Pedersen self energizing brakes.

Dirtroadie wrote:

wrote:

I think I understand your example, but we are talking about two
mechanisms here not one. If you were to take your thumb and push hard
on the trailing end of the brake pad without loading the lever/cable at
all, you would see (if your thumb was strong enough) the body of the
canti twist and move in towards the rim. similarly if you grab the
lever as hard as you can when the wheel is still the self energizing
mechanism doesn't come into play.. the only force you can exert on the
rim comes from the cable. The two mechanisms are related but
essentially independent in their operation.


OK when a brake is actuated it "pushes" on the rim. According to
Newton, the rim must also be "pushing" back i.e. trying to move the pad
away from the rim.
The quantity of these balanced forces is reflected in the tension on
the brake cable. Now when we add the additional force of the "self
energizing" effect, where does that additonal force go if not into
cable tension? Of course that is in a system where we are assuming
non-real things such as a friction-free system.

As we have been bantering with this I think I am seeing anther
explanation for why these brakes are generally perceived as having a
benefit in actual use. The additional force which is presumably applied
to the brake pad does not have to be applied through movement of the
cable, although the cable *system* still must counteract or balance any
such additional force. Since the cable does not need to move, some of
that force can be balanced by static friction within the system. It's a
bit like throwing a rope over a tree branch and being able to
statically support a weight (at the weighted end) using a force less
than the weight being supported (at the supporting end) by relying on
the friction of the rope on the branch to make up the difference. Under
that scenario it would also be possible to have situations where
additional weight could be added without any perceived change in the
force required at the supporting end. Sailors do the same thing when
handheld lines are wrapped around winches.

This may also explain the Pedersen brakes have been generally praised
for tandem use where longer cables runs may create greater cable
friction as well (but not necessarily so where cables are not housed).

DR


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. If the thread is steep enough not to be locked
by its own friction, torque results from the axial force. This is no
different in principle than other self-servoing brakes, such as
leading-shoe drum brakes. Hydraulic pressure does not rise due to
the self-servoing; the extra force comes from the component of the
reaction force to the pivot pin which is perpendicular to the brake
shoe.

This brake should have the same problems as other self-servoing
brakes; if designed to be safe it isn't very effective, and if
designed to be effective it isn't very safe. If you can predict the
highest coefficient of friction between the pads and rim that could
be generated in the worst case, you can design the brake to never
lock from self-servoing, but it won't give a lot of boost, and the
amount of boost changes as the coefficient of friction changes.

As for the effects of static friction, in the helical pivots or
elsewhere, it should make the brakes less linear and predictable,
with small changes in lever pressure making no difference, until the
static friction is broken and motion occurs, and the brake force
jumps up or down.

Dave Lehnen

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.

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

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

wrote:

A self-servo brake as you describe is useless because the user cannot
anticipate how much braking will occur from a given hand force.

DON'T USE THIS BRAKE!

I used Scott Pedersen Self-Energizing brakes for several years and tens
of thousands of miles before they became near-unobtainable. They
worked well and proved to be safer for me (because they gave me the
option of quick stops) than any other brake I used during the same span
of time.

You may be able to lift your rear wheel with ancient sidepulls, but I
couldn't do that even when I weighed 150 lbs less than I do now. With
SE cantilevers, I was able to slide back, put my chest on the saddle,
and decelerate hard (hard enough to bend unicrown forks, which I did
many times before getting Bontrager forks).

For such powerful brakes, Scott SE brakes were comparatively easy to
set up, since they required no toe-in.

The best linear-pull brakes have come to equal the stopping power of SE
cantilevers, and are even simpler to set up and more consistent in wet
weather. But SE brakes were vastly superior in their stopping power to
all other brakes available at the time, and would still be an excellent
choice for use with drop bars if they were available today.

Those who need abundant stopping power must be discriminating about
their brakes. For such riders, SE brakes are appropriate if they can
be had. Those who don't need much stopping power can make do with
whatever pleases them.

Chalo Colina

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

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

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.

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

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.