Re: [CR]TA "cottered" Cyclotourist

(Example: Framebuilders:Tony Beek)

From: <karl.frantz@juno.com>
To: CaptBike@sheldonbrown.com
Cc: karl.frantz@juno.com, jfbender@umich.edu, classicrendezvous@bikelist.org
Date: Mon, 11 Dec 2000 22:59:11 -0500
Subject: Re: [CR]TA "cottered" Cyclotourist


On Mon, 11 Dec 2000 22:18:21 -0500 Sheldon Brown <CaptBike@sheldonbrown.com> writes:
> Karl Frantz wrote:
>
> >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.

That's about what I figured would happen. Odd that SR would do that; they usually seem to make OK stuff. I guess the key word is "usually" :-)
> >(for that 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.

What I wondered about was whether the "jeu de grain" placed any preload of the spindle into the cornered pocket of the crank to prevent the joint moving. This is the main issue with the tapered-square spindle - it has to have enough preload to overcome the pedal forces. I guess the TA design, if it's not a press fit, doesn't work the same way. Hmm... as long as the forces from torque don't exceed the yield strength of the aluminum arm, you're OK - and there's a fair bit of area over which it is spread, plus the shape means the force doesn't necessarily all bear on a corner. It might wear through fretting though.

I can guess it was a bit more expensive to make, but shouldn't be too bad. I suppose it got dropped because everybody else used the square-taper?

Karl Frantz Sterling, Massachusetts -------------------------------------------------- Karl.Frantz@juno.com