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Roller brakes: running friction?



 
 
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  #21  
Old July 1st 05, 05:05 PM
Ted Bennett
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Default Roller brakes: running friction?

Pat Norton wrote:

Sheldon Brown wrote:
Seems a bit bizarre to me that a device that reduces
braking power could be considered a "safety feature" though.


I have no objection to the concept of modifying torque or preventing
skidding. ABS does both and is considered a safety feature.


As Sheldon would say, apples and locomotives. ABS has nothing at all to
do with the Shimano device, which arbitrarily cuts off braking with no
regard to the traction available, or whether or not the wheel is
skidding.

I am only concerned about the implementation. ABS modifies torque if
vehicle motion exceeds wheel motion and operates low or high friction
surfaces. The Shimano system merely puts an upper limit on torque.


That's the problem. If the aim is to prevent skidding by limiting
torque, the geometry of the situation (position of the center of mass
relative to the front tire contact patch) as well as the total mass
being braked, must be considered. For very many riders, the
torque-limiting device enters into play before maximum braking is
achieved.

Shimano quotes ISO 4120 and DIN 79100-2. This requires a weight of 100
kg and a brake lever force of 100 N. I understand that the requirement
is to exceed 2.2 m/s^2 wet and 3.4 m/s^2 dry.

Shimano quotes a deceleration capability of 3 m/s^2 for the SG-8c20
rear hub. Magura claims rim brakes can achieve 4 m/s^2 and discs 8
m/s^2. It would be interesting to know the torque or deceleration
values for the Shimano front hub.


An ordinary bike can achieve up to roughly 0.6g, or 5.8 m/s^2, before it
rotates on the contact patch. Any decent brake, whether rim or disc,
can do that. Where did you get those figures? They could not have been
generated by a rider on a bicycle.

--
Ted Bennett
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  #22  
Old July 31st 05, 02:28 AM
Jasper Janssen
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Default Roller brakes: running friction?

On Mon, 27 Jun 2005 23:50:20 +0100, "Pete Biggs"
wrote:

Roller brakes can be used up front as well. It's one way to make you ride
extremely carefully, I suppose ;-)


50-60% or more of the bikes on the road here have Sturmey, Sachs, or
Shimano 'roller' drum brakes on both wheels. Most of the rest are coaster
brake models. They work well enough for most people, although it's true
all drum brakes are fairly pathetic in absolute terms. I don't know,
though, if the Shimano roller front is much worse than a Sturmey Archer,
that could be a problem. OTOH, if it doesn't go out of adjustment as much
(and after a bit of wear, most Sturmeys are permanently out of
adjustment), it'd still be a net win.

Jasper
  #23  
Old July 31st 05, 08:21 AM
Simon Brooke
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Default Roller brakes: running friction?

in message , Jasper Janssen
') wrote:

On Mon, 27 Jun 2005 23:50:20 +0100, "Pete Biggs"
wrote:

Roller brakes can be used up front as well. It's one way to make you
ride extremely carefully, I suppose ;-)


50-60% or more of the bikes on the road here have Sturmey, Sachs, or
Shimano 'roller' drum brakes on both wheels. Most of the rest are
coaster brake models. They work well enough for most people


Where is 'here', and how high/steep are the local hills? I think this
sort of statement needs to be qualified in those terms. For values of
'here' which include much of southern Scotland, I think roller brakes
would be so inadequate as to be actively dangerous.

--
(Simon Brooke) http://www.jasmine.org.uk/~simon/

;; my other religion is Emacs
  #24  
Old August 11th 05, 10:18 PM
Jasper Janssen
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Default Roller brakes: running friction?

On Sun, 31 Jul 2005 08:21:30 +0100, Simon Brooke
wrote:
in message , Jasper Janssen
') wrote:

50-60% or more of the bikes on the road here have Sturmey, Sachs, or
Shimano 'roller' drum brakes on both wheels. Most of the rest are
coaster brake models. They work well enough for most people


Where is 'here', and how high/steep are the local hills? I think this
sort of statement needs to be qualified in those terms. For values of
'here' which include much of southern Scotland, I think roller brakes
would be so inadequate as to be actively dangerous.


'Here' would be the Netherlands. Hills? What are those? We've got
humpbacked bridges. That's about it. At least, while there are some hills
in the country, those would not usually be the place where these bikes go
in modern times, and they're fairly tame by any serious hillfolk's
standards anyway. Still, though, back in the 50s, 60s, and 70s there were
plenty of people riding these threespeeds, and even singlespeed
coaster-brakes, to the Ardennes mountains or even the Alps.

Jasper
  #25  
Old August 12th 05, 09:57 AM
Dan Gregory
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Default Roller brakes: running friction?

Jasper Janssen wrote:

Still, though, back in the 50s, 60s, and 70s there were
plenty of people riding these threespeeds, and even singlespeed
coaster-brakes, to the Ardennes mountains or even the Alps.


Yes, I remember my first trip round France in 1956. Somewhere between
Dijon & Lyon there was a large grassy bank next to the road and on it
were dozens of Dutch and German cylists (all teenagers like me) and they
were all on "sit up & beg" single speed bikes.
I met some on the Tourmalet too!
All the best
Dan Gregory

 




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