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Bicycle Stopping Distances



 
 
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  #21  
Old November 2nd 09, 10:15 PM posted to rec.bicycles.racing
DirtRoadie
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Posts: 2,915
Default Bicycle Stopping Distances

On Nov 2, 10:44*am, "Paul B. Anders" wrote:

Plug in 50 mph. Anyone who has done any high-speed descending who
believes a bike can stop from 50 mph in under 100 feet is smoking
weed. It's laughable. Go descend Carson or Monitor passes in the
Sierra's, where you can hit 50 mph easily, and do a full-on panic stop
and see if you can do this.


It's funny, I have ridden those passes, and 50 mph descents are also
common on many of my local roads. I honestly have no idea how I might
hope to stop under such conditions, although I suspect it would not be
that quickly. I DO know that I never want to be faced with finding
out nor am I likely to go out and do any significant experimenting
toward that end.

DR

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  #22  
Old November 2nd 09, 10:30 PM posted to rec.bicycles.racing
[email protected]
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Posts: 3,092
Default Bicycle Stopping Distances

On Nov 2, 10:44*am, "Paul B. Anders" wrote:
On Nov 1, 10:09*am, Anton Berlin wrote:



In a head to head test and in normal conditions a bike should be able
to stop faster than a car.


But that includes that the rider has both hands on the bars (and
brakes) which is hard to do when you're flipping someone off.


At 50 kmh


http://www.exploratorium.edu/cycling/brakes2.html


Bike stops in 10 meters


http://www.forensicdynamics.com/stopdistcalc


Car stops in 14 meters.


I hate proving Kunich wrong (again) at the expense of proving Magilla
right.


But Kunich may be right on an empirical basis. *It make take several
hundred meters to slow his fat ass to a stop.


Besides this is all theory as we know Kunich has never gone 30 mph on
a bike.


Plug in 50 mph. Anyone who has done any high-speed descending who
believes a bike can stop from 50 mph in under 100 feet is smoking
weed. It's laughable. Go descend Carson or Monitor passes in the
Sierra's, where you can hit 50 mph easily, and do a full-on panic stop
and see if you can do this.

Brad


Listen up monkeys,

The calculator is wrong because they did not
consider that a bike's braking ability is limited
by going over the bars.

The calculator, for 30 mph = 13.4 m/s, returns
a stopping distance of 10.4 meters. This
translates to a deceleration of 8.6 m/s^2, from
distance = 0.5 * (initial velocity)^2/ acceleration.

This is suspiciously close to 1 g = 9.8 m/s^2
times the "adhesion coefficient" of 0.85 that the
calculator suggests. So I think they assumed
that a bike can brake at slightly less than 1 g,
slightly less because it's limited by tire adhesion.

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.

If you use 0.6 g to calculate the stopping distance
from 30 mph, it's 15.2 meters. And that is assuming
absolutely perfect conditions and flat ground,
not down hill, which makes the endo problem worse.

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

Ben


  #23  
Old November 2nd 09, 10:41 PM posted to rec.bicycles.racing
RicodJour
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Posts: 3,142
Default Bicycle Stopping Distances

On Nov 2, 4:30*pm, "
wrote:

{snip of confusing facts and numbers that "smart" people use to
confuse the "stupid" people, and, yes, I'm talking about "you"}

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


It's not about the bike. It's about the guy _on_ the bike...unless
it's a really nice bike. If it's a hot girl on a bike, then it's a
toss up, or a tossed salad if she's feeling frisky.

R
  #24  
Old November 2nd 09, 10:45 PM posted to rec.bicycles.racing
marco
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Posts: 129
Default Bicycle Stopping Distances

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.

  #25  
Old November 2nd 09, 10:58 PM posted to rec.bicycles.racing
RicodJour
external usenet poster
 
Posts: 3,142
Default Bicycle Stopping Distances

On Nov 2, 4:45*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.


And those things have a negative impact on reaction time.

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


Somebody should do this test and post it on youtube.


  #26  
Old November 2nd 09, 11:03 PM posted to rec.bicycles.racing
RicodJour
external usenet poster
 
Posts: 3,142
Default Bicycle Stopping Distances

On Nov 2, 4:45*pm, "marco" wrote:

Somebody should do this test and post it on youtube.


Sorry, I hit send too quickly.

http://www.youtube.com/watch?v=tbroNClq7l4
That guy was still moving?!

http://www.youtube.com/watch?v=DvWJsxI8Vz4

And, if you're not humor impaired...
http://www.youtube.com/watch?v=2zeYdTjNrBg

R
  #27  
Old November 2nd 09, 11:53 PM posted to rec.bicycles.racing
Paul B. Anders
external usenet poster
 
Posts: 363
Default Bicycle Stopping Distances

On Nov 2, 2:30*pm, "
wrote:
On Nov 2, 10:44*am, "Paul B. Anders" wrote:





On Nov 1, 10:09*am, Anton Berlin wrote:


In a head to head test and in normal conditions a bike should be able
to stop faster than a car.


But that includes that the rider has both hands on the bars (and
brakes) which is hard to do when you're flipping someone off.


At 50 kmh


http://www.exploratorium.edu/cycling/brakes2.html


Bike stops in 10 meters


http://www.forensicdynamics.com/stopdistcalc


Car stops in 14 meters.


I hate proving Kunich wrong (again) at the expense of proving Magilla
right.


But Kunich may be right on an empirical basis. *It make take several
hundred meters to slow his fat ass to a stop.


Besides this is all theory as we know Kunich has never gone 30 mph on
a bike.


Plug in 50 mph. Anyone who has done any high-speed descending who
believes a bike can stop from 50 mph in under 100 feet is smoking
weed. It's laughable. Go descend Carson or Monitor passes in the
Sierra's, where you can hit 50 mph easily, and do a full-on panic stop
and see if you can do this.


Brad


Listen up monkeys,

The calculator is wrong because they did not
consider that a bike's braking ability is limited
by going over the bars.

The calculator, for 30 mph = 13.4 m/s, returns
a stopping distance of 10.4 meters. *This
translates to a deceleration of 8.6 m/s^2, from
distance = 0.5 * (initial velocity)^2/ acceleration.

This is suspiciously close to 1 g = 9.8 m/s^2
times the "adhesion coefficient" of 0.85 that the
calculator suggests. *So I think they assumed
that a bike can brake at slightly less than 1 g,
slightly less because it's limited by tire adhesion.

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.

If you use 0.6 g to calculate the stopping distance
from 30 mph, it's 15.2 meters. *And that is assuming
absolutely perfect conditions and flat ground,
not down hill, which makes the endo problem worse.

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


It's even worse than you cite. I used to coach riders in our club on
effective use of their brakes. Note that these weren't elites, they
were your typical club racers, probably like the guys who got brake-
checked. Many barely used their front brakes at all, due to fear of
doing an endo. We did drills in parking lots where we trained people
how to use their front brakes effectively by progressively braking
down from higher and higher speeds, and applying more front brake
bias. It took a LOT of passes before people could begin to properly
shift their weight, brake hard, and unweight their rear wheels at
all.

Add in the inherent limitation of the high CG on bicycle braking
performance, and your average rider can't begin to stop anywhere near
as quickly as a car where the driver has fully engaged the ABS. And
it's one thing to be fully prepared for a corner, shift your weight
back, and apply full braking, vs. having some hair-trigger lunatic ER
doc whip his car in front of you and slam on his brakes without
warning.
  #28  
Old November 3rd 09, 12:21 AM posted to rec.bicycles.racing
[email protected]
external usenet poster
 
Posts: 3,092
Default Bicycle Stopping Distances

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



  #29  
Old November 3rd 09, 12:50 AM posted to rec.bicycles.racing
Paul B. Anders
external usenet poster
 
Posts: 363
Default Bicycle Stopping Distances

On Nov 2, 4:21*pm, "
wrote:
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- Hide quoted text -

- Show quoted text -


I always use both brakes when stopping hard, modulating the rear to
avoid lock-up. Why? Because 99% of the braking I'm doing isn't
threshold braking at the max. You get shorter stopping distances with
more control by using both brakes. Also, most front brakes become
pretty tough to modulate when you're near the limit, so if I can shift
even 10% of the load to the rear brake and tire, it makes it easier to
control.

This video is pretty dry, but it gets the points across about using
your front and rear brakes effectively:

http://www.youtube.com/watch?v=TeJ1JH2ah00

Brad anders
  #30  
Old November 3rd 09, 07:01 AM posted to rec.bicycles.racing
Michael Press
external usenet poster
 
Posts: 9,202
Default Bicycle Stopping Distances

In article
,
Anton Berlin wrote:

On Nov 1, 6:50Â*pm, "Tom Kunich" wrote:
"Ryan Cousineau" wrote in message

]...





In article
,
Anton Berlin wrote:


In a head to head test and in normal conditions a bike should be able
to stop faster than a car.


But that includes that the rider has both hands on the bars (and
brakes) which is hard to do when you're flipping someone off.


At 50 kmh


http://www.exploratorium.edu/cycling/brakes2.html


Bike stops in 10 meters


http://www.forensicdynamics.com/stopdistcalc


Car stops in 14 meters.


I hate proving Kunich wrong (again) at the expense of proving Magilla
right.


But Kunich may be right on an empirical basis. Â*It make take several
hundred meters to slow his fat ass to a stop.


Besides this is all theory as we know Kunich has never gone 30 mph on
a bike.


The missing factor is essentially reaction time, which probably explains
how Dr. Evil managed to whomp two riders with his trunk.


Here's a claim that reaction times vary around 0.7-1.5 s for drivers in
braking situations.


That suggests that if the Doctor swerved and braked fast enough, the
riders would not have had time to react before hitting the car. He's
effectively got about a 1-second head start on braking, and at 50 km/h,
that's about 14 meters.


In other words, the car could be at zero km/h before the riders got to
their brakes, and the rest depends on how closely in front of them he
cut.


Considering he seems to have been trying to injure them, I'm going to
guess really close, like 5m.


I figure that scenario as being 14 metres of stopping distance but about
24 metres of rt+ideal stopping. In other words, physics says those
cyclists were gonna hit the car no matter how good their brakes, as long
as their reaction times were within human norms.


Gerbils or monkeys may have better reaction times than humans, though.


As usual, those who fail to think do the most talking.

The brakes on a modern car will stop the car at a rate of about one gee.
Race cars commonly brake well above one gee. Moreover, car tires, which
cover a large portion of the road and put more square inches of rubber on
the road per lb. of load, are less susceptible to road conditions, gravel
etc. on the road and other traction problems.

Because of the high center of gravity a bicycle has, the braking force you
can apply while sitting normally on the saddle is about 1/2 gee. Got that?
HALF the braking force of a car. You can increase your braking force to
perhaps .85 gees by sliding backwards and putting your stomach on the
saddle. This unfortunately greatly decreases your control of the bicycle
while increasing your ability to brake by lowering your center of gravity.
Note that normally the time to slide back like that would take more
time/distance than the slightly improved braking would justify.

The reaction time for both the driver and the rider are the same and so can
be ignored when discussing stopping distances at equal speeds.- Hide quoted text -

- Show quoted text -


Gee, where the **** did you get the idea G was gee?

Geesus ****ing christ you're an idiot.


I always write `g' or `g_n', because
the official nomenclature is `g' with a a subscript `n';
and the value is 9.806Â*65 m /s^2 exactly.

--
Michael Press
 




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