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Finding Your Ideal Tire Pressure

Erik Timmerman

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You aren't losing anything when you take it off. That is just the air coming out of the hose. I know what you mean though, it is an issue for sure with shocks. When you screw on the pump, some air fills the hose. How much? Either way, I would hope that nobody is changing their pressure as they ride up and down hills, that would just be stupid.
 

Ron

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I would hope that nobody is changing their pressure as they ride up and down hills, that would just be stupid.

totally agree so I agree with you about the gauge, lots of info that I wont do anything with :)
 
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martyg

martyg

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My issue is that i dont ride with a pump, just a co2 cartridge and I'm not going to keep stopping and inflating/deflating the tires as I climb/descend or if the temp spikes and then drops. I dont carry a gauge, i can never find one that actually works. I use a prestaflator with a digital gauge on a compressor at home that is very accurate but when you take the valve off or on, you lose a pound or two so just how accurate can you be?

It depends on the climb.... I have climbs on the mtn bike in the 5,000' / 2 - 3 hour realm and then a corresponding descent. My most efficient tire pressure is around 23 psi. if I get to the top of a 3 hour climb and my tire pressure is now at 35 pounds that descent is not going to be as enjoyable.
 
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Ron

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good point!:thumb::thumb::thumb: as long as you are staying at that altitude that makes sense but do you re-inflate for the descend?
 

Tom K.

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[QUOTE="epic, post: 249924, member: 113"... it is an issue for sure with shocks. When you screw on the pump, some air fills the hose. How much?[/QUOTE]

Screw on shock pump.

Note pressure. Call it A.

Remove shock pump again. Note new pressure. Call it B.

Original pressure = A + (A - B) or 2A - B.

Morning fun with math!

(pre-coffee, so somebody should check me! :)
 

Erik Timmerman

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It depends on the climb.... I have climbs on the mtn bike in the 5,000' / 2 - 3 hour realm and then a corresponding descent. My most efficient tire pressure is around 23 psi. if I get to the top of a 3 hour climb and my tire pressure is now at 35 pounds that descent is not going to be as enjoyable.

I find it very, very hard to believe that you could see such big difference. PV=nRT and all that. Have you really measured that big of a difference? If you are seeing that with the Tirewiz, I'd seriously question it's accuracy. Maybe @François Pugh will do the math for us and tell us what it would take to see a change that large.
 

Erik Timmerman

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[QUOTE="epic, post: 249924, member: 113"... it is an issue for sure with shocks. When you screw on the pump, some air fills the hose. How much?

Screw on shock pump.

Note pressure. Call it A.

Remove shock pump again. Note new pressure. Call it B.

Original pressure = A + (A - B) or 2A - B.

Morning fun with math!

(pre-coffee, so somebody should check me! :)[/QUOTE]

But does that math work the same at 100 psi as it does at 200 psi? I'm not sure that it does. Anyway, I figure it doesn't matter all that much if you can assume that whatever the gauge says is correct when you remove the shock pump. What I mean is if I check the pressure and it says 185, it was probably more like 190, but if I pump it up to 205, it is 205. I can be pretty sure about that.
 

Jersey Skier

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Like I stated above. Pressure makes a huge different on a fat bike. Temperature change will noticeably change the ride quality as pressure fluctuates. Not sure how accurate that Tirewhiz will be with less 1psi changes.

temp.jpg
 

Erik Timmerman

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It's good that your chart stops at -10. I was gonna say fat bikers aren't as dumb as us skiers, but that's not true. We had a couple hundred people for our fat bike festival when it was well colder than that.
 

Jersey Skier

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It's good that your chart stops at -10. I was gonna say fat bikers aren't as dumb as us skiers, but that's not true. We had a couple hundred people for our fat bike festival when it was well colder than that.

I was once loading my bike on to my hitch rack as the snow and wind were pelting me in the face. I was just thinking that my neighbors must think I am a complete idiot. Just then a jogger ran past wearing shorts and a sleeveless jersey and I though well he's the idiot, at least I was dressed.
 

Wilhelmson

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For off road I pump the tires up when I can feel I'm rolling too slowly for the effort I put in.
 

cantunamunch

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I find it very, very hard to believe that you could see such big difference. PV=nRT and all that. Have you really measured that big of a difference? If you are seeing that with the Tirewiz, I'd seriously question it's accuracy.

climbing 5000' from sea level would be a drop of approximately 150mb. So I can see a 2-3 psi pressure difference in a cold tire that was driven up to altitude, as measured by TyreWhiz. Note that at altitude climbing 5000' creates less pressure difference than at sea level.

If the sensor is truly measuring an 11-12 psi pressure difference, that tire got seriously hot - or the cyclist climbed into a hurricane.

How hot? The equation you wrote can be re-written as P/T= nR/V where nR/V is held constant, at which point it becomes Gay-Lussac's law. Taking the inflation temperature to be 20C, we get (23+14.7)/293 = (35+12.7)/x

or 47.7/x = 37.7/293

or x= 293*47.7/37.7

It is far easier for me to believe that TyreWhiz is off than that the tire is water-boiling hot.
 
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martyg

martyg

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good point!:thumb::thumb::thumb: as long as you are staying at that altitude that makes sense but do you re-inflate for the descend?

Ron - due to the 416 Fires I wasn't able to do a test of altitude - measure pressure in Durango at 6,500, wrap the wheel in a blanket to help moderate temp fluctuation, then drive to Purg at 8,500, then Molas Pass at almost 11,000.

In speaking to those who know this subject matter elevation changes would not result on profound changes in casing tension.

On days when you start out and it is 40 / 50 degrees and gets up to 80 and sunny during the day, the difference is profound. We did a one day lap of the White Rim this spring and tire pressures by 1300 were profoundly different than when we started.
 
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martyg

martyg

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climbing 5000' from sea level would be a drop of approximately 150mb. So I can see a 2-3 psi pressure difference in a cold tire that was driven up to altitude, as measured by TyreWhiz. Note that at altitude climbing 5000' creates less pressure difference than at sea level.

If the sensor is truly measuring an 11-12 psi pressure difference, that tire got seriously hot - or the cyclist climbed into a hurricane.

Read the article. The surface temps of the tire approached 140 degrees F as measured by a laser thermometer.
 
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martyg

martyg

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Like I stated above. Pressure makes a huge different on a fat bike. Temperature change will noticeably change the ride quality as pressure fluctuates. Not sure how accurate that Tirewhiz will be with less 1psi changes.

View attachment 48358

Solar gain plays a much larger factor in the West than ambient air temps.
 
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cantunamunch

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Read the article. The surface temps of the tire approached 140 degrees F as measured by a laser thermometer.

Then it is 70F short of where it would need to be, even correcting 2 psi for altitude gain, which is a sea level correction. At altitude the correction per foot climb is less, or much less.
 

Ken_R

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Rod9301

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On my mountain bike I squeeze the tires every week or so and if they feel soft I pump them up to about 36-40 according to the gauge on my floor pump.

On my road bike I check them almost every ride because I ride it a lot less. If they feel soft I pump them up to 90.

Not really planning on putting any more effort into finding the proper psi.

Then again I barely tune my skis either.
40 psi!!!!

Try lower pressures. I ride at 20 and the difference in traction is amazing.
 

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