Potential good news for Mt. Washington access. - Page 20 - Cycling Mob: Road Cycling, Mountain Biking and More

01-10-2005, 07:15 PM

[Only registered and activated users can see links. ] writes:

> Tim McNamara writes:
>
>> Hmm, well, if pressure increase from heating is the *only* cause of
>> blow-off, then one would expect to be able to blow off the tire at
>> the same pressure whether in the garage or rolling down the hill.
>> Or at least so it seems to me as a non-engineer. As I recall from
>> the prior discussion, heating the rim to the temperatures normally
>> seen doesn't cause a huge increase in pressure.
>
> As I said, the chafing strip on tire beads is not there for nothing,
> and from observation of aluminum on the strip and cloth marks in the
> aluminum, there is motion. Friction is not a consideration in tire
> retention when moving.
>
>> Or is there some other factor involved- the heat of the rim
>> affecting the coefficient of friction between the rim and tire?
>> There are different designs at the bead- some tires have a fabric
>> chafing strip over the bead, some are just a rubber coating over
>> the casing of the tire- could this make a difference and could it
>> be tested?
>
> Friction is not a consideration in tire retention when moving. I
> don't think rubber coatings last long at high pressure in this
> interface. What HP tires have no chafing strip? I'm considering
> road tires because fat tires have entirely different problems and
> rims.

Well, to answer this I had to look at the pile of tires in my
basement. All appeared to have some kind of chafing strip, although
most (Avocet Duro, Ritchey Tom Slick, Michelin Synergic) appeared to
be coated with the same rubber covering the sidewalls. The
Continental chafing strip seemed to have little or no rubber
coating. That was, I guess, the visual I had in mind when I wrote my
comment- the chafing strip is more visible on the Continentals.

>> Or perhaps the effect of braking causing some kind of pulling on
>> the bead on a line between the contact patch and the rim- drawing
>> the bead tight ahead of the contact patch and loosening it behind
>> the contact patch? Does rim have to be hot or can this happen cold
>> (if the latter is possible, I'd expect to see it happen in
>> criteriums or during panic stops).
>
> This would only be possible with an elastic bead because the entire
> circumference of the bead would have to slip for any of it to move.
> I have not seen any evidence of circumferential tire creep as I
> formerly saw on tubulars.
>
>> And out of this, if the mechanism can be determined, is the
>> question of how to prevent it. Closer tolerances for tire fit on
>> the rim? A change in the materials at the rim-tire interface? A
>> rim strip that insulates the tube?
>
> I think I mentioned that as part of the experiment.

The insulated rim strip, anyway. But I'm puzzled by the utility of
this, given your comments last August:

>> In a thread last August, Jobst stated:
>
>>> The idea that the tire bead gets soft occurred to me but I later
>>> rejected it because I have been in many situations where high rim
>>> temperatures occurred only for a short duration, not long enough
>>> to heat the air in the tube. There was no residual effect over
>>> many miles in which the tires wore out while repeating the rim
>>> heating. I get to review this every summer in riding over many
>>> mountain roads and have reduced the tire blow-off to air
>>> temperature in the tire alone.
>
>> This seems to me to suggest that temperature of the rim and not
>> pressure in the tube may be the culprit. What effect does heating
>> the rim have? The only thing I can think of is that there is some
>> effect on the interface between the tire and the rim- reduction of
>> friction, change in bead position at some point on the rim, etc.
>> And it also seems that the condition must be pretty specific and
>> difficult to achieve, or we'd have this happening on many rides,
>> not just a few..
>
> I don't understand. How do you draw that conclusion, one that is
> exactly the opposite from the one I derive from the cited paragraph
> that I wrote?

I misread, apparently. However, in the context of the thread back in
August it was suggested from the laws of physics that the temperature
increase in the tube would result in a small rise in pressure- from
100 psi to 126 psi with a temperature increase from about 60 F to 250
F, for example, well within the normal capacity of rims and tires to
withstand. This would seem to suggest that there is some other
mechanism other than increased pressure caused by heating the rims
from braking.

However, I'm obviously no physicist nor an engineer, and I'm looking
forward to your findings- one measurement being worth a thousand
opinions.

01-10-2005, 07:15 PM

[Only registered and activated users can see links. ] writes:

> Tim McNamara writes:
>
>> Hmm, well, if pressure increase from heating is the *only* cause of
>> blow-off, then one would expect to be able to blow off the tire at
>> the same pressure whether in the garage or rolling down the hill.
>> Or at least so it seems to me as a non-engineer. As I recall from
>> the prior discussion, heating the rim to the temperatures normally
>> seen doesn't cause a huge increase in pressure.
>
> As I said, the chafing strip on tire beads is not there for nothing,
> and from observation of aluminum on the strip and cloth marks in the
> aluminum, there is motion. Friction is not a consideration in tire
> retention when moving.
>
>> Or is there some other factor involved- the heat of the rim
>> affecting the coefficient of friction between the rim and tire?
>> There are different designs at the bead- some tires have a fabric
>> chafing strip over the bead, some are just a rubber coating over
>> the casing of the tire- could this make a difference and could it
>> be tested?
>
> Friction is not a consideration in tire retention when moving. I
> don't think rubber coatings last long at high pressure in this
> interface. What HP tires have no chafing strip? I'm considering
> road tires because fat tires have entirely different problems and
> rims.

Well, to answer this I had to look at the pile of tires in my
basement. All appeared to have some kind of chafing strip, although
most (Avocet Duro, Ritchey Tom Slick, Michelin Synergic) appeared to
be coated with the same rubber covering the sidewalls. The
Continental chafing strip seemed to have little or no rubber
coating. That was, I guess, the visual I had in mind when I wrote my
comment- the chafing strip is more visible on the Continentals.

>> Or perhaps the effect of braking causing some kind of pulling on
>> the bead on a line between the contact patch and the rim- drawing
>> the bead tight ahead of the contact patch and loosening it behind
>> the contact patch? Does rim have to be hot or can this happen cold
>> (if the latter is possible, I'd expect to see it happen in
>> criteriums or during panic stops).
>
> This would only be possible with an elastic bead because the entire
> circumference of the bead would have to slip for any of it to move.
> I have not seen any evidence of circumferential tire creep as I
> formerly saw on tubulars.
>
>> And out of this, if the mechanism can be determined, is the
>> question of how to prevent it. Closer tolerances for tire fit on
>> the rim? A change in the materials at the rim-tire interface? A
>> rim strip that insulates the tube?
>
> I think I mentioned that as part of the experiment.

The insulated rim strip, anyway. But I'm puzzled by the utility of
this, given your comments last August:

>> In a thread last August, Jobst stated:
>
>>> The idea that the tire bead gets soft occurred to me but I later
>>> rejected it because I have been in many situations where high rim
>>> temperatures occurred only for a short duration, not long enough
>>> to heat the air in the tube. There was no residual effect over
>>> many miles in which the tires wore out while repeating the rim
>>> heating. I get to review this every summer in riding over many
>>> mountain roads and have reduced the tire blow-off to air
>>> temperature in the tire alone.
>
>> This seems to me to suggest that temperature of the rim and not
>> pressure in the tube may be the culprit. What effect does heating
>> the rim have? The only thing I can think of is that there is some
>> effect on the interface between the tire and the rim- reduction of
>> friction, change in bead position at some point on the rim, etc.
>> And it also seems that the condition must be pretty specific and
>> difficult to achieve, or we'd have this happening on many rides,
>> not just a few..
>
> I don't understand. How do you draw that conclusion, one that is
> exactly the opposite from the one I derive from the cited paragraph
> that I wrote?

I misread, apparently. However, in the context of the thread back in
August it was suggested from the laws of physics that the temperature
increase in the tube would result in a small rise in pressure- from
100 psi to 126 psi with a temperature increase from about 60 F to 250
F, for example, well within the normal capacity of rims and tires to
withstand. This would seem to suggest that there is some other
mechanism other than increased pressure caused by heating the rims
from braking.

However, I'm obviously no physicist nor an engineer, and I'm looking
forward to your findings- one measurement being worth a thousand
opinions.

01-10-2005, 07:15 PM

[Only registered and activated users can see links. ] writes:

> Tim McNamara writes:
>
>> Hmm, well, if pressure increase from heating is the *only* cause of
>> blow-off, then one would expect to be able to blow off the tire at
>> the same pressure whether in the garage or rolling down the hill.
>> Or at least so it seems to me as a non-engineer. As I recall from
>> the prior discussion, heating the rim to the temperatures normally
>> seen doesn't cause a huge increase in pressure.
>
> As I said, the chafing strip on tire beads is not there for nothing,
> and from observation of aluminum on the strip and cloth marks in the
> aluminum, there is motion. Friction is not a consideration in tire
> retention when moving.
>
>> Or is there some other factor involved- the heat of the rim
>> affecting the coefficient of friction between the rim and tire?
>> There are different designs at the bead- some tires have a fabric
>> chafing strip over the bead, some are just a rubber coating over
>> the casing of the tire- could this make a difference and could it
>> be tested?
>
> Friction is not a consideration in tire retention when moving. I
> don't think rubber coatings last long at high pressure in this
> interface. What HP tires have no chafing strip? I'm considering
> road tires because fat tires have entirely different problems and
> rims.

Well, to answer this I had to look at the pile of tires in my
basement. All appeared to have some kind of chafing strip, although
most (Avocet Duro, Ritchey Tom Slick, Michelin Synergic) appeared to
be coated with the same rubber covering the sidewalls. The
Continental chafing strip seemed to have little or no rubber
coating. That was, I guess, the visual I had in mind when I wrote my
comment- the chafing strip is more visible on the Continentals.

>> Or perhaps the effect of braking causing some kind of pulling on
>> the bead on a line between the contact patch and the rim- drawing
>> the bead tight ahead of the contact patch and loosening it behind
>> the contact patch? Does rim have to be hot or can this happen cold
>> (if the latter is possible, I'd expect to see it happen in
>> criteriums or during panic stops).
>
> This would only be possible with an elastic bead because the entire
> circumference of the bead would have to slip for any of it to move.
> I have not seen any evidence of circumferential tire creep as I
> formerly saw on tubulars.
>
>> And out of this, if the mechanism can be determined, is the
>> question of how to prevent it. Closer tolerances for tire fit on
>> the rim? A change in the materials at the rim-tire interface? A
>> rim strip that insulates the tube?
>
> I think I mentioned that as part of the experiment.

The insulated rim strip, anyway. But I'm puzzled by the utility of
this, given your comments last August:

>> In a thread last August, Jobst stated:
>
>>> The idea that the tire bead gets soft occurred to me but I later
>>> rejected it because I have been in many situations where high rim
>>> temperatures occurred only for a short duration, not long enough
>>> to heat the air in the tube. There was no residual effect over
>>> many miles in which the tires wore out while repeating the rim
>>> heating. I get to review this every summer in riding over many
>>> mountain roads and have reduced the tire blow-off to air
>>> temperature in the tire alone.
>
>> This seems to me to suggest that temperature of the rim and not
>> pressure in the tube may be the culprit. What effect does heating
>> the rim have? The only thing I can think of is that there is some
>> effect on the interface between the tire and the rim- reduction of
>> friction, change in bead position at some point on the rim, etc.
>> And it also seems that the condition must be pretty specific and
>> difficult to achieve, or we'd have this happening on many rides,
>> not just a few..
>
> I don't understand. How do you draw that conclusion, one that is
> exactly the opposite from the one I derive from the cited paragraph
> that I wrote?

I misread, apparently. However, in the context of the thread back in
August it was suggested from the laws of physics that the temperature
increase in the tube would result in a small rise in pressure- from
100 psi to 126 psi with a temperature increase from about 60 F to 250
F, for example, well within the normal capacity of rims and tires to
withstand. This would seem to suggest that there is some other
mechanism other than increased pressure caused by heating the rims
from braking.

However, I'm obviously no physicist nor an engineer, and I'm looking
forward to your findings- one measurement being worth a thousand
opinions.

01-10-2005, 09:37 PM

Jim Smith points out that spoke tension is insufficient to
significantly restrain rim diameter from increasing due to thermal
expansion. While contemplating this on my evening walk, I wondered
whether the resulting spoke tension increase could be a significant,
and overlooked, factor affecting spoke life.

A few calculations are in order.

Let
dCr = (delta) increase in rim compressive force
dTs = (delta) increase in spoke tension
n = number of spokes = 36
R = wheel radius
As = spoke cross-sectional area ~ 2mm^2
ks = spoke elasticity ~ 30Mpsi
Fs = ks*As ~ 42,000kgf
a = coefficient of thermal expansion of rim ~ 25e-6/degC
Ar = rim cross-sectional area ~ 80mm^2
kr = rim elasticity ~ 10Mpsi
Fr = kr*Ar ~ 560,000kgf
dT = rim temperature increase
P = rim perimeter
dPt = change in P due to temperature increase in rim
dPc = change in P due to compressive force in rim

(1) dCr = n*dTs/2/pi
(2) dR/R = dTs/Fs
(3) dR/R = (dPt+dPc)/P
(4) dPt/P = a*dT
(5) dPc/P = -dCr/Fr

expanding (3) and plugging in (4) and 5 we get

(6) a*dT - dCr/Fr = dTs/Fs

Using (1) to eliminate dCr gives

(7) a*dT = dTs*(1/Fs + (n/2/pi)/Fr)
(7a) = dTs/Feff

where
1/Feff = 1/Fs + (n/2/pi)/Fr
~ 30,000kgF

From (7a) we find that

(8) dTs/dT = a*Feff ~ 0.75kgf/degC

So a 100degC rise in the rim temperature increases the spoke
tension by 75kgf (165lbf). This is not a trival amount, it
represents about 25% of the ultimate strength of a spoke.

Comments? I have ignored bending in the rim because
I couldn't easily compute its effect. Presumably it
significantly reduces the tension increase in the spokes.

Joe Riel

01-10-2005, 09:37 PM

Jim Smith points out that spoke tension is insufficient to
significantly restrain rim diameter from increasing due to thermal
expansion. While contemplating this on my evening walk, I wondered
whether the resulting spoke tension increase could be a significant,
and overlooked, factor affecting spoke life.

A few calculations are in order.

Let
dCr = (delta) increase in rim compressive force
dTs = (delta) increase in spoke tension
n = number of spokes = 36
R = wheel radius
As = spoke cross-sectional area ~ 2mm^2
ks = spoke elasticity ~ 30Mpsi
Fs = ks*As ~ 42,000kgf
a = coefficient of thermal expansion of rim ~ 25e-6/degC
Ar = rim cross-sectional area ~ 80mm^2
kr = rim elasticity ~ 10Mpsi
Fr = kr*Ar ~ 560,000kgf
dT = rim temperature increase
P = rim perimeter
dPt = change in P due to temperature increase in rim
dPc = change in P due to compressive force in rim

(1) dCr = n*dTs/2/pi
(2) dR/R = dTs/Fs
(3) dR/R = (dPt+dPc)/P
(4) dPt/P = a*dT
(5) dPc/P = -dCr/Fr

expanding (3) and plugging in (4) and 5 we get

(6) a*dT - dCr/Fr = dTs/Fs

Using (1) to eliminate dCr gives

(7) a*dT = dTs*(1/Fs + (n/2/pi)/Fr)
(7a) = dTs/Feff

where
1/Feff = 1/Fs + (n/2/pi)/Fr
~ 30,000kgF

From (7a) we find that

(8) dTs/dT = a*Feff ~ 0.75kgf/degC

So a 100degC rise in the rim temperature increases the spoke
tension by 75kgf (165lbf). This is not a trival amount, it
represents about 25% of the ultimate strength of a spoke.

Comments? I have ignored bending in the rim because
I couldn't easily compute its effect. Presumably it
significantly reduces the tension increase in the spokes.

Joe Riel

01-10-2005, 09:37 PM

Jim Smith points out that spoke tension is insufficient to
significantly restrain rim diameter from increasing due to thermal
expansion. While contemplating this on my evening walk, I wondered
whether the resulting spoke tension increase could be a significant,
and overlooked, factor affecting spoke life.

A few calculations are in order.

Let
dCr = (delta) increase in rim compressive force
dTs = (delta) increase in spoke tension
n = number of spokes = 36
R = wheel radius
As = spoke cross-sectional area ~ 2mm^2
ks = spoke elasticity ~ 30Mpsi
Fs = ks*As ~ 42,000kgf
a = coefficient of thermal expansion of rim ~ 25e-6/degC
Ar = rim cross-sectional area ~ 80mm^2
kr = rim elasticity ~ 10Mpsi
Fr = kr*Ar ~ 560,000kgf
dT = rim temperature increase
P = rim perimeter
dPt = change in P due to temperature increase in rim
dPc = change in P due to compressive force in rim

(1) dCr = n*dTs/2/pi
(2) dR/R = dTs/Fs
(3) dR/R = (dPt+dPc)/P
(4) dPt/P = a*dT
(5) dPc/P = -dCr/Fr

expanding (3) and plugging in (4) and 5 we get

(6) a*dT - dCr/Fr = dTs/Fs

Using (1) to eliminate dCr gives

(7) a*dT = dTs*(1/Fs + (n/2/pi)/Fr)
(7a) = dTs/Feff

where
1/Feff = 1/Fs + (n/2/pi)/Fr
~ 30,000kgF

From (7a) we find that

(8) dTs/dT = a*Feff ~ 0.75kgf/degC

So a 100degC rise in the rim temperature increases the spoke
tension by 75kgf (165lbf). This is not a trival amount, it
represents about 25% of the ultimate strength of a spoke.

Comments? I have ignored bending in the rim because
I couldn't easily compute its effect. Presumably it
significantly reduces the tension increase in the spokes.

Joe Riel

01-10-2005, 09:52 PM

Joe Riel <[Only registered and activated users can see links. ]> writes:

> where
> 1/Feff = 1/Fs + (n/2/pi)/Fr
> ~ 30,000kgF

That should be

Feff ~ 30,000kgF

Nothing else changes.

Joe

01-10-2005, 09:52 PM

Joe Riel <[Only registered and activated users can see links. ]> writes:

> where
> 1/Feff = 1/Fs + (n/2/pi)/Fr
> ~ 30,000kgF

That should be

Feff ~ 30,000kgF

Nothing else changes.

Joe

01-10-2005, 09:52 PM

Joe Riel <[Only registered and activated users can see links. ]> writes:

> where
> 1/Feff = 1/Fs + (n/2/pi)/Fr
> ~ 30,000kgF

That should be

Feff ~ 30,000kgF

Nothing else changes.

Joe

01-10-2005, 11:29 PM

>> Alfred Ryder wrote:
>> I would be interested in knowing whether the blow-off point is
>> affected by
>> whether someone is sitting on the bike or not. The only blow-off I
>> have had
>> was a few seconds after coming to a complete stop.

Frank Krygowski wrote:
> When our tandem was brand new (long, long ago) it suffered two blowouts
> as it sat alone in a bedroom, at least an hour after mounting and
> inflating the tires. I never did figure that out.

Most likely an installation issue.

--
Andrew Muzi
[Only registered and activated users can see links. ]
Open every day since 1 April, 1971