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Vented Discs



 
 
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  #21  
Old June 26th 09, 05:13 PM posted to rec.bicycles.tech,alt.rec.bicycles.recumbent
Opus[_2_]
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Posts: 414
Default Vented Discs

On Jun 25, 4:59 pm, Andre Jute wrote:
snip
I have seen bicycles that set their brakes on fire stopping after a
straight line run on level ground, at Battle Mountain NV.


I'll leave that one to the local scoffjaws, who're just engaging motor-
drive.

These were human powered bikes capable of 80km/h for an hour and
sprints in excess of 110 km/h, not assist bikes.

I have
personally experienced tire failure due to brake heat riding down Big
Cottonwood Canyon east of SLC UT.


Why don't you tell us how long and steep this ride is, how
irresponsibly you rode down there, how the tire failed, how you
determined that it was due to brake heat. Note that I never said it is
impossible, merely that the circumstances are rare and avoidable.


I don't have the info on that ride, and from what I have seen on
Google the road has been changed considerably since I did the race
with my friends. The tire exploded off the rim when the bead melted
and caused the tire to escape the rim. I was riding steel rims with
centerpull calipers in the mid 1970s and the tires were rated to 95
PSI. We would ride our bikes to the ski resort at the top of the
canyon, recover and then race to Wasatch Boulevard at the bottom of
the canyon. There used to be a C-store there and whoever got to the
door of the C-store first was the winner, bike had to be upright on
the kickstand so you couldn't just ride up to the door you had to stop
and park the bike like a normal person who had just exceeded the
national speed limit on a 10 speed bicycle. Nobody had tires go down
on the runs, just in the parking lot at the C-store, and sometimes
they would fail spectacularly, as in flip the bike over and set it
back upright on the seat and handlebars. When the tubes were replaced
after the blowouts the tire almost every time would not seat the bead
because of bead damage. Sometimes the damage was so easily spotted we
didn't even try to reinstall it we just bought another tire.

Now we crashed a whole bunch of different ways, but we never had a
tire let go from the rim on the road. I had some scary moments on that
bike with the steel rims in the rain, but never had a problem in the
dry. That race down Big Cottonwood Canyon would wear out a new set of
pads if you didn't save your brakes by sitting up with your hands on
top of the bars before the turns
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  #22  
Old June 26th 09, 05:15 PM posted to rec.bicycles.tech,alt.rec.bicycles.recumbent
Opus[_2_]
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Posts: 414
Default Vented Discs

On Jun 25, 10:24 pm, Tom Sherman _
wrote:
Opus aka DC wrote:
[...]
I have seen bicycles that set their brakes on fire stopping after a
straight line run on level ground, at Battle Mountain NV.[...]


Presumably the full streamliners that exceed 120 kph.

--
Tom Sherman - 42.435731,-83.985007
I am a vehicular cyclist.


aka DC? Who pray tell is DC. I'm Opus and have been using that name
since 1997
  #23  
Old June 26th 09, 07:24 PM posted to rec.bicycles.tech,alt.rec.bicycles.recumbent
Andre Jute[_2_]
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Posts: 8,828
Default Vented Discs

On Jun 26, 5:13*pm, Opus wrote:
On Jun 25, 4:59 pm, Andre Jute wrote:snip
I have seen bicycles that set their brakes on fire stopping after a
straight line run on level ground, at Battle Mountain NV.


I'll leave that one to the local scoffjaws, who're just engaging motor-
drive.


These were human powered bikes capable of 80km/h for an hour and
sprints in excess of 110 km/h, not assist bikes.

I have
personally experienced tire failure due to brake heat riding down Big
Cottonwood Canyon east of SLC UT.


Why don't you tell us how long and steep this ride is, how
irresponsibly you rode down there, how the tire failed, how you
determined that it was due to brake heat. Note that I never said it is
impossible, merely that the circumstances are rare and avoidable.


I don't have the info on that ride, and from what I have seen on
Google the road has been changed considerably since I did the race
with my friends. The tire exploded off the rim when the bead melted
and caused the tire to escape the rim. I was riding steel rims with
centerpull calipers in the mid 1970s and the tires were rated to 95
PSI. We would ride our bikes to the ski resort at the top of the
canyon, recover and then race to Wasatch Boulevard at the bottom of
the canyon. There used to be a C-store there and whoever got to the
door of the C-store first was the winner, bike had to be upright on
the kickstand so you couldn't just ride up to the door you had to stop
and park the bike like a normal person who had just exceeded the
national speed limit on a 10 speed bicycle. Nobody had tires go down
on the runs, just in the parking lot at the C-store, and sometimes
they would fail spectacularly, as in flip the bike over and set it
back upright on the seat and handlebars. When the tubes were replaced
after the blowouts the tire almost every time would not seat the bead
because of bead damage. Sometimes the damage was so easily spotted we
didn't even try to reinstall it we just bought another tire.

Now we crashed a whole bunch of different ways, but we never had a
tire let go from the rim on the road. I had some scary moments on that
bike with the steel rims in the rain, but never had a problem in the
dry. That race down Big Cottonwood Canyon would wear out a new set of
pads if you didn't save your brakes by sitting up with your hands on
top of the bars before the turns


Wow! An impressive ride. One would hope that components moved on a bit
from them. -- Andre Jute
  #24  
Old June 26th 09, 07:34 PM posted to rec.bicycles.tech,alt.rec.bicycles.recumbent
Andre Jute[_2_]
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Posts: 8,828
Default Heat soak on stoppping, was Vented Discs

On Jun 26, 2:57*am, somebody wrote:
On Thu, 25 Jun 2009 07:38:19 -0700 (PDT), Andre Jute

wrote:

(snip)

Hell, consider this sequence of facts. Jobst reported the other day
that he got up to 50mph or 80kph down some notorious mountain in
California. *If his brake pads faded, never mind failed to stop him
(or melted!), you can bet we would have heard of it. I needed truck
assistance to get up to 100kph, and had to stop in a hurry afterwards
to avoid crossing a dangerous intersection (or, as bad, hitting the
back of my own truck). Neither disc brakes nor roller brakes got too
warm to touch.. I didn't at the time have a bike with rim brakes but I
do now and often ride the brakes down long descents so as not to speed
ahead of my pedalpals -- result: at worst a slightly warmed rim. Hard
braking from over 50kph at the bottom of a hill makes no impression
either, as I've reported here before.


(snip)

Data point: *220 lb. rider, reasonable 700c rims (Alex DM-18). *Coming
down Gannett Hillhttp://www.mountainzone.com/mountains/detail.asp?fid=6276756
the road isn't too straight and there is a deep ditch on either side.
I kept speed down to 20 mph.


IOW you were riding the brakes almost constantly.

Both rims were very hot at the *bottom. *How hot? *Wish I knew. *It
wasn't boiling, but uncomfortable to the touch. *The temperature
measuring strips are a good idea.


Makes me wonder if "uncomfortable to the touch" is anywhere near the
failure of patches due to overheating beyond 80 degrees C found by the
the German researcher cited by Bernhard.

Now scale the hill up to Rocky Mountain size...


Did you read about Opus's ride on steel rims and narrow tyres with
bead failure on stopping? (Everything I've reported is on relatively
wide rims or very wide rims, and with tyres between 37 and 60mm wide.)
Again, it makes you wonder about heat build-up when you stop moving,
when the air no longer flows around the rim, IOW whether the danger
isn't the hard ride but cutting off the airflow. Maybe those recumbent
riders with fairings aren't doing their brakes much good either.

Andre Jute
Curioser and curioser
  #25  
Old June 26th 09, 10:47 PM posted to rec.bicycles.tech,alt.rec.bicycles.recumbent
Bill Bushnell
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Posts: 121
Default Heat soak on stoppping, was Vented Discs

In alt.rec.bicycles.recumbent wrote:
Andre Jute wrote:
Maybe those recumbent riders with fairings aren't doing their brakes
much good either.


Tires generally lift off while rotating under load because it requires
flexing of the tire bead o the rim to creep off.. Unless a tire is
already well on its way to blow off when the bicycle stops, the tire
doesn't come off.


On a recent ride in Sonoma County, California recently I experienced two blowoffs
that occurred on steep downgrades when I stopped to take photos. The first on my
front tire occurred after I had stopped about 2/3 of the way down a short but
steep downhill on Stewarts Point-Skaggs Springs Rd. between Tin Barn and Annapolis
Rds. for the express purpose of letting my rims cool, and the other on the rear
tire was near the bottom of Meyers Grade where I had stopped to take photos. In
the latter case the blowoff occurred shortly after I had resumed the descent
following a photo stop; in the former case, I had come to a complete stop before
the tire blew.

http://tinyurl.com/q889to

I also suffered a third blowoff earlier that day while I was dragging my rear
brake holding a camera in my free hand to record a video clip.

To Mr. Jute's point, aerodynamic fairings allow less aerodynamic braking,
requiring the brakes to be used more, resulting in hotter rims. My experiences
are not too unlike those of a moderately heavy tandem pair. Although I probably
end up replacing my pads and rims slightly more often than an upright bike rider
of similar weight, wear is not the main issue. Rim heating causing tire blowoff
is. Finding the right tire, tire pressure, and rim is the key to avoiding
blowoffs.

--
Bill Bushnell
http://pobox.com/~bushnell/
  #26  
Old June 27th 09, 01:51 AM posted to rec.bicycles.tech,alt.rec.bicycles.recumbent
Tom Sherman _
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Posts: 344
Default Vented Discs

Opus the Poet wrote:
On Jun 25, 10:24 pm, Tom Sherman _
wrote:
Opus aka DC wrote:
[...]
I have seen bicycles that set their brakes on fire stopping after a
straight line run on level ground, at Battle Mountain NV.[...]

Presumably the full streamliners that exceed 120 kph.

--
Tom Sherman - 42.435731,-83.985007
I am a vehicular cyclist.


aka DC? Who pray tell is DC. I'm Opus and have been using that name
since 1997


Oops, wrong C.

--
Tom Sherman - 42.435731,-83.985007
I am a vehicular cyclist.
  #27  
Old June 29th 09, 02:26 AM posted to rec.bicycles.tech,alt.rec.bicycles.recumbent
Radey Shouman
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Posts: 1,082
Default Vented Discs

Still Just Me... writes:

On Tue, 23 Jun 2009 20:49:34 -0400, Radey Shouman
wrote:

Also, the contents of the holes provide a bulk flow of air past the
*pad*. Without holes, when not braking, there is only a narrow space
for air to flow past the pad, which makes convection cooling very
slow. When braking there is no space for air to flow and cooling can
occur only by conduction, to the disk or to the caliper. With holes
there is significant flow of air past the pad, whether braking or not.
The air inside the holes will be well-mixed, and quickly approach
equilibrium with the surface of the pad. Once past the pad, air in
the holes will be exchanged with the free stream of air flowing past
the disk, thus cooling the pad.


This might apply in a system with a center vented rotor. The holes
would not cool the pads in a single, non vented disk configuration
except by virtue of the fact that they might lower the rotor
temperature while it's not between the pads. Even with the vented
rotor, the flow to the pad is minimal. I suspect that if the holes
have any cooling effect in either configuration, it's when the rotor
is NOT between the pads.


Holes that communicate with central vents would certainly have a
centrifugal pumping feature, as would the radial slots described
somewhere up thread. I think that even holes in a solid disk would
significantly increase air flow past the pad, since without them
air flow is near zero.

The disk is normally rotating, so there are two phases, when between
the pads the air in the holes is heated by the pads, and when not
between them the holes exchange air with the cooler free stream
around the brake. In order to cool the pads both are necessary,
which explains why clutch disks would not benefit from similar holes.

 




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