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Tire Pressure: How Much Before It Gets Dangerous?

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Tire Pressure: How Much Before It Gets Dangerous?

Old 05-17-06, 01:31 PM
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EnLaCalle
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Tire Pressure: How Much Before It Gets Dangerous?

I usually go about 10psi beyond the recommended max amount. Like 120psi on 110. How much is too much before you risk hazard, you think? I just got a pair of Vredstein Volante SE and they're spec'd to 130psi. I like to put in as much air as I can. The "sensitivity" to bumps and such doesn't really bother me.

Like most recommendations, I believe (and think I've read) that the manufacturers are lowballing us on the pressure tip to cover their asses in case something does happen. So that's why I feel okay about going beyond the "max". I just don't want the tires to accidentally explode. Does anyone have any hard facts?
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Old 05-17-06, 01:37 PM
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I use 10 psi over the suggested tire pressure on my TUFO's ... but ONLY on the track. lol =)

I've heard of people over-inflating the tufo's 30+ PSI over the max rating on the sidewall...

=)
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Old 05-17-06, 01:38 PM
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I always went to 120 on my 100 rated Kendas.
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Old 05-17-06, 01:38 PM
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Originally Posted by EnLaCalle
I usually go about 10psi beyond the recommended max amount. Like 120psi on 110. How much is too much before you risk hazard, you think? I just got a pair of Vredstein Volante SE and they're spec'd to 130psi. I like to put in as much air as I can. The "sensitivity" to bumps and such doesn't really bother me.

Like most recommendations, I believe (and think I've read) that the manufacturers are lowballing us on the pressure tip to cover their asses in case something does happen. So that's why I feel okay about going beyond the "max". I just don't want the tires to accidentally explode. Does anyone have any hard facts?
a lot of tires overstate the max because a lot of people think a higher psi = better tire. which is a lie. sheldonbrown has a page that describes the effects of over and under inflating tires if you search his page.
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Old 05-17-06, 01:39 PM
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i read on BF that the max pressure is actually somewhere around half of what it would take to create serious problems. i usually just pump my tires a little past the max, until it starts getting really hard. never had any problems doing that, and get very few flats.
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Old 05-17-06, 01:52 PM
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I think high PSI is pointless unless you are at the track. Uneven street surfaces ride better with lower pressure.
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Old 05-17-06, 01:54 PM
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so is it more easier to get a flat if your a lil underinflated??
 
Old 05-17-06, 01:55 PM
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i tend to go substantially past the max, because i weigh 220.
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Old 05-17-06, 01:57 PM
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Like overhead lifting hardware, recommended tire pressures have a healthy amount of safety overhead built into the numbers on the tire. I recall reading that the actual blowout pressure is in the ballpark of twice the sidewall recommended pressure. Another 10 or 20 PSI shouldn't cause a blowout, but it will strain your tire's beads and casing more, and put more pressure on your rim tape, possibly accelerating a potential spoke puncture.

The optimum tire pressure for efficiency is based on the shape and size of the contact patch while taking into consideration the tire size/flavor and the intended application. The recommended pressure is almost always at or past the optimum for a typical situation, so unless you dig really harsh ride characteristics or are on the track, keep 'em down around 100.

Sheldon has all of the basics on pressure recommendations and overinflation nailed here:
https://www.sheldonbrown.com/tires.html#pressure

If you really want a good idea of what pressure is best for your weight and tire situation, he has a table of recommended pressures based on load vs. width of tire. According to his chart, I should be running my rear tire around 100 to 110psi and my front around 60 to 70psi. You can measure the load on each wheel easily with a bathroom scale. Have someone help you balance perfectly still with one wheel at a time on the scale and see how much weight is on each wheel when you are in your most typical riding positions. To be more accurate, use a block of wood or a phone book to get the wheel that's not currently on the scale up to the same height as the one that is.
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Old 05-17-06, 02:09 PM
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i ride clinchers at about 125, but after a few days of riding they get back down to the more comfy (for them) 100-110.
you should always feel okay "going beyond the max," but watch your back beyond thunderdome.
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Old 05-18-06, 08:16 AM
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In a Rivendell Reader issue, Grant cut the beat of a clincher and it took quite a bit of PSI over the tire's rating to blow it. He then cut another tire's bead in 10 places...that one didn't last nearly as long.
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Old 05-18-06, 08:30 AM
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One thing to bear in mind here is that rim manufactures set pressure limits as well.

ie for a Mavic CXP-33

TIRE SIZE MAXIMUM PRESSURE
mm bars psi
19 10.00 146.00
23 9.50 138.00
25 9.00 131.00
28 8.00 117.00

Most clinchers are not spec'd for much more than 125 PSI. Some are rated lower than that (ie Corimas fancy carbon clinchers are spec'd at a max pressure of 115).
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Old 05-18-06, 08:31 AM
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Most pumps aren't 100% accurate, usually reading higher than what is really in the tire. 10 or even 20lbs over may only get you to the rec'd pressure.
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Old 05-18-06, 08:44 AM
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The "Maximum PSI" stamped on a tire is a safety rating. It is NOT the best PSI for a given rider. Companies base the "maximum PSI" on the assumption that a tire is mounting on a good quality rim of the correct width. They build in a safety margin of 50% or more.

That means that if (and ONLY if) a tire marked "100 PSI" is mounted on a good quality rim of the correct width, it could be inflated to at least 150 PSI without coming off the rim. However, if someone were to ride that tire at 150 PSI, it is highly likely that the first pot hole would rupture the inner tube.

The "best" PSI level depends on the rider's weight. A tire should slightly change the profile of its sidewalls when the rider gets on the bike. If the tire does not change its profile, the PSI is too high for the rider's weight. If it changes profile MORE than slightly, the PSI is too low for the rider's weight.

What does that mean in "real life"? Two guys own identical twenty pound bikes. They have 25mm tires that are marked "maximum PSI 110". One of those guys weights 120 pounds, and he will get "slight tire deflection" at a PSI level of around 80 PSI. So, 80 PSI is the best pressure level for him.

The other rider weighs 250 pounds. He gets "slight tire deflection" at 120 PSI. So, even though the tire is marked "110 PSI maximum", he can, and should, add the extra ten pounds of pressure and ride at 120 PSI.
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Old 05-18-06, 08:47 AM
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I usually just pump it up until it explodes then subtract 10% from whatever pressure I reached.
 
Old 05-18-06, 08:50 AM
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Originally Posted by fixedpip
One thing to bear in mind here is that rim manufactures set pressure limits as well.

ie for a Mavic CXP-33

TIRE SIZE MAXIMUM PRESSURE
mm bars psi
19 10.00 146.00
23 9.50 138.00
25 9.00 131.00
28 8.00 117.00

Most clinchers are not spec'd for much more than 125 PSI. Some are rated lower than that (ie Corimas fancy carbon clinchers are spec'd at a max pressure of 115).
Thanks for publishing that chart. Many rim makers "hide" that information on their web sites. Rarely is that information provided on or with a rim. It is basic safety information, yet many guys who work in bike shops (as well as the writers at "Bicycling Magazine") are unaware of the importance of matching the width of a tire to the width of the rim, and the fact that the "best" PSI is related to both the width of the tire, and the weight of the rider.

One tire company marks some of its tires with maximum PSI levels of 150, 160, and 170. Those maximum PSI levels may exceed the design limits of the rims that the tires are mounted on. Owners need to make sure they match the tire to the correct rim, and stay within the design limits of both the tire and the rim.

The BEST PSI level for most riders is often far below the maximum. A typical 21mm tire can be safely ridden at 140 PSI. Yet, many Pros would ride that tire at around 110 PSI on smooth, dry roads, and at PSI levels as low as 90 PSI on wet, water covered roads. Some folks confuse the words "best" and "maximum". They are NOT the same thing.
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Old 05-18-06, 09:23 AM
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i/r/t the rim issue, I spoke with Velocity on the phone a couple of weeks ago and they were pretty much like, "We don't rate our rims b/c a tire will blow before our rims will give out." Take that fwiw.
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Old 05-18-06, 09:30 AM
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Originally Posted by EnLaCalle
i/r/t the rim issue, I spoke with Velocity on the phone a couple of weeks ago and they were pretty much like, "We don't rate our rims b/c a tire will blow before our rims will give out." Take that fwiw.
I bet they said it in some tough-guy Australian accent, too.
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Old 05-18-06, 09:39 AM
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The rim strength is interesting. I just slapped a pair of Schwalbe Stelvio plus tires on my rig and they're rated for 145PSI. I run them about 120-130 because of my pump. 145 just scares me.
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Old 05-18-06, 02:35 PM
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The roadies i ride with all say not to go over 100psi unless the tire calls for it. Since a bunch of them saw a guy get airlifted from a race after he blew out a tire pumped up to 160psi on a descent.
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Old 05-18-06, 02:57 PM
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i ignore tire ratings

i pump to 110 and ride it there.. (23c).. 120 and above is uncomfy... 80 and below feels sluggish
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Old 05-18-06, 03:10 PM
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High volume, low pressure here. 32's, 90psi, 240#. Cushy. After riding 23's@ 110, it's lovely.

Tubulars at 120 are about the same as the 32's @ 100. Funny.
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Old 05-18-06, 03:38 PM
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Do you people use the 2 handed stand up pumps or gas station-LBS air? I just bought a small handheld one and can't seem to get my tires hard enough.
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Old 05-18-06, 03:48 PM
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Gas station pumps blow. LBS should be fine. Your handheld pump most likely needs to be chucked. Get a Topeak road morph.
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Old 05-18-06, 03:52 PM
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Operator, thanks for the input. Seems like a pretty good value ~28$
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