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uk.rec.cycling |
On 2007-09-05, David Damerell <damer...@chiark.greenend.org.uk> wrote: > Right. And what's got more heat capacity, a small metal disc or a large 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 500 * 0.897 * 80 = 35880J So 35880J can be soaked up in the rim before it gets to 100C. That E = 0.5*m*v^2 This seems to be saying that even if the rim doesn't manage to lose Of course this is cumulative, so ten 10kph retardations, with no loss to I think this probably explains why dissipation to the air isn't terribly Anyway, I couldn't see steel's SHC on Wikipedia but some other website 170 * 0.5 * 80 = 6800J So quite a bit worse it would seem. That's only four 10kph retardations I'm starting to wonder why disk overheating isn't a big problem with the
> Quoting Ben C <spams...@spam.eggs>: Does heat capacity help much?
>>On 2007-09-04, David Damerell <damer...@chiark.greenend.org.uk> 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.
> metal rim?
from 20C to 100C, on the assumption that 100C is very hot for a bicycle
rim (is that figure about right?)
corresponds to what change in speed of a 100kg bike+rider?
sqrt(2E/m) = v
sqrt((2 * 35880) / 100) = 27m/s, or 97kph.
_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.
the air, would cause the same buildup.
significant for one-off emergency stops, but is for repeated braking
during a long descent.
suggested it might be around 0.5 J/gK but it depends on the kind of
steel. A disk weighs around 170g.
sqrt((2 * 6800) / 100) = 12m/s or 43kph
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.
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).