On 2008-05-01,
[email protected] <
[email protected]> wrote:
[...]
>> It's not really the energy from your hands that's important, but the
>> force they can apply. That force can be geared up to give you as
>> much force as you want on the rim/disk.
>
> In what units are you measuring "energy" or do you mean "force"?
I'm not measuring energy but if I was I would use Joules.
I mean energy when I say "energy" and force when I say "force".
My point is that it takes very little energy from a rider to squeeze the
brakes. You have to move your fingers a short distance, but the main
requirement on your body is to maintain a force.
As discussed here previously continuous force maintenance by human
muscles does require the consumption of some energy, but it is only a
small amount.
>> You need four times as much force on a disk because it has about a
>> quarter the radius of the wheel. But in either case you just gear it
>> so it feels right for people with averagely normal hand
>> strength. This isn't the difficult part of brake design.
>
> That isn't the parameter of interest. Disk diameter and coefficient
> of friction both have a major effect on the ratio between application
> force and brake torque.
Yes that too, but the basic point is the same: you give the user enough
mechanical advantage to give the brakes a satisfactory feel whether they
are disk brakes or rim brakes.
An overgeared brake isn't a better brake.
>> Lots of cars didn't used to have servo assistance on the brakes.
>> All the force and work required to brake came from your leg. And
>> yet you could slow down a tonne of car from 70mph like that.
>
> Drum brakes used massive self servo action to achieve useful braking.
> Because that effect is highly unpredictable, they had both lock-up and
> fade. That is why we use disks today. You may recall that this was
> discussed here at great length.
There were plenty of cars with non-servo-assisted disks too.
>> The difficult part is getting rid of all the heat that the brakes
>> are converting the bike's kinetic energy into.
>
> Apparently the bicycle industry is not addressing that part of the
> problem when the surface area and thermal mass their gossamer rings of
> steel have.
This is why I think that disks don't really work as heat sinks but as
dissipators.
Here are my various guesses and estimates on which I'm basing this:
http://groups.google.co.uk/group/uk.rec.cycling/msg/991d32532f671264
> I have asked brake manufacturers why their disks are
> mostly air with a thin pattern of thin steel between. For cooling is
> the answer.
Ah but did they mean because they _look_ cool
> Maybe they should tell the automotive and railway people about their
> theory.
I remember a calculation here before about cross-drilled disks on cars.
I think your point was originally to slay a herring about "gas
bearings", which was fair enough, but I think it was concluded also that
a drilled disk will have a slightly higher surface area to volume ratio
than one that isn't drilled.
Same goes for a filigreed disk. But I don't really buy that "it cools
better" line either. It would have to be shown by how much and also what
the effect of a smaller surface area in contact with the pad has (for
car disks, I think those filigreed bike disks usually have a continuous
undrilled track that touches the pads).
>> Bicycle rims have enough heat capacity to just soak up the heat for
>> most stopping situations, but they can overheat badly if you need to
>> keep them on continuously because you want to go down a hill quite
>> slowly.
>
> It depends on gradient that gives (vertical) foot (rider weight
> (pounds) per second. The slower you go the less cooling and the less
> wind drag on the rider. There is a narrow trade-off between brake
> cooling and speed which was discussed here recently.
Indeed. For a given hill/bike there's a "worst speed" for heat buildup.
Descend either faster or slower than that speed and you're better off.
>> Disks have less capacity, but get hotter, so dissipate heat to the
>> air more rapidly. So I reckon they're better for sustainable
>> braking down long hills.
>
> By that measure, a wafer thin disk is all it takes, surface area and
> mass be damned.
Well surface area affects its dissipation rate, which does need to be
high by this measure.
But mass be damned, provided the thing's strong enough.
>> But for emergency stops either kind of brake (of decent quality and
>> level of maintenance) will be able to tip you over the handlebars if
>> you aren't careful.
>
> I doubt it. I don't know many riders who can raise the rear wheel
> while traveling at normal road speeds. In contrast, I have seen
> people go over the bars:
>
> http://tinyurl.com/3kunfl
Nice picture of a pedal, what's it got to do with going over the
handlebars?
I agree that it's hard to do though. I shouldn't have said "if you
aren't careful"-- it's not a problem practically speaking on any bike
I've ridden.
