Karl Frantz wrote:
>A while back on the I-BOB list, someone wondered if there had ever been any
>aluminum cottered cranks. I don't think this TA design was brough up, but
>about that time I had spotted a bike at the recycling center that
>appeared to have a cottered aluminum crank.
>
>That crank was made by SR (Japan), but I couldn't see the end of the axle
>because the arm was closed off - not even a dustcap. It didn't appear to
>be a two-piece type, but rather a regular cottered type, only with
>aluminum arms. I didn't look any closer because the bike was a dreadful
>peice of junk with no other interesting features.
>
>Anyway, does anybody know anything about this? Was it a regular cottered
>crank, as is usually done in steel,
Yes, it was.
>or could it have been a copy of the TA design?
Nope.
>I have my doubts about aluminum holding up with cotters
Bingo! These cranks didn't hold up well in practice. The cotter
hole would get deformed and stretched by the pressure of the cotter.
>(for athat matter, how did that TA design fare in service?).
The T.A. system doesn't use the cotter-like frob to resist torsional loads; that's handled by the shape of the axle and the matching hole in the crank. All the T.A. doodad does is hold the crank in position laterally so it won't slide on or off of the axle. There's very little stress in this direction, so there's no need to wail on the "jeu de grain" with an inertial impact alignment instrument.
Sheldon "An Idea Whose Time Has Gone" Brown
Newtonville, Massachusetts
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