Pros and cons of mechanical vs hydraulic disc brakes?

[Simon Brooke]

in message <[email protected]>, Tony Raven
('[email protected]') wrote:

> Simon Brooke wrote on 15/02/2007 16:35 +0100:
>>
>> Apart from the mud issue, disk brakes are less, not more, effective than
>> rim brakes, because of poorer leverage and less radiant surface.
>>
>
> I have cable and hydraulic rim brakes and hydraulic disk brakes and I
> can't believe that anyone who has ridden all three extensively could say
> what you have just said.

I also have both (cable) rim and (hydraulic) disk brakes (I don't have
hydraulic rim brakes). It's just basic physics. All brakes work by turning
kinetic energy into heat and then dumping that to the atmosphere. Disk
brakes have less surface to do this with.

--
[email protected] (Simon Brooke) http://www.jasmine.org.uk/~simon/

...but have you *seen* the size of the world wide spider?

 

[Clive George]

"Simon Brooke" <[email protected]> wrote in message
news:[email protected]...

> I also have both (cable) rim and (hydraulic) disk brakes (I don't have
> hydraulic rim brakes). It's just basic physics. All brakes work by turning
> kinetic energy into heat and then dumping that to the atmosphere. Disk
> brakes have less surface to do this with.

The basic physics you espouse only works if all other things are equal.
Which they aren't. The materials are different, the structures are
different, the forces are different, even the temperatures are different.

Disk brakes may have less surface to dump the heat to the atmosphere, but
they can safely run at a higher temperature than a rim (I've demonstrated
this in the past...), and this means they can dump heat quite well.

Basic physics...

cheers,
clive

 

[Tony Raven]

Simon Brooke wrote on 16/02/2007 00:58 +0100:
> in message <[email protected]>, Tony Raven
> ('[email protected]') wrote:
>>
>> I have cable and hydraulic rim brakes and hydraulic disk brakes and I
>> can't believe that anyone who has ridden all three extensively could say
>> what you have just said.
>
> I also have both (cable) rim and (hydraulic) disk brakes (I don't have
> hydraulic rim brakes). It's just basic physics. All brakes work by turning
> kinetic energy into heat and then dumping that to the atmosphere. Disk
> brakes have less surface to do this with.
>

I presume you have fitted your car with drum brakes all round.

Disk brakes may have less surface but the surface is much flatter and
can be heated to much higher temperatures without any problems. The
pads can run much closer and be of much harder material as can the disk.
So the pad movement to take up the gap is much less and you can
provide much greater mechanical advantage in the braking system so more
force for less effort. Because the disc is solid across its thickness
you can apply much more force without damaging the rim and the caliper
can be built much stronger so, unlike the brake bosses on a fork leg,
the two sides do not flex appreciably when you apply that force. Just
some of the reasons disks work better than rim brakes.

Hydraulics make a big difference as you would find out if you tried
hydraulic vs cable rim brakes but hydraulic disk brakes are appreciably
better still. As I say I cannot believe you can possibly find cable rim
brakes to be better than hydraulic disks.

--
Tony

"...has many omissions and contains much that is apocryphal, or at least
wildly inaccurate..."
Douglas Adams; The Hitchhiker's Guide to the Galaxy

 

[Tony Raven]

Clive George wrote on 16/02/2007 02:35 +0100:
>
> Disk brakes may have less surface to dump the heat to the atmosphere,
> but they can safely run at a higher temperature than a rim (I've
> demonstrated this in the past...), and this means they can dump heat
> quite well.
>

I know people with disks that have been blued by the temperature after a
long steep technical descent. The brakes still worked and work fine. I
also know people who have had tyres blown by the rim temperature doing
the same with rim brakes.

--
Tony

"...has many omissions and contains much that is apocryphal, or at least
wildly inaccurate..."
Douglas Adams; The Hitchhiker's Guide to the Galaxy

 

[Dave Larrington]

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

> Apart from the mud issue, disk brakes are less, not more, effective
> than rim brakes, because of poorer leverage and less radiant surface.


FSVO "effective". The far-from-bottom-end Avid linear-pull brake I have on
my old fixer is this: bloody awful. Even in the dry. The new one has a
GBFO four-pot disc (which is almost certainly overkill for road use) and
stops when requested.

--
Dave Larrington
<http://www.legslarry.beerdrinkers.co.uk>
The Real Slim Shady's gone to play tennis.

 

[Dave Larrington]

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

> Only thing I've not seen in the discussion to date is that mechanical
> are prone to cables frrzing if they get wet in them and it becomes
> cold. Presumably hydraulic don't (until it becomes very cold? What's
> the freezing point of the fluid?


Most hydraulic discs use automotive brake fluid, so the answer would seem to
be: low enough[1] not to worry about.

1 - Ferodo DOT 5.1 freezes at -55 deg. C, apparently

--
Dave Larrington
<http://www.legslarry.beerdrinkers.co.uk>
They came for Eamonn Holmes; I think I'm right in saying that I
applauded.

 

[Paul Boyd]

Clive George said the following on 16/02/2007 02:35:

> Disk brakes may have less surface to dump the heat to the atmosphere,
> but they can safely run at a higher temperature than a rim (I've
> demonstrated this in the past...), and this means they can dump heat
> quite well.

Re the brake tests in recent MTB magazines - they ran the brakes up on a
test rig and ran them to see what temperature they failed at. Most
worked quite happily with the discs glowing orange, so heat wouldn't
appear to be much of an issue for the average MTBer.

Now, try the same rim brakes with tyres fitted to see what temperature
something fails at

--
Paul Boyd
http://www.paul-boyd.co.uk/

 

[Paul Boyd]

Ian Smith said the following on 15/02/2007 18:58:

> Only thing I've not seen in the discussion to date is that mechanical
> are prone to cables frrzing if they get wet in them and it becomes
> cold. Presumably hydraulic don't (until it becomes very cold? What's
> the freezing point of the fluid?

Dunno, but over on (USA biased) alt.mountain-bike they've been talking
about forks locking up with the cold, plastic bits snapping with the
cold, but no-one's mentioned brakes not working. Maybe those whose
brakes failed in the cold were unable to report that fact

--
Paul Boyd
http://www.paul-boyd.co.uk/

 

[Paul Boyd]

> Now, try the same rim brakes with tyres fitted to see what temperature
> something fails at

What happened to "tests with"??

Let's try again:-

Now, try the same tests with rim brakes with tyres fitted to see what
temperature something fails at

--
Paul Boyd
http://www.paul-boyd.co.uk/

 

[p.k.]

Tony Raven wrote:
> I also know people who have had tyres blown by the rim
> temperature doing the same with rim brakes.

As happened to me on one of the very long steep decants on the C2C - I
cooked the wheel and the front tyre exploded!

pk

pk

 

[Tony Raven]

Paul Boyd wrote on 16/02/2007 09:02 +0100:
>
> Dunno, but over on (USA biased) alt.mountain-bike they've been talking
> about forks locking up with the cold, plastic bits snapping with the
> cold, but no-one's mentioned brakes not working. Maybe those whose
> brakes failed in the cold were unable to report that fact
>

What I have had in the cold is rims glazing with ice removing all
braking capability. Disks seem to be much less prone, presumably
because they both run hotter and are not being immersed and cooled in
puddles.

--
Tony

"...has many omissions and contains much that is apocryphal, or at least
wildly inaccurate..."
Douglas Adams; The Hitchhiker's Guide to the Galaxy

 

[Roger Merriman]

Tony Raven <[email protected]> wrote:

> Clive George wrote on 16/02/2007 02:35 +0100:
> >
> > Disk brakes may have less surface to dump the heat to the atmosphere,
> > but they can safely run at a higher temperature than a rim (I've
> > demonstrated this in the past...), and this means they can dump heat
> > quite well.
> >
>
> I know people with disks that have been blued by the temperature after a
> long steep technical descent. The brakes still worked and work fine. I
> also know people who have had tyres blown by the rim temperature doing
> the same with rim brakes.

and the whole if the rim gets knocked slightly out of true the brakes go
from good to rather poor.

where disks remain the same.

roger

 

[James Thomson]

"Duncan Smith" <[email protected]> a écrit:

> So, the bike came with some sleek carbon forks which only
> allowed up to 28mm width tyres. Cycling to work on un-gritted
> icy back roads made me a little twitchy, so I let the LBS fit some
> cross forks and either canties or a v-brake - they went for Vee
> in the end - Shimano Deore. I did mention about the 287V, but
> they thought that the front cable was short enough that it wasn't
> necessary - and kind of isn't, at least for one of my wheels. The
> other wheel has a slight, slight buckle that means I just have to
> slacken of the v-brake a little, but the result is that by the time I
> pull the lever (standard road lever) so that the pads bite the rims,
> there isn't all that much free travel left before touching the bars,
> which in the wet is definitely sub-par.

As you've discovered, you really need more cable pull to work a V-brake. The
287V will do that, and there are a few other options, like the "travel
agent" Rob Morley mentioned:

http://www.phred.org/~alex/bikes/brakes.html

I've run a time-trial style bar-end brake lever with V-brakes in a cowhorn
bar, and that gave enough travel to run a V-brake with more normal rim
clearances. Likewise, a Campag or Tektro style lever with a quick release in
the lever body can be run with the QR open to get more lever travel. That
doesn't solve the inherent leverage mismatch between brake and lever, but it
stops you running out of travel.

Another option would be a centre-pull canti, like the Avid Shorty or Empella
Froggleg. The Avids in particular are easy to adjust, and the cable pull
requirement is compatible with your levers.

> The idea was that it would be quick and easy to swap the wheels over
> depending upon the weather, one with a 28mm tread, the other a 35mm
> studded tyre, but that didn't really work out because the rims are
> different widths - and it takes longer to **** with the vees than it
> does to change the tyre over.

I ran 287V levers with XT V-brakes on a touring bike that I used with two
sets of wheels: one heavy, one light. It worked fine. The "parallel push"
system of XT and XTR V-brakes mean there's generally no need to adjust the
pads for different rims - you just adjust cable slack with the barrel
adjuster on the noodle supplied with the levers. Takes about two seconds.

> Strictly speaking I could probably make do without, but I wouldn't
> mind trying out some discs anyway. Never had discs before.

Fair enough. But consider you'll need a new fork, hubs (if your current hubs
aren't disk compatible), rotors, caliper,
lever, and the ensemble will weigh more than a matched lever and rim brake.

James Thomson

 

[Erik Sandblom]

Den 2007-02-16 10:02:47 skrev Paul Boyd <usenet.dont.work@plusnet>:

> Ian Smith said the following on 15/02/2007 18:58:
>
>> Only thing I've not seen in the discussion to date is that mechanical
>> are prone to cables frrzing if they get wet in them and it becomes
>> cold. Presumably hydraulic don't (until it becomes very cold? What's
>> the freezing point of the fluid?
>
> Dunno, but over on (USA biased) alt.mountain-bike they've been talking
> about forks locking up with the cold, plastic bits snapping with the
> cold, but no-one's mentioned brakes not working. Maybe those whose
> brakes failed in the cold were unable to report that fact


Cable brakes easily stop working overnight if you leave the poor bike out
in the cold. But I've never heard of them stop working while riding. Just
pull them once in a while to prevent ice from forming.

Don't know about hydraulic brakes.

Erik Sandblom
--
Oil is for sissies

 

[Ambrose Nankivell]

Tony Raven wrote:
> Clive George wrote on 16/02/2007 02:35 +0100:
>>
>> Disk brakes may have less surface to dump the heat to the atmosphere,
>> but they can safely run at a higher temperature than a rim (I've
>> demonstrated this in the past...), and this means they can dump heat
>> quite well.
>
> I know people with disks that have been blued by the temperature after a
> long steep technical descent. The brakes still worked and work fine. I
> also know people who have had tyres blown by the rim temperature doing
> the same with rim brakes.
>
Ditto: I've melted rim brake pads descending, and also got my rims up to
blueing temperature.

One of those brake technologies faded with the heat, and the other didn't.

A

 

[Tony Raven]

Ambrose Nankivell wrote on 16/02/2007 19:09 +0100:
>
> Ditto: I've melted rim brake pads descending, and also got my rims up to
> blueing temperature.
>

Since steel bluing temperature is 300C I doubt that very much.


--
Tony

"...has many omissions and contains much that is apocryphal, or at least
wildly inaccurate..."
Douglas Adams; The Hitchhiker's Guide to the Galaxy

 

[Ambrose Nankivell]

Tony Raven wrote:
> Ambrose Nankivell wrote on 16/02/2007 19:09 +0100:
>>
>> Ditto: I've melted rim brake pads descending, and also got my rims up
>> to blueing temperature.
>>
>
> Since steel bluing temperature is 300C I doubt that very much.
>
Quite rightly so. I meant to write discs.

 

[Rob Morley]

In article <[email protected]>, Simon
Brooke
[email protected] says...
> All brakes work by turning
> kinetic energy into heat and then dumping that to the atmosphere. Disk
> brakes have less surface to do this with.
>
But run at a higher temperature ...

 

[Dave Larrington]

In newsp.tnutmanmzubk0m@toshiba,
Erik Sandblom <[email protected]> tweaked the Babbage-Engine to tell us:

> Cable brakes easily stop working overnight if you leave the poor bike
> out in the cold. But I've never heard of them stop working while
> riding. Just pull them once in a while to prevent ice from forming.

A couple of weeks ago my front derailleur cable twice froze up on the way
into work in the morning; once after about 10 km and once after 4. It was
fine on the way home once it had thawed out again.

--
Dave Larrington
<http://www.legslarry.beerdrinkers.co.uk>
This Unit is a productive Unit.

 

[Rob Morley]

In article <[email protected]>, Dave Larrington
[email protected] says...
> In newsp.tnutmanmzubk0m@toshiba,
> Erik Sandblom <[email protected]> tweaked the Babbage-Engine to tell us:
>
> > Cable brakes easily stop working overnight if you leave the poor bike
> > out in the cold. But I've never heard of them stop working while
> > riding. Just pull them once in a while to prevent ice from forming.
>
> A couple of weeks ago my front derailleur cable twice froze up on the way
> into work in the morning; once after about 10 km and once after 4. It was
> fine on the way home once it had thawed out again.
>
>
Maybe you should give it a good squirt of moisture displacer.