How much tire pressure do I need?
10-09-06, 07:01 PM
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How much tire pressure do I need?
How much tire pressure do I need in these Hutchinson Flash 700x28 tires? According to the Hutchinson website (go to Road, then Flash) they list it at 87psi. Is this the maximum or recommended inflation pressure?
It's a 25 pound bike, add me, accessories and two fully loaded panniers and you're looking at about 200 pounds total at most.
It's a 25 pound bike, add me, accessories and two fully loaded panniers and you're looking at about 200 pounds total at most.
10-09-06, 07:16 PM
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87 psi would be the recommended pressure, a compromise between the engineers and the lawyers and the marketing people. Maximum would be somewhere up towards twice that but do not try it. Max is when the tire blows off of the rim; V. dangerous. I go with the sidewall listed pressure for normal use. Lower pressure for sand, snow and dirt. Higher pressure for time trailing on dead smooth pavement.
10-09-06, 07:36 PM
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Sheldon has a really good article on this. What I got out of it was that the listed numbers on the tyre are useless. Have a play round and see what works. Just check the tyre is busting off the rim, also as tyres warm up the pressure increases, I dont know by how much but if you live in a warmer area watch that one as well. What pressure the rim can take is important, dont go over that one.
10-09-06, 08:18 PM
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I don't like that graph.
As a matter of fact, the minimum recommended pressure for my tire is 115psi (Specialized Armadillo).
Take into aco**** that me and my bike weigh a combined 250lb. and the recommended pressure of 105psi is way off.
As a matter of fact, the minimum recommended pressure for my tire is 115psi (Specialized Armadillo).
Take into aco**** that me and my bike weigh a combined 250lb. and the recommended pressure of 105psi is way off.
10-10-06, 06:04 AM
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That chart says my SS road bike conversion with 27 x 1 1/4 tires should use an inflation of 55 pounds. At 55 pounds, I'll get a pinch flat for sure.
10-10-06, 06:37 AM
#7
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Originally Posted by mrkott3r
.. also as tyres warm up the pressure increases, I dont know by how much but if you live in a warmer area watch that one as well. ...
P = pressure, V = Volume, n = # of moles, R = gas constant, T = Temp in Kelvin
So, given that V, n & R are constant (ignoring the slow leak of gas out of the tire), a change in temperature is proportional to a change in pressure.
So, being sloppy with units, if the temp is 75 F (297 K) and rises to 100 F (310 K), the pressure in a 120 psi tire will increase as 310 / 297 = 4%, or about 5 psi.
For more exact calculations, go here
Last edited by DogBoy; 10-10-06 at 11:29 AM.
10-10-06, 06:43 AM
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It's about time someone brought the ideal gas law into our beloved forum!
10-10-06, 10:33 AM
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Just picked up some 28mm conti gater skins. Put in the recommended 95psi, and they felt good - significantly softer than the 23mm at 110 that I had been riding. Mine have both the recommended and the max (115psi) listed on the tire. How much pressure it takes to blow the tire off the rim, probably depends on a lot of factors beyond the control of the tire manufacturer. The listed max has a safety factor built in (for a reason).
Another point not brought up yet here: Running at a higher than recommended pressure is O.K., but the tires won't last as long. The wider tires have the rubber spread out over the intended contact area. The narrower tires are designed for higher pressure, and lower contact area. They have thicker rubber over a narrower band.
Another point not brought up yet here: Running at a higher than recommended pressure is O.K., but the tires won't last as long. The wider tires have the rubber spread out over the intended contact area. The narrower tires are designed for higher pressure, and lower contact area. They have thicker rubber over a narrower band.
10-10-06, 11:08 AM
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I always check the sidewall and run max pressure up front and max +5psi in the rear. Always worked for me.
According to my sidewalls I should have 85psi in my Sirrus's AllConditions. According to the graph I should be running 75psi. I think I'll stick with the sidewall.
According to my sidewalls I should have 85psi in my Sirrus's AllConditions. According to the graph I should be running 75psi. I think I'll stick with the sidewall.
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10-10-06, 12:20 PM
#11
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The graph I posted above is something I came across a year or two ago and seems to be a good starting point.
When I weigh myself and my road bike I come in around 210lbs and run my tires at about 110, no matter what the sidewalls say. This has worked well for me on that bike. No pinch flats and a decent ride.
Now on my commuter, the bike and I weigh about 220-240, depending on how much I have loaded. THe chart says about 65 to 70 PSI, but I run them at around 85-90 and again that seems to work great for me.
When I weigh myself and my road bike I come in around 210lbs and run my tires at about 110, no matter what the sidewalls say. This has worked well for me on that bike. No pinch flats and a decent ride.
Now on my commuter, the bike and I weigh about 220-240, depending on how much I have loaded. THe chart says about 65 to 70 PSI, but I run them at around 85-90 and again that seems to work great for me.
10-10-06, 12:41 PM
#12
GATC
I like to see the ideal gas law. I've been wondering about the relative volumes of my 26" x 1.5 tires vs 700 x 28mm tires (the former seem to take more pumping to fill to a lower pressure).
That figure looks... I don't know if it's even good for minimums. Sheldon Brown has good estimates for starting points, I think:
https://sheldonbrown.com/tires.html#pressure
(100 psi for 28s supporting 200# vs 75 psi on the figure above). But I go way past his starting point for my 26 x1.5s at least. I wonder if Sheldon has lawyers too.
That figure looks... I don't know if it's even good for minimums. Sheldon Brown has good estimates for starting points, I think:
https://sheldonbrown.com/tires.html#pressure
(100 psi for 28s supporting 200# vs 75 psi on the figure above). But I go way past his starting point for my 26 x1.5s at least. I wonder if Sheldon has lawyers too.
10-10-06, 12:43 PM
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Originally Posted by tuolumne
It's about time someone brought the ideal gas law into our beloved forum!
https://www.hull.ac.uk/php/chsajb/gen...nderwaals.html
You might also want to look at the heat capacity of the gas that you use to fill your tires, and consider the conditions under which you ride and how heat from the roads will transfer through the tires and tubes to affect said gas.
https://en.wikipedia.org/wiki/Heat_capacity
Happy calculating!
(Seriously: the graph seems like a good starting point, if a bit on the low side.)
