23MM AND 25MM tyres........any significant difference ?
Comments
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oldwelshman wrote:andrewgturnbull wrote:oldwelshman wrote:If your going to do a sportive with long decents in groups I woul lower pressure to at least 90lbs to stop punctures from "hot" rims from brealking
Hight pressures are ok for track and TT but not really for general riding.I use 100 psi for racing, and 120 for track.
Hi there.
No offence ow, but have you looked at the physics of exactly how hot the air inside your tyre would have to get to increase the pressure by the 10psi you're talking about?
It's never going to happen!
Cheers, Andy
Oh well I guess you know more than the pro riders
I guess Greg Lemond is also wrong in advising people to lower pressures for long decents whils in groups when your braking a lot and rim temperature gets very high.
I also suppose that was not the reason why most of the puncture in those types of rides are on decents?
I stand corrected.
By the way I never tried the calculation but I suspect that if the volume of the air remains costant, then an increase in temperature increases the pressure.
from what I can vaguely remember from school many moons ago, the pressure is directly proportional to the temperature provideing the volume remains constant? I think.
Hi there.
Yes, your recollection of Boyles law is about right, but the effect is very small - especially if you consider how badly air conducts heat.
Greg is most likely talking about melting the glue on tubulars.
Cheers, Andy0 -
To be fair, Andrew, the temperature increase to add 10psi is only about 30 degrees C, so it's not a totally silly argument (though I agree with you about the heat not actually conducting to the air).
To oldwelshman I'd point out that pros believe cycling folklore just as much as many on here do - if not more. Not many of them know much about physics.0 -
Have seen the effects of HOT Rims on a tandem on descents. Havent done the physics but the tyre seemed loose pressure very quickly and the rims was sufficiently hot that handling it was very uncomfortable. No I dont know for sure it was the heat build up but ...0
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Yes, but that's nothing to do with the pressure increasing, simply down to the heat weakening the tube.0
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Greg lemond just commented on the unusual number of punctures on decents during the Etape and mentione lowering the pressures, this was not to do with tubs rolling off
I can't find the article tho.
Glad my physics lnowledge is still correct after 30 years 0 -
Correct me if Im wrong but wouldnt the heat build up on the rim cause the rim to expand therefore counteracting the pressure increase cause by the heating of the air ??? Just a thought.0
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oldwelshman wrote:Greg lemond just commented on the unusual number of punctures on decents during the Etape and mentione lowering the pressures, this was not to do with tubs rolling off
I can't find the article tho.
Glad my physics lnowledge is still correct after 30 years
Ok I'll bite (again)...
Let's say, that I put 100psi in my tyres and hang on to the brakes all the way down a col. Somehow my rims transfer enough heat to cause a 30 degree celcius rise in temperature of the air inside the tyres (I doubt this bit, but I'll bear with you). In kelvin terms this represents roughly a 10% delta. Hence the pressure increases by 10% inside the tyre to 110psi.
Now, what exactly is the mechanism that will cause the puncture you're worried about at this point?
Cheers, Andy0 -
interesting discussion ...
my belief was that it is safer to ride with slightly lower pressure on long mountain descents, partly because it helps improve grip (perhaps slightly trading off speed on smoothest surfaces), and because it reduces the chance of a blow-out due to heat build up in the rim ... are people saying this last reason is a myth?
by "lower pressure" I mean something like 95-100 front and 100-105 rear, so only about 5psi below my normal pressures (23mm tyres, 73kg ish rider, 7kg bike).0
