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Braking while turning



 
 
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  #11  
Old July 21st 03, 04:45 AM
Joe Riel
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Default Braking while turning

(Chris Zacho "The Wheelman") writes:

If your front wheel slides out, it too will go in a straight line. With
disastrous results. Actually, the best thing is not to brake at all in a
turn, Do all your braking before.


It may be the safest thing to do, but if you want to descend fast,
it's poor technique.

The tires have a limited amount of "grip" on the pavement, any that's
diverted towards slowing the bike down is being taken away from the
amount holding it in a curved path.


Not really. First off, it is unlikely that you will be riding
right at the limits of adhesion. For a simple test, with your
bicycle beside you, lean the bike over, with one hand on the
stem and the other on the seat. Push into the bike. See how
how far over you can lean the bike until the tires slip. You
should be able to get the bike to around 45 degrees. Do you
corner anywhere near that lean angle? Few do.

Second, because the total force on the contact patch is the vector
sum of the cornering force and the braking force, which are
applied perpendicular to each other, it is possible to
apply considerable braking force while barely changing the total
force. For example, let the braking force (fb) be 20%
of the cornering force (fc). The total force is then

ftot = sqrt(fc^2 + fb^2)
= fc*sqrt(1+(fb/fc)^2)
~ fc*(1+(fb/fc^2)/2) for fb fc
= fc*(1+(2/10)^2/2) = 1.01*fc

With a mere 1% increase in contact patch force,
you can be slowing considerably.

If you must apply brakes in a turn, do so very gently, and it's best, in
this case, to favor the rear. If that gives away, you have a much better
chance of riding it out.


This is poor advice. Better to learn and practice skills so that
you can corner at a comfortable speed with safety. If you only
learn to brake with the rear, you won't be able to stop or slow
down when you really need to.

Joe Riel
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  #12  
Old July 21st 03, 03:20 PM
Christopher Brian Colohan
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Default Braking while turning

Joe Riel writes:
... Do you
corner anywhere near that lean angle? Few do.


Ok, so you first establish that on flat pavement with no surface gunk
friction is very high. And you argue that few folks ride anywhere
near this limit on corners. So for most people fc is not that high.

Second, because the total force on the contact patch is the vector
sum of the cornering force and the braking force, which are
applied perpendicular to each other, it is possible to
apply considerable braking force while barely changing the total
force. For example, let the braking force (fb) be 20%
of the cornering force (fc). The total force is then


You then argue that 20% of fc (which we established is not that high)
is a "significant braking force". For some reason, I disagree. :-)

ftot = sqrt(fc^2 + fb^2)
= fc*sqrt(1+(fb/fc)^2)
~ fc*(1+(fb/fc^2)/2) for fb fc
= fc*(1+(2/10)^2/2) = 1.01*fc


Here is a much easier way of solving (and understanding) this, with
the bonus of getting a more accurate answer:

ftot = sqrt(fc^2 + fb^2)

Assume that fc = 1unit. Assume that fb = .2*fc = .2units

ftot = sqrt(1^2 + .2^2) = 1.02

Now, if you assume that the rider is cornering somewhat
conservatively, because they fear that there might be a patch of sand
or tar on the corner somewhere, then you can assume that fc is not too
high. What happens if they are also going down a steep hill, and want
to brake to maintain their speed? If fc=fb, then this changes to:

ftot = sqrt(1^2 + 1^2) = 1.41

You have just lost 40% of the margin of safety you planned to have in
the corner.

I do agree that the best way to learn this is not through math, but
through practice. I found that a great way to learn how to deal with
low traction conditions in corners is to go out and ride just after a
fresh snowfall (before the plows come by). Very low traction, loads
of fun, and if you happen to fall then the snow offers some padding...

Chris
--
Chris Colohan Email: PGP: finger
Web:
www.colohan.com Phone: (412)268-4751
  #13  
Old July 21st 03, 03:31 PM
Joe Riel
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Default Braking while turning

"Mike S." mikeshaw2@coxDOTnet writes:

" It may be the safest thing to do, but if you want to descend fast,
it's poor technique.

Actually, its better to brake and accelerate in straight lines. Don't ask
me to explain, its physics...

I have a nice scar on my left bicep from braking in a corner and running
wide into a barbed wire fence. Grab brakes in the middle of a turn and the
bicycle will have a tendency to stand up and widen the arc of the corner
you're taking. If you don't know its going to happen, you too can end up
with a nifty scar like mine.


I doubt that you would have been better off without braking. The point
being, unless you are willing to go slow all the time, you are better
off learning how to brake in turns. The time to learn is not when
you need it...rather practice under known conditions.

If you have to brake, do it just like in cars and motorcycles: brake before
the corner, corner, then accelerate as you're exiting.


You think cars and motorcycles don't brake in turns? That's nonsensical.
Furthermore, the amount of acceleration available to a cyclist is trivial
compared to the deceleration. Learn to use.

If you can help it, don't brake. That's the fastest way down the
mountain...


Not one with real turns.

Joe Riel
  #14  
Old July 21st 03, 09:39 PM
Chalo
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Default Braking while turning

"Phil, Squid-in-Training" wrote:

Chalo wrote:
Furthermore, slipping the rear can cause a highside,
flinging you into the land of broken clavicles.


Really now? In my experience, highsides have been limited to the domain of
motorcycles, where the speeds of the bikes and massive grips of the tires
have the potential to cause a highside.

Uh... wait. I recall highsiding on my MTB commuter once. I landed on my
forearms, so nothing broken. I now have a dolphin-shaped scar on my arm.
Nevermind what I said.


Though I've not actually seen video of it, the crash that took Beloki
out of the Tour this year (which commands its own thread right now)
has been described as a highside.

I've highsided both pushbikes and motorbikes, and motorbikes are
worse. ;^)

Chalo Colina
  #15  
Old July 22nd 03, 04:28 AM
Joe Riel
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Default Braking while turning

Christopher Brian Colohan writes:

Joe Riel writes:
... Do you
corner anywhere near that lean angle? Few do.


Ok, so you first establish that on flat pavement with no surface gunk
friction is very high. And you argue that few folks ride anywhere
near this limit on corners. So for most people fc is not that high.

Second, because the total force on the contact patch is the vector
sum of the cornering force and the braking force, which are
applied perpendicular to each other, it is possible to
apply considerable braking force while barely changing the total
force. For example, let the braking force (fb) be 20%
of the cornering force (fc). The total force is then


You then argue that 20% of fc (which we established is not that high)
is a "significant braking force". For some reason, I disagree. :-)





ftot = sqrt(fc^2 + fb^2)
= fc*sqrt(1+(fb/fc)^2)
~ fc*(1+(fb/fc^2)/2) for fb fc
= fc*(1+(2/10)^2/2) = 1.01*fc


Here is a much easier way of solving (and understanding) this, with
the bonus of getting a more accurate answer:


The reason I expanded the term was so that the math would be easier
(i.e. I wouldn't have to do the square root in my head). Alas,
I apparently can't do squares in my head. My formula gives 1.02
when done properly.

ftot = sqrt(1^2 + .2^2) = 1.02

Now, if you assume that the rider is cornering somewhat
conservatively, because they fear that there might be a patch of sand
or tar on the corner somewhere, then you can assume that fc is not too
high. What happens if they are also going down a steep hill, and want
to brake to maintain their speed? If fc=fb, then this changes to:

ftot = sqrt(1^2 + 1^2) = 1.41

You have just lost 40% of the margin of safety you planned to have in
the corner.


That's why it's called margin 8-).

Consider this. The maximum braking force on dry asphalt is limited
to approximately 0.7g, to prevent flipping over. Assume that the
maximum contact force (where sliding initiates) is about 1g. It
is then possible to be simulateously braking at the maximum (0.7g),
while cornering at 0.7g! Cornering at 0.7g corresponds to a lean
angle, from upright, of approximately 35 degrees. I have crudely
measured my lean angles and know that 35 degrees feels quite aggressive;
I consider myself a reasonably fast descender---certainly one of
the faster in my club. The point being, the typical margins
riders deal with are wide indeed. Naturally, stuff in road
can reduce the margins in a hurry, but one of the points of practicing
is to learn to maneuver at speed.


I do agree that the best way to learn this is not through math, but
through practice.


Agreed, but I like doing the math anyway...

I found that a great way to learn how to deal with
low traction conditions in corners is to go out and ride just after a
fresh snowfall (before the plows come by). Very low traction, loads
of fun, and if you happen to fall then the snow offers some padding...


I guess that's one of the downsides to living in San Diego; we don't
get much snowfall here 8-). To improve my cornering I have
used stiff wire attached to the rear dropout and set to touch the
ground at a preset lean angle. Start with a lean angle of about 30 degrees,
ride and corner progressively harder until you just hear the wire
scraping on the ground. Then gradually increase the lean angle.

Joe Riel
  #16  
Old July 22nd 03, 07:31 AM
Chris B.
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Default Braking while turning

On Tue, 22 Jul 2003 03:28:19 GMT, Joe Riel wrote:

Christopher Brian Colohan writes:


[...]

I found that a great way to learn how to deal with
low traction conditions in corners is to go out and ride just after a
fresh snowfall (before the plows come by). Very low traction, loads
of fun, and if you happen to fall then the snow offers some padding...


I guess that's one of the downsides to living in San Diego; we don't
get much snowfall here 8-). To improve my cornering I have
used stiff wire attached to the rear dropout and set to touch the
ground at a preset lean angle. Start with a lean angle of about 30 degrees,
ride and corner progressively harder until you just hear the wire
scraping on the ground. Then gradually increase the lean angle.

Joe Riel


I have to try this...

--

Chris Bird
  #17  
Old July 22nd 03, 01:48 PM
TJ Poseno
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Default Braking while turning

MY suggestion is to brake like others said before a corner, but if
your in the corner, i use a small combination of both brakes.

Recently I got to learn this, i was going down a dirt road pushing
25mph and went into a dirt corner, my bike has slicks in the middle of
the tire and small knobs on the outsides. Well I pushed the tires too
far and started to lose traction, so I tried to feather both brakes in
a no turning back attemp to slow down. The rear end broke lose which I
thought i could handle because me and friends do it all the time for
fun, it was all working out but with the bike sideways like I wanted
it, I hit the raised portion of the road, and all hell broke loose.


No damage to the bike or me!
  #18  
Old July 22nd 03, 04:37 PM
David Damerell
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Default Braking while turning

TJ Poseno wrote:
MY suggestion is to brake like others said before a corner, but if
your in the corner, i use a small combination of both brakes.
far and started to lose traction, so I tried to feather both brakes in
a no turning back attemp to slow down. The rear end broke lose which I
thought i could handle because me and friends do it all the time for
fun, it was all working out but with the bike sideways like I wanted
it, I hit the raised portion of the road, and all hell broke loose.


"I had an accident using both brakes, so I suggest you use both brakes in
the corners, too."
--
David Damerell Distortion Field!
  #19  
Old July 22nd 03, 04:51 PM
Michael
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Default Braking while turning

wrote ...
One thing I have noticed is how differently bicycles handle when the rear
brake is applied compared to the front brake.

While braking with the front wheel and turning, the rear wheel naturally
swings around follows through the turn. The steering feels the same as if
the brake wasn't applied.

When the rear brake is applied while turning it feels like a force is
pulling rear tire in a straight line backwards causing it to not want to
swing around and follow the path of the front tire. The bike plows though
the turn. Even when the bike is traveling in a straight line the bike
doesn't feel nearly as controllable as it would with the front brake is
applied.

Is there a physics explanation for this? Even the seasoned riders at the
bike shop give me a wierd look when I tell them I hardly ever use the rear
brake.


Another thing to consider is changing the bike's track through the
turn WITHOUT braking. I use 2 techniques to tighten my turn radius:

1) Countersteer. It's counter-intuitive, but if you slightly steer
toward the OUTSIDE of the turn, the bike leans IN more and tracks INTO
the apex. It's a bit scary to try at first, but you get used to it,
and I find it corrects my line throught the turn - allowing me to hit
apexes I can't hit merely by leaning - and it feels like I retain all
my speed through the turn.

2) Lean the bike. Just push the handlebars down toward the ground
underneath you. This also tightens my line, but feels like it reduces
speed more than countersteering. But it's easier and safer.

Michael
  #20  
Old July 23rd 03, 01:55 AM
Douglas Landau
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Default Braking while turning

Actually, its better to brake and accelerate in straight lines. Don't ask
me to explain, its physics...


The physics have been explained. It doesn't support your claim.


Actually, the physics do, if 'better' is defined as 'safer'. One has
less chance of losing traction if one stays off the brakes.

As others have said, though, it's not the fastest.

Doug
 




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