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M2 Racer Orb Ti at Supergo...(4 posts)

M2 Racer Orb Ti at Supergo...Akirasho
Feb 21, 2003 1:04 PM
... here's your chance to try the Orb minimalist pedal system

http://www.supergo.com/itemdisplay.asp?itempageitemid=21716

Be the bike.
Minimalist pedals at maximal price...Brooks
Feb 21, 2003 1:21 PM
And several references to reducing rotational weight, which isn't that significant in the crank but can be noticeable in the wheels.
Rotational weight: why less significant at the cranks?torquer
Feb 21, 2003 2:03 PM
I have no opinion pro or con about these pedals (happy with my Speedplays), but I don't get your reasoning about RW at cranks vs. wheels.

As I understand it, you want to minimize RW where the component is most subject to accelerations (and decelerations, of course).

If anything, cranks accelerate/decelerate more often than wheels (unless you're on a fixie), although across a smaller radius, granted.

Any engineers want to enlighten us?
Rotational weight: why less significant at the cranks?jamesau
Feb 22, 2003 4:26 AM
As a rotating system, cranks have a lower 'polar moment' of inertia than a wheelset. It's the high polar moment that tends to resist accelerations / decelerations. Polar moment is the rotational analog to mass; heavier objects resist speeding up / slowing down as objects with higher polar moment resist changes in rpm.

For two rotating systems of the same mass, the system with mass concentrated at the outside (larger radius) has a higher polar moment. Polar moment of a system is proportional to the mass of the rotating system times the radius squared (radius of where the mass is concentrated relative to its rotational axis).

The two main rotating systems are wheelsets (including tires, tubes, cassettes), and cranks (including BB, cranks, pedals, cleats, shoes, socks, toe warmers, etc).

Your point, "If anything, cranks accelerate/decelerate more often than wheels (unless you're on a fixie), although across a smaller radius, granted." is probably meaningful, but don't know if it's been quantified.

Jim