Drum-Brake Reliable for Long, Steep Descents?



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Risto Varanka writes:

>> It depends on how fast you are going. If you are holding the brakes hard enough to only go 1 or 2
>> kph, they won't get very hot because there is lots of time to dissipate heat, and if you are
>> going 30+ not using the brakes they obviously won't get hot. The max heat generation is somewhere
>> between these two extremes.

> I thought we were talking the extreme where you have to do constant and significant braking just
> to keep from going faster than the 30 km/h.

> If you go faster, you lose altitude at a greater rate, ie. you convert potential energy to kinetic
> at a greater rate. To me this would appear to mean that you need to apply the brakes more, in
> order to control the resulting acceleration.

To hold the bicycle at constant speed below 30km/h requires practically the same brake force
regardless of speed, wind being an insignificant brake on such steep gradients at least up to that
speed. The greatest problem is air cooling and heating rate. At less than 15km/h, convection cooling
becomes miserable and it is for this reason that the cautious rider is most likely to blow a tire if
it is going to happen at all.

With straight sections between curves, letting the bicycle roll free between curves produces far
lower temperatures than continuous braking at a slow speed.

http://tinyurl.com/pd86

Jobst Brandt [email protected]
 
Matt J wrote:
> On a really long/steep descent with rim brakes, it's possible to blow off tires due to the extreme
> heat generated from braking.

Are you sure about that? How hot do the rims get and how much can the tire pressure increase? It
doesn't seem likely to be all that much. There's not that much air in the tires. I think this notion
comes from tubulars which can get hot enough that the glue melts and the tire can roll of the rim.

--Bill Davidson
--
Please remove ".nospam" from my address for email replies.

Support the Electronic Frontier Foundation http://www.eff.org Petition Congress to stop the RIAA
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[email protected] wrote:
> The only time I blew a tire off the rim was while waiting for a friend on a descent as I crept
> downhill on the rear brake. I have never had a problem on that grade because I usually only brake
> entering hairpin turns, of which there were many.

Jobst:

A couple of Sundays ago (temps around 95F) I was descending Kings Mtn. Rd. and felt my rear tire
(Avocet City 1.5) going soft as I entered the "S" curves just prior to the first right-hand hairpin
as one descends. I usually don't significantly brake until just prior to these curves. I heard no
"pop". An inspection of the butyl tube revealed a very short
(3mm) but clean tear on the side of the tube that faces the rim, and the rim was too hot to handle
for about a minute after I stopped. I suspect a blowoff. I noticed that the rim strip (Velox)
had shifted slightly, allowing the tube at the failure to contact the metal rim directly (but
not the edge of a spoke hole). I readjusted the rim strip, replaced the tube, and continued
down the hill without further incident.

Last Sunday I descended Loma Almaden, followed without pause by the New Almaden side of Hicks
without incident. As you know both of these roads (8-12% grade) are steeper than Kings Mtn. Rd.
(6-7% grade) and require more constant braking. Temperature was in the low-70's. When I stopped at
the preserve gate on Loma Almaden Rd. part-way down the hill, I felt the rim, which was too hot to
touch for longer than an instant. I decided to press on anyway after I passed through the
walk-through portal.

I had put about the same air pressure in the same tire/rim (85 psi) on both days and was using the
same size of tube both days. The only difference is that I was braking more, it was 25 degrees
cooler, and I had a properly positioned rim strip last Sunday. What do you think led to the tube
failure on the first Sunday?

A couple of general questions:

4) Does ambient air temperature increase the likelihood of a tube failure when the rims get hot
from braking?

5) Does an undersize rim strip (or one that allows the tube to contact the aluminum rim bed)
increase the likelihood of tube failure when the rims get hot from braking?

--
Bill Bushnell
 
Bill Davidson writes:

>> On a really long/steep descent with rim brakes, it's possible to blow off tires due to the
>> extreme heat generated from braking.

> Are you sure about that? How hot do the rims get and how much can the tire pressure increase? It
> doesn't seem likely to be all that much. There's not that much air in the tires. I think this
> notion comes from tubulars which can get hot enough that the glue melts and the tire can roll of
> the rim.

You ought to have that experience at least once to appreciate the effect. As I have related before
on occasion, I once crossed the San Giacomo pass in the Alps in snow and warm weather after which I
waded through the upper reaches of the Ticino to get to the Nufenen Pass road over which I then
rode. On the descent, I got a flat in my front tubular tire as I braked into the first hairpin turn
but noticed as I pulled over that the tire was no longer leaking and was still hard.

I repeated this a couple of times and then decided to brake hard and stop on a straight section.
What I found was steam coming from the stem hole from water that had gotten in the rim over the
long snow trek and stream crossing. If you recall, to raise a cc of water one degree Celsius takes
one calorie, to convert it to steam takes 450 calories. This 550 calories of heat is a lot of
thermal energy concentrated in a small aluminum rim. This "experiment" gave me a great appreciation
of brake heating.

Tire blow-offs are common with tandems and over cautious descents if tires are at normal high
inflation of high performance bicycles. As I said, I blew a tire descending Grosse Scheidegg pass
because I waited for a friend who was taking pictures, as I coasted slowly down the hill with the
rear brake on, never thinking of the effect at that speed.

Jobst Brandt [email protected]
 
Elisa Francesca Roselli <[email protected]> wrote:

> ...hydraulic disk brakes by Shimano. These brakes were an important component in its hefty price
> tag (nearly 1300 Euros). I was told they are the highest performing brakes on the market today.

No, but they offer braking comparably strong to a rim brake along with other advantages. Namely,
they allow the broadest choice of rims and tires for a given frame, they allow effective braking
when the rim is bent, and they are almost as resistant to water and mud as drum brakes.

Disadvantages include the fact that they are heavier than rim brakes, they weaken the front wheel
structurally, they are subject to rotor warpage, and they are expensive.

> I am interested in a Nexus gear-shifter of the type that can be shifted when you are at a
> standstill. This shifter is available for 3, 4 and 7 speeds. But from what I understand (through a
> glass darkly, as the Shimano brochure is written in Parseltongue ....) this kind of shifter
> _requires_ a hub-brake on the rear wheel.

No, the drum brake can be left off in favor of a rim brake. The coaster (back-pedal) brake is not
retrofittable.

> ...the hydraulic disk brake.
>
> So is this luxury brake the one to rely on for long steep descents? What are the reasons for its
> high price?

Hydraulic brakes are easy to maintain, but difficult to repair. They can offer a lighter lever pull
to deliver a given amount of braking power, but they offer much less choice of levers or pads than
cable-operated rim brakes.

Linear-pull or "V-brakes" offer the most braking power with the simplest mechanism at the
lowest price.

If you wish to use a gearhub, I recommend a linear-pull brake in front and a drum brake in the rear.
That gives you the braking power advantages of the linear-pull with the wet-weather predictability
and cleanliness of the drum brake, and the cost is reasonable. Both of these brakes are more
reliable than disc brakes IMO. The disc brake's rotor is subject to heat warpage and impact damage,
and cannot be trued like a wheel can.

Chalo Colina
 
Bill Bushnell <[email protected]> writes:

>> The only time I blew a tire off the rim was while waiting for a friend on a descent as I crept
>> downhill on the rear brake. I have never had a problem on that grade because I usually only brake
>> entering hairpin turns, of which there were many.

> A couple of Sundays ago (temps around 95F) I was descending Kings Mtn. Rd. and felt my rear tire
> (Avocet City 1.5) going soft as I entered the "S" curves just prior to the first right-hand
> hairpin as one descends. I usually don't significantly brake until just prior to these curves. I
> heard no "pop". An inspection of the butyl tube revealed a very short (3mm) but clean tear on the
> side of the tube that faces the rim, and the rim was too hot to handle for about a minute after I
> stopped. I suspect a blow-off. I noticed that the rim strip (Velox) had shifted slightly, allowing
> the tube at the failure to contact the metal rim directly (but not the edge of a spoke hole). I
> readjusted the rim strip, replaced the tube, and continued down the hill without further incident.

I don't know what to make of such incidents. If there is a 3mm tear, it is possible that it was a
manufacturing flaw, a cold seam that did not properly weld in the mold and that the heat was all it
needed. 85psi is a safe margin from blow-off. Tubes do not move in the tire or on the rim, unless
the tire moves, and then the tube moves with the tire. A blow-off usually causes a 3 to 6 inch slash
and makes a loud bang.

> Last Sunday I descended Loma Almaden, followed without pause by the New Almaden side of Hicks
> without incident. As you know both of these roads (8-12% grade) are steeper than Kings Mtn. Rd.
> (6-7% grade) and require more constant braking. Temperature was in the low-70's. When I stopped at
> the preserve gate on Loma Almaden Rd. part-way down the hill, I felt the rim, which was too hot to
> touch for longer than an instant. I decided to press on anyway after I passed through the
> walk-through portal.

I don't believe air temperature has much influence, considering that the working temperature is
above 100C (200F). That descent was a difficult challenge in the days of tubulars for melting rim
glue. Only those who were willing to let the speed get high (little braking) were spared.

> I had put about the same air pressure in the same tire/rim (85 psi) on both days and was using the
> same size of tube both days. The only difference is that I was braking more, it was 25 degrees
> cooler, and I had a properly positioned rim strip last Sunday. What do you think led to the tube
> failure on the first Sunday?

> A couple of general questions:

> 1) Does ambient air temperature increase the likelihood of a tube failure when the rims get hot
> from braking?

Oops, I jumped ahead on that. It does if you lay your bicycle in the grass at the Hellyer Park
Velodrome. Tires have gone bang there but you couldn't touch the rims they were so hot. I loaned my
tire pump to a rider who fried the pump leather by laying on the lawn and found he couldn't pick it
up after it heated up sheltered by the grass from cooling air currents.

> 2) Does an undersize rim strip (or one that allows the tube to contact the aluminum rim bed)
> increase the likelihood of tube failure when the rims get hot from braking?

It transfers more heat. I know that the heat of interest has to come from the rim to the tube and
that the kapok filled fabric rim strips of old were precisely for insulation although we as tubular
riders had no idea that clinchers had heat problems. That's why no one knew why the French made such
"stupid" cumbersome rim strips.

Jobst Brandt [email protected]
 
Chalo:

> The disc brake's rotor is subject to heat warpage and impact damage, and cannot be trued like a
> wheel can.

Yes it can, and easier than a rim. A simple adjustable wrench or smooth-faced pliers will do it.
 
[email protected] wrote:

> Tire blow-offs are common with tandems and over cautious descents if tires are at normal high
> inflation of high performance bicycles. As I said, I blew a tire descending Grosse Scheidegg pass
> because I waited for a friend who was taking pictures, as I coasted slowly down the hill with the
> rear brake on, never thinking of the effect at that speed.

Interesting discussion. I feel right in the firing line as I am a heavy, very cautious rider who
doesn't know how to change or repair a tyre (I saw it being done once, but from there to practice is
a long way).

The Giant Melbourne also is supposed to have "pneus anti-crevaison", "unpuncturable" tyres. What are
those, and would they make a difference in the kind of blow-off situation you're describing?

Another question: if a blow-off should happen, what does it feel like? Do you lose control? Or is
it a decline in perfs sufficiently slow that a person with very slow reflexes can get possession
of herself?

Elisa Francesca Roselli Ile de France
 
Chalo wrote:

> If you wish to use a gearhub, I recommend a linear-pull brake in front and a drum brake in the
> rear. That gives you the braking power advantages of the linear-pull with the wet-weather
> predictability and cleanliness of the drum brake, and the cost is reasonable.

Thanks for that reply, which saves me a lot of money. So far, the Giant Melbourne, with the V-brake
drum-brake combination, is my front runner, and it's only 2/3 the price of the Cannondale.

EFR
 
Jose Rizal <_@_._> wrote:

> Chalo:
>
> > The disc brake's rotor is subject to heat warpage and impact damage, and cannot be trued like a
> > wheel can.
>
> Yes it can, and easier than a rim. A simple adjustable wrench or smooth-faced pliers will do it.

The stresses introduced in doing what you describe lead upon heating to further distortion, and you
begin the cycle over again.

Brake rotors are stress relieved by heat treating and then wet ground to a flat surface. If it were
possible to simply bend them flat and have them stay that way, such measures would not be necessary.

Chalo Colina
 
Elisa Francesca Roselli wrote:
> [email protected] wrote:
>
>> Tire blow-offs are common with tandems and over cautious descents if tires are at normal high
>> inflation of high performance bicycles. As I said, I blew a tire descending Grosse Scheidegg pass
>> because I waited for a friend who was taking pictures, as I coasted slowly down the hill with the
>> rear brake on, never thinking of the effect at that speed.
>
> Interesting discussion. I feel right in the firing line as I am a heavy, very cautious rider who
> doesn't know how to change or repair a tyre (I saw it being done once, but from there to practice
> is a long way).
>
> The Giant Melbourne also is supposed to have "pneus anti-crevaison", "unpuncturable" tyres. What
> are those, and would they make a difference in the kind of blow-off situation you're describing?
>
> Another question: if a blow-off should happen, what does it feel like? Do you lose control? Or is
> it a decline in perfs sufficiently slow that a person with very slow reflexes can get possession
> of herself?
>
> Elisa Francesca Roselli Ile de France

Elisa:

High up in this thread, you wrote: "My descent goes on for almost 1.5 km and is _seriously_ steep."

First, the descents that Jobst is talking about are mountain descents that are much longer than
this. Second, hub-brake heating doesn't affect the rims, so there's no risk of tire blow-off from a
rear hub brake. Third, I live in Paris and although I don't know all the triple-chevron hills in the
Ile de France, none of the ones I do know are long enough to cause a problem with rim brakes *when
properly used.* It seems to me that even if you were to opt for a bike with only rim brakes, it
would be possible to descend safely with proper technique. That said, it sounds to me like a
rear-hub front-rim braking system would be ideal for you, and relieve you of some worries.
 
In article <emJeb.43329$Ms2.29510@fed1read03>, [email protected]=20 says...
> Matt J wrote:
> > On a really long/steep descent with rim brakes, it's possible to blow off tires due to the
> > extreme heat generated from braking.
>=20
> Are you sure about that? How hot do the rims get and how much can the ti=
re
> pressure increase? It doesn't seem likely to be all that much. There's =
not that
> much air in the tires. I think this notion comes from tubulars which can=
get
> hot enough that the glue melts and the tire can roll of the rim.

I had my rims become uncomfortably hot to the touch on a 70=B0 day just=20 testing my brakes in the
road in front of my house. I could easily=20 see how keeping them on down a large hill could heat
them 50 to 75=20 degrees or more, which might be plenty to blow off a tire if you=20 already had it
at max rated pressure.

--=20 Dave Kerber Fight spam: remove the ns_ from the return address before replying!

REAL programmers write self-modifying code.
 
"Chalo" <[email protected]> wrote:

> If you wish to use a gearhub, I recommend a linear-pull brake in front and a drum brake in the
> rear. That gives you the braking power advantages of the linear-pull with the wet-weather
> predictability and cleanliness of the drum brake, and the cost is reasonable.

One disadvantage of a rear drum brake is that it can complicate rear wheel removal. Hub gear/ drum
brake is the least convenient combination to deal with when mending a puncture at the roadside, and
for me the advantages of a drum brake don't outweigh this inconvenience.

James Thomson
 
In article <emJeb.43329$Ms2.29510@fed1read03>, [email protected] says...
> Matt J wrote:
> > On a really long/steep descent with rim brakes, it's possible to blow off tires due to the
> > extreme heat generated from braking.
>
> Are you sure about that? How hot do the rims get and how much can the tire pressure increase? It
> doesn't seem likely to be all that much. There's not that much air in the tires. I think this
> notion comes from tubulars which can get hot enough that the glue melts and the tire can roll of
> the rim.
>
> --Bill Davidson
>

This is ony of those things that has two points of view, some people say the tires blow off and
others say tyhe tubes blow and if the tire is off the rim it happened after the fact. I think one
thing is known to be true, it is possible to heat the rim so much that it cases the tube to fail. I
did it once, but it was a mountian bike so it just kind of deflated like any other puncture.
--
_________________________
Chris Phillipo - Cape Breton, Nova Scotia http://www.ramsays-online.com
 
"Chris Phillipo" <[email protected]> wrote in message
news:[email protected]...

> This is ony of those things that has two points of view, some people say the tires blow off and
> others say tyhe tubes blow and if the tire is off the rim it happened after the fact. I think one
> thing is known to be true, it is possible to heat the rim so much that it cases the tube to fail.
> I did it once, but it was a mountian bike so it just kind of deflated like any other puncture.

Perhaps there are two points of view on what *might* have happened, but the situation I witnessed
was clear. A friend of mine and I commuted to school together for several years. One day, he topped
off his rear tire with his dad's air compressor just before the ride. On the way in, he started
slowing down with a funny noise coming from his bike. We stopped and watched in amazement as his
rear tube crawled out from under the tire. The noise was the tube rubbing the frame as we rolled
along. After a minute or so, the tube had stretched enough to finally pop, making us both jump in
surprise. The cause wasn't overheating due to braking because we weren't braking at the time. It was
just overinflation.

Other than being a 27", I have no idea what kind of tube he had. Back in those days, we didn't
bother to carry tools or spare tubes because flats never happened. Seriously. This was the only flat
we ever experienced during years of commuting together. Either we were extremely lucky, or they just
made tubes a lot tougher back then.

-Buck
 
Chris Phillipo writes:

> This is one of those things that has two points of view, some people say the tires blow off and
> others say the tubes blow and if the tire is off the rim it happened after the fact. I think one
> thing is known to be true, it is possible to heat the rim so much that it cases the tube to fail.
> I did it once, but it was a mountain bike so it just kind of deflated like any other puncture.

You didn't actually say it but you suggest that the tube can explode inside a tire that does not
become unseated. I can assure you that if you heard the bang, the tire was off the rim. What do you
mean by: "it is possible to heat the rim so much that it cases the tube to fail"? We have often
heard from writers who claim their tube got a long straight slash, made a loud bang and that the
tire did not loft off the rim. This of course ignores that a tire often lifts of in an over heating
situation and falls back into place. Tubes do not develop slashes inside a tire.

Jobst Brandt [email protected]
 
Chalo:

> Jose Rizal <_@_._> wrote:
>
> > Chalo:
> >
> > > The disc brake's rotor is subject to heat warpage and impact damage, and cannot be trued like
> > > a wheel can.
> >
> > Yes it can, and easier than a rim. A simple adjustable wrench or smooth-faced pliers will do it.
>
> The stresses introduced in doing what you describe lead upon heating to further distortion, and
> you begin the cycle over again.
>
> Brake rotors are stress relieved by heat treating and then wet ground to a flat surface. If it
> were possible to simply bend them flat and have them stay that way, such measures would not be
> necessary.

In motorcycle discs maybe, but in bicycle discs such distortion from frictional heat that you
describe is not so great that this happens. Bicycle brake rotors don't always come perfectly flat
when new and it's common to true them prior to use, without adverse effect after hard usage.
 
"James Thomson" <[email protected]> wrote:

> One disadvantage of a rear drum brake is that it can complicate rear wheel removal. Hub gear/ drum
> brake is the least convenient combination to deal with when mending a puncture at the roadside,
> and for me the advantages of a drum brake don't outweigh this inconvenience.

If you are equipped and prepared to remove/replace a Shimano Nexus 7 gearhub wheel and its shift
cable, which is the kind of wheel the OP is discussing, then detaching the brake hardware does not
substantially complicate that process.

If removing a screw to release the brake torque arm is too much of an operation for you, you will
probably not be changing out a nutted hub or gearhub either.

Chalo Colina
 
[email protected] wrote:

: I think you are hypothesizing rather than working from experience.

That's correct, we don't have real hills so I'm descending in an armchair, if you will. Thanks for
corrections ;)

: The real reason for stopping is if the primary front brake, rim or drum failed. Without a front
: brake, hard braking is gone on a single. Descending fast uses primarily the front brake, the rear
: one being largely ineffective in any hard braking because it skids so easily.

How about a straight, long descent that isn't too steep? That wouldn't require hard braking. One
would pick up speed, one would have to brake for an extended period. Still, could overheating be a
problem - would the braking cause more heating than the cooling factors would handle? What if we
have HPVs with different aerodynamics: a touring bike vs road bike (guess one doesn't need to
descend in the drops though :) vs a recumbent vs a fully faired recumbent?

As a side note, bents could really make a difference as comes to overheating. A two-wheeled bent
with a front rim brake and 20" front wheel would be especially prone to the problem because of
faster descending and the rim is smaller so it takes less energy to heat it up. On the other hand a
tadpole trike with disc brakes on the front wheels would be quite well off on a descent that
requires hard braking.

Hmm anybody suggested air brakes as drag brakes yet? :)

--
Risto Varanka | http://www.helsinki.fi/~rvaranka/hpv/hpv.html varis at no spam please iki fi
 
Risto Varanka writes:

>> The real reason for stopping is if the primary front brake, rim or drum failed. Without a front
>> brake, hard braking is gone on a single. Descending fast uses primarily the front brake, the rear
>> one being largely ineffective in any hard braking because it skids so easily.

> How about a straight, long descent that isn't too steep? That wouldn't require hard braking. One
> would pick up speed, one would have to brake for an extended period. Still, could overheating be a
> problem - would the braking cause more heating than the cooling factors would handle?

As I pointed out, steep hills with straight runs between curves give enough cooling to prevent over
heating as on this road:

http://tinyurl.com/len5 http://tinyurl.com/lenc

the side view give a better sense of the 8-10% grade. The bottom of the picture is only half the
hill... there's more. This is not a blow-off grade except for the faint of heart who are unwilling
to let the bicycle roll free between curves. In contrast, some passes have closely packed curves
that seem safer to some but are in fact more capable of causing a blow-off.

> What if we have HPVs with different aerodynamics: a touring bike vs road bike (guess one doesn't
> need to descend in the drops though :) vs a recumbent vs a fully faired recumbent?

I suppose that would add doubly, first for the lack of wind drag and secondly for the grater weight.
Besides that, I have never seen a recumbent descent fast on a curvy road. The Stelvio, in the above
pictures, also has rough pavement over which on must stand on the pedals intermittently. That also
puts a crimp into recumbent descending.

> As a side note, bents could really make a difference as comes to overheating. A two-wheeled bent
> with a front rim brake and 20" front wheel would be especially prone to the problem because of
> faster descending and the rim is smaller so it takes less energy to heat it up. On the other hand
> a tadpole trike with disc brakes on the front wheels would be quite well off on a descent that
> requires hard braking.

I don't think designing a bicycle for single purpose does it for me. I use my bicycle on all kinds
of trails and roads, mountains and flatlands. It is a good compromise.

> Hmm anybody suggested air brakes as drag brakes yet? :)

We've been there.

Jobst Brandt [email protected]
 
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