Ritchey compact crank problem (chain rubbing)

Mark McMaster wrote in message ...
Trevor Jeffrey wrote:
wrote in message Dear Booker,

Maybe the chain angle causes problems?

Since the chain doesn't mate squarely with the gear teeth,
maybe pressures and wear are concentrated on the edges
instead of across the faces?

And perhaps the sideways flex of the chain opens it up for
more grit to enter?

Even when aligned, smaller gears may wear faster than larger
gears because fewer teeth are cranking the chain at a
sharper angle. On the big-front/small-rear, the 11-tooth
wears out long before the 53-tooth.

It would be nice if someone worked out (or gave general
guidelines) for the average force per tooth for speeds and
pressures. A small-front/big-rear, for example, puts very
little strain on things idling along on level pavement at 10
mph, but things change if the rider is using the same gear
combination to struggle up a steep slope.

I seem to recall Richard Feynmann writing that before he
went to New Mexico, he spent a little time working out the
best gear ratios for the war effort, but never improved on
the advice that he was given on his first day: pick the two
largest gears that give the ratio that you want.



Some years back I did work out bearing pressure within the chain to find
out
whether it would be advantageous to use high pressure gearbox oil.
Cannot
recall how many tons/sq inch but was not sufficient to make gear oil
advantageous. From an engineers handbook of a century ago it was
suggested
that sprocket sizes be above 17 teeth for minimal chain and sprocket
wear.
This is likely to be counting block and link for each tooth, so is the
equivalent of 34 teeth on a bicycle sprocket. Some divergence of chain
was
also accounted for. So using the largest available sprocket sizes seems
appropriate for minimal wear.

As usual Trevor, you are disremembering (or more likely,
just making things up). The recommendation from the
engineers handbook for not using a sprocket less then 17
teeth is to minimize chordal action, not to minimize wear.
In fact, the handbook recommends using a sprocket of no
greater than 67 teeth to minimize chain wear. Here is the
excerpt from the handbook:

http://www.sdp-si.com/D757/pdf/D757P478.pdf

I know not which you refer to, the handbook in my possession is dated 1897
and is clear in its advice.

Trevor