Re: Exploding tires II

Joe Riel said:

Quoted message said:

Quoted message said:

Well, a quick back of the email calculation shows that for a bike and
rider weighing 1000N (225lbs) and a 10% grade, maximum energy
dissipation by the brakes will occur at a speed of around 50 km/h. At
that speed the bike is losing gravitational potential energy at a rate
of 1388 watts and it is taking 465 watts for wind and rolling
resistance, so that leaves 923 watts for the brakes to get rid of. If
we are using only one brake that is 923 watts into a 450 gm piece of
Al with a surface area of a few hundred cm^2. I suspect it could get
pretty hot.

Hold the phone! I'll propose that the rim will get hotter at 1/10
that speed because there is practically no convection cooling when
riding slowly. Convection is the primary coolant of rims in the
absence of rain water. Besides, the rate of conversion of kinetic
energy to heat increases with speed until the brake pad fails. Where
do you get 50km/h?

Quoted message said:

I should let Jim respond, but from my post it is clear that this
is the speed that maximizes the power dissipation in the brakes.

You are assuming constant speed braking rather than braking for
hairpin turns at the end of free rolling straights. My experience is
that the tube is well insulated by the tire and that heat comes in
mostly by contact with the rim (strip).

Quoted message said:

As you suggest, this may not be the worst-case for rim temperature.

Quoted message said:

What's a reasonable model for convection losses vs speed?

I have no idea. I'm sure there is data on wind speed air temperature
and heat transfer, but that requires a known rim temperature and
speed. As I said, steep slow descents are the bane of tires. These
are where I've seen the blowouts.

Jobst Brandt
[email hidden]

Quoted message said:

Carl Fogel writes:

[snip]

Quoted message said:

Quoted message said:

Quoted message said:

The "physics lesson" used an inappropriately low temperature as I
see it.

Quoted message said:

So what's an appropriate rim temperature for your physics
lesson?

Quoted message said:

Elsewhere in this thread, people are calculating rim
temperatures of 570 degrees K (or F, take your pick).

How about starting a 110psi and, following Mike's test of at what
pressur a tire comes off wehn ridden at room temperature, re-evaluate
what temperature that takes. As I said, I could make plenty of steam
on a not-so-steep Alpine pass. I know that you can burn hands
(blisters) at the tempreature I developed on an 18% grade in 500m
using both brakes at 40km/h. I had no way of measuring it but it was
hot enough that we stopped at the runaway lane for the rims to cool to
warm-to-the-touch. There was a breeze on the W?rzen Pass.

Jobst Brandt
[email hidden]

Dear Jobst,

So what's an appropriate temperature?

I see numbers for grade, distance, speed, and tire pressure,
but nothing about what temperature you think would be
"appropriate."

If you're going to dismiss other posts for using an
"inappropriately low temperature," why not tell us what
temperature you have in mind?

Carl Fogel

Quoted message said:

Did you try riding that wheel with the tire at 150psi? I suspect that
riding it on flatland, with no braking, that it would come off. I am
not convinced by a static test. Would you test ride that?

Not yet. What are the forces that would, without any braking action, cause
it to come off the rim while being ridden (but not otherwise)? We've
already been through threads explaining that there are virtually no "g
forces" exerted on tires (and thank goodness, since I wouldn't want to be
testing one for blowing off a rim while doing a slalom!).

But maybe I'm looking at the wrong thing. I keep thinking about what would
change in a tire or tube at higher temps that would cause one to blow off a
rim, but maybe it's not the tire that has trouble at (what I consider)
mildly-elevated pressures, but the rim? As pressure on the sides of the rim
increases (due to increasing tire pressure), the sides will bulge out
slightly. Perhaps enough to loosen their grip on the tire bead? And if
higher temperatures turn out to make the sidewall material more slippery?

--Mike-- Chain Reaction Bicycles
www.ChainReactionBicycles.com

Quoted message said:

Curiously, 570 F is almost the same as 570 K, so I doubt
that this high figure is due to the kind of mistake that I
often make.

But do bicycle rims really reach 570 degrees F or K?

Or is this just a pardonably rough approximation that tends
toward the high side, sort of like taking Chalo Colina as
having a roughly normal weight for a human being?

What happens to rubber tires and butyl inner tubes and rim
tape pressing firmly against 570 degree F aluminum rims?

I doubt rims get as hot as 570 degrees F. The calculations were just
to get an idea of the size of the numbers involved. In particular, I
ignored heat transfer by convection, which is most certainly wrong.
Unfortunately, there is no good way to calculate how much heat is lost
by convection, one really has to measure or simulate it. I think the
figure for air can run from about 10 - 100 W/m^2*K.

The estimate for the energy input should be pretty good though.

Quoted message said:

Jim Smith said:

Well, a quick back of the email calculation shows that for a bike and
rider weighing 1000N (225lbs) and a 10% grade, maximum energy
dissipation by the brakes will occur at a speed of around 50 km/h. At
that speed the bike is losing gravitational potential energy at a rate
of 1388 watts and it is taking 465 watts for wind and rolling
resistance, so that leaves 923 watts for the brakes to get rid of. If
we are using only one brake that is 923 watts into a 450 gm piece of
Al with a surface area of a few hundred cm^2. I suspect it could get
pretty hot.

Hold the phone! I'll propose that the rim will get hotter at 1/10
that speed because there is practically no convection cooling when
riding slowly. Convection is the primary coolant of rims in the
absence of rain water. Besides, the rate of conversion of kinetic
energy to heat increases with speed until the brake pad fails. Where
do you get 50km/h?

I was just looking at the power input to the rim, which I think we
agree reaches a maximum for this set of numbers somewhere around 50
kmh. At 5 kmh the power to the brakes is about 135 watts.

I am curious how you know that convection is the primary method of
heat transfer from the rims? Has someone measured this? If so, it
seems they would know exactly how hot rims get.

[email hidden] wrote in message ...

Quoted message said:

Trevor Jeffrey said:

[email hidden] wrote in message ...

Quoted message said:

Frank Krygowski writes:

>>> There are commercially available stick-on temperature indicators that
>>> record maximum temperatures of the surface to which they're stuck.
>>> http://www.tempil.com/Tempilabel.htm

>> Forget about that, it's both temperature and duration that cause
>> dangerous heating and overpressure blow-offs. I've done it as have
>> others with whom I have ridden on steep roads. Unobservant riders
>> might attribute the blow-off to a faulty tire or poor tire mounting
>> but it is heat.

> As a guy who likes data, I'd still be interested in a "maximum
> temperature" reading. If someone wanted to log temperature versus
> time instead, that would be even more interesting, but much more
> difficult.

>>> ISTR someone checking tandem rims on mountain descents with these
>>> things, but I don't recall what the maximum temperature was.

>> After you have ridden a few hundred miles in mountainous terrain
>> and then have a blow-off should be definitive.

> I've done the first, but not the second. I'd hope to find a less
> scary way of learning about this!

OK, nothing lost if you put a few heat indicators on your rear rim,
find a steep road (steeper than 12%) in your area and with a tire
inflated over 100psi, roll down the hill at between 5 and 8 mph with
only the rear brake applied. When the tire blows off, use the front
brake to stop and check the highest temperature reached. This will
cost you no more than one tube, preferably one that had a few patches.
There is no hazard here if you don't have any tight turns.

This is an easy test that I have done inadvertently without
instrumentation. I'm satisfied that it occurs easily. I have had an
opportunity to testify on a tandem case where the rider concocted a
story that was immediately apparent to me, because the scenario was
based on the belief that brake heating did not cause the tire to bow
off. I could prove by the evidence on the bicycle that his story was
false and also how the failure actually occurred.

It may predominantly be the braking in itself which causes the tyre to be
pulled around the rim and so release the tyres grip on the rim, it is only
typical tube pressure which then lifts the tyre from the rim. Degreasing

of

Quoted message said:

Quoted message said:

the rim and tyre before fitting will ensure satisfactory adhesion to

prevent

Quoted message said:

Quoted message said:

this lifting. Alternatively use tubulars.

Trevor

Dear Trevor,

Since the tire's braking is confined to a small contact
patch, it seems unlikely that there is enough traction to
spin a normally inflated tire on its rim by braking.

There's about 160 linear inches of tire striping the insides
of the rim, perhaps a quarter of an inch wide, all under the
same pressure per square inch as the contact patch. That's
roughly 40 square inches versus one to two square inches of
contact patch.

The tire would skid under braking long before it began to
creep around the rim, barring impressive greasing. And on
dry pavement, you'd flip a normal bicycle before the front
tire began to skid.

My suggestion is that the tyre creeps due to the braking force is valid, use
low pressure within the front tyre and brake down a 1 in 5 the tyre creeps
around the rim evident from the valve resting at a skew angle. High
pressure makes tyres grip rim with a greater tenacity whether hp tyres or
tubulars. Certain rim/tyre combinations do not grip so well, and higher
pressure will lift the tyre so causing a blow-out. A good tyre/rim combo
with elevated pressure reduces this creep even with softening glue under
tubulars.

Trevor

[email hidden] wrote in message ...

Quoted message said:

If your idea of discussion is the classic: "Tires blow off because..."

You should have used tubulars.

Trevor

[email hidden] wrote in message ...

Quoted message said:

Mike Jacoubowsky said:

That it happens isn't questionable. Why it happens is. A tire
ought to be able to handle a pretty significant amount of pressure
over what it's rated for. A typical tire will easily handle 150psi
before blowing off a rim (easily proven in the shop). But I'm not
seeing the physics (yet) that show a high-enough temperature
differential to accomplish this. That's why I'm thinking there's
more to it than just an increase in pressure.

Did you try riding that wheel with the tire at 150psi? I suspect that
riding it on flatland, with no braking, that it would come off. I am
not convinced by a static test. Would you test ride that?

So what pressure does a track cyclist use with 20mm high pressures?
I think it would be near 150psi in most cases.

Trevor

dianne_1234 wrote in message ...

Quoted message said:

On Wed, 25 Aug 2004 01:10:37 GMT, [email hidden]

Quoted message said:

dianne_1234567890? said:

> I have seen a far greater number of tubes exhibiting
> snake-bite-type damage (as you'd see on a compression cut) on
> "blowouts" on steep descents, making me wonder if the material
> properties of the rubber itself change (for the worse) as
> temperatures rise.

Quoted message said:

I wonder if the tube gets the snakebite cut after the blowout,
before the rider comes to a stop?

That's a red herring! If the tire got a flat first, then what made it
blow off the rim and go BANG! This makes no sense.

Jobst Brandt
[email hidden]

Sorry, let me try to describe it differently:
Maybe the tube gets the snakebite cut *after* the blowout, while the
rider rides on the flat tire, the rim bumping along on the tire and
tube, while slowing to a stop?

No requirement for apology, your posting was clear, Jobst just jumping down
peoples throats as usual.

Trevor

[email hidden] wrote in message ...

Quoted message said:

Mike Jacoubowsky said:

Quoted message said:

On the other hand, I wasn't aware that there was any doubt that rim
heating cause blow-offs, especially among bicycle shop operators.

Quoted message said:

There is no question that rim heating causes tube/tire failure.
There is a question as to the mechanism that causes this.
Previously, before the physics was made clear, I had assumed it was
simply high pressure... but since that same amount of pressure
(according to the physics lessons presented in this thread, which
nobody has seriously questioned yet) doesn't accomplish this in the
shop, that causes me to question the assumption.

The "physics lesson" used an inappropriately low temperature as I see
it.

Why is it inappropriately low? Is it because the answer does not conform
with your theory?

Trevor

[email hidden] wrote in message ...

Quoted message said:

Jim Smith said:

Well, a quick back of the email calculation shows that for a bike and
rider weighing 1000N (225lbs) and a 10% grade, maximum energy
dissipation by the brakes will occur at a speed of around 50 km/h. At
that speed the bike is losing gravitational potential energy at a rate
of 1388 watts and it is taking 465 watts for wind and rolling
resistance, so that leaves 923 watts for the brakes to get rid of. If
we are using only one brake that is 923 watts into a 450 gm piece of
Al with a surface area of a few hundred cm^2. I suspect it could get
pretty hot.

Hold the phone! I'll propose that the rim will get hotter at 1/10
that speed because there is practically no convection cooling when
riding slowly. Convection is the primary coolant of rims in the
absence of rain water. Besides, the rate of conversion of kinetic
energy to heat increases with speed until the brake pad fails. Where
do you get 50km/h?

You are being ignorant of radiation. And if there is no convection, then
the heat raised is insignificant.

Trevor

[email hidden] wrote in message ...

Quoted message said:

Carl Fogel said:

Still, I agree with your conclusion that it could get pretty hot. I
think that what's interesting the other posters is whether all that
heat expands a rim enough to contribute to its tire coming off.

Than test has been done. Descending the same hard braking hill with
about 90psi inflation pressure does not blow the tire off while 110psi
does. Anyway, expansion is trivially small in the rim, especially
because it doesn't change it's shape and it is the shape of the rim
bead that holds the tire.

At 90 pi the tyre will have crept around the rim if at 110 it blew off.

Trevor

Quoted message said:

Quoted message said:

What's a reasonable model for convection losses vs speed?

I have no idea. I'm sure there is data on wind speed air temperature
and heat transfer, but that requires a known rim temperature and
speed.

Which brings us back to: collecting data. To understand this
phenomenon, it makes sense to measure rim temperatures under various
braking conditions.

--
Frank Krygowski [To reply, remove rodent and vegetable dot com.
Substitute cc dot ysu dot
edu]

Quoted message said:

Frank Krygowski said:

Jobst, it's a discussion. I'm just curious about the details of the
phenomenon! Saying "Accept it!" about tires blowing off the rim
this is like saying "Accept it, people die of cholera!"

You use an odd way to ask. Telling us what else could cause a tire to
come off the rim, raises specters that attain a life of their own. We
have enough myth and lore to quash without you introducing new ones.
You've been around this newsgroup long enough to have read all this
about tire blow-off several times. That's why I ask "what are you
trying to generate?"

Quoted message said:

If the discussion sheds light to enable someone to design a
blow-off-proof tire, that would be wonderful. If the discussion
merely teaches us something about the physics of the situation, that
would at least be beneficial.

If your idea of discussion is the classic: "Tires blow off because..."
followed by a series of remotely plausible reasons, followed by a
belated,"that's right isn't it?" somewhere in a disconnected
sentence, you are not asking. That is a typical "end run" offered by
folks who have pet ideas but have no way to prove them or substantiate
that they ever occur.

As I recall, my contribution so far was to demonstrate that the increase
in pressure from a given air temperature is not so great as many people
think. If you want to dispute that, do it mathematically.

Beyond that, I merely pointed to some material properties data and
wondered if they may be relevant. And I suggested actual measurement of
rim temperature to learn more about the problem.

If you prefer we exclude all information and ideas beyond what you
choose to contribute, that puts a burden on you to explain the
phenomenon completely.

Being a man of supreme self-confidence, I'm sure you wouldn't mind
taking on that job - but up to this point, you haven't delivered. And
until you do, you may have to put up with people doing annoying things
like generating hypotheses, posting data, and doing calculations, rather
than just ignorantly "accepting" things, without wondering why.

Quoted message said:

Quoted message said:

As I see it, this is at least as valuable as harping about whether
the word "stretch" should be used to describe elongation of chain
pitch.

Only if you say stretch without defining what you mean by the word.
From misuse of stretch, we have seen many posts in which riders
believe they stretched their chain from riding on too steep a hill.

And the importance of that illusion is greater than the importance of a
blowout on a mountain descent? I think many would disagree.

--
Frank Krygowski [To reply, remove rodent and vegetable dot com.
Substitute cc dot ysu dot
edu]

Jim Smith said:

Quoted message said:

Curiously, 570 F is almost the same as 570 K, so I doubt
that this high figure is due to the kind of mistake that I
often make.

But do bicycle rims really reach 570 degrees F or K?

Or is this just a pardonably rough approximation that tends
toward the high side, sort of like taking Chalo Colina as
having a roughly normal weight for a human being?

What happens to rubber tires and butyl inner tubes and rim
tape pressing firmly against 570 degree F aluminum rims?

I doubt rims get as hot as 570 degrees F. The calculations were just
to get an idea of the size of the numbers involved. In particular, I
ignored heat transfer by convection, which is most certainly wrong.
Unfortunately, there is no good way to calculate how much heat is lost
by convection, one really has to measure or simulate it. I think the
figure for air can run from about 10 - 100 W/m^2*K.

The estimate for the energy input should be pretty good though.

Dear Jim,

Darn!

Once again, a calculating engineer pours cold water on an
comforting illusion that was going to keep me warm at night.

Elsewhere in this thread, someone suggested attaching some
kind of temperature-sensitive strips to the rim to show how
hot they get. I suppose that I'll have to pin my hopes on
that approach.

But thanks for the effort that you put into the
calculations.

Carl Fogel

Jim Smith said:

Quoted message said:

Quoted message said:

Well, a quick back of the email calculation shows that for a bike
and rider weighing 1000N (225lbs) and a 10% grade, maximum energy
dissipation by the brakes will occur at a speed of around 50
km/h. At that speed the bike is losing gravitational potential
energy at a rate of 1388 watts and it is taking 465 watts for wind
and rolling resistance, so that leaves 923 watts for the brakes to
get rid of. If we are using only one brake that is 923 watts into
a 450 gm piece of Al with a surface area of a few hundred cm^2. I
suspect it could get pretty hot.

Quoted message said:

Quoted message said:

Hold the phone! I'll propose that the rim will get hotter at 1/10
that speed because there is practically no convection cooling when
riding slowly. Convection is the primary coolant of rims in the
absence of rain water. Besides, the rate of conversion of kinetic
energy to heat increases with speed until the brake pad fails.
Where do you get 50km/h?

Quoted message said:

I was just looking at the power input to the rim, which I think we
agree reaches a maximum for this set of numbers somewhere around 50
kmh. At 5 kmh the power to the brakes is about 135 watts.

Quoted message said:

I am curious how you know that convection is the primary method of
heat transfer from the rims? Has someone measured this? If so, it
seems they would know exactly how hot rims get.

OK. What other method of cooling comes into play. Radiation is way
down at the temperatures involved. Besides, as I have mentioned,
creeping downhill on a road that otherwise poses no problem, I blew a
tire off the rim as did a colleague on another occasion.

Jobst Brandt
[email hidden]

Carl Fogel said:

Elsewhere in this thread, someone suggested attaching some kind of
temperature-sensitive strips to the rim to show how hot they get. I
suppose that I'll have to pin my hopes on that approach.

I've done that and all the temperature spots turned from silver to
black, indicating that the strips don't go high enough and their
sensitivity to crude.

This was a red tape with silver spots calibrated in degrees. They
turn black when their temperature is reached. I did this long ago and
do not recall the name of the product or how high a temperature they
could read, only that they did not go high enough. Not much was
learned from this experiment.

Jobst Brandt
[email hidden]

In article <[email hidden]>,
"Mike Jacoubowsky/Chain Reaction Bicycles" <[email hidden]>

Quoted message said:

I knew that Skywriting planes trail smoke, but figured there must be
something that could be added to water that would evaporate and leave a
colored trail of some kind. But of course, if there was, you would just use
that and not water, right? So the question becomes what boils at a relevant
temperature and trails darkly-colored smoke.

Probably an easy way to generate a colored puff would be with colored
chalk dust in the inner tube--no water needed. You might need a fair
amount to make sure the cloud is visible.

--
B.B. --I am not a goat! thegoat4 at airmail.net

Quoted message said:

Carl Fogel said:

Elsewhere in this thread, someone suggested attaching some kind of
temperature-sensitive strips to the rim to show how hot they get. I
suppose that I'll have to pin my hopes on that approach.

I've done that and all the temperature spots turned from silver to
black, indicating that the strips don't go high enough and their
sensitivity to crude.

This was a red tape with silver spots calibrated in degrees. They
turn black when their temperature is reached. I did this long ago and
do not recall the name of the product or how high a temperature they
could read, only that they did not go high enough. Not much was
learned from this experiment.

Maybe it's time for a rerun.

One brand of these is Tempilabel, and at least some of them read up to
500 deg. F. That ought to cover things. See
http://www.tempil.com/pdf/Tempilabel.pdf
or see Omega's offerings at http://www.omega.com/ppt/pptsc.asp?ref=TL-10

As an alternative, an infrared thermometer could be used.
http://www.omega.com/toc_asp/section.asp?book=temperature&section=j

These are non-contact, and might even be rigged to get readings while
riding the bike. These things can measure up to the melting temperature
of aluminum, which is probably enough range.

--
Frank Krygowski [To reply, remove rodent and vegetable dot com.
Substitute cc dot ysu dot
edu]

Benjamin Weiner said:

"Microinches"? Barf! What kind of units are those, join the 20th
century already! Anyway, "microinches per inch" are just parts per
million.

The tire is given shape by the cords, not the rubber - I think the
rubber's expansion is not relevant. I'm not sure which Matweb
listing applies to cotton or nylon fibers that might make it into
a tire casing, but

http://www.matweb.com/search/SpecificMaterial.asp?bassnum=O2406
http://www.matweb.com/search/SpecificMaterial.asp?bassnum=PDUNOM1

say extruded nylon has a CTE of 45 microinches/inch/deg F, while
aramid (Kevlar) yarn has a CTE of only 10 microinches/inch/deg F.

^^^^^^^^^^^

You forgot to barf! ;-)

But in any case: I wasn't proposing that the whole tire expanded by that
amount. Rather, I was wondering if the different expansion rates of
the various elements (rubber, bead wire, cord material) might cause some
distortion or warping of the shape of the bead, which could cause the
bead to lose its grip on the rim - especially if, say, the rubber lost
hardness at high temperatures.

--
Frank Krygowski [To reply, remove rodent and vegetable dot com.
Substitute cc dot ysu dot
edu]