mancello@acf4.UUCP (Homeboy#1) (09/12/85)
I do most of my riding on Specialized Touring II's which have a recommended tire pressure of 100 psi. I usually inflate them up to about 120 psi with no problems. My question is, how high can I safely inflate Specialized tires before they blow off the rim? I have heard that Specialized claims they can take upto ~ 200 psi before blowing. Although, I doubt if they endorse such pressures for their tires. Also I have heard that in hot weather or excessive braking one should lower the pressure in their tires because of the air expansion in the tube do to heat. Is this significant? How much can the pressure increase due to air temperature or braking? Are there any valid formulas for determining the increase in pressure? -Dominic Mancello (cmcl2!acf4!mancello)
fred@varian.UUCP (Fred Klink) (09/17/85)
> Also I have heard that in hot weather or excessive braking one should lower > the pressure in their tires because of the air expansion in the tube do to > heat. Is this significant? How much can the pressure increase due to > air temperature or braking? Are there any valid formulas for determining > the increase in pressure? Gases behave such that pv/T is constant, where p= pressure, v= volume, T= absolute temperature (degrees K=degrees C + 273). This is idealized, but its close enough for this use. Therefore p(1) v(1)/ T(1) = p(2) v (2)/ T(2). Plugging in p(1)= 7 atm (~100 psi), T(1)= 294 K (70 F), T(2)= 311 K (100 F) and v(1)= v(2) we get p(2)= 7.4 atm or about 104 psi. So, if you inflate your tires to 100 psi at 70, they will rise to about 104 psi at 100 degrees F. Be sure to use absolute (Kelvin) temperature or the result is way off. In any case, the rise is insignificant. I have never worked this out before, but I suspected it from my experience riding around here (hot part of SF Bay Area) in summmer. As for excessive braking, I know some people who have blown tires this way. Given the equation above, the only explanation for this is a result of actual heating of the rim metal and transfer of this heat to the inner tube where it contacts the rim metal. The localized heating of the tube causes it to weaken and burst rather than an overall increase in the tire's air pressure. This should be less of a problem for sew-ups since the inner tube is completely isolated from the rim metal by the tire casing and rim tape. Fred Klink Varian, Walnut Creek
leimkuhl@uiucdcsp.Uiuc.ARPA (09/17/85)
I was using some of the new Turbo/Rs in a century last weekend and pumped them to about 110 (rating is 115 max). After about two hours of riding in the sun (hot that day!) the rear tire blew up. The bead was blown right off the rim. The tube was punctured and the force of escaping air blew the bead off. Some friends of mine who routinely run their tubulars at 140+psi tell me that if you can put 120psi into them, they hold 120psi, but I follow the advice of Michelin's tire experts who say that the optimum pressure is around 100psi for road racing. Above that, rolling resistance can actually increase as the tire ends up bouncing around on minute debris instead of rolling over it, and higher pressure can make the tires more flat-prone, too. Also, those very high pressures may be more than your rims can stand. -Ben Leimkuhler
eirik@tekchips.UUCP (Eirik Fuller) (09/19/85)
I have my doubts that Turbos would fail to meet their pressure rating; in my experience, Specialized tires are very conservatively rated; I have also read of Turbo S's holding 240 psi, nough to crumple some rims. I regularly ride my 27 by 1 Touring II's (rating: 95 psi) at 120 psi, with no problems. The single most likely reason for blowing a tire off of the rim is a pinched tube. I have seen brand new bicycles with pinched tubes. One in particular worked fine until I inflated the front tire to its rated pressure (which, apparently, had not been done in the shop that sold it); the result was a deafening boom. It never had the same problem again, once the tire was properly mounted. I suspect that your Turbo was pinching the tube slightly, so that it took a bit of shifting around for the tube to work its way into such a position that it could lift the bead. I have seen such a delayed reaction many a time, especially with high temperatures to help out. To help in avoiding pinched tubes, it is a good idea, when mounting a tire, to get the tube entirely inside of both the tire and the rim before working the second bead over the rim (this also makes it easier to mount the second bead). This situation is easy to detect because the inside of the tire will be touching the rim all the way around the wheel on the side left to be mounted. I don't know about what pressure is optimal for rolling resistance (though I would expect it to vary greatly with the properties of the road surface), but I do know that rim damage is much less likely with fully inflated tires when hitting curbs and such; a rim is only likely to dent when the tire bottoms out, which is less likely at high pressures. I would rather repair a blowout then take a flat spot out of a rim (though I have done both often; both require removing at least one bead of the tire, but the latter requires a tool which I don't generally carry on the road). The flat tires I do get (which never result from pinched tubes) probably aren't much influenced by tire pressure. My last puncture was on a 26 by 1.5 tire at 50 psi. A determined piece of schrapnel will work its way into anything.
meister@linus.UUCP (Phillip W. Servita) (09/22/85)
In article <4200023@uiucdcsp> leimkuhl@uiucdcsp.Uiuc.ARPA writes: >I was using some of the new Turbo/Rs in a century last weekend and Question: why in the world would you use turbo/Rs in a century? i would never use turbo/Rs in anything but a time trial or criterium ride, where the road surface is known. Rs have no tread on them. hardly the tire to use on 100m of random road. -- ----------------------------------------------- "Of course the Nielsen ratings are accurate... We proved it in a taste test!" ----------------------------------------------- -the venn buddhist
leimkuhl@uiucdcsp.CS.UIUC.EDU (09/24/85)
Some of us like to do fast centuries. After all, we're not talking about some six-day tour--what's 100 miles but a long training ride? There's a real myth around right now that these smoothies are less durable or more slippery than there treaded cousins. All tests seem to say the opposite. My feeling is that they bite the road on corners appreciably better than my turbo/s's and not much worse than my cotton tubulars. As for durability, well I had a couple of flats that day, but I suspect it was my use of metal tools that caused the second and my pinching the tube that caused the first. My brother has no problems with his Avocet 30s, and another guy I know says he didn't have a flat in six months of riding smoothies. Face it, we've probably all been hoodwinked by Specialized's tire tread ads, and the only reason they're not recommending turbo/rs for all surfaces is because they would be admitting that they've been selling us extra rubber for years. -Ben Leimkuhler
wew@mgweed.UUCP (Ed Weiss) (09/30/85)
ed Ben In the last month or so I road the Turbo R's in two road races a 100 K Team Time Trial, and a criterium. All conditions were dry and swept. The tires (and I) did fine. HOWEVER, last weekend during a club training ride on the tires on WET roads I experienced fish tailing on off the seat climbs and sprints and most exciting of all, coming back into town at the finish of the ride, I had to jump a curb take to the sidewalk in order to avoid real trouble as I slid through a turn I've made many times on Turbo S's. Needles to say, I'll use the S's in wet weather. . w . w w . n e q . ed n. . w . ,n