer.co.uk

schlepcycling

Here's a video guide using the big sprocket to big ring +1 link method.


http://bicycletutor.com/calculate-chain-length/

trig1

cheers