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#32
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Pedersen self energizing brakes.
hatrack & millinery writes:
When a RR wheel skids, it loses traction as it glides on molten metal. ITYM molten rubber. Not so. Rubber may melt but it is left behind while the skidding rail wheel was originally round and now had a flat spot. Liquid metal is noted for its lubrication and in the days of steam engines the sound of spinning drivers on a steam engine, long after the throttle was shut, were common. Once the skid starts [on steel rails], traction is lost in any event. |
#33
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Pedersen self energizing brakes.
Benjamin Lewis wrote: Imagine brake pads in the form of wedges between the fork and rim, with the narrow end of the wedge pointing forward. If the wedges are pushed forward so that they jam between the fork and rim, motion of the wheel will pull them forward even farther without any additional outside force, and the wheel will lock up. No question. But you have added the assumption that you are now bracing the pads against an essentially immovable surface - the fork. It is also clear that in this scenario the pads exert a force against the fork in order to be able to "wedge" the wheel to a stop. It is exactly that force (that *additional* force) that must also be resisted by the arms of the cantilevers which are NOT immovable but are braced against movement by the brake cable, which is, in turn, braced by the grip of the rider. Picture gripping a rim as tightly as possible with your thumb and forefinger as a brake caliper. Can you then grip the rim *tighter* if you create a similar wedging effect between your finger tips and the rim? If so, where do you get the extra strength? DR |
#34
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Pedersen self energizing brakes.
Dave Lehnen wrote: The total force on the pad is not just from cable tension, it's also from torque resulting from pushing a female helical thread against a male helical thread. Just how is this helicallly created force applied to the pad/rim if there is no corresponding tension in the cable? DR |
#35
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Pedersen self energizing brakes.
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#36
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Pedersen self energizing brakes.
Dirtroadie wrote:
Dave Lehnen wrote: The total force on the pad is not just from cable tension, it's also from torque resulting from pushing a female helical thread against a male helical thread. Just how is this helicallly created force applied to the pad/rim if there is no corresponding tension in the cable? The reaction force is exerted by the bolt heads on the ends of the brake studs, since the helix's axis is perpendicular to the tension on the wire. They work. Find some and try them if you don't believe it. I bent many forks under braking force alone, and it did not take unusual lever input to do so. Chalo Colina |
#37
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Pedersen self energizing brakes.
Chalo wrote:
The reaction force is exerted by the bolt heads on the ends of the brake studs, since the helix's axis is perpendicular to the tension on the wire. That doesn't account for any force component which would create an increased force against the rim. They work. I have acknowledged that the general consensus is that they work. What I am not entirely willing to accept is the commonly accepted REASON why they work. It makes little sense despite being oft repeated. Find some and try them if you don't believe it. I have some but I have never used them. I bent many forks under braking force alone, and it did not take unusual lever input to do so. I'm not sure what that establishes. I am reasonably sure that I would have difficulty bending a fork, yet I have locked up front wheels quickly enough to sommersault gracefully over the bars. Might it be that you are harder on equipment than I am? DR |
#38
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Pedersen self energizing brakes.
Dirtroadie wrote:
Chalo wrote: The reaction force is exerted by the bolt heads on the ends of the brake studs, since the helix's axis is perpendicular to the tension on the wire. That doesn't account for any force component which would create an increased force against the rim. They work. I have acknowledged that the general consensus is that they work. What I am not entirely willing to accept is the commonly accepted REASON why they work. It makes little sense despite being oft repeated. One consideration is that brake systems are not entirely efficient since some of the force applied at the lever is lost to friction in the brake handle and in the cable. When you apply force (F) to a normal brake then only some percentage of that force is effectively applied to the rim brakes. But in the case of self-energizing brakes, the friction in the system actually makes the brakes work 'better' (or at least apply more stopping force - controllability is impaired) since you're still applying force F to the lever, but now that force plus the friction in the system act together to balance the self braking force of the brake. I don't have any numbers on how much of a factor such frictional losses are, but let's assume 30%. In that case the normal brake only sees 0.7 x F applied while the self-energizing brake would be applying 1.43 x F to the rim brake which is then reduced by a factor of 0.7 to F at the lever. So if friction is this high then the self-energizing brake could apply just over twice as much stopping power assuming other factors (pad material, leverage, etc.) are equal. |
#39
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Pedersen self energizing brakes.
Dan Burkhart wrote:
Every day is an adventure for a new bike mechanic. There seems to be no end to the exotic componentry out there. I just had a unit come in with the Pedersen cantis. It was easy enough to figure out how they work, the only real mystery is, well, why? I am certainly no engineer,so maybe someone who is can explain the advantage if there is one. How does moving the pads forward as the brake arms rotate toward rim contact energize the brake? Was this just another answer to a question no one asked? There have been several versions of that idea over the years, from your SE brakes to a CLB ramped shoe/holder. All are dangerous IMHO but some riders insist on them (??!?!?) -- Andrew Muzi www.yellowjersey.org Open every day since 1 April, 1971 |
#40
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Pedersen self energizing brakes.
Quoting Matt O'Toole :
IIRC Scott-Pedersen sold these brakes for mountain bikes with a disclaimer not to use them on the front. Of course no one needs the extra power on the rear, which is usually skidding along already anyway. No. A disclaimer would be of no use, because a brake of this type for the front is differently arranged to one for use on the rear; a front one is only ever intended for front use. Pedersen made front and rear; Suntour only rear. -- David Damerell Kill the tomato! Today is First Leicesterday, June. |
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