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#21
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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
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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
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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
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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
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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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