Hard braking down hill blowouts

[Ben Kaufman]

On Sat, 29 Mar 2008 12:21:05 -0700 (PDT), datakoll <[email protected]> wrote:

>On Mar 27, 10:06 pm, Ben Kaufman <spaXm-mXe-anXd-paXy-5000-
>[email protected]> 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.
>>
>> Thanks.
>>
>> Ben
>
>How many times do you bake from X to Y ? and how long are the times
>from braking point A to BP B?

This is a continuous downhill slope for about 1 mile. To keep my speed at about
10mph braking is continuous using a pulsing technique. The longest the brakes
are off is about (roughly) 5 seconds. At the steepest part they are off for
perhaps 1 second.
>
>I wrote the following last night. Sidewall dirt at the bead is
>relatively invisible under normal shop conditions.
>a thorough all surfaces cleaning before reassembly helps (and before
>disassembly) then pull and push nipple in and out to seat and pinch
>pinch pinch sidewalls inward thoroughly all around before and then
>maybe during the first pounds going in.
>once in a while when placing a new tube in, I'll soap, soak and brush
>sidewalls clean. I cover the sidewalls with FL teflon wax on the bead
>then overspray the area with belt conditioner as brake prep so the
>dirt is on that surface, floats away when soaping. after it gets
>beaten with a stick.

How does dirt on the bead and lubricating the bead help avoid this type of
blow-out? Wouldn't a lubricant actually make it easier for a tube bubble to
get around the bead?

Ben

 

[Ben Kaufman]

On Sat, 29 Mar 2008 12:40:24 -0700 (PDT), datakoll <[email protected]> wrote:

>On Mar 27, 10:06 pm, Ben Kaufman <spaXm-mXe-anXd-paXy-5000-
>[email protected]> 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.
>>
>> Thanks.
>>
>> Ben
>
>btw. BEN, do you know what reconditioned Panasonics are fetching on
>Ebay?

No idea. I have two of them. While mine can pass for heavily used my wife's
(sport - model below mine) has about 25 miles and the original Panasonic brand
tires.

Ben

 

[Hank]

On Mar 29, 6:06 pm, Ben Kaufman <spaXm-mXe-anXd-paXy-5000-
[email protected]> wrote:
> On Sat, 29 Mar 2008 12:21:05 -0700 (PDT), datakoll <[email protected]> wrote:
> >On Mar 27, 10:06 pm, Ben Kaufman <spaXm-mXe-anXd-paXy-5000-
> >[email protected]> 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.
>
> >> Thanks.
>
> >> Ben
>
> >How many times do you bake from X to Y ? and how long are the times
> >from braking point A to BP B?
>
> This is a continuous downhill slope for about 1 mile. To keep my speed at about
> 10mph braking is continuous using a pulsing technique. The longest the brakes
> are off is about (roughly) 5 seconds. At the steepest part they are off for
> perhaps 1 second.
>
>
>
> >I wrote the following last night. Sidewall dirt at the bead is
> >relatively invisible under normal shop conditions.
> >a thorough all surfaces cleaning before reassembly helps (and before
> >disassembly) then pull and push nipple in and out to seat and pinch
> >pinch pinch sidewalls inward thoroughly all around before and then
> >maybe during the first pounds going in.
> >once in a while when placing a new tube in, I'll soap, soak and brush
> >sidewalls clean. I cover the sidewalls with FL teflon wax on the bead
> >then overspray the area with belt conditioner as brake prep so the
> >dirt is on that surface, floats away when soaping. after it gets
> >beaten with a stick.
>
> How does dirt on the bead and lubricating the bead help avoid this type of
> blow-out? Wouldn't a lubricant actually make it easier for a tube bubble to
> get around the bead?
>
> Ben

Why 10mph? Even if it's winding, surely 15 doesn't seem too out of the
realm of sanity, IMO. I guess I'd have to see the road before passing
judgment on that, but 10mph just seems you're trading one danger for
another, if you're having frequent blowouts.

 

[A Muzi]

>> Ben Kaufman <[email protected]> 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.

> datakoll <[email protected]> wrote:
>> How many times do you bake from X to Y ? and how long are the times
>>from braking point A to BP B?

Ben Kaufman wrote:
> This is a continuous downhill slope for about 1 mile. To keep my speed at about
> 10mph braking is continuous using a pulsing technique. The longest the brakes
> are off is about (roughly) 5 seconds. At the steepest part they are off for
> perhaps 1 second.

>> I wrote the following last night. Sidewall dirt at the bead is
>> relatively invisible under normal shop conditions.
>> a thorough all surfaces cleaning before reassembly helps (and before
>> disassembly) then pull and push nipple in and out to seat and pinch
>> pinch pinch sidewalls inward thoroughly all around before and then
>> maybe during the first pounds going in.
>> once in a while when placing a new tube in, I'll soap, soak and brush
>> sidewalls clean. I cover the sidewalls with FL teflon wax on the bead
>> then overspray the area with belt conditioner as brake prep so the
>> dirt is on that surface, floats away when soaping. after it gets
>> beaten with a stick.

Ben Kaufman wrote:
> How does dirt on the bead and lubricating the bead help avoid this type of
> blow-out? Wouldn't a lubricant actually make it easier for a tube bubble to
> get around the bead?

Unless something's awry with your particular equipment (torn tire,
dented rim), a nondestructive lubricant just makes proper, even seating
quicker. Car shops use soapy water on a brush which works equally well,
albeit messy. Once the tire is mounted straight, the manufacturer's
pressure ratings become relevant; If uneven, tires will creep over the edge.
--
Andrew Muzi
www.yellowjersey.org
Open every day since 1 April, 1971

 

[Bill Bushnell]

[email protected] wrote:
> 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.

I stopped getting blowoffs when I reduced cold inflation pressure from 85 to 75
psi (Ritchey Tom Slick, 559x36 on Ritchey rim). I believe that the likelihood of
blowoff may be especially sensitive to inflation pressure because even though
I've been riding around lately with an extra 45 lbs. on the bike, I still don't
get blowoffs, even on the roads where they are likely to occur (Hicks, Vista
Verde/Ramona, etc.)

Perhaps you would consider repeating your test with incrementally more air
pressure in your tire.

--
Bill Bushnell
http://pobox.com/~bushnell/

 

[sergio]

On Mar 31, 7:17 am, Bill Bushnell <[email protected]> wrote:

> Perhaps you would consider repeating your test with incrementally more air
> pressure in your tire.

Obviously unecessary.
What matters is the ultimate, not the starting, pressure.

Sergio
Pisa

 

[Ben Kaufman]

On Sat, 29 Mar 2008 21:39:19 -0700 (PDT), Hank <[email protected]> wrote:

>On Mar 29, 6:06 pm, Ben Kaufman <spaXm-mXe-anXd-paXy-5000-
>[email protected]> wrote:
>> On Sat, 29 Mar 2008 12:21:05 -0700 (PDT), datakoll <[email protected]> wrote:
>> >On Mar 27, 10:06 pm, Ben Kaufman <spaXm-mXe-anXd-paXy-5000-
>> >[email protected]> 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.
>>
>> >> Thanks.
>>
>> >> Ben
>>
>> >How many times do you bake from X to Y ? and how long are the times
>> >from braking point A to BP B?
>>
>> This is a continuous downhill slope for about 1 mile. To keep my speed at about
>> 10mph braking is continuous using a pulsing technique. The longest the brakes
>> are off is about (roughly) 5 seconds. At the steepest part they are off for
>> perhaps 1 second.
>>
>>
>>
>> >I wrote the following last night. Sidewall dirt at the bead is
>> >relatively invisible under normal shop conditions.
>> >a thorough all surfaces cleaning before reassembly helps (and before
>> >disassembly) then pull and push nipple in and out to seat and pinch
>> >pinch pinch sidewalls inward thoroughly all around before and then
>> >maybe during the first pounds going in.
>> >once in a while when placing a new tube in, I'll soap, soak and brush
>> >sidewalls clean. I cover the sidewalls with FL teflon wax on the bead
>> >then overspray the area with belt conditioner as brake prep so the
>> >dirt is on that surface, floats away when soaping. after it gets
>> >beaten with a stick.
>>
>> How does dirt on the bead and lubricating the bead help avoid this type of
>> blow-out? Wouldn't a lubricant actually make it easier for a tube bubble to
>> get around the bead?
>>
>> Ben
>
>Why 10mph? Even if it's winding, surely 15 doesn't seem too out of the
>realm of sanity, IMO. I guess I'd have to see the road before passing
>judgment on that, but 10mph just seems you're trading one danger for
>another, if you're having frequent blowouts.

I could handle the road at 20 mph but my assumption has been that higher speeds
generate greater rim temperatures due to the square of the velocity for the
kinetic energy formula. If the hill wasn't so steep then taking it faster would
probably require a lot less use of the brakes but based upon other safer hills
that take me to about 40 mph, I would say this should be a significantly faster
decent.

Ben

 

[Michael Press]

In article <[email protected]>,
Ben Kaufman <[email protected]> wrote:

> On Sat, 29 Mar 2008 21:39:19 -0700 (PDT), Hank <[email protected]> wrote:
> >Why 10mph? Even if it's winding, surely 15 doesn't seem too out of the
> >realm of sanity, IMO. I guess I'd have to see the road before passing
> >judgment on that, but 10mph just seems you're trading one danger for
> >another, if you're having frequent blowouts.
>
> I could handle the road at 20 mph but my assumption has been that higher speeds
> generate greater rim temperatures due to the square of the velocity for the
> kinetic energy formula. If the hill wasn't so steep then taking it faster would
> probably require a lot less use of the brakes but based upon other safer hills
> that take me to about 40 mph, I would say this should be a significantly faster
> decent.

I explained this several times. The amount of energy dissipated
is the same for a given descent and initial and terminal speeds.
Forget kinetic energy as a function of speed. Concentrate on
total energy dissipated. The goal is to reduce the heating of
the tire bead and the air in the tube. If the rim is always hot
as with constant braking you are guaranteed to heat the bead
an inflation air. If you allow for free runs downhill with no
braking, the rim will cool quickly, very much more quickly if
you allow the free runs to have maximum speed before braking.
Heavy braking decreases the amount of time to heat the rim.
Then you get off the brakes and allow the rim to cool. The
energy dissipation rate for a hot rim increases with the
temperature, so you are spending more time dissipating heat to
the air at a high rate; dissipating heat that will not go to
heating the bead and inflation air. Another thing you have not
considered is that when coasting downhill at speed you dissipate
more energy to wind drag the faster you go; the rate of dissipation
increases more than linearly with speed.

--
Michael Press

 

[Ben C]

On 2008-03-31, Michael Press <[email protected]> wrote:
> In article <[email protected]>,
> Ben Kaufman <[email protected]> wrote:
>
>> On Sat, 29 Mar 2008 21:39:19 -0700 (PDT), Hank <[email protected]> wrote:
>> >Why 10mph? Even if it's winding, surely 15 doesn't seem too out of the
>> >realm of sanity, IMO. I guess I'd have to see the road before passing
>> >judgment on that, but 10mph just seems you're trading one danger for
>> >another, if you're having frequent blowouts.
>>
>> I could handle the road at 20 mph but my assumption has been that higher speeds
>> generate greater rim temperatures due to the square of the velocity for the
>> kinetic energy formula. If the hill wasn't so steep then taking it faster would
>> probably require a lot less use of the brakes but based upon other safer hills
>> that take me to about 40 mph, I would say this should be a significantly faster
>> decent.
>
> I explained this several times. The amount of energy dissipated
> is the same for a given descent and initial and terminal speeds.

Yes, but the rate at which it is dissipated (power) is not. I think Ben
has a point: braking power is minimized by descending as slowly as
possible, even though total braking energy will be higher (the air will
do less for you at a lower speed, and you will have less k.e. at the
bottom).

If the brakes are overheating, going slowly will help I think. Yes if
you get them hotter they will cool faster, but with rim brakes I suspect
that's a losing battle-- the temperature at which the tyres blow off is
not that high.

 

[Michael Press]

In article <[email protected]>,
Ben C <[email protected]> wrote:

> On 2008-03-31, Michael Press <[email protected]> wrote:
> > In article <[email protected]>,
> > Ben Kaufman <[email protected]> wrote:
> >
> >> On Sat, 29 Mar 2008 21:39:19 -0700 (PDT), Hank <[email protected]> wrote:
> >> >Why 10mph? Even if it's winding, surely 15 doesn't seem too out of the
> >> >realm of sanity, IMO. I guess I'd have to see the road before passing
> >> >judgment on that, but 10mph just seems you're trading one danger for
> >> >another, if you're having frequent blowouts.
> >>
> >> I could handle the road at 20 mph but my assumption has been that higher speeds
> >> generate greater rim temperatures due to the square of the velocity for the
> >> kinetic energy formula. If the hill wasn't so steep then taking it faster would
> >> probably require a lot less use of the brakes but based upon other safer hills
> >> that take me to about 40 mph, I would say this should be a significantly faster
> >> decent.
> >
> > I explained this several times. The amount of energy dissipated
> > is the same for a given descent and initial and terminal speeds.
>
> Yes, but the rate at which it is dissipated (power) is not. I think Ben
> has a point: braking power is minimized by descending as slowly as
> possible, even though total braking energy will be higher (the air will
> do less for you at a lower speed, and you will have less k.e. at the
> bottom).
>
> If the brakes are overheating, going slowly will help I think. Yes if
> you get them hotter they will cool faster, but with rim brakes I suspect
> that's a losing battle-- the temperature at which the tyres blow off is
> not that high.

Why do you choose to silently excise the technical material in
the message to which you replied?

--
Michael Press

 

[[email protected]]

On Mar 31, 12:50 pm, Ben C <[email protected]> wrote:
> On 2008-03-31, Michael Press <[email protected]> wrote:
>
> > In article <[email protected]>,
> >  Ben Kaufman <[email protected]> wrote:
>
> >> On Sat, 29 Mar 2008 21:39:19 -0700 (PDT), Hank <[email protected]> wrote:
> >> >Why 10mph? Even if it's winding, surely 15 doesn't seem too out of the
> >> >realm of sanity, IMO. I guess I'd have to see the road before passing
> >> >judgment on that, but 10mph just seems you're trading one danger for
> >> >another, if you're having frequent blowouts.
>
> >> I could handle the road at 20 mph but my assumption has been that higher speeds
> >> generate greater rim temperatures due to the square of the velocity forthe
> >> kinetic energy formula.  If the hill wasn't so steep then taking it faster would
> >> probably require a lot less use of the brakes but based upon other safer  hills
> >> that take me to about 40 mph, I would say this should be a significantly  faster
> >> decent.  
>
> > I explained this several times. The amount of energy dissipated
> > is the same for a given descent and initial and terminal speeds.
>
> Yes, but the rate at which it is dissipated (power) is not. I think Ben
> has a point: braking power is minimized by descending as slowly as
> possible, even though total braking energy will be higher (the air will
> do less for you at a lower speed, and you will have less k.e. at the
> bottom).
>
> If the brakes are overheating, going slowly will help I think. Yes if
> you get them hotter they will cool faster, but with rim brakes I suspect
> that's a losing battle-- the temperature at which the tyres blow off is
> not that high

Dear Ben,

A few years ago, I wondered about fixie braking downhill.

On the same hill, the braking watts needed to maintain a particular
speed rise and fall in a curve, up from 0 watts at a standstill (no
movement) to a maximum value and then back to 0 watts again at top
speed (no braking).

For a 40 mph coasting-speed hill, the peak of the braking power curve
was around 23 mph. Faster or slower than 23 mph meant less braking
effort. It would take considerable (and impractical) calculations to
discover which two points on either side of the peak of the curve
correspond to each other.

That is, for 15 mph on the hill, the same braking power is needed at
some speed between 23 mph and 40 mph, but the precise speed is not
practical.

Here's where Peter Cole cleared things up:
http://groups.google.com/group/rec.bicycles.tech/msg/20e679ab31dbf07d

Even going very slowly, Jobst blew a tire off. I did the same thing,
even though I'd read his post:

http://groups.google.com/group/rec.bicycles.tech/msg/e7b83b9ccefa849f

Cheers,

Carl Fogel

 

[Ben C]

On 2008-03-31, [email protected] <[email protected]> wrote:
> On Mar 31, 12:50 pm, Ben C <[email protected]> wrote:
[...]
>> Yes, but the rate at which it is dissipated (power) is not. I think Ben
>> has a point: braking power is minimized by descending as slowly as
>> possible, even though total braking energy will be higher (the air will
>> do less for you at a lower speed, and you will have less k.e. at the
>> bottom).
>>
>> If the brakes are overheating, going slowly will help I think. Yes if
>> you get them hotter they will cool faster, but with rim brakes I suspect
>> that's a losing battle-- the temperature at which the tyres blow off is
>> not that high
>
> Dear Ben,
>
> A few years ago, I wondered about fixie braking downhill.
>
> On the same hill, the braking watts needed to maintain a particular
> speed rise and fall in a curve, up from 0 watts at a standstill (no
> movement) to a maximum value and then back to 0 watts again at top
> speed (no braking).
>
> For a 40 mph coasting-speed hill, the peak of the braking power curve
> was around 23 mph. Faster or slower than 23 mph meant less braking
> effort. It would take considerable (and impractical) calculations to
> discover which two points on either side of the peak of the curve
> correspond to each other.
>
> That is, for 15 mph on the hill, the same braking power is needed at
> some speed between 23 mph and 40 mph, but the precise speed is not
> practical.
>
> Here's where Peter Cole cleared things up:
> http://groups.google.com/group/rec.bicycles.tech/msg/20e679ab31dbf07d
>
> Even going very slowly, Jobst blew a tire off. I did the same thing,
> even though I'd read his post:
>
> http://groups.google.com/group/rec.bicycles.tech/msg/e7b83b9ccefa849f

Thanks for the links. From what I can understand of that, the 23mph peak
arises because of wind drag. If you go faster you get more help from the
wind. But if you go slower, you are dumping the given amount of
gravitational potential energy you have into the brakes at a lower rate,
which helps them not to get so hot.

The OP's hill is good for 50mph coasting-speed and he's talking about
going 10mph rather than 20mph. I think he might be better off at 10mph,
although he might be doomed either way (see below).

You overheated a wheel descending at 10mph, but would the situation have
been better at 20mph? It depends on the hill-- if it was very steep,
10mph might have been better than 20mph.

But there is one factor I'm not sure of here, which is the rate of
cooling of a rim. The fastest you can safely descend (given that you
want to be on the slow side of the peak) is the speed at which the rim
can remain constantly at the temperature just below that required to
give you tyre problems. In other words the speed at which which braking
power is equal to cooling power.

You can also ask what's the slowest speed at which you can stably
descend while being on the fast side of the peak. But if the peak is too
fast for safety, then you'll have to choose the slow side.

There is a slow and a fast stable descending speed for any given
indefinitely long hill and given bike+rider mass. I suspect that the
slow stable descending speed is rather low, but I don't know what it is.

If you go faster than the slow stable speed (but less than the peak, or
slower than the fast stable speed but faster than the peak) then it's
only a matter of time before heat builds up to dangerous levels. So it
depends how long the hill goes on for.

It would be interesting to know just what that the slow stable speed is
for a typical steep hill. It might be unreasonably slow, in which case
the OP is doomed unless the hill is short and will have to bite the
bullet and go at the fast stable speed, get disk brakes, or walk.

http://www.spiraxsarco.com/resource...es-and-heat-transfer/heat-transfer.asp#head33

or

http://tinyurl.com/2vpv7v

describes a "general heat transfer equation" which looks quite easy to
deal with.

Does anyone know the "overall heat transfer coefficient" for aluminium
in air?

 

[Ben C]

On 2008-03-31, Michael Press <[email protected]> wrote:
> In article <[email protected]>,
> Ben Kaufman <[email protected]> wrote:
>
>> On Sat, 29 Mar 2008 21:39:19 -0700 (PDT), Hank <[email protected]> wrote:
>> >Why 10mph? Even if it's winding, surely 15 doesn't seem too out of the
>> >realm of sanity, IMO. I guess I'd have to see the road before passing
>> >judgment on that, but 10mph just seems you're trading one danger for
>> >another, if you're having frequent blowouts.
>>
>> I could handle the road at 20 mph but my assumption has been that
>> higher speeds generate greater rim temperatures due to the square of
>> the velocity for the kinetic energy formula. If the hill wasn't so
>> steep then taking it faster would probably require a lot less use of
>> the brakes but based upon other safer hills that take me to about 40
>> mph, I would say this should be a significantly faster decent.
>
> I explained this several times. The amount of energy dissipated
> is the same for a given descent and initial and terminal speeds.

Yes. But the power at which the rim is heated and at which it cools is
not.

> Forget kinetic energy as a function of speed.

Why? It seems to me it is quite important.

> Concentrate on total energy dissipated. The goal is to reduce the
> heating of the tire bead and the air in the tube. If the rim is always
> hot as with constant braking you are guaranteed to heat the bead an
> inflation air.

If you heat the rim at the same rate at which it is also cooling, then
you're going to be all right aren't you?

> If you allow for free runs downhill with no braking, the rim will cool
> quickly, very much more quickly if you allow the free runs to have
> maximum speed before braking.

Yes. But it might not be safe to descend with no braking.

> Heavy braking decreases the amount of time to heat the rim.
> Then you get off the brakes and allow the rim to cool. The
> energy dissipation rate for a hot rim increases with the
> temperature, so you are spending more time dissipating heat to
> the air at a high rate; dissipating heat that will not go to
> heating the bead and inflation air.

I agree that non-continuous braking may help.

> Another thing you have not considered is that when coasting downhill
> at speed you dissipate more energy to wind drag the faster you go; the
> rate of dissipation increases more than linearly with speed.

This is all good advice, but it doesn't answer the question: is it
better to brake continuously to maintain 10mph or continuously to
maintain 20mph on a hill with a coasting speed of 50mph?

If my tyre blows off either way, do I get further before it blows off if
I go at 10mph or if I go at 20mph?

 

[Ben C]

On 2008-03-31, Michael Press <[email protected]> wrote:
> In article <[email protected]>,
> Ben C <[email protected]> wrote:
>
>> On 2008-03-31, Michael Press <[email protected]> wrote:
>> > In article <[email protected]>,
>> > Ben Kaufman <[email protected]> wrote:
>> >
>> >> On Sat, 29 Mar 2008 21:39:19 -0700 (PDT), Hank <[email protected]> wrote:
>> >> >Why 10mph? Even if it's winding, surely 15 doesn't seem too out of the
>> >> >realm of sanity, IMO. I guess I'd have to see the road before passing
>> >> >judgment on that, but 10mph just seems you're trading one danger for
>> >> >another, if you're having frequent blowouts.
>> >>
>> >> I could handle the road at 20 mph but my assumption has been that higher speeds
>> >> generate greater rim temperatures due to the square of the velocity for the
>> >> kinetic energy formula. If the hill wasn't so steep then taking it faster would
>> >> probably require a lot less use of the brakes but based upon other safer hills
>> >> that take me to about 40 mph, I would say this should be a significantly faster
>> >> decent.
>> >
>> > I explained this several times. The amount of energy dissipated
>> > is the same for a given descent and initial and terminal speeds.
>>
>> Yes, but the rate at which it is dissipated (power) is not. I think Ben
>> has a point: braking power is minimized by descending as slowly as
>> possible, even though total braking energy will be higher (the air will
>> do less for you at a lower speed, and you will have less k.e. at the
>> bottom).
>>
>> If the brakes are overheating, going slowly will help I think. Yes if
>> you get them hotter they will cool faster, but with rim brakes I suspect
>> that's a losing battle-- the temperature at which the tyres blow off is
>> not that high.
>
> Why do you choose to silently excise the technical material in
> the message to which you replied?

I often excise material if it doesn't seem relevant to the particular
quibble I am making.

But since you mention it, I have just posted another followup with no
snipping which I hope will be clearer.

 

[Bill Bushnell]

sergio <[email protected]> wrote:
> On Mar 31, 7:17 am, Bill Bushnell <[email protected]> wrote:

> > Perhaps you would consider repeating your test with incrementally more air
> > pressure in your tire.

> Obviously unecessary.
> What matters is the ultimate, not the starting, pressure.

Both are interesting. The cold pressure matters because it's something the user
can easily measure and set. I know that if I inflate my tire to no more than 75
psi I'm unlikely to have a blowoff on any local descent. This is useful
information.

Based on my own experience (1 sample), I found I no longer had blowoffs on my
particular tire/rim combination after reducing my cold inflation pressure from 85
to 75 psi. Even after I started carrying more weight on the bike, and presumably
generating more rim heat on the descents, I still did not get any blowoffs. Jobst
doesn't say what cold pressure he started with, but I'm suggesting that he might
yet achieve a blowoff if he were to start with more pressure in the tire.

--
Bill Bushnell
http://pobox.com/~bushnell/

 

[Ben Kaufman]

On Mon, 31 Mar 2008 11:29:49 -0700, Michael Press <[email protected]> wrote:

>In article <[email protected]>,
> Ben Kaufman <[email protected]> wrote:
>
>> On Sat, 29 Mar 2008 21:39:19 -0700 (PDT), Hank <[email protected]> wrote:
>> >Why 10mph? Even if it's winding, surely 15 doesn't seem too out of the
>> >realm of sanity, IMO. I guess I'd have to see the road before passing
>> >judgment on that, but 10mph just seems you're trading one danger for
>> >another, if you're having frequent blowouts.
>>
>> I could handle the road at 20 mph but my assumption has been that higher speeds
>> generate greater rim temperatures due to the square of the velocity for the
>> kinetic energy formula. If the hill wasn't so steep then taking it faster would
>> probably require a lot less use of the brakes but based upon other safer hills
>> that take me to about 40 mph, I would say this should be a significantly faster
>> decent.
>
>I explained this several times. The amount of energy dissipated
>is the same for a given descent and initial and terminal speeds.
>Forget kinetic energy as a function of speed. Concentrate on
>total energy dissipated. The goal is to reduce the heating of
>the tire bead and the air in the tube. If the rim is always hot
>as with constant braking you are guaranteed to heat the bead
>an inflation air. If you allow for free runs downhill with no
>braking, the rim will cool quickly, very much more quickly if
>you allow the free runs to have maximum speed before braking.
>Heavy braking decreases the amount of time to heat the rim.
>Then you get off the brakes and allow the rim to cool. The
>energy dissipation rate for a hot rim increases with the
>temperature, so you are spending more time dissipating heat to
>the air at a high rate; dissipating heat that will not go to
>heating the bead and inflation air. Another thing you have not
>considered is that when coasting downhill at speed you dissipate
>more energy to wind drag the faster you go; the rate of dissipation
>increases more than linearly with speed.

As I have explained in previous messages, I cannot to go very fast due to
concerns about cars.

Ben.

 

[Ron George]

On Mar 27, 10:06 pm, Ben Kaufman <spaXm-mXe-anXd-paXy-5000-
[email protected]> 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.
>
> Thanks.
>
> Ben

Ben,

Here are some tubulars popping off wheels. Famous accidents of the
Tour de France. http://cozybeehive.blogspot.com/2007/11/tubulars-exploding-and-peeling-off.html

This is an interesting discussion, I'm watching where its leading.

Ron
http://cozybeehive.blogspot.com

 

[Ben Kaufman]

On Mon, 31 Mar 2008 12:19:06 -0700 (PDT), [email protected] wrote:

>On Mar 31, 12:50 pm, Ben C <[email protected]> wrote:
>> On 2008-03-31, Michael Press <[email protected]> wrote:
>>
>> > In article <[email protected]>,
>> >  Ben Kaufman <[email protected]> wrote:
>>
>> >> On Sat, 29 Mar 2008 21:39:19 -0700 (PDT), Hank <[email protected]> wrote:
>> >> >Why 10mph? Even if it's winding, surely 15 doesn't seem too out of the
>> >> >realm of sanity, IMO. I guess I'd have to see the road before passing
>> >> >judgment on that, but 10mph just seems you're trading one danger for
>> >> >another, if you're having frequent blowouts.
>>
>> >> I could handle the road at 20 mph but my assumption has been that higher speeds
>> >> generate greater rim temperatures due to the square of the velocity for the
>> >> kinetic energy formula.  If the hill wasn't so steep then taking it faster would
>> >> probably require a lot less use of the brakes but based upon other safer  hills
>> >> that take me to about 40 mph, I would say this should be a significantly  faster
>> >> decent.  
>>
>> > I explained this several times. The amount of energy dissipated
>> > is the same for a given descent and initial and terminal speeds.
>>
>> Yes, but the rate at which it is dissipated (power) is not. I think Ben
>> has a point: braking power is minimized by descending as slowly as
>> possible, even though total braking energy will be higher (the air will
>> do less for you at a lower speed, and you will have less k.e. at the
>> bottom).
>>
>> If the brakes are overheating, going slowly will help I think. Yes if
>> you get them hotter they will cool faster, but with rim brakes I suspect
>> that's a losing battle-- the temperature at which the tyres blow off is
>> not that high
>
>Dear Ben,
>
>A few years ago, I wondered about fixie braking downhill.
>
>On the same hill, the braking watts needed to maintain a particular
>speed rise and fall in a curve, up from 0 watts at a standstill (no
>movement) to a maximum value and then back to 0 watts again at top
>speed (no braking).
>
>For a 40 mph coasting-speed hill, the peak of the braking power curve
>was around 23 mph. Faster or slower than 23 mph meant less braking
>effort. It would take considerable (and impractical) calculations to
>discover which two points on either side of the peak of the curve
>correspond to each other.
>
>That is, for 15 mph on the hill, the same braking power is needed at
>some speed between 23 mph and 40 mph, but the precise speed is not
>practical.
>
>Here's where Peter Cole cleared things up:
> http://groups.google.com/group/rec.bicycles.tech/msg/20e679ab31dbf07d
>
>Even going very slowly, Jobst blew a tire off. I did the same thing,
>even though I'd read his post:
>
>http://groups.google.com/group/rec.bicycles.tech/msg/e7b83b9ccefa849f
>
>Cheers,
>
>Carl Fogel

FWIW, i used my GPS's topo map to roughly calculate the grade for the steepest
part which is 12.9%. That is in .16 miles (845 ft) it drops 109 ft.

The average grade for the entire decent is .79 miles (4,171ft) it drops 303ft
for grade of 7.26%


Ben

 

[Ben Kaufman]

On Mon, 31 Mar 2008 13:50:08 -0500, Ben C <[email protected]> wrote:

>On 2008-03-31, Michael Press <[email protected]> wrote:
>> In article <[email protected]>,
>> Ben Kaufman <[email protected]> wrote:
>>
>>> On Sat, 29 Mar 2008 21:39:19 -0700 (PDT), Hank <[email protected]> wrote:
>>> >Why 10mph? Even if it's winding, surely 15 doesn't seem too out of the
>>> >realm of sanity, IMO. I guess I'd have to see the road before passing
>>> >judgment on that, but 10mph just seems you're trading one danger for
>>> >another, if you're having frequent blowouts.
>>>
>>> I could handle the road at 20 mph but my assumption has been that higher speeds
>>> generate greater rim temperatures due to the square of the velocity for the
>>> kinetic energy formula. If the hill wasn't so steep then taking it faster would
>>> probably require a lot less use of the brakes but based upon other safer hills
>>> that take me to about 40 mph, I would say this should be a significantly faster
>>> decent.
>>
>> I explained this several times. The amount of energy dissipated
>> is the same for a given descent and initial and terminal speeds.
>
>Yes, but the rate at which it is dissipated (power) is not. I think Ben
>has a point: braking power is minimized by descending as slowly as
>possible, even though total braking energy will be higher (the air will
>do less for you at a lower speed, and you will have less k.e. at the
>bottom).
>
>If the brakes are overheating, going slowly will help I think. Yes if
>you get them hotter they will cool faster, but with rim brakes I suspect
>that's a losing battle-- the temperature at which the tyres blow off is
>not that high.

Next time I am going to try 5mph. I am game for anything that will get me to
crash slower.

Ben

 

[Ben Kaufman]

On Mon, 31 Mar 2008 15:40:20 -0500, Ben C <[email protected]> wrote:

>On 2008-03-31, [email protected] <[email protected]> wrote:
>> On Mar 31, 12:50 pm, Ben C <[email protected]> wrote:
>[...]
>>> Yes, but the rate at which it is dissipated (power) is not. I think Ben
>>> has a point: braking power is minimized by descending as slowly as
>>> possible, even though total braking energy will be higher (the air will
>>> do less for you at a lower speed, and you will have less k.e. at the
>>> bottom).
>>>
>>> If the brakes are overheating, going slowly will help I think. Yes if
>>> you get them hotter they will cool faster, but with rim brakes I suspect
>>> that's a losing battle-- the temperature at which the tyres blow off is
>>> not that high
>>
>> Dear Ben,
>>
>> A few years ago, I wondered about fixie braking downhill.
>>
>> On the same hill, the braking watts needed to maintain a particular
>> speed rise and fall in a curve, up from 0 watts at a standstill (no
>> movement) to a maximum value and then back to 0 watts again at top
>> speed (no braking).
>>
>> For a 40 mph coasting-speed hill, the peak of the braking power curve
>> was around 23 mph. Faster or slower than 23 mph meant less braking
>> effort. It would take considerable (and impractical) calculations to
>> discover which two points on either side of the peak of the curve
>> correspond to each other.
>>
>> That is, for 15 mph on the hill, the same braking power is needed at
>> some speed between 23 mph and 40 mph, but the precise speed is not
>> practical.
>>
>> Here's where Peter Cole cleared things up:
>> http://groups.google.com/group/rec.bicycles.tech/msg/20e679ab31dbf07d
>>
>> Even going very slowly, Jobst blew a tire off. I did the same thing,
>> even though I'd read his post:
>>
>> http://groups.google.com/group/rec.bicycles.tech/msg/e7b83b9ccefa849f
>
>Thanks for the links. From what I can understand of that, the 23mph peak
>arises because of wind drag. If you go faster you get more help from the
>wind. But if you go slower, you are dumping the given amount of
>gravitational potential energy you have into the brakes at a lower rate,
>which helps them not to get so hot.
>
>The OP's hill is good for 50mph coasting-speed and he's talking about
>going 10mph rather than 20mph. I think he might be better off at 10mph,
>although he might be doomed either way (see below).
>
>You overheated a wheel descending at 10mph, but would the situation have
>been better at 20mph? It depends on the hill-- if it was very steep,
>10mph might have been better than 20mph.
>
>But there is one factor I'm not sure of here, which is the rate of
>cooling of a rim. The fastest you can safely descend (given that you
>want to be on the slow side of the peak) is the speed at which the rim
>can remain constantly at the temperature just below that required to
>give you tyre problems. In other words the speed at which which braking
>power is equal to cooling power.
>
>You can also ask what's the slowest speed at which you can stably
>descend while being on the fast side of the peak. But if the peak is too
>fast for safety, then you'll have to choose the slow side.
>
>There is a slow and a fast stable descending speed for any given
>indefinitely long hill and given bike+rider mass. I suspect that the
>slow stable descending speed is rather low, but I don't know what it is.
>
>If you go faster than the slow stable speed (but less than the peak, or
>slower than the fast stable speed but faster than the peak) then it's
>only a matter of time before heat builds up to dangerous levels. So it
>depends how long the hill goes on for.
>
>It would be interesting to know just what that the slow stable speed is
>for a typical steep hill. It might be unreasonably slow, in which case
>the OP is doomed unless the hill is short and will have to bite the
>bullet and go at the fast stable speed, get disk brakes, or walk.
>
>http://www.spiraxsarco.com/resource...es-and-heat-transfer/heat-transfer.asp#head33
>
>or
>
>http://tinyurl.com/2vpv7v
>
>describes a "general heat transfer equation" which looks quite easy to
>deal with.
>
>Does anyone know the "overall heat transfer coefficient" for aluminium
>in air?

How about steel? The wheels on my old panasonic are pretty heavy.

Ben