Bicycle Stopping Distances

" 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

Hey dumbass,

When you brake hard, your suppose to transfer your weight as far rearward as
possible in a way that is proportional to the braking effort. You don't just sit
on your seat like a Liz Hatch dodo bird and then wonder why you go over your
bars.

You people better stop talking like children.

Magilla

On Nov 3, 6:01*pm, MagillaGorilla wrote:
"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

Yes, I believe I can stop a bike from 50 mph in about 94 feet. *

Cool, then I'm sure you're willing to prove it. And like you said,
downhill isn't the way to go, You're a pro, so you have access to
motorpacing, just have your guy pace you to 50 mph, and get one of
your friends to shoot it on the Flip video his dad bought for him.
Now, since you're so sure you can do it, you can really rub our faces
in it by painting a white starting line, and then positioning a "nice
Infiinti" 95 feet away, so that you've got an additional 1 foot of
stopping distance.

This will make a great Youtube video. I'll bet you'll get over a
million hits when people see how this turns out.

Brad Anders

In article ,
Susan Walker wrote:

Michael Press wrote:
and the value is 9.806 65 m /s^2 exactly.

Depends.

No, it does not.

--
Michael Press

On Nov 3, 6:34*pm, MagillaGorilla wrote:
" wrote:

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

Hey dumbass,

When you brake hard, your suppose to transfer your weight as far rearward as
possible in a way that is proportional to the braking effort. *You don't just sit
on your seat like a Liz Hatch dodo bird and then wonder why you go over your
bars.

You people better stop talking like children.

Moron,

Put down the road bike and say Sir when you
speak to your betters.

Unlike you, I used my MTB for more than riding
into the woods to smoke dope, and know about
shifting your weight back. It's a great idea, but
you can't really get more than an extra 5-10 cm
back. To get any further back, you have to push
your monkey ass so far back it is off the seat,
your fat master aerobelly is resting on the seat,
your useless gonads are all the way behind the
saddle and your ass is close to rubbing the rear
tire.

This is doable on an MTB, but if you can do
it easily on a road bike and get more than 10cm
of shift, your bars are too close, your saddle isn't
high enough, and you normally ride with your knees
sticking out like a newbie century rider or half-triathlon
dork that is still worried about being able to put a
foot on the ground without shifting his ass from
the saddle when stopping at traffic lights. You
probably still worry abut falling down when you can't
clip out of your pedals too.

Shifting your weight is good for marginal traction
situations like pointing your bike downhill on dirt
and acting out that Missy Giove fantasy of yours
(and I don't want to hear about your or Bruce's other
Missy Giove fantasies). It is not going to save you
from malevolent ER doctors driving penis-compensator
sportscars.

So let me rephrase my statement. _I_ can't
stop a bike faster than a car. _You_, on the other
hand, are such a crap descender that you can't
get the bike up to the same speed as a car
to begin with, so the point is moot. "Moot" is a
fancy English word that means it doesn't matter
to monkeys because they have already been
distracted by their own feces.

With kindest regards,
Ben

On Nov 3, 6:25*pm, MagillaGorilla wrote:
" wrote:

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.

"Most people agree"...WRONG. *most people have no ****ing clue what .6 vs.
1.3 G's on a bike is. *I just love how you claim that people know the
difference between G forces measured in the hundredths of a G! * You've got
to be kidding me with this bull**** sales pitch, Jimmy Mays.

Chimp Chump,

I see you found my post where I explained that
this is due to simple geometry: the vertical distance
from your center of mass to the ground is larger
than the horizontal distance from your center of
mass to the front contact patch. That sets the
ratio of torques and is where the ~0.6 factor comes
from.

The rest is just you being embarrassed because
you believed the online calculator (which showed
less of its reasoning than I did) even though it
gave a result that is unphysical.



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

Yeah, like you really proved it with statements like "most people agree" and
declaing what the maximum deceleration is on a bike without showing how you
came up with those numbers.

Nice ****ing "data." (your own declarations that show no work)

It's not data. It's reasoning. Try it sometime.

Ben

Paul B. Anders wrote:
Cool, then I'm sure you're willing to prove it. And like you said,
downhill isn't the way to go, You're a pro, so you have access to
motorpacing, just have your guy pace you to 50 mph, and get one of
your friends to shoot it on the Flip video his dad bought for him.
Now, since you're so sure you can do it, you can really rub our faces
in it by painting a white starting line, and then positioning a "nice
Infiinti" 95 feet away, so that you've got an additional 1 foot of
stopping distance.

This will make a great Youtube video. I'll bet you'll get over a
million hits when people see how this turns out.

Yes but does his team have a healthcare plan or will he have to ask Liz
Hatch for a contribution towards the hospital bill ?

" 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

Hey dumbo,

In a maximum braking effort, you transfer your weight as far back and as low as
possible. You bury your head into your stem...you even your pedals so no leg is
higher than it has to be....you push your entire center of mass down into your
top tube. All of this is done instinctually and in a fraction of a second. So
all your numbers are wrong.

What you are talking about is the physics of how a ****ing Cat. 5 girl brakes her
bike on the Saulsalito Kenny Pap Smear group ride and then crashes into 6 riders
in front of her while claiming she "couldn't stop in time."

That's not how I stop my bike if I need to dig deep into the pro suitcase of
courage.

In fact, I would say a good proportion of maximum braking effort comes not from
how hard or quick you pull on the brake levers, but how quickly, how low, and how
rearward you shift your weight.

Your equation treats one of the most important aspects of maximum braking (i.e.
the lowering and shifting of center of mass rearward) as being a constant instead
of a rather large variable. That's the fundamental mistake of your equation.

You wrote the equation for how Liz Hatch stops her bike, and not for how the
monkey stops his bike.

QID.

Magilla

"Paul B. Anders" wrote:

On Nov 3, 6:01*pm, MagillaGorilla wrote:
"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

Yes, I believe I can stop a bike from 50 mph in about 94 feet. *

Cool, then I'm sure you're willing to prove it. And like you said,
downhill isn't the way to go, You're a pro, so you have access to
motorpacing, just have your guy pace you to 50 mph, and get one of
your friends to shoot it on the Flip video his dad bought for him.
Now, since you're so sure you can do it, you can really rub our faces
in it by painting a white starting line, and then positioning a "nice
Infiinti" 95 feet away, so that you've got an additional 1 foot of
stopping distance.

This will make a great Youtube video. I'll bet you'll get over a
million hits when people see how this turns out.

Brad Anders

It would be very difficult to precisely measure the stopping distance because
how do you measure exactly where the onset of braking was applied without
guessing? Actually, I suppose it could be done to a reasonable degree of
certainty, but you'd have to trust me as to the mark on the road where I said
I applied the brakes.

Also, I would never do a maximum braking effort because you would run the risk
of going over the bars or locking your front/rear wheel up and sliding out. I
would only do a max effort braking if I had to do it to avoid being seriously
injured - i.e. hitting a guardrail or cross-traffic. So you'd have to give me
that safety fudge factor and allow for say a 90% brake effort of around 110
feet for 50 mph. I would be willing to shoot for that stopping distance. If
I can do 110 safely, then 94 is really not a far fetch if you had to risk it
all to save your life.

Magilla

Donald Munro wrote:

Paul B. Anders wrote:
Cool, then I'm sure you're willing to prove it. And like you said,
downhill isn't the way to go, You're a pro, so you have access to
motorpacing, just have your guy pace you to 50 mph, and get one of
your friends to shoot it on the Flip video his dad bought for him.
Now, since you're so sure you can do it, you can really rub our faces
in it by painting a white starting line, and then positioning a "nice
Infiinti" 95 feet away, so that you've got an additional 1 foot of
stopping distance.

This will make a great Youtube video. I'll bet you'll get over a
million hits when people see how this turns out.

Yes but does his team have a healthcare plan or will he have to ask Liz
Hatch for a contribution towards the hospital bill ?

Can you believe that...Liz Hatch thinking that her team would foot her
medical bills when it's not in her contract? That chick is not lucid.
I'm on the edge of my seat to find out which team signed this train wreck
for 2010. It should be coming any week now.....the excitement is killing
me.

Magilla

On Nov 4, 7:42*am, MagillaGorilla wrote:

Also, I would never do a maximum braking effort because you would run the risk
of going over the bars or locking your front/rear wheel up and sliding out. *I
would only do a max effort braking if I had to do it to avoid being seriously
injured - i.e. hitting a guardrail or cross-traffic.

I am truly impressed with the articulation you are able to derive with
your nether fundamental orifice, but you're still talking out yer
ass. You argue that maximum braking would stop a cyclist faster than
a car (ignoring reaction time entirely, which makes your contention
totally without merit), then argue that maximum braking is dangerous
and just as likely to launch the cyclist.

Well done! Remember to wipe next time.

R