Pedersen self energizing brakes.

[Dan Burkhart]

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?

 

[[email protected]]

On Mon, 6 Jun 2005 13:18:52 +1000, Dan Burkhart
<[email protected]> 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?

You answered one of your own questions here:

"How does moving the pads forward as the brake arms
rotate toward rim contact energize the brake?"

Look at this statement backwards - the brake arms rotate towards the
rim as they move forwards.

When the rider squeezes the lever, the brake blocks make contact with
the rim. Friction between the rim and the block means that the block
and its attached arm are dragged forward. The forward motion causes
the arms to move inwards somewhat, causing an increase in the force
holding the block againast the rim.

Other manufacturers produced similar designs - the only one I could
remember off the top of my head being Scott. Interestingly, the first
Google hit for "Scott self energizing brake" is a section on John
Forester's site relating to a lawsuit:
http://www.johnforester.com/Articles/BicycleEng/sebrake.htm

Hit No. 2 is Sheldon's site:
http://www.sheldonbrown.com/gloss_sa-o.html#self

As to they're being an answer to an unasked question, probably so.
So much "innovation" and "development" in bicycle tech seems to be a
marketing (rather than market) led drive to obsolete current
components in order to keep customers on the purchasing treadmill.

 

[Zog The Undeniable]

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?

I have a pair sitting around in the spares box. I never thought they
had any more power than regular brakes when I used them (they came with
a bike).

Anyway, there's a helix in there which applies the brake harder as the
pads are dragged forward by the rim.

 

[davidof]

Zog The Undeniable wrote:

> I have a pair sitting around in the spares box. I never thought they
> had any more power than regular brakes when I used them (they came with
> a bike).
>
> Anyway, there's a helix in there which applies the brake harder as the
> pads are dragged forward by the rim.

I have a pair made by Sun Tour - they were only for back brakes I think.
I replaced with V brakes which work better, although my old U-Brake on
my Muddy Fox is pretty good too.

 

[[email protected]]

the answer is!!!

http://www.vintagebike.co.uk/Bike Directories/Scott Bikes/pages/Scott Flying Squirrel 1929.htm

and the question is!!!

why does it not work??

 

[[email protected]]

check the Velocette Venom 500!!!

 

[Dirtroadie]

Zog The Undeniable wrote:

> Anyway, there's a helix in there which applies the brake harder as the
> pads are dragged forward by the rim.

OK, I understand that is the theory. What I don't grasp is how the
brake can be applied "harder" without a corresponding increase in the
force at the brake lever (through the cable), where the riders grip is
the only thing securing the brake lever.

Is this perhaps just a matter of anatomy where it is easier for the
hand to resist a force statically than it is to dynamically apply the
same force?

Am I missing something somewhere?

DR

 

[[email protected]]

Dirtroadie wrote:
> Zog The Undeniable wrote:
>
> > Anyway, there's a helix in there which applies the brake harder as the
> > pads are dragged forward by the rim.
>
> OK, I understand that is the theory. What I don't grasp is how the
> brake can be applied "harder" without a corresponding increase in the
> force at the brake lever (through the cable), where the riders grip is
> the only thing securing the brake lever.
>
> Is this perhaps just a matter of anatomy where it is easier for the
> hand to resist a force statically than it is to dynamically apply the
> same force?
>
> Am I missing something somewhere?
>
> DR

The rotating force of the wheel pulls the pad forward.. the mechanism
then forces the pads tighter into the rim..

they worked OK when installed correctly.. except on some bikes where
most of the extra force went into forcing the canti pegs outward (the
stays would twist) the GT triple triangle were the worst.
Adding a brake booster would fix the stay flex and then they worked
quite well (assuming they were installed correctly)

 

[David Damerell]

Quoting Zog The Undeniable <[email protected]>:
>I have a pair sitting around in the spares box. I never thought they
>had any more power than regular brakes when I used them (they came with
>a bike).

On a normal bike, of course, any decent rear brake will lock the rear
wheel and the SE cantilevers offer no advantage.

However, on a tandem the cable run is much longer and the potential
maximum rear braking is much greater because the centre of gravity is much
further behind the rear wheel but no higher off the ground. That's the
real use of these brakes.
--
David Damerell <[email protected]> flcl?
Today is First Gloucesterday, June.

 

[Dirtroadie]

[email protected] wrote:

> The rotating force of the wheel pulls the pad forward.. the mechanism
> then forces the pads tighter into the rim..
>
You are missing my point.
The leverage between the brake lever and the cantilever arm does not
change in any meaningful fashion once the brake pad is in contact with
the rim. If the brake shoe is pushed against the rim with more force by
the wedging action which results from the helical movement, something
must resist that force, i.e., the tension in the brake cable must
increase. If the tension in the cable increases then the force required
at the lever must also increase. The seemingly erroneous assumption is
that the cantilever arms are somehow immovably anchored.

Most comments about these brakes indicate that they seem to increase
braking performance. However for the reasons noted above the
explanation which is given does not entirely make sense in that
it suggests that "free" extra force is generated.

I have a set stashed away somewhere that I acquired for use on a tandem
but I have never used them (anybody want to make an offer?).

 

[[email protected]]

Dirtroadie wrote:
> [email protected] wrote:
>
> > The rotating force of the wheel pulls the pad forward.. the mechanism
> > then forces the pads tighter into the rim..
> >
> You are missing my point.
> The leverage between the brake lever and the cantilever arm does not
> change in any meaningful fashion once the brake pad is in contact with
> the rim. If the brake shoe is pushed against the rim with more force by
> the wedging action which results from the helical movement, something
> must resist that force, i.e., the tension in the brake cable must
> increase.

What is resisting the force is the spring mechanism in the canti
itself.. to some extent all you are doing with the brake lever is
positioning the pads in such a way as to allow the rotating force of
the wheel to engage the brake.. (not actually true.. there is sort of
a shared increase between the spring and the cable but after the
mechanism is engages the cable doesn't need to change in length of
pull).


If the tension in the cable increases then the force required
> at the lever must also increase. The seemingly erroneous assumption is
> that the cantilever arms are somehow immovably anchored.
>
> Most comments about these brakes indicate that they seem to increase
> braking performance. However for the reasons noted above the
> explanation which is given does not entirely make sense in that
> it suggests that "free" extra force is generated.

I agree nothing is free... the spring mechanism is using the force of
the wheels rotation to help cancel it.. that is where the "free" energy
comes from.


>
> I have a set stashed away somewhere that I acquired for use on a tandem
> but I have never used them (anybody want to make an offer?).

 

[Zog The Undeniable]

Dirtroadie wrote:

> OK, I understand that is the theory. What I don't grasp is how the
> brake can be applied "harder" without a corresponding increase in the
> force at the brake lever (through the cable), where the riders grip is
> the only thing securing the brake lever.
>
> Is this perhaps just a matter of anatomy where it is easier for the
> hand to resist a force statically than it is to dynamically apply the
> same force?
>
> Am I missing something somewhere?

Yes. The energy required to pull the pads in harder comes from rotation
of the wheel. It's like drum brakes on a car [1] - there's a "self
servo" effect where the brake pulls itself on harder after the initial
contact between drum and shoe.

I'm pretty sure that self-servo is the reason ABS on cars was initially
only offered with discs. It's quite hard to tune the feedback loop
properly when the brake has a "mind of its own". Nowadays the engineers
seem to have got round it though.

[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.

 

[Dirtroadie]

[email protected] wrote:

> now I'm guessing you are being sarcastic here.. but what heck I'll
> reply anyway..

Yes I am, but not in any vicious fashion. I'm just trying to get you
to see the point you seem to be missing.

> all the elastic is doing is rotating the canti around its post...just
> like the cable does..what you are missing is the fact that as the pad
> engages the rim the friction pulls the pad in the direction rotation of
> the wheel...that is added force .. some of that force is captured by
> the cam in the brake mechanism and used to drive the brake in to the
> rim..
> there needs to be enough force to engage the cam, you can't start with
> nothing so there needs to be some bias towards the rim but thats pretty
> much it..

Try this illustration.
This is a free pivoting horizontal lever with an anchored pivot at its
left end and a weight supported on the right end creating a downward
force D. The center of the the lever rests against a solid brace.
Presumably the lever exerts a downward force P (=2*D) against this
solid brace. And the solid brace exerts an equal upward force U = 2 * D
if everything is in equilibrium, which it is.

D
o-------P-------
[U^]

Now knowing that a wedge is a means of applying a substantial force, we
are going to forcefully drive a wedge between the center of the lever
and the solid brace.

D
o-------P-------
<wedge
[U^]

Once the wedge is driven (and the system is in equilibrium) how much
have we increased force P? We haven't - P must still equal 2*D since
the only force on the right end of the lever is still the force D.

Now if the right end were anchored instead of freely pivoting, it would
be a whole different story.

Similarly, providing a means of wedging a brake pad against a rim
cannot increase the force applied against the rim without increasing
the force at the end of the lever (cantilever arm).
DR

 

[[email protected]]

Dirtroadie wrote:
> [email protected] wrote:
>
> > now I'm guessing you are being sarcastic here.. but what heck I'll
> > reply anyway..
>
> Yes I am, but not in any vicious fashion. I'm just trying to get you
> to see the point you seem to be missing.
>
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.

 

[[email protected]]

Dan Burkhart writes:

> 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 cantilever. 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?

I think brake subjects on this NG easily become lost. We had a long
discussion about this a while back showing that a non-linear brake
(one that has a variable brake force to hand force ratio) is dangerous
and reduces control. It was for this reason that the Campagnolo Delta
brake was scrapped. It had a greatly variable mechanical advantage as
pads wore, even varying within brake pad compression. This caused
crashes when riders over-braked unintentionally.

The reason we don't use drum brakes on passenger cars any more is that
they are also "self-servo" (self activating) brakes. Those who recall
driving with them may recall that on foggy wet days, car brakes
occasionally locked solid with a light tough of the pedal. It is for
this non-linearity that we use disc brakes today even though disc
brakes have less heat capacity than drum brakes and why highway trucks
still use drums... often leaving long dual skid marks on highways.

A self-servo brake as you describe is useless because the user cannot
anticipate how much braking will occur from a given hand force. The
brake can lock up since it is its own application force multiplier.
The multiplier depends on the coefficient of friction of brake pads,
which most riders are aware, changes with temperature and humidity.
Because self servo brakes are unpredictable, you will not see a drum
brake on railways where a skid permanently damages wheels. When a RR
wheel skids, it loses traction as it glides on molten metal.

DON'T USE THIS BRAKE!

[email protected]

 

[Zog The Undeniable]

[email protected] wrote:

> DON'T USE THIS BRAKE!

If you try one you'll realise they're not very powerful anyway, so it's
nothing to worry about (in any case, a locked back wheel is rarely an
emergency).

My main objection to them, and why I don't use them any more, was the
horrible plain-stud pad mounting system. I use ordinary Tektro Oryx
cantilevers now.

St John Street Cycles (makers of Thorn bikes) insist they're the best
rear rim brake available, but I suspect Robin Thorn has the world's
remaining stock of SunTour SE in his warehouse and needs to shift them ;-)

 

[[email protected]]

wheel motion causes the drum's brake shoes to rotate outward against
the inner drum surface.
the shoes slide with the drum rotation and lever tighter. right?

 

[[email protected]]

Dan Burkhart writes:

> 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 cantilever. 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?

I think brake subjects on this NG easily become lost. We had a long
discussion about this a while back showing that a non-linear brake
(one that has a variable brake force to hand force ratio) is dangerous
and reduces control. It was for this reason that the Campagnolo Delta
brake was scrapped. It had a greatly variable mechanical advantage as
pads wore, even varying within brake pad compression. This caused
crashes when riders over-braked unintentionally.

The reason we don't use drum brakes on passenger cars any more is that
they are also "self-servo" (self activating) brakes. Those who recall
driving with them may recall that on foggy wet days, car brakes
occasionally locked solid with a light touch of the pedal. It is for
this non-linearity that we use disc brakes today even though disc
brakes have less heat capacity than drum brakes and why highway trucks
still use drums... often leaving long dual skid marks on highways.

A self-servo brake as you describe is useless because the user cannot
anticipate how much braking will occur from a given hand force. The
brake can lock up since it is its own application force multiplier.
The multiplier depends on the coefficient of friction of brake pads,
which most riders are aware, changes with temperature and humidity.
Because self servo brakes are unpredictable, you will not see a drum
brake on railways where a skid permanently damages wheels. When a RR
wheel skids, it loses traction as it glides on molten metal.

DON'T USE THIS BRAKE!

[email protected]

 

[[email protected]]

your reflexes are shot JB

 

[Dirtroadie]

[email protected] 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