Progressive brakes?

I frequently read road tests or component tests that talk approvingly
about how nicely "progressive" the brakes are.

I have trouble with this concept. To me, that means the more force I
apply to the levers, the more torque the brakes apply to the wheel,
with the relationship between force and brake torque being not greatly
different from direct proportion. It seems to me that would be very
normal. And while I don't have much of the connoisseur in me, I'd
think that most deviations from that ideal would be small and hard to
detect.

(Of course, I also want sufficient stopping force, I don't want brakes
to drag when released, I don't want to lose all braking when wet, etc
- but those have nothing to do with that "progressive" adjective.)

So, absent real oddities like parallelogram linkages (ala Campy Delta
brakes), coaster brakes or band brakes, what would make bike brakes
_not_ be progressive? Seems to me friction in the cable would be the
main thing, followed by friction in the pivots. But cables are not
part of a brake set, and I'd think pivot friction wouldn't vary
much.

So besides those sources of friction... what matters?

BTW, almost all my bikes have cantilever brakes of one type or
another. They all seem "progressive" to me.

- Frank Krygowski

Frank Krygowski wrote:
I frequently read road tests or component tests that talk approvingly
about how nicely "progressive" the brakes are.

I have trouble with this concept. To me, that means the more force I
apply to the levers, the more torque the brakes apply to the wheel,
with the relationship between force and brake torque being not greatly
different from direct proportion. It seems to me that would be very
normal. And while I don't have much of the connoisseur in me, I'd
think that most deviations from that ideal would be small and hard to
detect.

(Of course, I also want sufficient stopping force, I don't want brakes
to drag when released, I don't want to lose all braking when wet, etc
- but those have nothing to do with that "progressive" adjective.)

So, absent real oddities like parallelogram linkages (ala Campy Delta
brakes), coaster brakes or band brakes, what would make bike brakes
_not_ be progressive? Seems to me friction in the cable would be the
main thing, followed by friction in the pivots. But cables are not
part of a brake set, and I'd think pivot friction wouldn't vary
much.

So besides those sources of friction... what matters?

BTW, almost all my bikes have cantilever brakes of one type or
another. They all seem "progressive" to me.

- Frank Krygowski


Add that to the "carve through corners" phrasebook.

--
Andrew Muzi
www.yellowjersey.org/
Open every day since 1 April, 1971

On 10/2/2010 12:47 AM, A. Muzi wrote:
Frank Krygowski wrote:
[...]
BTW, almost all my bikes have cantilever brakes of one type or
another. They all seem "progressive" to me.

- Frank Krygowski


Add that to the "carve through corners" phrasebook.


I watched a guy carve through a corner today on a CAT D-4 with an
articulated blade.

--
Tom Sherman - 42.435731,-83.985007
I am a vehicular cyclist.

On 02/10/10 05:39, Jobst Brandt wrote:

thread highjack

The progression is self defeating because it is not normal to the
braking surface and has other drawbacks that are killers. Cantilever
brakes have such serious cosine error that they pop under the braking
surface into the spokes, never to return, while Dual pivot have cosine
error that sweeps the pads into the tire for a blowout.

Stick with the centrally pivoted caliper brake and the side pull lever
mechanism that have close to zero cosine error.

Jobst Brandt

I saw a set of Weinmanns yesterday. Normal boss mount, but looked like
hydraulic pipe going to them. Brake actuation caused some parts of the
brake to rotate as the pads moved in. Couldn't get the number, they
might have been PBS3000's, but they looked very new. Any ideas?

Oh, it was the cosine error that reminded me btw.

On Oct 2, 12:39*am, Jobst Brandt wrote:
Frank Krygowski wrote:


So besides those sources of friction... what matters?

Change in mechanical advantage and self servo, both are serious
intruders in braking.

I understand. Self servo, especially, would make braking much
different from straight "progressive" - that is, it would make brake
torque far from proportional to applied lever force. And it would
make braking action inconsistent, dependent on brake shoe friction
coefficient. But self-servo is very rare in bike brakes.

Yet in comparisons or road tests in Buycycling and other enthusiast
magazines, I find statements implying that (perhaps) Paul cantilevers
are more progressive and controllable than (say) Shimano cantilevers.
Or than Shimano dual pivot brakes. They're not talking about any self-
servo action in either brake type.

I'm left to assume that only pivot friction really matters - that
absent rolling element bearings at various pivots, a small amount of
stick-slip friction is possible, and that this might effect response.
Consequently, I assume that brakes with similar pivots will have
similar "progressiveness" or controllability.

So is it entirely, or almost entirely, marketing hype?

- Frank Krygowski

Op 2-10-2010 17:43, Frank Krygowski schreef:
On Oct 2, 12:39 am, Jobst wrote:
Frank Krygowski wrote:


So besides those sources of friction... what matters?

Change in mechanical advantage and self servo, both are serious
intruders in braking.

I understand. Self servo, especially, would make braking much
different from straight "progressive" - that is, it would make brake
torque far from proportional to applied lever force. And it would
make braking action inconsistent, dependent on brake shoe friction
coefficient. But self-servo is very rare in bike brakes.

Yet in comparisons or road tests in Buycycling and other enthusiast
magazines, I find statements implying that (perhaps) Paul cantilevers
are more progressive and controllable than (say) Shimano cantilevers.
Or than Shimano dual pivot brakes. They're not talking about any self-
servo action in either brake type.

I'm left to assume that only pivot friction really matters - that
absent rolling element bearings at various pivots, a small amount of
stick-slip friction is possible, and that this might effect response.
Consequently, I assume that brakes with similar pivots will have
similar "progressiveness" or controllability.

What do you think of stiffness?


So is it entirely, or almost entirely, marketing hype?

I use on several bikes, lets see,

- dual pivot brakes,
- single pivot brakes,
- mechanical diskbrakes,
- hydraulic dis brakes,
- V-brakes,
- Magura hydraulic rim brakes,

and they all have a different feeling and 'progressiveness' so no it is
entirely marketing hype IMO.

Single pivot brakes on my ATB's? No thanks Jobs.

Lou

On Oct 1, 8:57*pm, Frank Krygowski wrote:
I frequently read road tests or component tests that talk approvingly
about how nicely "progressive" the brakes are.

I have trouble with this concept. *To me, that means the more force I
apply to the levers, the more torque the brakes apply to the wheel,
with the relationship between force and brake torque being not greatly
different from direct proportion. *It seems to me that would be very
normal. *And while I don't have much of the connoisseur in me, I'd
think that most deviations from that ideal would be small and hard to
detect.

(Of course, I also want sufficient stopping force, I don't want brakes
to drag when released, I don't want to lose all braking when wet, etc
- but those have nothing to do with that "progressive" adjective.)

So, absent real oddities like parallelogram linkages (ala Campy Delta
brakes), coaster brakes or band brakes, what would make bike brakes
_not_ be progressive? *Seems to me friction in the cable would be the
main thing, followed by friction in the pivots. *But cables are not
part of a brake set, and I'd think pivot friction wouldn't vary
much.

So besides those sources of friction... what matters?

BTW, almost all my bikes have cantilever brakes of one type or
another. *They all seem "progressive" to me.

- Frank Krygowski

Brake forces produce a torque that tilts the the brake pads out of
alignment to the rim, a contributor to less-than-linear brake
performance, shudder, etc. For cantilever brakes, the stiffness of the
bosses, fork and seatstays, and the particular geometry of the brake
(which, independently of mechanical advantage, affects the amount of
levering-out which you can feel if you wrap your hand around your seat
stays then apply the brake). Misalignment reduces the contact area
between brake shoe and rim, reducing performance.

I already hear the objection brewing from somewhere, isn't frictional
force just the product of a coefficient of friction and the normal
force, and not a function of contact area? This is the first model of
friction that many people are introduced to -- but it's a model that
tends not to be true for compliant materials, such as bicycle brake
shoes are made of. For compliant materials, the _real_ (i.e.
microscopic) area of contact saturates at a relatively low pressure,
so that the amount of available friction at a given normal force
becomes dependent on (macroscopic) area of contact.

http://depts.washington.edu/nanolab/...0Tribology.htm

-pm

Jobst, you need to occassionally look at a bicycle..;-) The angle of
a cantilever is more or less the same as a center pivot side-pull.
There is, however, an r-multiplier...

Per Frank Krygowski:
I have trouble with this concept. To me, that means the more force I
apply to the levers, the more torque the brakes apply to the wheel,
with the relationship between force and brake torque being not greatly
different from direct proportion. It seems to me that would be very
normal. And while I don't have much of the connoisseur in me, I'd
think that most deviations from that ideal would be small and hard to
detect.

Maybe they're talking about granularity or "modulation".

Going down a steep hill on loose gravel, I can play my disc front
brake without locking it up and losing control. I think I would
have trouble doing that with at least some rim brakes.
--
PeteCresswell

On Oct 2, 5:39*am, Jobst Brandt wrote:

Stick with the centrally pivoted caliper brake and the side pull lever
mechanism that have close to zero cosine error.

Jobst Brandt

Or buy hydraulic rim brakes to shortcut all these problems and gain
perfect predictability.

I must confess, when I first saw Krygowski's post I wondered what sort
of newbie or cheapskate or incompetent he might be. Surely the problem
with bicycle brakes is to avoid being thrown headfirst over the
handlebars, except for those too cheap to buy brakes that work, and
those too cackhanded to change brake blocks and make simple
adjustments.

Andre Jute
Feed a tree today, produce more CO2!