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Old 06-14-07, 07:57 PM   #1
JFlitt
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Tire pressure question

Hi, this may be a stupid question,

when a tire says the recommended inflation pressure is 110 psi, does it really go up to 110? reason i'm asking is because I got my tire up to 60psi and the tire seems very bulky as if it is gonna explode, I can barely pump any more air in.

Am I missing something? or should I just keep pumping till it reaches 110psi. I really don't want it to explode lol
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Old 06-14-07, 08:23 PM   #2
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The stamped on pressure is the working pressure of the tire.

First check the tube is well set in the tire,the tire is seated well on the rim,Pump away.

On my racing bike,the first time trying to get a tire at 100 psi up to 120 psi was terrifying(especiallly in an apartment)I spent the night thinking the tire was going to blow any time,but, no problems

Last edited by Burningman; 06-15-07 at 07:26 PM.
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Old 06-14-07, 08:44 PM   #3
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Quote:
Originally Posted by JFlitt
I can barely pump any more air in.
If you're using a hand-held mini-pump, it may be the pump that tops out around 60: sometimes they are the limiting factor.
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Old 06-14-07, 09:25 PM   #4
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Thanks for the reply.

I did manage to get it up to 90psi but ran out of breath. my pump only goes to 100 psi .


I'm just gonna take it to my LBS and borrow their pump or just purchase one that goes to 110 and up.
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Old 06-14-07, 09:38 PM   #5
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Are you airing the tire via mouth?
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Old 06-14-07, 10:19 PM   #6
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Originally Posted by Portis
Are you airing the tire via mouth?
lol no. I'm using a foot pump but I can't for the life of me get it up to 100psi.

I must be doing something wrong or it's just that i'm weak. arghh
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Old 06-14-07, 11:33 PM   #7
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I've got an old fashioned kind that maxes out about 60 PSI. Enough for my 26" tires.
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Old 06-15-07, 12:23 AM   #8
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Quote:
Originally Posted by JFlitt
Hi, this may be a stupid question,

when a tire says the recommended inflation pressure is 110 psi, does it really go up to 110? reason i'm asking is because I got my tire up to 60psi and the tire seems very bulky as if it is gonna explode, I can barely pump any more air in.

Am I missing something? or should I just keep pumping till it reaches 110psi. I really don't want it to explode lol
Any chance that the air pressure guage isn't working correctly?
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Old 06-15-07, 04:23 AM   #9
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Road Morph G is a great pump, about $30.


I respect the opinions of seasoned bikers on here, however, when the tire says MAX PRESSURE 125 PSI that over rules any opinion given to me. It means MAX pressure of 125 PSI while cold. Unless the meaning of MAX PRESSURE stamp on the tire is somehow different for bikes than cars.
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Old 06-15-07, 05:26 AM   #10
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Quote:
Originally Posted by djSlvt
Road Morph G is a great pump, about $30.


I respect the opinions of seasoned bikers on here, however, when the tire says MAX PRESSURE 125 PSI that over rules any opinion given to me. It means MAX pressure of 125 PSI while cold. Unless the meaning of MAX PRESSURE stamp on the tire is somehow different for bikes than cars.
I've read that the manufcturers rate their tires at a lower pressure than they will actually take, mainly for liability reasons. The tires on my old MTB are rated to 50psi but I run them at 65 to decrease the rolling resistance.
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Old 06-15-07, 08:44 AM   #11
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Quote:
Originally Posted by djSlvt
Road Morph G is a great pump, about $30.


I respect the opinions of seasoned bikers on here, however, when the tire says MAX PRESSURE 125 PSI that over rules any opinion given to me. It means MAX pressure of 125 PSI while cold. Unless the meaning of MAX PRESSURE stamp on the tire is somehow different for bikes than cars.
You raise a good point, djSlvt. I have experienced pumping up a tire to capacity in the cool morning, and then gone on a long road trip where the ambiant temperature increased 25 degrees by noon. The road surface temperature was much more than that and the sun was beating down on us like a heat lamp on french fries.

About 40 miles into the trip, while we were taking a break, KA BLAM! One of the tires exploded. It blew the tire right off the rim.

I figured it was because the heat increased the tire pressure to the extent that it was way over-inflated. So, I if you are riding under similar extreme changes in temperature or maybe even altitude, it is a good idea to check the air pressure along the way.
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Old 06-15-07, 09:06 AM   #12
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Quote:
Originally Posted by Burningman
The real blow it off the rim pressure is much higher.
That's often true but it's still dangerous advice. Half of the tire/rim interface is the rim. Bad combinations of tires and rims = blow offs.
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Old 06-15-07, 11:03 AM   #13
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The PSI level on most tires is a safety rating that assumes you are using a rim of the correct width and design. It is the MAXIMUM safe PSI for a given tire if mounted on the correct rim.

If you buy a new bike, the tire's width and design will be a good match for the width and design of the rim. However, I have seen older bikes where the tire installed was much too narrow or much too wide for the rim because the owner did not know rims are designed for a specific width of tire. If a tire is mismatched to a rim, there is a good chance the tire will come off the rim, usually at the worst possible moment.

The BEST PSI for a given tire and given rider is usually a far lower number than the maximum. Tires are designed to provide the best blend of shock absorption, handling, cornering, and braking when the rider's weight and load causes a slight deflection of the sidewalls (about 15% deflection).

With a given tire, a 250 pound rider might obtain 15% deflection of the sidewalls at 110 PSI, and a 140 pound rider might obtain 15% deflection at just 75 PSI or 85 PSI.

As a rule of thumb, if you weigh more than 200 pounds, you will usually get the best performance from a tire riding at about the maxiumum PSI stamped on the tire. If you weigh 140 pounds, use about 70% of the maximum PSI. Riders who weigh 150 pounds to 180 pounds can experiment with PSI levels that are 80% or 90% of the maximum PSI.
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Old 06-15-07, 11:13 AM   #14
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External temperature don't cause that much pressure change, unless it's a dramatic 25 degrees change, think +/- 5 PSI.. The biggest effect is from speed and weight. I don't know on bike, but on car, driving 1 mile at 55 mph, increases temperature such that new pressure is measured 5 PSI more.


It's how most tires get worn and accidents happen. People drive to get air, and them inflate hot tires. They get cold and tire becomes underinflated. Less traction, less responsive steering.
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Old 06-15-07, 04:07 PM   #15
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You won't see anything like +/-5 psi difference between hot and cold in your bike tires. You can't compare the operating parameters of the tires on your bike to the ones on your car, it's totally different. There's no way you can put anything like the energy and loads through your bike tires that your car tires have to deal with. you can top up your bike tires any time, even after the hardest ride of your life and it won't make a practical difference to when they're cold.
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Old 06-15-07, 04:42 PM   #16
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I've heard of loaded touring bikes going down long steep hills having a tire explode from braking heat...
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Old 06-15-07, 05:08 PM   #17
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Quote:
Originally Posted by Retro Grouch
That's often true but it's still dangerous advice. Half of the tire/rim interface is the rim. Bad combinations of tires and rims = blow offs.
Agreed,but without knowing more(rims shacky,8 yrs old tires,etc.) all we can help with is "my tire feels full at 60psi the side wall says 110psi,can it take more?"

If things do get hot out the pressure can also rise enough to blow the tire off.

(I've had this happen on tour,luckly I was having lunch under a near by tree when it let go___BANG)

Last edited by Burningman; 06-15-07 at 07:18 PM.
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Old 06-15-07, 05:49 PM   #18
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Quote:
Originally Posted by mike
You raise a good point, djSlvt. I have experienced pumping up a tire to capacity in the cool morning, and then gone on a long road trip where the ambiant temperature increased 25 degrees by noon. The road surface temperature was much more than that and the sun was beating down on us like a heat lamp on french fries.

About 40 miles into the trip, while we were taking a break, KA BLAM! One of the tires exploded. It blew the tire right off the rim.

I figured it was because the heat increased the tire pressure to the extent that it was way over-inflated. So, I if you are riding under similar extreme changes in temperature or maybe even altitude, it is a good idea to check the air pressure along the way.
Most likely this was caused by an improperly seated tube that was pinched between the rim and tyre. The rated sidewall pressure is typically 1/2 the blow-off pressure. In order to increase internal pressure to that much, you'd have to heat up the inside air to something like 400-F or so...

And yes, auto-tyres are completely different due to their operating parameters. They experience much higher range of stresses than a bike tyre due to the weight and cornering/braking forces. When I go to tracks like Laguna Seca, Willow Springs, California Speedway, etc, I'll typically measure tyre-temps across the tread with a pyrometer. It's not unusual to get temps in the 180-220F range and have tyre-pressures go from 40psi when cold to 50psi hot at those temps. The primary contribution to the pressure-rise is water-vapour which has a drastic increase in partial vapour-pressure at 212-F. Without the water-vapor (using pure nitrogen), tyre-pressure increases are typically only 2-3psi to give much more consistent performance and easier tuning.

Last edited by DannoXYZ; 06-15-07 at 10:42 PM.
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Old 06-15-07, 07:49 PM   #19
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My tanksess compressor will only get them up to 92 p.s.i. I don't know how you hand pump guys are doing it. bk
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Old 06-15-07, 10:21 PM   #20
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Quote:
Originally Posted by JFlitt
lol no. I'm using a foot pump but I can't for the life of me get it up to 100psi.

I must be doing something wrong or it's just that i'm weak. arghh
By "foot pump," do you mean the kind you operate with your foot? Like a bellows with a hose coming out? If so, save that for air mattresses--I'm shocked, SHOCKED you could even get that to 60psi. It's designed for low-pressure, high volume stuff like beach balls. You need a real bike pump, a frame-fit or mini to carry when you ride and a FLOOR pump for the garage.
It's also possible your gauge is bogus. A lot of them are. I check mine against a good digital gauge I keep in my car, and bike pump gauges are often 20psi or more off.
When you say the tire is "bulky," whazzat? Bulky is a size thing. If it feels HARD, that's OK--it takes a lot of practice to tell a tire's pressure by feel, and 50-60 psi can feel pretty firm. If it's bulging or sticking out anywhere, then you have other problems.
Finally, 110psi or whatever it says on the tire is the maximum. If you're lighter than average and running on smooth roads, you can go lower for a more comfortable ride and better handling.
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Old 06-16-07, 05:07 PM   #21
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Sheldon Brown on pressure: http://www.sheldonbrown.com/tires.html#pressure
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