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Over-inflating tires?

Old 12-26-07, 02:31 PM
  #1  
special_k
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Over-inflating tires?

Sheldon said that the maximum tire pressure printed on the side of tire walls are rather "conservative".
'"They commonly shoot for half of the real blow-off pressure."

If the max. tire pressure says it's 65 psi, and I want the tire firmer for a faster ride,
would it be dangerous to pump it up to say, 80 psi?
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Old 12-26-07, 02:56 PM
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https://www.google.com/search?hl=en&q...+bicycle+tires

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Old 12-26-07, 03:03 PM
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Originally Posted by special_k View Post
Sheldon said that the maximum tire pressure printed on the side of tire walls are rather "conservative".
'"They commonly shoot for half of the real blow-off pressure."

If the max. tire pressure says it's 65 psi, and I want the tire firmer for a faster ride,
would it be dangerous to pump it up to say, 80 psi?
no as long as the tire doesn't show any rot
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Old 12-26-07, 03:18 PM
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You got only half the issue covered when only the tires are considered. The other part are the rims and how much they can handle. Some CF wheels are rated for max pressure below many tires. Another thing is the tire beags will slip and blow the tire/tube off the rims. So if you have something like Vredenstein that are rated at 145 psi, I wouldn't trust that rule and try inflating to 218.
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Old 12-26-07, 03:27 PM
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Originally Posted by StanSeven View Post
You got only half the issue covered when only the tires are considered. The other part are the rims and how much they can handle. Some CF wheels are rated for max pressure below many tires. Another thing is the tire beags will slip and blow the tire/tube off the rims. So if you have something like Vredenstein that are rated at 145 psi, I wouldn't trust that rule and try inflating to 218.
I'll wager any rim will take 80 psi, even half dead ones from ancient hardware store bikes
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Old 12-26-07, 04:22 PM
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Originally Posted by special_k View Post
Sheldon said that the maximum tire pressure printed on the side of tire walls are rather "conservative".
'"They commonly shoot for half of the real blow-off pressure."

If the max. tire pressure says it's 65 psi, and I want the tire firmer for a faster ride,
would it be dangerous to pump it up to say, 80 psi?
Had a set of Conti Town and Country 1.9x26 inflated to 90psi. Sidewall blew out above the bead.
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Old 12-26-07, 04:49 PM
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Originally Posted by special_k View Post
Sheldon said that the maximum tire pressure printed on the side of tire walls are rather "conservative".
'"They commonly shoot for half of the real blow-off pressure."

If the max. tire pressure says it's 65 psi, and I want the tire firmer for a faster ride,
would it be dangerous to pump it up to say, 80 psi?
If you want a faster ride (less rolling resistance) try going with a narrower tire. A 25mm tire is good for any paved road and you can easily run that tire at 100 psi.
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Old 12-26-07, 06:39 PM
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Originally Posted by Fredmertz51 View Post
Had a set of Conti Town and Country 1.9x26 inflated to 90psi. Sidewall blew out above the bead.
could never do that, pump sucks. only can use my friends pump to do that. and i dont even want to. i keep it at 40 psi, 26x1.5
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Old 12-27-07, 04:14 AM
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Originally Posted by San Rensho View Post
If you want a faster ride (less rolling resistance) try going with a narrower tire. A 25mm tire is good for any paved road and you can easily run that tire at 100 psi.
Yeah, narrower tyres will allow you to go faster. Not necessarily from just the tyre-pressure, but you get even bigger benefits from lower weight and less aero-drag. The smoother tread also contributes more to lower rolling-resistance than the increased pressure.
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Old 12-27-07, 06:24 AM
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The more that I learn about bicycle tire air pressure the more I realize how much I don't know.

I've heard that bicycle tire companies set the max tire pressure at half the actual blow off pressure for decades. I've never heard it from a bicycle tire company person so I don't know it's origin or even if it's true. Could this be one of those things like "There's more suicides at Christmas time" that everybody knows and repeats yet turns out to be false?

Half of your tire and rim interface is the rim. There's lots of older bikes still around that don't have hook bead rims. You can put any tire you want onto a straight sided rim but it'll blow off if you pump it up to the max sidewall pressure.

How old are your rims? Rim brakes gradually erode the rim sidewall making it weaker over time. If you pump your tire up to 100psi, that means for every square inch of tire sidewall you have 100# of pressure trying to push the two rim flanges apart. Narrower tires can be rated for higher air pressures because they have fewer square inches of sidewall.

Finally, while there's truth in the statement "Harder tires are faster." It's only true to a point. A rock hard tire, when it encounters any tiny little pebble bounces upward. Lifting the bicycle and rider upward requires energy that ultimately has to come from the rider. A slightly softer tire will compress as it rolls over the pebble and spring back, returning much of that energy to push you down the road. Every road surface, tire and rider combination has it's own optimum tire pressure. A bike that feels like it's dancing around under you may feel faster but it's not.
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Old 12-27-07, 09:26 AM
  #11  
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Originally Posted by Retro Grouch View Post

Half of your tire and rim interface is the rim. There's lots of older bikes still around that don't have hook bead rims. You can put any tire you want onto a straight sided rim but it'll blow off if you pump it up to the max sidewall pressure.

How old are your rims? Rim brakes gradually erode the rim sidewall making it weaker over time. If you pump your tire up to 100psi, that means for every square inch of tire sidewall you have 100# of pressure trying to push the two rim flanges apart. Narrower tires can be rated for higher air pressures because they have fewer square inches of sidewall.
Even with hooked rims, the weak part of the wheel is still the rim itself...even new. Aluminum makes a really lousy pressure vessel. It's soft and prone to fracture. The weak link in the system isn't the tire or the tube but the actual rim itself. The rim/tire interface is even weaker. Sometimes I'm amazed...and a little frightened...at how well it actually works

The casing of the tire is actually a pretty strong vessel given the overlapping layers of fabric and the rubber binder.
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Old 12-27-07, 02:51 PM
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My 26x1.50" Serfas Drifters are rated for 65 max. I usually run them around 70-75. I really don't want them any "harder", so I haven't pushed them beyond that. Maybe a little fear factor too?
It did surprise me how much a "few" extra PSI made them roll easier.
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Old 12-28-07, 07:48 PM
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Originally Posted by special_k View Post
Sheldon said that the maximum tire pressure printed on the side of tire walls are rather "conservative".
'"They commonly shoot for half of the real blow-off pressure."

If the max. tire pressure says it's 65 psi, and I want the tire firmer for a faster ride,
would it be dangerous to pump it up to say, 80 psi?
Are you riding on or off road. If you're riding off road, ridiculously high pressures are counterproductive.
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Old 12-28-07, 07:49 PM
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Originally Posted by BearSquirrel View Post
Are you riding on or off road. If you're riding off road, ridiculously high pressures are counterproductive.
They're counterproductive everwyhere.
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Old 12-28-07, 08:14 PM
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My rule of thumb has always been to inflate 20 psi above the marked pressure. I do that for two reasons, I am heavy, and I like to reduce pedaling effort. Now I can not guarantee that will work OK for you, but I have never had a problem and I have been doing it since the early 1950's.

I would like to point out that you can not blame the manufacturer though if something fails when you are using it beyond their specs.
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Old 12-28-07, 08:46 PM
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Read the thread by the Retro Grouch above. End of thread. Next subject.
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Old 12-28-07, 09:19 PM
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Retro Grouch is probably referring to a long series of scientific tests on tire rolling resistence conducted by Bicycle Quarterly. Previous tests done by a tire manufacturer were conducted with tires rolling on steel drums where high pressure tires showed lower rolling resistance. BQ did its tests on real world asphalt pavement with much different results. To summarize the findings, tire construction has the biggest affect on rolling resistance and not tire pressure or tire size. Some of the fastest rolling tires were 28mm or larger and had pressures well below 100 psi. Tires usually had an optimum pressure and rolling resistance would increase with higher pressures. Identical tires of the same manufacturer and model had increasing resistance as the tire width got smaller.

Finally, and this is most important. The testers were not able to identify the tires with less rolling resistance by riding them. Some faster tires felt slower and some slower tires felt faster. So when someone tells you to choose narrower tires or increase your air pressure to make you go faster; asking him for his testing methodology, and see if it stacks up to BQ.
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Old 12-29-07, 01:18 AM
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"So when someone tells you to choose narrower tires or increase your air pressure to make you go faster; asking him for his testing methodology, and see if it stacks up to BQ."

I use the speedometer reading vs amount of sweat on my brow! You got a better method?
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Old 12-29-07, 10:58 AM
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Tires are designed to provide their best performance when the sidewalls can flex a bit under the weight of the rider. If a tire is over-inflated, relative to the rider's weight, the sidewalls don't flex, the tires does not aborb road shock, and the performance and lifespan of the tire is harmed.

If (and only if) a high quality tire in good condition is perfectly matched to a rim that is in good condition) you could exceed the maximum PSI stamped on the tire by 30% or so without any problems. But, if a tire is a bit too narrow or too wide for the rim, mismatched to the rim, or the tire has sidewall problems...yikes.

The goal is to obtain 15% deflection of the sidewalls (a slight sag of the rim toward the pavement) when you get on the bike. The PSI that provides 15% deflection is a factor of the tires true width (not the advertised width) and the weight of the rider.

Because of the 15% deflection principle, a 110 pound rider might get 15% deflection with a given tire at 70 PSI, and a 220 pound rider might get 15% deflection at 110 PSI.

Riders who weigh 250 or 270 pounds have another problem. If they are rider 21mm or 23mm tires, the PSI that provides 15% deflection may be well above the safety rating of that tire. The solution for such heavy riders is to switch to 28mm or 32mm tires that have very high safety ratings. The size of the air cushion with a 32mm tire is about double the size of the cushion of a 23mm tire...and a 270 pound rider needs that extra protection for his rims.

Trial and error is helpful in finding the best PSI level. My bike and I weigh 200 pounds (a VERY heavy) bike. When I used 23mm tires (most of my bikes now have 28mmm tires) I learned that the most comfortable PSI level was about 100 PSI in front and 110 PSI in back. Some of my tires were rated for 140 PSI, but at the level, it felt as if the tires were made out of wood.
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Old 12-30-07, 04:17 AM
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Originally Posted by cyccommute View Post
Even with hooked rims, the weak part of the wheel is still the rim itself...even new. Aluminum makes a really lousy pressure vessel. It's soft and prone to fracture. The weak link in the system isn't the tire or the tube but the actual rim itself. The rim/tire interface is even weaker. Sometimes I'm amazed...and a little frightened...at how well it actually works

The casing of the tire is actually a pretty strong vessel given the overlapping layers of fabric and the rubber binder.
I ride Mountain bikes and occasionally fit slicks to it for road rides. These are high pressure tyres at 120psi and work well. One proviso is the rims. My rims only last about 2 years before they get a bit too worn for safety and the tell tale sign of a worn rim is that when you inflate the tyre- The rim takes on a concave profile. Running at 50 psi may not do this- but at the high pressure I use for the slicks-it is frightening how much that rim will deform if the rim has been worn away by mud and grit.

And as to over-inflation of a tyre- They will take a lot more pressure that stated on the side. I also ride a Tandem and a sidewall pressure max of 65 psi will only give me snakebites. Have to run them at 85 psi minimum to take the weight.
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