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



Guest
On 5 Sep,
Peter Clinch <[email protected]> wrote:

> Tony Raven wrote:
>
> > I stand corrected then but going back to the OP, do you think the
> > situation of "on an MTB pointing down a 60 degree loose slope you need
> > very precise braking control" is realistic. I recognise there are 60
> > degree slopes but there is unlikely to be any loose material on them and
> > I somehow doubt the rear wheel would stay down if you tried to brake.

>
> I can see such a slope existing if one takes "loose" as "stuck on... but
> not at all well, and not enough to stay put when a bike hits it", but I'd
> agree that anyone going down 60 degree slopes on a bike is pretty much
> wasting their time with the brakes! 60 degrees is actually typical of
> lower grade rock climbing, and IME people very often over-estimate true
> steepness of steep slopes (where "steep" starts at about 20 degrees).


A slope of 45 degrees would require a coefficient of friction of one just to
stay on it. Coefficients higher than one are unusual, perhaps limiting at
about 1.1 with sticky tyres. It would be impossible to stay put, nevermind
brake to a halt, on a 60 degree slope, unless there was an interlocking rack
and pinion arrangement.


--
BD
Change lycos to yahoo to reply
 
A

Ace

Guest
On Wed, 05 Sep 2007 09:18:13 +0100, Peter Clinch
<[email protected]> wrote:

>I'd agree that anyone going down 60 degree slopes on a bike is pretty
>much wasting their time with the brakes! 60 degrees is actually typical
>of lower grade rock climbing, and IME people very often over-estimate
>true steepness of steep slopes (where "steep" starts at about 20 degrees).


Same happens in skiing - many people will overestimate the angle of
slope they've just skied, and the main reason isn't one of boasting,
but just that it _feels_ like that. In practice, marked runs over
40degrees are very rare, although I know a couple that I have
personally measured at 42=43deg. I have a clinometer needle on my
compass which can amuse or embarrass folk who will insist that the
slope's steeper than it really is.

As for 'loose' material on such a steep slope - we're taught in
avalanche training that slopes above 55deg are outside the danger
angles, as it's not possible for enough snow to accumulate on them.

38deg is normally accepted as the most dangerous angle, as this is the
maximum slope that a truly granular substance will stick to.

--
Ace in Alsace - brucedotrogers a.t rochedotcom
 
P

Peter Clinch

Guest
Ace wrote:

> As for 'loose' material on such a steep slope - we're taught in
> avalanche training that slopes above 55deg are outside the danger
> angles, as it's not possible for enough snow to accumulate on them.


You need to be careful about qualifying assumptions like that... how
about a convex slope where the bottom is too steep for accumulation, but
the top isn't?

Friends of mine have been caught in a cornice collapse in a gully where
loose snow wasn't accumulating, but was unfortunately passing through at
some speed :-(

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/
 
A

Ace

Guest
On Wed, 05 Sep 2007 11:01:46 +0100, Peter Clinch
<[email protected]> wrote:

>Ace wrote:
>
>> As for 'loose' material on such a steep slope - we're taught in
>> avalanche training that slopes above 55deg are outside the danger
>> angles, as it's not possible for enough snow to accumulate on them.

>
>You need to be careful about qualifying assumptions like that... how
>about a convex slope where the bottom is too steep for accumulation, but
>the top isn't?


Oh yes, you're quite right - clearly one needs to be looking at the
slope above you as well as the bit you're planning to ski on.

>Friends of mine have been caught in a cornice collapse in a gully where
>loose snow wasn't accumulating, but was unfortunately passing through at
>some speed :-(


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

--
Ace in Alsace - brucedotrogers a.t rochedotcom
 
T

Tony Raven

Guest
Ace <[email protected]> wrote in news:[email protected]
4ax.com:

> On Wed, 05 Sep 2007 09:18:13 +0100, Peter Clinch
> <[email protected]> wrote:
>
>> [...]

>
> Same happens in skiing - many people will overestimate the angle of
> slope they've just skied, and the main reason isn't one of boasting,
> but just that it _feels_ like that. In practice, marked runs over
> 40degrees are very rare, although I know a couple that I have
> personally measured at 42=43deg. I have a clinometer needle on my
> compass which can amuse or embarrass folk who will insist that the
> slope's steeper than it really is.


Saudan's Couloir/Couloir Extreme in Blackcomb is reputedly 50 degrees and a
marked double diamond run. It feels vertical but they run an annual GS
race down it.

--
Tony

" I would never die for my beliefs because I might be wrong."
Bertrand Russell
 
P

Peter Clinch

Guest
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/
 
M

Marz

Guest
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 <[email protected]> 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.
 
D

David Damerell

Guest
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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D

David Damerell

Guest
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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D

David Damerell

Guest
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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D

David Damerell

Guest
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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D

David Damerell

Guest
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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D

Dane Buson

Guest
In rec.bicycles.misc CoyoteBoy <[email protected]> wrote:
> On 30 Aug, 11:42, "Clive George" <[email protected]> wrote:
>>
>> cheers,
>> clive

>
> Clive - well summarised! We did rather go off on a tangent didnt we!
>
> In essence, buy the best brake you can afford and make sure its set up
> correctly (a decent shop should sort it for you) and you will have few
> problems, be they cable or hydro. Any half decent shop should be able
> to repair / maintain them for you but they are generally simple enough
> to "tweak" on your own - if you have any problems pop back on here and
> someone will answer you in 30 seconds - then no doubt we'll go off at
> a tangent and argue about the shape of the moon :)


Well, one point. Hasn't the OP had rather a lot of problems with her
local shops being rubbish? If so, I wouldn't trust them to work on
hydraulics since IIRC they are racer (road) oriented. I think the cable
brakes are probably a safer choice in terms of keeping her trike
actually on the road and operating.

--
Dane Buson - [email protected]
"Drinking through your hair is neither tasty nor nutritious."
-Anonymous
 
B

Ben C

Guest
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).
 
C

Clive George

Guest
"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
 
D

David Damerell

Guest
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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?

_

Guest
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?
 
B

Ben C

Guest
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.
 
?

_

Guest
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.
 
A

Alan Braggins

Guest
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.