From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.tech
Subject: Re: Truvativ cranks? Opinions
Date: 11 Oct 2000 23:29:07 GMT

Mark Chou writes:

> Can you elaborate why you think the ISIS design wouldn't be subject
> to the same loosening and backlash issues that the "other" splined
> interface suffers?

The ISIS spline has a one degree taper per side that has a preload on
a 16mm long spline on which the crank sits on the spindle.  The spline
diameter in the cranks has a major diameter of 22mm and a minor
diameter of 18mm diameter.  Made with the precision that is specified,
something that is practical with current machines, the crank will
achieve its press fit before stopping against a fixed shoulder that
prevents lateral creep that is the bane of the square taper and it has
sufficient engagement so it does not suffer from spline overload or
elastic and clearance backlash that the Shimano design has.

Because the Shimano spline allows no significant press fit, having
only 3mm engagement, it has clearance that together with elastic
compression of the spline teeth, under pedaling load, cause enough
backlash to move the retaining bolt.

> I'm not an ME, went over to the ISIS site and downloaded (and read)
> their spec., but aside from positioning benefits I was at a loss why
> the ISIS spline design would be any "better/less prone to failure" than
> the existing ones.

The cranks is firmly seated on the spindle, something the Shimano
design achieves with a conical seat on a smooth bore.  Because the
torque is transmitted by the end of the shaft into the far end of the
crank, this conical bore is subjected to torsional movement comprised
of deformation of both spindle and crank.  To visualize this, metal
parts can be seen as varying harnesses of rubber.  The distortions you
visualize are real and have fretting effects on the interface.  I
think it is evident that the ISIS interface is more stable than any
others that have been proposed, the two elements, crank and spindle
having no areas where the creep or slide.

To see the results of such fretting motions, a square taper hardened
steel spindle shows erosion and rouge in the center of its flat faces.

Jobst Brandt      <jbrandt@hpl.hp.com>