Index Home About Blog
From: (Jobst Brandt)
Subject: Re: The weight question...
Date: 26 Mar 2000 02:30:27 GMT

Andre  Charlebois writes:

> The most important weight is that which is at the periphery of a
> rotating mass.  In other words, pedals, rims, tires, tubes are more
> important than cogs, BB's, hubs, etc.

This is an age old fable in bicycling and useful to cite when an
excuse for some new wheels is needed.  If they are aerodynamic, then
that's easy to justify even though they are heavier, but just a bit
lighter ones are less so, so out comes the rotational inertia bit.
Inertia is important for acceleration but not at constant speed where
it is probably beneficial, although I don't know of any study that has
quantified this.  Francesco Moser used a large flywheel rear wheel in
his last attempt at increasing his Hour Record but the flesh wouldn't
respond.  I am sure they analyzed the effects however.

Although it may seem daunting, when another rider pulls away on a hill
or in a bike race on the flat, these accelerations, except in standing
starts, are so small as to make the rotating mass story a hoax.  Sure,
the mass counts twice as much when accelerating but two times zero is
still zero, and how long does a rider accelerate.  Weight of bicycle
components for climbing is the main consideration, not acceleration.
The rotating mass story is a fable that sounds good and has just
enough technical truth to be one that will probably sustain itself
indefinitely.  Making equipment choices by it are a matter of faith,
not fact.

Jobst Brandt      <>

From: (Jobst Brandt)
Subject: Re: My proposed wheelset....opinions please.
Date: 5 Jan 2001 18:30:36 GMT

Mike Krueger writes:

>>> When one pushes down on the pedals, and the bike speeds up, that
>>> is acceleration.

>> sure.  but no acceleration worth mentioning when talking about
>> inertia of rotating mass.

> OK, I'm not an engineer. Let's put this in layman's terms, so that I
> can understand it. Are you saying that it does not require more work
> to increase the rotating speed of a heavy wheel vs. a light wheel?
> (aerodynamics, tire rolling resistance, and all else being equal)

For bicyclists the main effort is overcoming wind resistance, the
second is raising weight up climbs (unless this is your specialty),
the two are distinctly different in human response as most of us
recognize.  Way down the list for better bicycles lies tire rolling
resistance and friction in the chain.  After that, at a great
distance, comes acceleration of changing speed... affected primarily
by the mass of the rider and bicycle.  The difference of whether that
weight is at the periphery of the wheel or not is triflingly small as
was demonstrated by another writer.  However, bicycles do not
accelerate at any perceptible rate from one speed to another, and when
they do, its at startup from the standing start.  It has almost
nothing to do with riding once underway where change of speed occurs
at glacial accelerations (aka <1/20g).

Therefore, although one gram more or less on the tread (periphery) of
a tire is equivalent to two grams on the hub *for acceleration only*,
this is a greatly exaggerated physical effect that enables bicycle
shops to sell unnecessarily expensive wheels on the basis of this true
but irrelevant physics.

Jobst Brandt      <>

Index Home About Blog