The consequences for sprockets of running worn chain are well-known, but what are the effects on rings? I ask because I'm running a 9,500 mile screw-on block with carefully-chosen, partly-worn chain, and it's working well enough. Unfortunately, though, I'm going to have to change the needle-like front rings soon. Will running 3/16 worn chain result in accelerated wear of new (T. A. Cyclotouriste) rings?
Thanks for any comments.
David
Sorry, this might have been better in "Bikes and Bits".
Chainring wear
Re: Chainring wear
yes it will wear the chainrings prematurely, but you can work out where exactly by measuring the chain for stretch and taking a view on it.
Say the chain is 0.5% elongated, the working radius of the chainring teeth (i.e. where the roller bear) is also increased by a similar amount.
Suppose the chainring is 42 teeth it has a circumference of 21" . Ignoring the fact that there is an error due to the 21" circumference being a 42 sided polygon, it nominally has a radius of 84.89mm, which increases to 85.32mm, i.e. the rollers bear just over 0.4mm further up each tooth than they should. This radius increase would be larger (at the same proportion of chain wear) if the chainring is larger to start with, and would be pro rata increased if the chain is more worn to start with.
I wouldn't worry overly about that much wear or perhaps a touch more. But I'd be tempted to use the old chainrings until the block and chain are worn out and then change the whole lot together. Can the chainring be turned 90 degrees?
BTW when you say the chain is 3/16" elongated, over what length are you measuring that elongation? If it is over the full chain length then maybe a new chain would work on the sprockets. If it is over a 12" length then I certainly wouldn't use that chain on new chainrings and expect them not to suffer.
The worst (fastest) wear occurs when the chain rollers start moving around on the teeth when the rollers are under load. This happens easily on sprockets (because they are small) but normally the chain/chainring has to be quite worn before this happens on chainrings.
cheers
Say the chain is 0.5% elongated, the working radius of the chainring teeth (i.e. where the roller bear) is also increased by a similar amount.
Suppose the chainring is 42 teeth it has a circumference of 21" . Ignoring the fact that there is an error due to the 21" circumference being a 42 sided polygon, it nominally has a radius of 84.89mm, which increases to 85.32mm, i.e. the rollers bear just over 0.4mm further up each tooth than they should. This radius increase would be larger (at the same proportion of chain wear) if the chainring is larger to start with, and would be pro rata increased if the chain is more worn to start with.
I wouldn't worry overly about that much wear or perhaps a touch more. But I'd be tempted to use the old chainrings until the block and chain are worn out and then change the whole lot together. Can the chainring be turned 90 degrees?
BTW when you say the chain is 3/16" elongated, over what length are you measuring that elongation? If it is over the full chain length then maybe a new chain would work on the sprockets. If it is over a 12" length then I certainly wouldn't use that chain on new chainrings and expect them not to suffer.
The worst (fastest) wear occurs when the chain rollers start moving around on the teeth when the rollers are under load. This happens easily on sprockets (because they are small) but normally the chain/chainring has to be quite worn before this happens on chainrings.
cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~