Re: Commuting wheels

In article <slrnfju7nh.ufr.armb@xxxxxxxxxxxxxxxxxxxxxx>,
Alan Braggins <armb@xxxxxxxxxxxxxxxxxxxxxx> wrote:
In article <Qef*hf8Zr@xxxxxxxxxxxxxxxxxxxxxxxxxxx>, Ian Jackson wrote:
The reasons to try to reduce the mass of a bike are IME largely to do
with the ease of acceleration (and braking).

If you can brake hard enough to lock your wheels/get thrown over
your handlebars, then reducing the rotating weight isn't going to
make any difference (and reducing the static weight of both wheels
isn't going to shift the centre of gravity much). So it's only for
acceleration that weight at the rims is (nearly) twice as important
on the flat as weight elsewhere.

I haven't done any detailed calculations on the situation for braking
but I think you're barking up some kind of wrong tree.

When braking at a particular deceleration at a particular speed, more
rotating mass means more energy needs to be absorbed in the brakes;
the energy calculations are the same for acceleration and the forces
between the wheel and brake are corresponding increased. Ie, you have
to press the brakes more firmly and any overheating or lack of rim
grip (eg due to wet rims) is likely to be more of a problem. For all
of these effects rotating mass is worth about twice nonrotating mass.

When braking rapidly, ie with the front brake, AFAICS it is true that
increased wheel rim mass will (with typical bike geometries and
compared to nonrotating mass) produce a lower proportional increase in
the friction required to sustain the deceleration less per increase in
the vertical force, so a naive interpretation suggests that the risk
of front wheel slip will be reduced with rotating mass compared to an
similar nonrotating mass. However in practice increased forces at the
tyre contact patch would seem likely to exacerbate the consequences of
any brief problems with the road contact. Quantifying these effects
will be very difficult.

If we're considering pivoting about the front contact patch as a
failure mode then rotating mass is unambiguously worse than a similar
nonrotating mass.

But all of this is perhaps too much analysis of a parenthetical
comment! Acceleration is the key thing I think.

--
Ian Jackson personal email: <ijackson@xxxxxxxxxxxxxxxxxxxxxx>
These opinions are my own. http://www.chiark.greenend.org.uk/~ijackson/
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