On Nov 2, 2:45*pm, "marco" wrote:
bjw wrote:
The calculator is wrong because they did not
consider that a bike's braking ability is limited
by going over the bars.
...snip...
However, everyone who thinks about this says that
a bike can't do that because of the high center of mass.
Most people agree that just from geometry (height of
the center of mass relative to how far forward the front
wheel contact patch), a bike is limited to at most
0.6 g deceleration, or 5.9 m/s^2.

A naive dynamics question: isn't it possible to modulate the front and rear
brakes to offset the high-center-of-gravity problem? Without really thinking
about it, that's what it feels like you do instinctively when trying to stop
really quickly. And of course you also push your weight back as much as you
can.

So no, a bike cannot stop faster than a car.

Somebody should do this test and post it on youtube.

No, you can't get extra braking power from the rear brake.
Braking transfers weight forward, as we all have felt;
another way of thinking about it is that the deceleration
of your center of mass generates a torque which wants
to pivot the bike forward around the front contact patch,
and the force of gravity pulling you down is what
counteracts that torque.

The limit of about 0.6 g is when the bike is just about
to start pivoting about the front contact patch by lifting
the rear wheel. At that point, it doesn't matter what
you do with the rear brake because there's almost no
weight on the rear wheel, so no friction. If you grab
it hard you may skid the rear wheel.

When trying to stop quickly, I grab both brakes, but I
think the rear is psychological. If your brakes are weak
or squishy or the road is wet, grabbing both may help.
I don't like braking real hard on the rear because it
skids - if you hit a wet patch or a bit of sand on the
road, very easy to skid and lose it. As Anders said,
newbies never get this right. It's tough to brake hard
enough on the front to endo, unless you are MTB'ing
and drop the front wheel into a rut or hole.

BTW, the 0.6 g number isn't magical, it's just based
on the angle from your center of mass to the front
contact patch. If the center of mass is around your
belly button, then (on my bike) the height off the ground
is about 1.2 m and the horizontal distance to the
contact patch is about 0.75 m. The geometry of the
opposing torques from deceleration and gravity means
that the bike starts to endo when the deceleration is
more than (0.75/1.2) ~ 0.63 g. All fairly approximate.

Ben