At 11/29/2005 09:50 PM -0500, Sheldon wrote:
>Ted Ernst wrote:
>
>>I was taught to have the round heavy part of the pin facing forward
>>so that the heavy thrust of force would be against the strongest
>>cross section of the pin lessening the possibility of loosening.
Sheldon's right about the direction only mattering WRT trouser cuffs, the spindle only transmits the torque from the left arm.
But you are also wrong about which is the strongest part of the
cotter. Cotters don't shear off. (At least I've never seen one
sheared.) They fail because of surface distortion. So the thinner,
wider end can take more load with out distorting.
>This is an old, oft repeated fallacy. If one pin is loaded on the
>wide end the other pin will be loaded on the narrow end. Since the
>loading is essentially equal on the two sides, it really doesn't
>matter which way you do it.
Theoretically, the most durable way to make a cottered crank would be to have the flats on the spindle oriented at something like 190 and 170 degrees instead of 180/180. Then with the left crank forward, insert two 10 degree tapered pins with the threads down. Not practical of course, without some way to ensure that pins with the proper taper angle would always be used.
Even then, you would never get the British, Italians & French to
agree on a common angle.
The Brits would want 9.57 degrees because that's what Kirkpatrik
Macmillan used.
The Italians would use 6 degrees figuring that only their superior
machining and metallurgy could withstand the higher stresses. And to
prevent anyone from using inferior (ie. non-Italian) cotters with
their cranks.
The French would use 9 degrees because 9 deg. = 10 Grads and
they're.. well, French.
>As I said, it really doesn't matter, but I have a slight preference
>for the nut being down when the crank is horizontal and facing
>forward, because you're less liable to snag your trouser cuff on it.
>
>Sheldon "http://sheldonbrown.com/cotters" Brown
BTW Sheldon, Would you mind updating your Cotters page? Park quit making cotter presses late in the last millenium. I understand http://bikesmithdesign.com has some nice ones :-)
Mark Stonich;
Minneapolis Minnesota
http://mnhpva.org
http://bikesmithdesign.com