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#12
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Tire pressure/recommended psi and rider's weight
On 9 Jul 2005 21:33:29 -0700, "41"
wrote: wrote: 41 wrote: Joss Winn wrote: Thanks for everyone's advice. I'm running them at 110psi and will try to get used to the softer feel. They are generally more comfortable than my old 120psi 23mm tyres but I kind of miss the sound of rock hard tyres rolling on the road and the feel of smooth tarmac underneath the tyre. These new tyres dampen this experience. For fun & revelation, why not time yourself, over several attempts, over a fixed course at say 100, 110, and 120psi. Dear 41, Unfortunately, it's unlikely that a series of real-world test-runs will show any speed difference between 100, 110, and 120 psi inflation on high quality tires. Why would that be unfortunate? You seem to have missed the point of the suggestion. This calculator uses Jobst's rolling resistance data to predict exactly that kind of effect: http://www.analyticcycling.com/ForcesTires_Page.html The range of difference in a 1-hour 40 km ride at about 25 mph is less than thirteen seconds for inflations from 120 to 100 psi. Th is kind of tiny change is why drum spin-down tests are necessary to provide rolling-resistance data and why our impromptu roll-outs are so unreliable. The poster did not ask about rolling resistance, he asked what pressure he should run his tires at. These are completely different questions. Furthermore, the question of which pressure will produce the fastest time over a particular course is also different from which pressure produces the lowest rolling resistance, as it is considered classically and as tested on JB's drum apparatus. In real life one does not ride on a polished drum but on a road surface with irregularities. The lower the pressure, the less these irregularities retard the motion, giving an effect opposite to that of pressure on rolling resistance. This is traditionally not called rolling resistance because variously deforming or lifting off over obstacles is not precisely rolling. The roads in England can be very rough and a lower pressure may very well produce a significantly faster time. Potentially large changes of this kind are why systematic "roll-outs" are useful and interesting to provide pressure effectiveness data and why drum spin-down tests are so irrelevant for this purpose. Dear 41, Tom Compton's calculator predicts less than 13 seconds per hour if we reduce a premimum-quality clincher from 120 to 100 psi--0.36%. If his prediction is off by an order of magnitude, that would be about two minutes and ten seconds per hour--3.6%. What do you predict? (You cut most of the figures from the calculator without providing any of your own.) And how would you demonstrate this difference on rough English roads? Where would such an effect not be obscured by the effects of wind, rider, posture, temperature, barometric pressure, and traffic, which vary considerably between 25-mile rides in England? Curiously, Carl Fogel |
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Tire pressure/recommended psi and rider's weight
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#14
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Tire pressure/recommended psi and rider's weight
On 9 Jul 2005 23:56:38 -0700, "41"
wrote: wrote: Tom Compton's calculator predicts less than 13 seconds per hour if we reduce a premimum-quality clincher from 120 to 100 psi--0.36%. If his prediction is off by an order of magnitude, that would be about two minutes and ten seconds per hour--3.6%. As I said the problem is an effect different from rolling resistance that varies with tire pressure in the opposite sense. If you think about it you will realize that this effect has a very large potential for variation, much greater than that typical for rolling resistance. What do you predict? (You cut most of the figures from the calculator without providing any of your own.) The problem is undefined and therefore there is no prediction to be made. Predict what? I've never met him and I don't know where he rides. Besides, why shouldn't he just find out for himself? Where would such an effect not be obscured by the effects of wind, rider, posture, temperature, barometric pressure, and traffic, which vary considerably between 25-mile rides in England? That is the reason for multiple attempts, if he is sensible under reasonably similar conditions, perhaps over a much shorter circuit. If in fact the rider cannot discern any difference in this way for his typical riding, then he has the information he needs and should feel free to make the pressure whatever he finds most comfortable or entertaining. e Dear 41, I suppose that I should have predicted your answer. Cheers, Carl Fogel |
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Tire pressure/recommended psi and rider's weight
"41" wrote:
For fun & revelation, why not time yourself, over several attempts, over a fixed course at say 100, 110, and 120psi. snip You might be very surprised at which produces the fastest ride average. Another experiment might be to ride a century at each of those tire pressures. The comfort factor might turn out to be more significant than any slight differences in rolling resistance. Art Harris |
#16
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Tire pressure/recommended psi and rider's weight
Joss
How did you manage to get some Avocets imported?! I haven't had a reply (so far) from either Avocet or Harris Cycles about getting them sent to the UK. I would happily act as a distributor as I'm sure many others would! cccrider |
#17
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Tire pressure/recommended psi and rider's weight
wrote:
How did you manage to get some Avocets imported?! I haven't had a reply (so far) from either Avocet or Harris Cycles about getting them sent to the UK. We're happy to send tyres to the U.K. or anywhere else, we'll even spell it with a "y" for your benefit! Not sure how you tried to contact us, but we send stuff to the U.K. all the time. Before I got my anti-spam filters set up I was averaging 4,000 emails daily, so I really need to have filters...however, sometimes they intercept legitimate email. This might have happened to you. Safest way to make sure you get through is to use one of the encrypted email links on my Website. For actual orders, it's best to use our secure Web based shopping cart system. http://harriscyclery.com/tires Sheldon "Avocet Advocate" Brown +------------------------------------------------+ | Every normal man must be tempted, at times, | | to spit on his hands, hoist the black flag, | | and begin slitting throats. -- H.L. Mencken | +------------------------------------------------+ Harris Cyclery, West Newton, Massachusetts Phone 617-244-9772 FAX 617-244-1041 http://harriscyclery.com Hard-to-find parts shipped Worldwide http://captainbike.com http://sheldonbrown.com |
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