Why does a short chain fail?

stevec205gti
stevec205gti Posts: 366
edited February 2008 in Workshop
As per the title - it's mentioned numerous times if your chain is too short it'll fail. Can anyone explain to me why
Has the head wind picked up or the tail wind dropped off???

Comments

  • redddraggon
    redddraggon Posts: 10,862
    All chains fail eventually.
    I like bikes...

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  • dennisn
    dennisn Posts: 10,601
    stevec205gti wrote:
    As per the title - it's mentioned numerous times if your chain is too short it'll fail. Can anyone explain to me why

    I have never heard this. Could you give me an example from any kind of source
    other than word of mouth.

    Dennis Noward
  • Garybee
    Garybee Posts: 815
    If your chain is too short and you shift to a combination of chainring/sprocket that it is unable to reach round it will do a fair bit of damage. If you're lucky the chain will break.
    Hypocrisy is only a bad thing in other people.
  • stevec205gti
    stevec205gti Posts: 366
    I have never heard this. Could you give me an example from any kind of source
    other than word of mouth.

    Dennis Noward

    I've heard it given out as advice on this forum on numerous occasions, and am interested in trying to understand the engineering principle of this failure (if there actually is one). I'd have thought that the most extreme short chain is that of a single speed - and they don't break. Why does a pair of jockey wheels introduced into this system cause failure?
    Has the head wind picked up or the tail wind dropped off???
  • dennisn
    dennisn Posts: 10,601
    Garybee wrote:
    If your chain is too short and you shift to a combination of chainring/sprocket that it is unable to reach round it will do a fair bit of damage. If you're lucky the chain will break.

    And by the same token too long can create a real mess also. I not sure if Steve is
    talking about this kind of problem. Ya know, tearing parts off your bikes. Seemed to
    me like he is saying that too short of chain would just wear out quicker. Who knows?

    Dennis Noward
  • aracer
    aracer Posts: 1,649
    I hink you need to give us a link to somebody saying that Steve, so we can understand your point. I've certainly suggested sizing your chain so it is long enough to use big/big to avoid breaking things, but that doesn't seem to be what you're on about.
  • Garybee
    Garybee Posts: 815
    dennisn wrote:
    Garybee wrote:
    If your chain is too short and you shift to a combination of chainring/sprocket that it is unable to reach round it will do a fair bit of damage. If you're lucky the chain will break.

    And by the same token too long can create a real mess also.

    Yes it can. However, as the original poster was asking about a problem caused by a chain that was too short, I didn't think that was relevant.
    Hypocrisy is only a bad thing in other people.
  • stevec205gti
    stevec205gti Posts: 366
    I'm ignoring general wear on the chain and more interested in catastrophic failure. I've had a look back at some old posts but have not found what I'm after - the closest is aracer's post in ths thread http://www.bikeradar.com/forums/viewtop ... ain+length
    Has the head wind picked up or the tail wind dropped off???
  • dennisn
    dennisn Posts: 10,601
    stevec205gti wrote:
    I'm ignoring general wear on the chain and more interested in catastrophic failure. I've had a look back at some old posts but have not found what I'm after - the closest is aracer's post in ths thread http://www.bikeradar.com/forums/viewtop ... ain+length

    Well, if you don't put the new pin in correctly you are talking about a real possibility
    of "catastrophic" failure. When a chain breaks and Big Jim and the Twins get slammed
    into the top tube. I'm feeling faint already. I think this kind of thing happens more with
    pins being replaced than with using master links. I have seen the aftermath of chains
    getting caught in the deraillers but usually this results in the deraillers getting pretty
    tore up but not breaking the chain. I can't remember ever breaking a chain.

    Dennis Noward
  • synchronicity
    synchronicity Posts: 1,415
    Why does it fail?

    Because if you mount a chain using one gear combination (such as 39T - 12T), the circumference (length of the chain) going around those sprockets is shorter than other combinations, such as 53 / 21 (you shouldn't use either of these combinations in practice though)

    Theoretically, the derailleur cage + pulley arrangement is supposed to take up all of this slack. If the chain is too short, what happens is that there will be a certain combination of sprocket sizes, above which, the chain simply won't bloody reach around. Short cage, long cage or otherwise.

    How can I put this simply? It's like wearing an un-interruptable necklace that won't even make it over your forehead... something has to give... :wink:

    :arrow: And then what will happen is that, generally speaking, you're forcing it to do something it can't do, so something has to break. I've seen rear derailleurs ripped back and over themselves. Those flimsy alloy cages aren't designed for those kinds of forces. It's not a nice way to go. The transmission literally self destructs, and the weakest link isn't normally the chain.

    Yes of course the same thing can happen on a single speed, but it doesn't because it's obvious what the correct chain length is. Just try re-mounting a chain with two or three fewer links back onto your single speed... and then try turning the cranks. The chain tension will skyrocket, the rhs chainstay will be forced into compression, the axle + BB bearings will grind towards eachother...

    It's not the introduction of the jockey wheels that does it :!:
    vertebr.ae | precision braking & shifting
  • aracer
    aracer Posts: 1,649
    Well if it is my post there you don't understand, the issue is that if the chain's not long enough to wrap big/big and you try to change into it you're liable to break something (not necessarily the relatively cheap chain) if you accidentally try to change into that combination as you will put huge forces through your whole drivetrain. The one time I did this I was fortunate that stuff just flexed rather than broke, but the rear wheel did lock up, which was rather scary at the time. People might say they will never use this gear, but that's what I thought with the bike I did it on.
  • stevec205gti
    stevec205gti Posts: 366
    I think i've been viewing these posts too deeply - to myself with a decent engineering background it's obvious that a chain so short it's physically impossible to get big/big combinations would fail - I've always read the posts as a short chain meaning one that results in a long stretch out of the rear mech
    Has the head wind picked up or the tail wind dropped off???

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