On 12/19/2017 8:21 PM, James wrote:
On 18/12/17 08:43, Emanuel Berg wrote:
It seems after putting on a new chain, it turns
really slack after only 2-3 weeks. When you
then restraighten it, it remains tight
considerably longer. Is this so or does
intuition play a trick on me? In general, how
many bike hours does a chain last?


2-3 weeks seems like only a short amount of time under "normal"
conditions.Â* There are too many variables to quantify exactly "how long".

Is it possible the rear axle nuts were simply not done up quite tight
enough and the axle slipped forward in the drop out slots?

I'd expect at least 2000-6000km from a new chain to wear 1%, depending
on the conditions and the rider.Â* The Wippermann chain test shows more
than a 3:1 wear rate from the best to worst chains they tested.Â* (36 to
114 hours of their abusive test.)

http://www.cantitoeroad.com/Wipperma...ained_b_2.html

Standard bicycle chains measure 1/2 inch from pin centre to pin centre.
There are something close to 100 half links or 50 inches of chain, so 1%
is one half link or 1/2 inches of wear.Â* That would mean the rear wheel
would need to be moved 1/4 inches rearward to take up the slack from a
worn out chain.

(Actual number of links or half links depends on the length of chain
stays and the crank and hub gear diameters.)

Another factor: The geometry of the situation means it takes only a tiny
change in length (or position of the axle relative to the cranks) to
generate a noticeable amount of slack.

This has a couple possible implications. Here's one: sprockets
(especially chainrings on less expensive bikes) are seldom perfectly
round. Usually you can notice the change in tension (i.e. change in
visible slack) as you turn the cranks with the bike on a stand. I
suppose it's possible that the first estimate of chain "tightness"
happened with the chain on a high spot, the next one on a low spot.

Conversely, if someone installed a chain and happened to have it at a
low spot on the sprocket, then bolted the rear axle in place, the
crank's high spot might over-tension the chain. It might even have
enough leverage to pull the axle forward when a properly adjusted chain
would never do so. Or the over-tensioned chain might wear more quickly,
I suppose. (I learned about this problem in a related way back in my
pre-BMW days, when my motorcycles had drive chains.)

Finally, no matter what the situation, it seems the chain must have some
microscopic roughness - peaks and valleys - on mating parts. The peaks
would tend to wear off in the beginning generating at least some
clearance. Once the peaks wore, further wear would be much slower. This
is partly why engines are (or were) supposed to be carefully "run in" or
"broken in" when new.


--
- Frank Krygowski