Pedersen self energizing brakes.

Quoting Werehatrack :
On Mon, 06 Jun 2005 20:52:53 GMT,
... When a RR
wheel skids, it loses traction as it glides on molten metal.
ITYM molten rubber.

In my experience railway trains rarely have any rubber to melt.
--
David Damerell Kill the tomato!
Today is First Leicesterday, June.

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.

Be cool. There is a force vector that combines the rotational
component of the thrust on the helix and the rotational component of
the pull on the wire. That's how self-servo systems work. The drum
brakes in an old car don't kick your foot back off the pedal when they
engage, right?.

In a normal cantilever brake, the force generated by friction against
the rim is wholly transferred to the heads of the pivot bolts. In an
SE brake, some amount of this rim drag force is transferred to the pad
against the face of the rim. The amount transferred depends on the
pitch of the helix and its change in friction as it is loaded.

If you acknowledge that SE brakes exhibit power boosting compared to
normal cantilevers, then you must accept that the added braking force
is attributable to deflection of axial force at the pivot. There is no
other mechanism that distinguishes SE brakes from other cantis.

The trait of these brakes I have wondered about is why they never ever
fail to release when the lever is released. It doesn't matter how hard
you brake, they always stop biting when you let go of the lever.
That's a good thing, I just wonder why. I guess it would be possible
to make SE brakes that self-locked by steepening the pitch of the helix
past a certain angle.

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?

The point is that SE brakes will supply enough braking torque to bend
the fork, unlike almost all other species of bicycle brake. The fact
that you, specifically, go over the bars first just means that you
would never be able to use that amount of braking torque, because you
reach another kind of limit. My statement turns out to be irrelevant
to your question; I mentioned it because I thought you were suggesting
that self-energizing did not take place at all.

If you are not able to "max out" a brake, then you can only evaluate
its response curve and not its gross power. I think this partially
accounts for the popular rejection of SE brakes. Their response curve
tends to be somewhat abrupt and inconsistent-- it is their gross power
that is unusually good. If you can't use such a brake's primary
benefit, then you are just coping with its tradeoffs.

Chalo Colina

Chalo wrote:
The trait of these brakes I have wondered about is why they never ever
fail to release when the lever is released. It doesn't matter how hard
you brake, they always stop biting when you let go of the lever.
Precisely! They can't remain locked once cable tension is released
because there is nothing to hold them in place. This is the point I
have been raising through this whole thread.
I also speculated earlier that the reason they seem to work is that
friction in the system helps resist the increased braking force and
limit its transfer through the cable to the lever. Another poster has
provided some additional figures in support of this analysis.

That's a good thing, I just wonder why. I guess it would be possible
to make SE brakes that self-locked by steepening the pitch of the helix
past a certain angle.
Again, that's like changing the angle of a wedge without taking into
account whether the wedge has a solid support to brace against. It is
not going to matter how much you tinker with the geometry of the helix,
the cable is still required to prevent the brake arm (and pad) from
moving away from the rim.
Also note that the function of cantilever brake (having opposing pads
on either side of a braking surface) has more in common with that of an
automotive disk brake than it does with the function of a drum brake.

DR

Chalo wrote:
The trait of these brakes I have wondered about is why they never ever
fail to release when the lever is released. It doesn't matter how hard
you brake, they always stop biting when you let go of the lever.

Yes, precisely! They can't remain locked once cable tension is released
because there is nothing to hold them in place. This is the point I
have been raising through this whole thread.

I also speculated earlier that the reason they seem to work is that
friction in the system helps resist the increased braking force and
limit its transfer through the cable to the lever. Another poster has
kindly provided some additional figures in support of this analysis.

That's a good thing, I just wonder why. I guess it would be possible
to make SE brakes that self-locked by steepening the pitch of the helix
past a certain angle.
I doubt it. Changing the geometry is not going to change the fact that
the brake cable is still absolutely necessary if any force is to be
provided to the rim. Similarly you can play all you want with the
pitch or shape of a wedge (give it a curved surface to change its
"response" as it is driven) but it can only have a wedging effect when
it is braced against a surface that can resist whatever wedging force
is created.

Despite the suggestions throughout this thread that these brakes are
like automotive drum brakes, these (and any rim other brake) are far
more similar to a disk brake i.e. opposing brake pads which clamp
against opposite sides of an essentailly disk shaped surface.

DR

wrote:



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.

The brake cable is not the only force bracing them against movement. If
you apply an inward force on the brakes, the mechanism transforms some of
this force into force in the forward direction; conversely, if you (or in
this case, the rim), applies a forward force to the brakes, the mechanism
transforms some of that force to an inward force.

From a given fore-aft position, the pads are *not* free to move inwards and
outwards.

--
Benjamin Lewis

Seeing is deceiving. It's eating that's believing.
-- James Thurber

add a smallish gas shock each side?
equalize or reduct the tendency to lock up and send the rider into the
trees?
old stuff adapts to new stuff and revives!

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?

DR


I didn't say there was no tension in the cable, only that there was
no increase in cable tension as the brake self-energizes. The force
of the pad against the rim is the sum of that from the cable acting
through the lever of the brake arm, and the force from the torque
generated by axial force on the helical threads, acting through the
part of the brake arm between the helical pivot and the pad.

If the brake is designed properly, reducing tension in the cable
lessens the force of the pad against the rim, which in turn reduces
the axial force on the helix, and its torque, and the additional or
boost force on the pad, until the brake arm is back in equilibrium.
If the brake is designed with an unsafely high amount of boost, the
brake can lock, and stay locked with a complete release of cable
tension.

Dave Lehnen

Dave Lehnen wrote:
I didn't say there was no tension in the cable, only that there was
no increase in cable tension as the brake self-energizes. The force
of the pad against the rim is the sum of that from the cable acting
through the lever of the brake arm, and the force from the torque
generated by axial force on the helical threads, acting through the
part of the brake arm between the helical pivot and the pad.

That is similar to placing a 10 lb weight on top of a jack and
measuring the force between the jack and the weight (10 lbs- agreed?).
Using your analysis, if you then operate the jack and remeasure the
force you will see an increased force equal to the sum of the weight
PLUS whatever force is applied by the jack, when in actuality ther is
no increase in force since you cannot ever get above the initial 10
lbs. without providing a brace above the weight.
In our brake example, that brace is the cable at the upper end of the
cantilever arm, which gains tension as the brake pad presses harder
against the rim regardless of HOW the increased force against the rim
is created. It is a balanced system- you can't arbitrarily change force
at one point without a corresponding change which keeps the system
balanced.

Now if the brake pad were directly in line between the cantilever post
and the rim instead of being in the middle a lever supported at one end
by the cantilever post and at the other by the cable it would be
possibly to have the post take all of the increased force, but as it
is, the cable MUST take some of it.

DR

Chalo Colina writes:

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.

For trucks, this may work, but if you are riding in mountains with
braking descents, you are engaging in a dangerous gamble because you
cannot predict what brake effect will result from your grip on the
lever. Do you descend curvy mountain roads. I was thinking of this
as I descended the Sierra this weekend on Ebbetts and Sonora Passes
where banking into turns at high speed while braking would be
disastrous with a servo brake.

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).

This isn't about brake force but rather control. The way you say that
I see only a truck braking in a straight line. I'm sure you have seen
the picture of descending while leaning at speed with two fingers on
both brakes. It is there that servo brakes are out of place, as was
the Campagnolo Delta brake that was not as hazardous as a servo brake.

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.

Stopping power and control are separate concepts. The servo brake has
miserable control.

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.

Don't use them unless you don't descend mountain roads... and even
then they aren't advisable for the reasons stated.

In article 42a48b10.0@entanet,
Zog The Undeniable wrote:

[...]

[1] I understand American Bendix brakes and European drum brakes were a
little different in their action; the degree of self-servo depends
whether the shoes are "leading" or "trailing" respective to their pivot
point.

Yeah, the drum brakes with a cam action instead have one pad that is
self-energizing, and another that is just the opposite. They feed each
other back and forth through the cam, so the self-energizing effect of
one pad cancels out the self-deenergizing effect in the other. And the
other way around. You wind up with a wash and the drum brakes behave
similarly to disc brakes.
Large vehicles like trucks and buses typically have drum brakes all
around, and they're cam style, so no real SE effect.

--
B.B. --I am not a goat! thegoat4 at airmail dot net
http://web2.airmail.net/thegoat4/