Hard braking down hill blowouts

Is it normal to have blow-outs on a road bike from prolonged hard braking going
about one mile down a steep hill or should superior wheels and tires be able
to deal with the generated heat? I have an old panasonic but keep the tires and
tubes up to date (PerfomanceBike GT2 Kevlar, rated at 105 lbs, 26TPI, which are
not the best in the world but a heck of a lot better than what my LBS sold me.
It is a 27 inch rim and I could not find any better quality tires). But is it
the tire/wheel quality at issue? I have been thinking about getting a new bike
rather than trying to upgrade this one for a number of reasons (I don't think
it's even possible to switch to the current wheel size) but the blow-outs are
my biggest justification of expenditure to my wife.

Thanks.

Ben

Ben Kaufman wrote:

Is it normal to have blow-outs on a road bike from prolonged hard
braking going about one mile down a steep hill or should superior
wheels and tires be able to deal with the generated heat? I have an
old Panasonic but keep the tires and tubes up to date
(Performance Bike GT2 Kevlar, rated at 105 lbs, 26TPI, which are not
the best in the world but a heck of a lot better than what my LBS
sold me. It is a 27 inch rim and I could not find any better
quality tires). But is it the tire/wheel quality at issue? I have
been thinking about getting a new bike rather than trying to upgrade
this one for a number of reasons (I don't think it's even possible
to switch to the current wheel size) but the blow-outs are my
biggest justification of expenditure to my wife.

This is a traditional problem, so much so that bicycles are prohibited
down certain roads in the Alps. Zirlerberg above Innsbruck is a
classic with six runaway escape ramps for brake failures of trucks.
The Würzen Pass from Slovenia to Austria has a long 18% grade.

Locally in the Santa Clara Valley we have several tire blow-off
descents on Metcalf Road and Hicks and Almaden Roads, each of which
has had fatalities on descents. Tandems are noted for this problem,
especially if the driver isn't careful to use front and rear brakes
equally. For this reason tandems used for mountain touring are best
equipped with hub brakes (drums or disks).

In the days of tubular tires, that were glued onto rims, glue melted
allowing tires to creep and tear off the inflation stem causing rapid
deflation. I experienced that effect in the alps ans later had
clincher blow-offs on the same stretch of the Gotthard Pass.

I instrumented a wheel with pressure sensor and temperature
thermocouple to monitor a blow-off but was unsuccessful getting the
tire to separate even though temperature reached 150 degC and pressure
125 psi. The mechanism for blow-off is still unclear but it seams the
bead material softens and creeps of the hooked bead of the rim.

On steep descents after riding through water, I have generated steam
that came hissing out of the rim from around the stem as I braked for
successive hairpin turns on the Nufenen Pass in the Alps.

Jobst Brandt

Zirlerberg:

http://www.sagen.at/texte/gegenwart/...zirlerberg.htm

Jobst Brandt

On 28 Mar, 06:35, wrote:

In the days of tubular tires, that were glued onto rims, glue melted
allowing tires to creep and tear off the inflation stem causing rapid
deflation. *I experienced that effect in the alps ans later had
clincher blow-offs on the same stretch of the Gotthard Pass.

Not to dispute or anything, but where was that?
Down the south side of the old Tremola, perhaps?

Sergio
Pisa

On 2008-03-28, Ben Kaufman wrote:
Is it normal to have blow-outs on a road bike from prolonged hard
braking going about one mile down a steep hill or should superior
wheels and tires be able to deal with the generated heat? I have an
old panasonic but keep the tires and tubes up to date (PerfomanceBike
GT2 Kevlar, rated at 105 lbs, 26TPI, which are not the best in the
world but a heck of a lot better than what my LBS sold me. It is a
27 inch rim and I could not find any better quality tires). But is it
the tire/wheel quality at issue? I have been thinking about getting a
new bike rather than trying to upgrade this one for a number of
reasons (I don't think it's even possible to switch to the current
wheel size) but the blow-outs are my biggest justification of
expenditure to my wife.

Some math I did earlier:

http://groups.google.co.uk/group/uk....1d32532f671264

It's probably all wrong off course, but my tentative conclusion from it
is that the rims act as heat sinks-- they can soak up quite a lot of
energy, enough for most purposes, but they can't really cope with
sustained braking.

Disks on the other hand have much less heat capacity (because they're
smaller) and probably work hotter, so I reckon they might get up to
temperature quite quickly and then dissipate more because of the bigger
temperature difference with the surrounding air. This would mean they
should be better for continuous braking. In any case they won't make the
tyres pop off.

On Mar 28, 9:49 am, Ben C wrote:

Disks on the other hand have much less heat capacity

Nor do they exchannge heat with the tyre.

Sergio
Pisa

On 2008-03-28, sergio wrote:
On Mar 28, 9:49 am, Ben C wrote:

Disks on the other hand have much less heat capacity

Nor do they exchannge heat with the tyre.

Indeed. I wonder if they ever melt the grease in the hub though.

Sergio Servadio wrote:

In the days of tubular tires, that were glued onto rims, glue
melted allowing tires to creep and tear off the inflation stem
causing rapid deflation. Â*I experienced that effect in the alps ans
later had clincher blow-offs on the same stretch of the Gotthard
Pass.

Not to dispute or anything, but where was that? Down the south side
of the old Tremola, perhaps?

Yes, on the last hairpin turn leading to the Fortezza.

Jobst Brandt

Ben C? wrote:

Disks on the other hand have much less heat capacity

Nor do they exchange heat with the tyre.

Indeed. I wonder if they ever melt the grease in the hub though.

Most bicycle disks are mounted on support spiders so flimsy that heat
conduction to the bearings is insignificant.

Jobst Brandt

wrote:
Ben C? wrote:

Disks on the other hand have much less heat capacity

Nor do they exchange heat with the tyre.

Indeed. I wonder if they ever melt the grease in the hub though.

Most bicycle disks are mounted on support spiders so flimsy that heat
conduction to the bearings is insignificant.


"flimsy"??? is that stanford parlance for "stainless [disk] steel is a
poor conductor"? or is it presumptive nonsense from someone that
doesn't know what they're talking about?