Re: Which Brakes? Avid Mechanical Disc? Magura Big or Marta HydraulicDisc?

[Peter Clinch]

Ace wrote:

> Known as a "terrain trap". Hope they weren't hurt.

Not terminally, but one heel smashed into enough pieces to never be
quite right again and a leg broken well enough to acquire some bonus
titanium :-(

I ended up helping the MRT a year later where the same thing had
happened to a group in exactly the same place (Cinderella Gully on Craeg
Smeggy) just before I was passing, which was a broken ankle on one and
chipped vertebrae on another.

Pete.
--
Peter Clinch Medical Physics IT Officer
Tel 44 1382 660111 ext. 33637 Univ. of Dundee, Ninewells Hospital
Fax 44 1382 640177 Dundee DD1 9SY Scotland UK
net [email protected] http://www.dundee.ac.uk/~pjclinch/

 

[Marz]

On Sep 4, 2:26 pm, Tony Raven <[email protected]> wrote:
> Marz <[email protected]> wrote innews:[email protected]:
>
>
>
>
>
> > On Sep 4, 3:01 am, Tony Raven <[email protected]> wrote:
> >> Paul Boyd <usenet.is.worse@plusnet> wrote
> >> innews:[email protected]:
>
> >> > Less likely on a
> >> > tandem, but on an MTB pointing down a 60 degree loose slope you
> >> > need very precise braking control
>
> >> <pedant>
>
> >> You can't have a 60 degree loose slope - its beyond the maximum angle
> >> of repose for loose material.
>
> >> </pedant>
>
> >> --
> >> Tony
>
> >> " I would never die for my beliefs because I might be wrong."
> >> Bertrand Russell
>
> > You can very easily have a 60 degree slope, rock face, rooty sections,
> > washed out river banks, but it's not something you're ever going to
> > attempt to brake on. When things get that steep all you're hoping to
> > do is roll it out without crashing.
>
> But it can't be a loose slope - the loose material will just roll off it
> if the slope is steeper than the maximum angle of repose which is
> typically IIRC around 30-40 degrees.
>
> --
> Tony
>
> " I would never die for my beliefs because I might be wrong."
> Bertrand Russell- Hide quoted text -
>
> - Show quoted text -

Ahh, 'loose' slope. I should read every word and yes there is a limit
to the slope of loose material.

 

[David Damerell]

Quoting Roger Merriman <[email protected]>:
>David Damerell <[email protected]> wrote:
>>V-brakes (alone) on a tandem have the difficulty that they will blow off
>>tyres, so they are not without issue.
>if they are not with out issue they why are tamdems being sold with
>them? and very pricey ones at that?

You will find those are not intended for anything like loaded touring
or additionally have a drum or disc [1] brake intended for use as a
drag brake. For example, a racing tandem doesn't have these issues.

>>If you can't lift the rear wheel, adjust your brakes properly. Note that a
>>downhill gradient makes this easier, not harder.
>yes if don't shift my weight back, but shifting ones weight back, can be
>very effective.

Wrong again. Regardless of shifting weight, a downhill gradient makes it
easier to lift the rear wheel.

[1] optimistically.
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[David Damerell]

Quoting Peter Clinch <[email protected]>:
>David Damerell wrote:
>>No, there hasn't. With a well adjusted cable rim brake you can lift the
>>rear wheel (or skid the front on a poor surface). How can a hydraulic disc
>>offer more braking than that? It can't.
>*** isn't the *more* it offers you from locking,

You have excised the quoted context where I say;

"Because mountain bikes can't actually use any more braking than any other
solo bike" and Roger Merriman disputes that specific claim.

I know you want to leap back on the fingertips hobbyhorse but that's not
what I'm talking about there.
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[David Damerell]

Quoting Ben C <[email protected]>:
>On 2007-09-04, David Damerell <[email protected]> wrote:
>>No, there hasn't. With a well adjusted cable rim brake you can lift the
>>rear wheel (or skid the front on a poor surface). How can a hydraulic disc
>>offer more braking than that?
>Easily, it might have better power dissipation and therefore not
>overheat and become useless after prolonged and/or repeated use.

Right. And what's got more heat capacity, a small metal disc or a large
metal rim? What's got more surface area?

Read the earlier thread; we started talking about tandems, where
overheating actually is an issue.
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[David Damerell]

Quoting Roger Merriman <[email protected]>:
>David Damerell <[email protected]> wrote:
>>No, there hasn't. With a well adjusted cable rim brake you can lift the
>>rear wheel (or skid the front on a poor surface). How can a hydraulic disc
>>offer more braking than that? It can't.
>one at lower speeds it offers more control ie your not just janking on
>the lever.

If you're just "yanking on the lever" with a well-adjusted cable rim
brake, that's your poor technique.

>2nd at higher speeds you will have a job to lock the wheel, unless you
>delbertly unweight a wheel etc.

This one won't wash with anyone who has made emergency stops downhill on
road. We might not brake as much, but the speeds are higher.
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[David Damerell]

Quoting Peter Clinch <[email protected]>:
>David Damerell wrote:
>>Yes, an unmaintained brake can be dangerous. Not a new fact.
>You've gloriously and spectacularly missed the fact that Clive's
>hydraulic brakes haven't been maintained for the precise reason that
>they haven't /needed/ maintaining in order not to degrade unacceptably,

Well, no. There's a dodge going on here where the real advantage of not
needing as much maintenance is twisted into better performance (because
the performance is better when unmaintained). That's double accounting; if
you're going to argue based on the unmaintained performance, you can't
argue that they need less maintenance.
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[Ben C]

On 2007-09-05, David Damerell <[email protected]> wrote:
> Quoting Ben C <[email protected]>:
>>On 2007-09-04, David Damerell <[email protected]> wrote:
>>>No, there hasn't. With a well adjusted cable rim brake you can lift the
>>>rear wheel (or skid the front on a poor surface). How can a hydraulic disc
>>>offer more braking than that?
>>Easily, it might have better power dissipation and therefore not
>>overheat and become useless after prolonged and/or repeated use.
>
> Right. And what's got more heat capacity, a small metal disc or a large
> metal rim?

Does heat capacity help much?

I suppose I could attempt the math...

Aluminium has an SHC of 0.897 J/gK according to Wikipedia.

A rim weighs about 500g. So how much energy to raise its temperature
from 20C to 100C, on the assumption that 100C is very hot for a bicycle
rim (is that figure about right?)

500 * 0.897 * 80 = 35880J

So 35880J can be soaked up in the rim before it gets to 100C. That
corresponds to what change in speed of a 100kg bike+rider?

E = 0.5*m*v^2
sqrt(2E/m) = v
sqrt((2 * 35880) / 100) = 27m/s, or 97kph.

This seems to be saying that even if the rim doesn't manage to lose
_anything_ to the air, you can still brake to a stop from 97kph (using
only one brake) and not get the rim hotter than 100C.

Of course this is cumulative, so ten 10kph retardations, with no loss to
the air, would cause the same buildup.

I think this probably explains why dissipation to the air isn't terribly
significant for one-off emergency stops, but is for repeated braking
during a long descent.

Anyway, I couldn't see steel's SHC on Wikipedia but some other website
suggested it might be around 0.5 J/gK but it depends on the kind of
steel. A disk weighs around 170g.

170 * 0.5 * 80 = 6800J
sqrt((2 * 6800) / 100) = 12m/s or 43kph

So quite a bit worse it would seem. That's only four 10kph retardations
before the disk reaches 100C (assuming zero dissipation to the air).

> What's got more surface area?

In the case of bicycle disks, a rim I should think, since the disks are
usually so filigreed and of course have smaller diameter.

I'm starting to wonder why disk overheating isn't a big problem with the
lower heat capacity and probably lower dissipation rate to the air.
Perhaps the operating temperature range of a disk is higher as I
suggested earlier. I pulled this figure of 100C out of the air as "high"
for a rim brake, but perhaps they don't really get that hot. A disk on
the other hand can probably afford to get a bit hotter than that-- the
fluid won't boil until perhaps 170C. If the disk is hotter then its
dissipation to the air will be better (bigger temperature gradient).

 

[Clive George]

"Ben C" <[email protected]> wrote in message
news:[email protected]...

> I'm starting to wonder why disk overheating isn't a big problem with the
> lower heat capacity and probably lower dissipation rate to the air.
> Perhaps the operating temperature range of a disk is higher as I
> suggested earlier. I pulled this figure of 100C out of the air as "high"
> for a rim brake, but perhaps they don't really get that hot. A disk on
> the other hand can probably afford to get a bit hotter than that-- the
> fluid won't boil until perhaps 170C. If the disk is hotter then its
> dissipation to the air will be better (bigger temperature gradient).

Yup, that's pretty much it. It's quite easy to get a drum brake to the
temperature where water merrily boils away when sprayed onto it - discs
aren't that different. Can't remember if I've got rims to that point -
definitely too hot to touch, but a precise answer is harder.

cheers,
clive

 

[David Damerell]

Quoting Ben C <[email protected]>:
>I think this probably explains why dissipation to the air isn't terribly
>significant for one-off emergency stops, but is for repeated braking
>during a long descent.

So far, so good.

>Anyway, I couldn't see steel's SHC on Wikipedia but some other website
>suggested it might be around 0.5 J/gK but it depends on the kind of
>steel. A disk weighs around 170g.

I must admit, shamefully, I quite forgot the question of which material
we were using.

>I'm starting to wonder why disk overheating isn't a big problem with the
>lower heat capacity and probably lower dissipation rate to the air.
>Perhaps the operating temperature range of a disk is higher as I
>suggested earlier.

That's certainly true, but we're conflating two issues here. It's fair to
say that discs don't suffer as badly from overheating in the sense that a
catastrophic failure will occur at a much higher temperature, so much
higher that (I guess) this more than offsets the effect of their lower
heat capacity. But the question asked was about brake fade - and, given
that disc pads aren't made of anything magic, if anything they'll suffer
_that_ first because they get hotter faster.

[Of course, they don't, but I think that's because brake fade with heat is
not a serious issue on either type of brake, as opposed to drum brakes...]
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[_]

On Wed, 5 Sep 2007 19:23:37 +0100, Clive George wrote:

> "Ben C" <[email protected]> wrote in message
> news:[email protected]...
>
>> I'm starting to wonder why disk overheating isn't a big problem with the
>> lower heat capacity and probably lower dissipation rate to the air.
>> Perhaps the operating temperature range of a disk is higher as I
>> suggested earlier. I pulled this figure of 100C out of the air as "high"
>> for a rim brake, but perhaps they don't really get that hot. A disk on
>> the other hand can probably afford to get a bit hotter than that-- the
>> fluid won't boil until perhaps 170C. If the disk is hotter then its
>> dissipation to the air will be better (bigger temperature gradient).
>
> Yup, that's pretty much it. It's quite easy to get a drum brake to the
> temperature where water merrily boils away when sprayed onto it - discs
> aren't that different. Can't remember if I've got rims to that point -
> definitely too hot to touch, but a precise answer is harder.
>


There is research out there on the inter-webby-thing showing rim brakes
reaching something like 175-200 degrees farrenheit, and coaster-brakes
reaching (depending on the part measured) around 800 degrees.

The enrgy has to go somewhere, and that somewhere eventually is the air
around the wheel. The pad and surface heat up; the heat is transmitted to
the air. The rates of heating and of tranmission determine the temperture
reached - a larger dissipative area, all else being equal, will reach a
lower temperature.

A typical cycle rim has around 150 inches of area exposed to the air. How
much area does a typical hub-brake-disc have?

 

[Ben C]

On 2007-09-05, _ <[email protected]> wrote:
> On Wed, 5 Sep 2007 19:23:37 +0100, Clive George wrote:
>
>> "Ben C" <[email protected]> wrote in message
>> news:[email protected]...
>>
>>> I'm starting to wonder why disk overheating isn't a big problem with the
>>> lower heat capacity and probably lower dissipation rate to the air.
>>> Perhaps the operating temperature range of a disk is higher as I
>>> suggested earlier. I pulled this figure of 100C out of the air as "high"
>>> for a rim brake, but perhaps they don't really get that hot. A disk on
>>> the other hand can probably afford to get a bit hotter than that-- the
>>> fluid won't boil until perhaps 170C. If the disk is hotter then its
>>> dissipation to the air will be better (bigger temperature gradient).
>>
>> Yup, that's pretty much it. It's quite easy to get a drum brake to the
>> temperature where water merrily boils away when sprayed onto it - discs
>> aren't that different. Can't remember if I've got rims to that point -
>> definitely too hot to touch, but a precise answer is harder.
>>
>
> There is research out there on the inter-webby-thing showing rim brakes
> reaching something like 175-200 degrees farrenheit, and coaster-brakes
> reaching (depending on the part measured) around 800 degrees.
>
> The enrgy has to go somewhere, and that somewhere eventually is the air
> around the wheel. The pad and surface heat up; the heat is transmitted to
> the air. The rates of heating and of tranmission determine the temperture
> reached - a larger dissipative area, all else being equal, will reach a
> lower temperature.

In a given amount of time-- eventually they will both reach the
temperature of the surrounding air.

But the other point is that the greater the temperature gradient, the
greater the rate of dissipation.

I think rate of dissipation is proportional both to temperature
difference and to area.

http://www.spiraxsarco.com/resource...rinciples-and-heat-transfer/heat-transfer.asp

> A typical cycle rim has around 150 inches of area exposed to the air. How
> much area does a typical hub-brake-disc have?

It's about 1/4 the radius, and I think therefore about 1/4 the area
because in both cases the brake track area is something like
circumference * a few cm.

If the disk was a complete disk it would be more, but most of them
consist of a slender spider's web arrangement supporting a sort of
"rim". Perhaps that rim is a bit deeper than the wheel rim, so there
could be as much as 1/2 the area. And the spider's web bit will make a
small contribution.

I wonder if the spider's web is also intended to insulate the hot disk
from the hub where it would start cooking the grease. That and to look
cool and like it's saving a few grams (which also reduces the
effectiveness as a heat sink).

Anyway, if the disk is 1/4 the area of a rim but operates at four times
the temperature it ought to dissipate at the same rate.

 

[_]

On Wed, 05 Sep 2007 15:08:53 -0500, Ben C wrote:

> On 2007-09-05, _ <[email protected]> wrote:
>> On Wed, 5 Sep 2007 19:23:37 +0100, Clive George wrote:
>>
>>> "Ben C" <[email protected]> wrote in message
>>> news:[email protected]...
>>>
>>>> I'm starting to wonder why disk overheating isn't a big problem with the
>>>> lower heat capacity and probably lower dissipation rate to the air.
>>>> Perhaps the operating temperature range of a disk is higher as I
>>>> suggested earlier. I pulled this figure of 100C out of the air as "high"
>>>> for a rim brake, but perhaps they don't really get that hot. A disk on
>>>> the other hand can probably afford to get a bit hotter than that-- the
>>>> fluid won't boil until perhaps 170C. If the disk is hotter then its
>>>> dissipation to the air will be better (bigger temperature gradient).
>>>
>>> Yup, that's pretty much it. It's quite easy to get a drum brake to the
>>> temperature where water merrily boils away when sprayed onto it - discs
>>> aren't that different. Can't remember if I've got rims to that point -
>>> definitely too hot to touch, but a precise answer is harder.
>>>
>>
>> There is research out there on the inter-webby-thing showing rim brakes
>> reaching something like 175-200 degrees farrenheit, and coaster-brakes
>> reaching (depending on the part measured) around 800 degrees.
>>
>> The enrgy has to go somewhere, and that somewhere eventually is the air
>> around the wheel. The pad and surface heat up; the heat is transmitted to
>> the air. The rates of heating and of tranmission determine the temperture
>> reached - a larger dissipative area, all else being equal, will reach a
>> lower temperature.
>
> In a given amount of time-- eventually they will both reach the
> temperature of the surrounding air.
>
> But the other point is that the greater the temperature gradient, the
> greater the rate of dissipation.
>
> I think rate of dissipation is proportional both to temperature
> difference and to area.
>
> http://www.spiraxsarco.com/resource...rinciples-and-heat-transfer/heat-transfer.asp
>
>> A typical cycle rim has around 150 inches of area exposed to the air. How
>> much area does a typical hub-brake-disc have?
>
> It's about 1/4 the radius, and I think therefore about 1/4 the area
> because in both cases the brake track area is something like
> circumference * a few cm.

On a cycle rim you also have the rest of the rim contiguous with the
braking track - giving a much larger dissipative area.

 

[Alan Braggins]

In article <4lA*[email protected]>, David Damerell wrote:
>heat capacity. But the question asked was about brake fade - and, given
>that disc pads aren't made of anything magic, if anything they'll suffer
>_that_ first because they get hotter faster.

They might not be magic, but nor are they they same material as
rim brake pads, so they could still resist fade better.

 

[Peter Clinch]

David Damerell wrote:

> Well, no. There's a dodge going on here where the real advantage of not
> needing as much maintenance is twisted into better performance (because
> the performance is better when unmaintained). That's double accounting; if
> you're going to argue based on the unmaintained performance, you can't
> argue that they need less maintenance.

Two things:

1) maintenance is an improved feature

2) performance, via better control through more sensitivity, is an
improved feature.

It's in the books as two things because it's two things. The
maintenance angle is better, and the performance is better, quite
independently. But if you don't do all the maintenance on a mech cable
then the hydraulic performs much, much better, rather than merely a
(tangibly) bit better.

Pete.
--
Peter Clinch Medical Physics IT Officer
Tel 44 1382 660111 ext. 33637 Univ. of Dundee, Ninewells Hospital
Fax 44 1382 640177 Dundee DD1 9SY Scotland UK
net [email protected] http://www.dundee.ac.uk/~pjclinch/

 

[Dave Larrington]

In news:[email protected],
Ben C <[email protected]> tweaked the Babbage-Engine to tell us:

> I'm starting to wonder why disk overheating isn't a big problem with
> the lower heat capacity and probably lower dissipation rate to the
> air. Perhaps the operating temperature range of a disk is higher as I
> suggested earlier. I pulled this figure of 100C out of the air as
> "high" for a rim brake, but perhaps they don't really get that hot. A
> disk on the other hand can probably afford to get a bit hotter than
> that-- the fluid won't boil until perhaps 170C.

A bit of googling suggests that the boiling point of DOT5.1 brake fluid is
about 260 deg. C. The mineral oils used in Magura systems, and LHM used by
the parsimonious instead of the Magura stuff, seems to be in the same
region.

--
Dave Larrington
<http://www.legslarry.beerdrinkers.co.uk>
Hoc ardur vincere docet.

 

[Roger Merriman]

David Damerell <[email protected]> wrote:

> Quoting Roger Merriman <[email protected]>:
> >David Damerell <[email protected]> wrote:
> >>V-brakes (alone) on a tandem have the difficulty that they will blow off
> >>tyres, so they are not without issue.
> >if they are not with out issue they why are tamdems being sold with
> >them? and very pricey ones at that?
>
> You will find those are not intended for anything like loaded touring
> or additionally have a drum or disc [1] brake intended for use as a
> drag brake. For example, a racing tandem doesn't have these issues.
>
maybe so but seems plenty about. which seems a odd saving when the
tamdem can be the price of a new car.

but yes from what you've said (and others) and just plain comman sence.
ie how well would those tamdems cope with two large men let alone their
gear...warm i guess at best.

i can see that Santana have spent a lot of time and effort into tamdem
brakes. consdering the possible weight, that still looks quite under
powered when you consider the weights.

> >>If you can't lift the rear wheel, adjust your brakes properly. Note that a
> >>downhill gradient makes this easier, not harder.
> >yes if don't shift my weight back, but shifting ones weight back, can be
> >very effective.
>
> Wrong again. Regardless of shifting weight, a downhill gradient makes it
> easier to lift the rear wheel.
>
if you don't shift your weight about yes, the greatest possiblity for a
endo is the emergency stop on the flat as you will be luckly to beable
to shift your weight fully, in the time it takes to stop.

decending a hill even a hill over 30% at least on a mountain bike you
can get a lot of weight over the rear, if fact going the other way i can
un weigh the rear enought to start to scrabble on the tarmack. i do
though acknowedge that one growing up in a steep sided gorge i'm used to
shifting one's weight around for hills, and that a mountain bike is
easyer to do that than on road bike.

> [1] optimistically.

roger
--
www.rogermerriman.com

 

[Roger Merriman]

David Damerell <[email protected]> wrote:

> Quoting Roger Merriman <[email protected]>:
> >David Damerell <[email protected]> wrote:
> >>No, there hasn't. With a well adjusted cable rim brake you can lift the
> >>rear wheel (or skid the front on a poor surface). How can a hydraulic disc
> >>offer more braking than that? It can't.
> >one at lower speeds it offers more control ie your not just janking on
> >the lever.
>
> If you're just "yanking on the lever" with a well-adjusted cable rim
> brake, that's your poor technique.
>
no it is easyer to make a controlled emergency stop if the lever
requires less force.

> >2nd at higher speeds you will have a job to lock the wheel, unless you
> >delbertly unweight a wheel etc.
>
> This one won't wash with anyone who has made emergency stops downhill on
> road. We might not brake as much, but the speeds are higher.

if your locking managing to lock the wheels at high speeds in the dry
etc then you either have tires with not enought grip or are not
countering with your weight. which a hill would help with.

i use my old mounatin bike on road, with road tires yes they have lot
more of footprint on the road, and i'm used to shifting weight. only the
other week i had to do a emergency stop down hill, no i didn't lock the
tires, i came close there was bit of slewing but not attaul locking.

roger
--
www.rogermerriman.com

 

[Ben C]

On 2007-09-06, Dave Larrington <[email protected]> wrote:
> In news:[email protected],
> Ben C <[email protected]> tweaked the Babbage-Engine to tell us:
>
>> I'm starting to wonder why disk overheating isn't a big problem with
>> the lower heat capacity and probably lower dissipation rate to the
>> air. Perhaps the operating temperature range of a disk is higher as I
>> suggested earlier. I pulled this figure of 100C out of the air as
>> "high" for a rim brake, but perhaps they don't really get that hot. A
>> disk on the other hand can probably afford to get a bit hotter than
>> that-- the fluid won't boil until perhaps 170C.
>
> A bit of googling suggests that the boiling point of DOT5.1 brake fluid is
> about 260 deg. C. The mineral oils used in Magura systems, and LHM used by
> the parsimonious instead of the Magura stuff, seems to be in the same
> region.

Yes, although I also got that "perhaps 170C" from a bit of googling. The
page I found made a distinction between "wet" and "dry" boiling points,
"wet" being what you get when your brake fluid has absorbed 3% of water.

Here it is: http://www.afcoracing.com/tech_pages/fluid.shtml

I went for the "wet" boiling point of DOT 4 since I heard somewhere else
that bicycle brake fluid boils much more easily than the car stuff
(although it strips the paint less).

Brake fluid should never boil in a car unless it's very old. You should
be able to get the pads practically on fire before the fluid boils. But
I have heard tell of fluid boiling occurring with MTB disks, implying to
me that the stuff people are putting in boils at a rather lower temp
than DOT4 or DOT5.1.

Dry for DOT5.1 is 288 deg C according to them.

Anyway you can get special paint for testing this. You get three or four
colour-coded paints with different, known melting points. You dab a
series of spots on the disk somewhere they won't get in the way, take
your favourite MTB route down Mont Blanc, and see which spots have
melted.

 

[Dave Larrington]

In news:[email protected],
Ben C <[email protected]> tweaked the Babbage-Engine to tell us:

> Yes, although I also got that "perhaps 170C" from a bit of googling.
> The page I found made a distinction between "wet" and "dry" boiling
> points, "wet" being what you get when your brake fluid has absorbed
> 3% of water.
>
> Here it is: http://www.afcoracing.com/tech_pages/fluid.shtml
>
> I went for the "wet" boiling point of DOT 4 since I heard somewhere
> else that bicycle brake fluid boils much more easily than the car
> stuff (although it strips the paint less).
>
> Brake fluid should never boil in a car unless it's very old. You
> should be able to get the pads practically on fire before the fluid
> boils. But I have heard tell of fluid boiling occurring with MTB
> disks, implying to me that the stuff people are putting in boils at a
> rather lower temp than DOT4 or DOT5.1.
>
> Dry for DOT5.1 is 288 deg C according to them.


I use DOT 5.1 from Halfrauds in all my hydraulic discs and can't recall any
problems which might have been caused by boiling fluid. This is 99.9%
on-road, admittedly, but even try to keep a recumbent down to a sane speed
on a steep wet twisty downhill in the middle of the night using only the
rear brake[1] it's been OK.

1 - the front one decided to lose interest after a minor off - no sign of a
fluid leak and rebleeding seems to have restored normal functionality.
Colour me baffled.

--
Dave Larrington
<http://www.legslarry.beerdrinkers.co.uk>
Apparently Guy has now got a Brompton. I'd never have guessed.