On 09/22/2009 07:32 PM, David Snyder wrote:
> The 93 cranks, their 86bcd cousins, as well as the similar SR Apex
> cranks are possibly the 3 most troublesome '70's cranks in terms of
> using modern chain.
> Credit the thick spider tabs and rings themselves, which lack the
> beveled inner side below the teeth.
>
> Even 1950's steel Stronglight cranksets handle 9-speed chains in my
> limited experience, but not the above 3, which can have the chain
> skating and/or getting wedged between rings (causing immediate
> bending/dishing of the rings) even when used with common 7/8 speed
> Shimano HG chain.
>
> Since I've used 93's for lots of my riding, I've have to remove the
> inner ring and bend each tooth <1mm or so to the right.
> Simply beveling the inner ring's teeth, which similarly re-locates the
> tips of the teeth rightward, also curiously led to the chain getting
> tossed to the inside (but ultimately might have been simply a matter of
> too much beveling on my part).
> For what it's worth, I can bend all of a ring's teeth within a few
> minutes and have yet to break one using a 6" adj wrench.
The 86mm BCD SR Apex chainrings have another problematic feature: the middle chainrings have a ramp bevelled into inner aspect of the ring profile to facilitate shifting from a "granny" inner ring to the middle ring. If, like me, you like to run an outer ring in the mid-40s tooth count, you cannot use one of these relatively abundant middle rings as an outer ring unless you mount it with the bevel on the inside -- that is, with the counter-bore for the bolt head facing the arm rather than accepting the bolt head. I've tried it the other way, and it makes it almost impossible to shift. :(
--
-John Thompson (john@os2.dhs.org)
Appleton WI USA