From: Bernd.Kischnick@launchpad.unc.edu (Bernd Kischnick)
Subject: Re: Chain Maintenance Question
Summary: sources of chain wear
Keywords: chain wear elongation
Organization: University of North Carolina Extended Bulletin Board Service
Date: Sat, 12 Dec 1992 14:24:45 GMT

In article <1992Dec10.003047.13270@kestrel.edu> king@reasoning.com (Dick King) writes:
>In article <1992Dec8.194012.25353@samba.oit.unc.edu> Bernd.Kischnick@launchpad.unc.edu (Bernd Kischnick) writes:
>>In article <1460033@hplred.HPL.HP.COM> jbrandt@hplred.HPL.HP.COM (Jobst Brandt) writes:
>>>
>>>Wear and Gear Slippage  (Jobst Brandt  jobst_brandt@hplabs.hp.com)
>>[...]
>>>As was mentioned on the net, chains don't stretch in the sense
>>>that the metal elongates, but the parts wear.  Wear in the pins
>>>and sleeves change the length of the chain as the pins fit more
>>>loosely. 
>>
>>There has to be at least the tiniest bit of elongation of the side plates
>>of a chain. I conclude this from the fact that the side plates of Rohloff
>>chains (german brand also sold as "Campagnolo Contax" chain) are purposely
>>produced being 1/100 mm too _short_ . Rohloff claims this is done to
>>prolong the chain's life, and that would be pointless if elongation is not
>>a limiting factor to a chain's life at all. Indeed without elongation the
>>side plates would never match their norm length of 1/2 inch.
>
>Rohloff claims that making the pitch initially short prolongs chain life by
>requiring the chain to wear more before the pitch becomes so long that you must
>discard the chain.  Even if the claim is true, however, it says nothing about
>HOW the pitch increases.

I am convinced now your interpretation is correct. I was assuming that the
shortening of the side plates was done to compensate for plastic
elongation because Rohloff has implemented another strategy against the
pitch increase mechanism you mention:
>
>Indeed, the reason the pitch increases is that the pins and sleeves, which are
>essentially concentric when you buy the chain, come to be non-concentric.  Pins
>are attached to even numbered plates, and cylinders to odd numbered ones.  The
>plates come to be further apart when the gaps between the pins and sleeves get
>larger; as you pull on the chain the pitch increases.

The gaps between pins and sleeves get larger because the sleeves deform to
adapt the same diameter as the pins on that side where the pins pull on the
sleeves. In the case of ideally concentric pins and holes, the contact area
between them would be a line; small contact area => high pressure => a lot
of friction when the joint turns => abrasion, wear. The holes of Rohloff
chains are not circular from the start, they provide a 60 degree segment
of the same diameter as the pin. This increases the contact area between
pins and sleeves. The total friction force is _not_ reduced
(Coulomb-Amonton's law), but the force per area is, and this is the
important value to determine the effect of abrasion.

So I thought, they use segmented bearings against wear in the joints, then
the shortened side plates are probably to compensate for another effect.

>Maybe Rohloff is right, but only because the chain must wear from "pitch too
>short" through "pitch normal" before it gets to "pitch too long".

D'accord.

>Another mechanism Rohloff might be counting on to reduce chain wear is the
>possibility that underpitched chains ride in sprockets differently, reducing
>wear in a subtle way.

Well, at least Rohloff says the underpitch does not prevent the chain to
ride in the sprockets in a decent way, and that it does not introduce
additional wear for the sprockets. My experience is that a Rohloff chain
is distinctly more silent than a Sedis (whatever that may mean for the
performance, but I take it for a positive sign).

>I'm from Missouri.  One joint is flexing at a time as
>the chain comes off the freewheel sprocket [and, to a lesser extent as the
>chain is winding up on the chainring].  That joint must be under tension,
>because one of its links is no longer touching the sprocket [or not yet
>touching the chainring].  That joint is between two plates, at least one of
>which is under tension.  If the plate that is under tension is the one
>containing pins, than the joint is under tension as well because the sprocket
>holds it by the sleeve.

I can't see whether shorter plates should be any better from your
description? (Nor do I see whether being from Missouri should change the
performance of a chain in any way :-) ?!)

>-dk
>> Bernd.
>>> Jobst Brandt

-   Bernd.

home address bernd.kischnik@cdc2.ifum.maschinenbau.uni-hannover.dbp.de


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