Retro Bob said:Rider weight vs. bike weight ?
OK... I'm sure this has been hacked around here quite a bit already,
but I'm new here. Let me start with what I know: In automobile
suspension design, you attempt to limit the "unsprung" weight
because it's uncontrollable. The sprung weight you can control
and tune.But, I'm having trouble applying this concept to biking. It seems
to me like the entire bike/rider on a road bike is unsprung.
Therefore, it does not matter if the weight is on the rider or
the bike. Therefore, with the exception of those of use in perfect
condition (me, of course) you are just as wise to cut your own
weight by a pound as you are to spend $500+ cutting the bike's
weight by the same amount. Of course, the rider is movable, so in
a sense it's "sprung" weight".But, practical experience seems to challenge this. I know that getting
on a light bike makes me go much faster... and if doesn't seem to be
the 5 lbs difference... or is it ? Is it just an illusion ? If I
hung a 5lb weight on me and then got on a light bike would I feel
the same as the heavy bike (mechanical smoothness of the more
expensive bike aside) ?Comments appreciated.
Dear Bob,
The unsprung weight on suspended vehicles matters because
the suspension has to drive more mass back down to keep the
tires in contact with the ground. As you say, this is a
matter of control. A heavier tire, wheel, and lower forks on
the front end of a motorcycle will not follow rough ground
as well.
If you (in perfect condition and weighing 180 pounds) switch
from a 20-pound bike to a 15-pound bike, you'll probably
notice the light bike immediately in terms of low-speed
handling as you roll it out the door--it probably tips back
and forth more easily, being 25% lighter.
You may also notice some improvement in climbing and
acceleration, and descents, which are where the weight
matters.
It's only 2.5% less weight, but most people will go faster
with 5 pounds less to haul to the top of a hill. You might
need a stopwatch to notice this:
http://www.kreuzotter.de/english/espeed.htm
For a a 7% climb, hands on the drops, 90 cadence, 200 watts,
180/20 pound bike/rider weights, this calculator predicts
6.3 mph.
Ride a 15-pound bike up the same 7% slope and your speed is
expected to increase to 6.5 mph--0.2 mph, suggesting that
you'll be a thousand feet further up the climb in an hour.
(You'll also accelerate 2.5% faster for the same effort on
level ground, again an amount that may take a stopwatch to
confirm the notion in your head.)
But you'll coast slower downhill:
http://www.kreuzotter.de/english/espeed.htm
For hands on the drops, -10 for the grade, 0 cadence, 0
slope, and 180/20 rider/bike weight, this calculator
predicts 50.3 mph coasting downhill. Drop the bike weight 5
pounds from 20 to 15 and the speed drops to 49.7 mph. It's
only 0.6 mph. Few descents at these speeds last anywhere
near an hour, so don't expect much of a gain.
Carl Fogel