Brake pad efficiency

On Sat, 30 Jan 2016 14:04:56 -0800, Joe Riel wrote:

AMuzi writes:

On 1/30/2016 3:07 PM, Barry Beams wrote:
On Saturday, January 30, 2016 at 5:38:29 AM UTC-8, AMuzi wrote:
On 1/29/2016 9:20 PM, Jeff Liebermann wrote:
On Fri, 29 Jan 2016 17:44:48 -0800 (PST), wrote:

I've been thinking some more and realize I'm barking up the wrong tree a bit.
The rider's hand grip energy is not translated into the deceleration torque.
That's coming from the rotational energy in the wheel rubbing the pad.

Nope. The kinetic coefficient of friction between the disk/rim and
the pads requires that a force normal to the direction of the
frictional drag be applied. This normal force comes from the rider
squeezing the brake levers. The frictional drag force is proportional
to this normal force as:
Frictional_drag_force = Normal_force * kinetic_coeff_of_friction
http://www.engineeringtoolbox.com/friction-coefficients-d_778.html

Ignoring hystersis, deceleration, inertia, heating, ablation, filth,
dirt, grease, water, grip strength, cable stretch, and all the other
variables that make such calculations a pain, if one knows the normal
force, disk diameter, rim diameter, and coefficient of friction, one
should be able to obtain a ratio of the rotational force to the lever
grip force. I hope.

The method is partly outlined he
https://www.sensorprod.com/news/white-papers/2010-03_ctb/wp_ctb-2010-03.pdf
Notice the hysteresis in the Fig 10 (bottom of last page) and the lack
of linearity, which should make comparisons rather awkward.

I tried to think of a way to easily measure the braking forces on a
functional bicycle instead of a test stand. Something like a force
sensor on the brake cables and a speed sensor. Generate a graph of
speed versus time at various brake lever grip pressures. That should
produce families of curves, where the fastest rate of deceleration at
a given lever grip pressure is the most efficient.

I'm feeling rather lousy due to a cold or flu, so I'll spare you my
bad math and deranged ideas.




I think you want deceleration rate and not speed per se
although you could, given enough speed samples in time,
chart deceleration probably.

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

Discs also cause torque twist from the spokes winding up under load
which dampens some of the force the wheel transfers. Spokes stretch
and break sooner than on a rim brake wheel form all this extra
fatigue. Disc wheels may need more often trueing.

Taking the same dynamics as cars, discs and pads function better
when heated up, all will have an ideal operating temperature up to
the point of material failure. Rim brakes have greater heatsinking
because the greater mass of the rim absorbs the excess heat more
than a disc can. But carbon rims pose problems when the carbon +
binder resin ends up acting as an insulator.
There's no easy answer, the choice needs to be application specific.
I like my brake pads and am sticking with them for the foreseeable future. If I rode on mud and crud a lot, I might give discs a try.
Helmet side up,
Barry

PS. About trusting MTBR, please keep in mind they are an instrument, sometimes a puppet, of an advertising buying bicycle equipment industry. Francis Cebedo makes a ton of money from his ad sales and sponsors.
He is not above compromising his integrity when necessary.

Please see:
https://drive.google.com/folderview?...&usp=sha ring

for how MTBR deliberately misrepresented my Barry Beam versus a Light and Motion Taz. L&M advertises and provides testing facilities to MTBR.

If the exposures were identical, the background blue sky and city lights below should be identical in both pictures. But the Taz picture is at least three f-stops overexposed compared to my light.

If you envision three f-stops brighter exposure in your mind, then
you can see how my light easily will outshine a Taz if the test
conditions were the same. Francis also tilted my beam sideways and
mis-aimed it so it wouldn't light up the same light field as the Taz
is shining on.



MTBR? Acronym finder suggests 'mean time between ( various)'

Pretty sure he means Mountain Bike Review, www.mtbr.com.

I thought it was "Mean Time Between Rides", or perhaps "Repairs".
--

Cheers,

John B.

On Sun, 31 Jan 2016 01:53:27 +0000, Mike Causer
wrote:

On Fri, 29 Jan 2016 12:47:04 -0800 (PST)
wrote:

I've been looking at the difference between disk and rim brakes in
terms of efficiency (energy from hand to deceleration torque). It
looks like it depends upon friction losses in the delivery system, the
spring constant of the deliver system (how much the calipers are
[snip]


Short answer: If the radius of application causes 25% less "efficiency"
how come the pro peleton are worried about the better braking from disks
causing crashes?


Longer answer: You missed out that the travel of the pads is a lot less
than the travel of calliper pads. That makes the ratio between lever
movement and pad movement much greater for the same hand pressure.
Therefore for the same hand pressure the pads are pressing a lot harder
than pad for a rim brake. Pad material can be selected for higher
temperatures too. The constraints on braking are the coefficient of
friction between tyres and ground, and where the Centre of Gravity of
bicycle & rider combined is.




Various books, and calculations I've done, will tell you that the
maximum braking you can get on almost every bicycle on the road is
somewhere between 0.60 and 0.66 G. If the tyre/roadway friction (mu)
is lower than that you will get lower deceleration -- and should be
using the back brake as well as the front.

I'm sure that someone will now chime in that recumbents are much better,
but for a short-wheelbase it's not true. On mine the CoG has moved
forward enough to almost cancel out the lower position. A low-rider or
long wheelbase would be better than 0.66g, but I don't know by how much
because I haven't got my hands on one to measure up.


Mike

There you go, ruining my big chance for proving that My Brakes are
better then anyone!

Letting the cat out of the bag that the determining factor is simply
how well the tires stick to the road :-)
--

Cheers,

John B.

On 31/01/16 13:04, John B. wrote:
On Sun, 31 Jan 2016 09:54:19 +0100, Tosspot
wrote:

On 31/01/16 00:40, Joerg wrote:

snip

Long story short I am totally sold on disc brakes and will never look
back. Any bike I might buy in the future will have disc brakes.
Yeah, the hydraulics maintenance is a hassle but much less so than a
detonated rim because it wore through.

With oil filled hydraulic system the maintenance is less than cable
rims. The pads last longer, no replacing/lubing/capping cables.

Or an "Oh dang!" situation when trying to slow down with rim brakes
in the rain like I had to do on Thursday. Or that awful grinding
noise when slowing down on muddy turf where you can literally hear
the rim eat itself up.


Out of curiosity why do the pads last longer with a hydraulic system?

No idea. I used to get about 2,000 miles out of rim pads, more than
double that from hydraulic pads based on the fact I've not changed them yet!

On Saturday, January 30, 2016 at 9:12:48 PM UTC-8, Mike Causer wrote:
On Sat, 30 Jan 2016 19:30:47 -0800 (PST)
jbeattie wrote:

The pros are using hydraulic road disks, so all the assumptions about
cable drag are out the window as are the old lever/caliper ratios
(mechanical advantage).

Nope, take a look into the way hydraulic brakes work on motor vehicles.
For split F/R systems (without ABS natch) there are going to be
differences in the ratios of front and back master-slave cylinder
relationships. Or pressure limiters on the back, or all manner of
gizmos.

And I haven't mentioned "cable drag" whatever that may be.

That reference to cable drag goes back to the original post. Yes, hydraulics have ratios, too, but they are not subject to the same conventions as cable brakes (obviously). You have piston bore size, system pressure and a bunch of other things that affect the over-all power of the system. The OP took a pretty simple approach to comparing discs and rim brakes that doesn't really apply when you get into hydraulic discs.



If you operate a hydraulic road disc like an
old Campy NR brake, you'll throw yourself over the bars.

I only have one bike with disks: they're bottom of the line cable Avids
and work OK-ish, but I live in flat, very flat, country. And I have no
bikes with Campag brakes, mainly Weinmann or GB. And a couple with (hush
non-period) dual-pivot Tektro. Could you put your point into
engineering terms?

In human engineering terms, heavy hand pressure will lock up your front wheel using a hydraulic road disc brake. Depending on pad compound, the same pressure would be a pretty tame stop on an old NR brake.



Frankly, I don't see any reason for road discs unless you ride in the
rain and crap. That's where they shine.

Where I live "road" includes a lot of crap because it's a farming area,
and the tractors haul a lot of crap out of the fields onto the road.
And "road" includes a lot of off-road to follow the provided cycle
routes. No, it's not in Africa, it's England.

Another good thing about discs -- fender clearance.

-- Jay Beattie.

On 2016-01-31 00:54, Tosspot wrote:
On 31/01/16 00:40, Joerg wrote:

snip

Long story short I am totally sold on disc brakes and will never look
back. Any bike I might buy in the future will have disc brakes.
Yeah, the hydraulics maintenance is a hassle but much less so than a
detonated rim because it wore through.

With oil filled hydraulic system the maintenance is less than cable
rims. The pads last longer, no replacing/lubing/capping cables.


But bleeding DOT4 is nasty and I can't get a bleed kit with the extra
long 5mm nipple for my Promax Decipher brakes. So I do it by removing
the reservoir cap which is a bit of a kludge.

Anyhow, the operational advantages of hydraulic disc brakes are so great
that I won't complain about that.

What I found interesting was that the expensive ($15) pads from the
bike shops only last 500mi while the ceramic-based ones directly from
China for $3 last 1000mi. This is on a mountain bike that is heavily
used for XC.

--
Regards, Joerg

http://www.analogconsultants.com/

On 31/01/16 17:08, Joerg wrote:
On 2016-01-31 00:54, Tosspot wrote:
On 31/01/16 00:40, Joerg wrote:

snip

Long story short I am totally sold on disc brakes and will never look
back. Any bike I might buy in the future will have disc brakes.
Yeah, the hydraulics maintenance is a hassle but much less so than a
detonated rim because it wore through.

With oil filled hydraulic system the maintenance is less than cable
rims. The pads last longer, no replacing/lubing/capping cables.


But bleeding DOT4 is nasty and I can't get a bleed kit with the extra
long 5mm nipple for my Promax Decipher brakes. So I do it by removing
the reservoir cap which is a bit of a kludge.

Anyhow, the operational advantages of hydraulic disc brakes are so great
that I won't complain about that.

What I found interesting was that the expensive ($15) pads from the
bike shops only last 500mi while the ceramic-based ones directly from
China for $3 last 1000mi. This is on a mountain bike that is heavily
used for XC.

DOT fluid is a pita, because of the annual change, and imho, the
hygroscopic reasons for employing DOT fluid over oil are negligible for
a bike system. The performance reasons less so.

I'm daily commuting a couple of thousand miles a year over flat, but
sandy terrain so rim pads did not like wet conditions. Disc pads
probably don't either, but they last longer.

If I had my way I'd have hydraulic shifters as well, having just
replaced a cable because the end cap came off and it frayed.

On Sun, 31 Jan 2016 17:28:46 +0100
Tosspot wrote:

DOT fluid is a pita, because of the annual change, and imho, the
hygroscopic reasons for employing DOT fluid over oil are negligible for
a bike system.


Yup, I have yet to see bicycle disks glowing red[*], I've seen plenty
of automobile ones that way. The "hygroscopic" part is that DOT brake
fluid absorbs water over time and when heated the water turns to steam,
the steam expands and the brakes can longer apply the pressure to the
pad. Another failure mode in brakes is that the pad material has a
temperature range in which it works and above, or below, that does not.
For applications where the brakes are going to get really hot you have
to accept that they will be useless when cold and need warming up. The
converse is that the car you take to the supermarket needs instant-on
brakes when they're cold. The downside of that is that they maybe have
one stop in them from top speed then they're overheated and have to be
left to cool. Not what you want on a race-track really.


If I had my way I'd have hydraulic shifters as well, having just
replaced a cable because the end cap came off and it frayed.

Wouldn't it be simpler and cheaper to go electronic?

[*] I'll be looking for it at this year's TdF though.


Mike

On 2016-01-31 08:28, Tosspot wrote:
On 31/01/16 17:08, Joerg wrote:
On 2016-01-31 00:54, Tosspot wrote:
On 31/01/16 00:40, Joerg wrote:

snip

Long story short I am totally sold on disc brakes and will never look
back. Any bike I might buy in the future will have disc brakes.
Yeah, the hydraulics maintenance is a hassle but much less so than a
detonated rim because it wore through.

With oil filled hydraulic system the maintenance is less than cable
rims. The pads last longer, no replacing/lubing/capping cables.


But bleeding DOT4 is nasty and I can't get a bleed kit with the extra
long 5mm nipple for my Promax Decipher brakes. So I do it by removing
the reservoir cap which is a bit of a kludge.

Anyhow, the operational advantages of hydraulic disc brakes are so great
that I won't complain about that.

What I found interesting was that the expensive ($15) pads from the
bike shops only last 500mi while the ceramic-based ones directly from
China for $3 last 1000mi. This is on a mountain bike that is heavily
used for XC.

DOT fluid is a pita, because of the annual change, and imho, the
hygroscopic reasons for employing DOT fluid over oil are negligible for
a bike system. The performance reasons less so.


Mineral oil would sure be nicer but AFAIK my brakes do not like that.
Also, one must consider brake fade which is a dangerous condition with
hydraulic brakes. At least when offroad. A friend of mine had the front
fade out at the end of a long downhill. He managed to avoid a nasty
crash but probably only because he is an experienced dirt bike rider.


I'm daily commuting a couple of thousand miles a year over flat, but
sandy terrain so rim pads did not like wet conditions. Disc pads
probably don't either, but they last longer.


Disc do not care about the weather, in contrast to rim brakes they
always work. The only thing that happens is that they can sound like
stopping a big truck ... phseeee ... HOOOOOO ...and everybody turns around.


If I had my way I'd have hydraulic shifters as well, having just
replaced a cable because the end cap came off and it frayed.


No way :-)

--
Regards, Joerg

http://www.analogconsultants.com/

On 31/01/16 18:15, Mike Causer wrote:
On Sun, 31 Jan 2016 17:28:46 +0100 Tosspot
wrote:

snip

If I had my way I'd have hydraulic shifters as well, having just
replaced a cable because the end cap came off and it frayed.

Wouldn't it be simpler and cheaper to go electronic?

Then I'd have to worry about more bloody batteries. Have you seen my
collection of dead CR2032s!?

OTOH, the ability to put as many shifter switches/buttons anywhere you
want them might be attractive, other than an inadvertant shift while
scratching your bum :-)

On Sun, 31 Jan 2016 20:14:37 +0100
Tosspot wrote:

Wouldn't it be simpler and cheaper to go electronic?

Then I'd have to worry about more bloody batteries. Have you seen my
collection of dead CR2032s!?

Quicker to change a battery than bleed the hydraulics. I have a moto
which takes a couple of hours of actual labour in the garage and then
an overnight wait to do the front two. The wait has the lever strapped
to the bars. Why this gets the last air out I don't know, but it works.

[MikeC admits there's something he doesn't know! Film at 11!]


OTOH, the ability to put as many shifter switches/buttons anywhere you
want them might be attractive, other than an inadvertant shift while
scratching your bum :-)

Quite why you'd put a gearchange switch under your saddle....

.... ahh you're joining the Dark Side with an Under Seat Steering
'bent. Go for it!


Mike