Date: Thu, 9 Sep 2004 15:17:32 -0500

Author: "Cliff Bettis"

Subject: Re: bike wheel



I (since moving around too much is painful on the road rash, for those of
you who don't know, I had a close encounter with a Doberman this morning)
had time to fool around with my bike's front wheel. I first balanced it as
best I could, placing a small counterweight opposite the valve stem. Then I
mounted a laser switch photogate so that the spokes would interrupt the
beam. I then made some measurements of angular acceleration, starting the
wheel with various initial angular velocities. I found the frictional
deceleration approached 0.030 rad/s**2 at slower speeds. You could actually
see that the net frictional torque was greater for higher initial speeds. I
think this might be the effect of the aerodynamic drag and it would be real
interesting if you would notice a difference with your wheel covered and
uncovered making the same measurement.

I then applied a torque, (you should have seen my setup I had my bike
hanging from some ring stands; it looked like Mr. Goldberg had come calling)
using some light string partially wrapped on the OD of the wheel and a 50 g
mass on the other end. I found I got an angular acceleration of 1.39
rad/s**2. Using my known frictional deceleration, I figure a rotational
inertia of 0.14 kg m**2. This compares with what you would crudely calculate
assuming the wheel was a hoop with all its mass concentrated on the rim of
0.221 kg m**2 (my wheel has a mass of 1.458 kg and a diameter of 0.775 m).

----- Original Message -----
From: "sampere"
Sent: Tuesday, September 07, 2004 9:28 AM
Subject: Re: bike wheel

> No, all I wanted to know what how much longer it would take to slow
> down. Dang! It took about the same with my crude measureing
> techniques. Remember, the cyclocomputer takes a finite time to update
> the display. I figured my 10 readings per were enough to average out
> those errors and have reasonable stats. But you're right, this means
> that the wheel lost more energy with the cover than without.
> My wheel is in excellent condition and is a good quality wheel and hub.
> It's not a $900 wheel though, with those fancy new silicon carbide
> bearings and all.
> The cover is very light. The mass of the cover is small compared to the
> mas of the wheel. Also, the mass of the wheel is distributed at the rim
> while the cover is essentially a thin disc. So the rotational inertia of
> the wheel >> rotational inertial of the disc. To a first approximation,
> we can ignore the KE of the disc.
> I think we need to add some air moving past the wheel and see what
> effect that has. When I get around to it, I'll let you know the results.
> Cliff Bettis wrote:
> >Sam,
> >
> >Did you take into account the increased rotational inertia with the cover
> >on? It would mean there was a little more stored energy at the same speed
> >which would make the lack of a difference even less impressive.
> >
> >Sammy sans sarcasm won't be the same.
> >
> I'm trying to improve the quality of person that I am. We'll see just
> how long this lasts. It's been tough so far.
> >
> >Cliff
> >
> >P.S. Sam, the mechanic at the bike shop in Ithaca told my son that the
> >threading on the pedals was to prevent ankle injury if the pedals should
> >seize up (they would unscrew). I'm skeptical and more inclined to believe
> >the Wright brothers.
> >
> I'm in your boat. Remember, this threading convention came into style
> long before clipless shoes or even toe clips exitsted. I don't think a
> free foot would be injured if the bearings seized up. Come to think of
> it, I've never heard of a pedal bearing seizing up. I think Marc needs
> to find a new bike shop. They aren't giving him very good service or
> even believable information!
> Sam
From Thu Sep 9 16:48:13 2004