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Discussion Starter · #1 ·
When I regularly check my tires with an electronic guage, I never get the same readings. I know why, but never bothered to find out how big the pressure changes could be. This morning it was cold here at just above freezing. So I rechecked pressures set the day before. The tires had been recently mounted and checked for leaks. The pressures were both 3psi lower.

Some riders say they run tyres 2-3 psi higher to reduce wear. I wondered what might happen if you set this pressure at different times of year or were a hard rider. I read a post recently where the guy ran pressures close to maximum for the tire.

For UK members, there's a rough formula. A 100 deg C increase in tire air temperature will increase tire pressure by 33%. For US members, a 10 deg F increase in tire air temperature increases your tire pressure by 1 psi. Everything is proportional, so a tire starting 30 psi 0 deg C gets up to 35 psi at 50 deg C. I don't know exactly how hot a tire can get, the attached article I found gives some clues.

Motorcycle tires have a relatively small air volume and profile compared to cage tires. Any air leak or change in temperature will have a greater effect on the tire profile in contact with the road.

In my own mind I now have some specific do's and don'ts:

Setting tire pressures on hot tires after riding is bad news - when the tires cool down, pressure can drop a lot! e.g 5-10 psi depending on your outside temperature.

Checking pressures cold and hot with/without pillion occasionally, before and after a normal ride is useful, since it gives you a typical pressure change and a crude estimate of tire air temperature for your type of riding at that ambient temperature. You can then adjust your figures to keep tire pressure closer to your chosen ride pressure.

Always using the same figures for your tire pressure setting may lead to under/over pressures if you don't consider variations in temperature for your climes and riding style. It's no good posting your figures as which fixed figure is best either, because the temperature has such a big effect on tire pressure. I worked the theory back from simple physics, then found a NA article which made sense for you guys. There's lots of other interesting tire stuff as well.

I hope you enjoy the article, those that get the biggest start to stop pressure changes on a ride, win the prize for bike thrashing, those that get the smallest change get the pussey prize!

http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=73



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Vox,
I use a Smartire on my R11RS and will install one soon on my K12RS. So I can only relate some data from the R bike. Today coming to work the rear tire temp went from 55 to 99 and the pressure went from 38 to 42--pretty consistent with the Tirerack predictions. The rear tire always heats up the most. The highest I've ever seen it was about 120 deg f riding at about 90 mph in 80 deg heat with full bags. I didn't have it installed last summer when I was going 100 mph thru the desert in 120 deg heat. We'll try that again this summer.

I don't use cold pressure any more. I now decide what pressure I want warm and set for that. I've decided this set of tires (just mounted) to shoot for 40 -42 warm.

Cheers,
Jerry
 

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Discussion Starter · #3 ·
Now I know a more about how the pressures change, I'm taking a bit more account of hot and cold. I always thought there was just one pressure that riders picked to suit them, now I think more about time of year and type of ride.

I guess you shouldn't exceed the manufacturer recommended maximum pressure under any condition, but I'm not sure yet if they allow for temperature rise in their spec. or not. It just surprised me how pressure can change so much whilst many riders seem religiously to try to hit a pressure figure, but not taking account of temperature rise.

Track riders must know all about this. Cheers - vox



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I'm pretty sure they are talking about cold pressure and I'm pretty sure they have to make conservative assumptions. So if they specify a max pressure of 43 psig for example, they probably assume you check your pressure at 30 deg F in the morning, go riding hard into the afternoon and your tire goes to 130 deg F and your pressure goes to 53 psig...or something like that. The lawyers wouldn't let them make non-conservative assumptions. --Jerry
 

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Oh ya. tires hold air, air holds up the vehicle.
I have tried to tell folks that for years. I am in the BIg rig business.

Lettme add this for your thought. Altitude! Yep. that will have an affect on tire pressure too.

I have to trailer my bike (poor me) around the country and I have seen what altitude can do.
But I have been in this hotel room in a foreign country for too long and I forget what the formula is. It might be the beer.

But altitude will affect pressure.

Bazra
aka: The Flash
 

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I used to be a mountain climber. I remember that at 18,000 ft air pressure is half an atmosphere or about 7.4 psia. At lower altitudes, it is not more than a pound or two. I've been looking for a formula for years to do just the calculation you're talking about and can't find it. Anyway, I found a link that will calculate the boiling temperature of water at any altitude:

http://www.biggreenegg.com/boilingPoint.htm

then you plug that number into saturated steam tables and it will give you the pressure:

http://www.efunda.com/materials/water/steamtable_sat.cfm

So at 10000 ft the pressure is about 10 psia or 4.7 psid less than sea level.

At 5,000 ft the pressure is 12.25 psia or about 2.5 psid less than sea level.

However, you don't want to change your air pressure if you live in Denver. All pressure gauges used on tires measure pressure in units of PSIG. In other words, 42 psi means 42 psi ABOVE local air pressure. The Absolute pressure inside your tire will be 42 plus 14.7 at seal level but it will be 42 plus 12.25 in denver. The tire pressure gauge will read 42 both places.

Now if you fill up your tires at sea level and then ride up a tall mountain, you might want to adjust your tire pressure. I wouldn't bother, a couple of psi high won't hurt anything.

I have a degree in physics so this seems simple to me. I know there are lots of laymen reading this so if I haven't made it clear I'd be glad to explain more.

Cheers,
Jerry
 

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Hey thanks for that chart...question though: I'm planning a cross country trip that will involve riding for 14 hours a day. Do I need to adjust my tire pressure for this type of sustained riding?
 

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might want to find out where your tire press gauges are made...if it was made in denver and used in boston...well now we have more things add....must calibrate gauges to alltitude...blow the damn thing up and ride!!!..ha ha
 

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voxmagna said:
For UK members, there's a rough formula. A 100 deg C increase in tire air temperature will increase tire pressure by 33%. For US members, a 10 deg F increase in tire air temperature increases your tire pressure by 1 psi. Everything is proportional, so a tire starting 30 psi 0 deg C gets up to 35 psi at 50 deg C. I don't know exactly how hot a tire can get, the attached article I found gives some clues.
It's not worth worrying about "rules of thumb" here, when an exact number is so easy to get. For example, for a temperature increase from (say) 20°C to 30°C, and a tire pressure (measured at the original temperature of 20°C) of 35psi, the new pressure at 30° is simply in proprtion to the change in absolute temperature.

So the new pressure at 30° (or 303°K) is 303°K/293°K x 35psi = 36.2psi

But if you want a rule of thumb, then for tire pressures around 30 to 40 psi, and typical ambient temperatures, a 10º increase (or decrease) in temperature results in a pressure increase (or decrease) of about 1-1/2 psi. Give or take!

Bob.
 

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I got it

Hey, I calculated the tire pressure as a function of altitude and temperature a different way, I took the first letter from each chapter in the Da Vinci Code, converted that to a hexadecimal number, added them together like a Fibonacci Sequence, then plotted that in 3 dimensions using pressure, the numbers and the altitude, calculated the 3D Fourier transform of that, displayed all that in frequency domain, then determined the Eigen vectors from that, popped those into a multiple-regression inverse non-linear matrix and got the Christoffel coefficients, calculated the harmonic resonance series from that, got my buddy at CERN to use that result to fire alpha particles at the aluminum rim to match the resonance frequencies, solved Fermats Theorem, introduced the Bouguer gravity at my location using Plancks constant, the Coriolis effect, Chandlers wobble, global warming, string theory, the psychic hotline, tide corrections, barometric pressure, humidity, and my lat and long, performed error analysis, and the result was, wait, I think I left something out, Heisenbergs Uncertainty, will get back,
Jack
 

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frisco99 said:
Hey, I calculated the tire pressure as a function of altitude and temperature a different way, I took the first letter from each chapter in the Da Vinci Code, converted that to a hexadecimal number, added them together like a Fibonacci Sequence, then plotted that in 3 dimensions using pressure, the numbers and the altitude, calculated the 3D Fourier transform of that, displayed all that in frequency domain, then determined the Eigen vectors from that, popped those into a multiple-regression inverse non-linear matrix and got the Christoffel coefficients, calculated the harmonic resonance series from that, got my buddy at CERN to use that result to fire alpha particles at the aluminum rim to match the resonance frequencies, solved Fermats Theorem, introduced the Bouguer gravity at my location using Plancks constant, the Coriolis effect, and my lat and long, performed error analysis, and the result was, wait, I think I left something out, Heisenbergs Uncertainty, will get back,
Jack
I'm glad someone correctly figured this out!!
 

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frisco99 said:
Hey, I calculated the tire pressure as a function of altitude and temperature a different way, I took the first letter from each chapter in the Da Vinci Code, converted that to a hexadecimal number, added them together like a Fibonacci Sequence, then plotted that in 3 dimensions using pressure, the numbers and the altitude, calculated the 3D Fourier transform of that, displayed all that in frequency domain, then determined the Eigen vectors from that, popped those into a multiple-regression inverse non-linear matrix and got the Christoffel coefficients, calculated the harmonic resonance series from that, got my buddy at CERN to use that result to fire alpha particles at the aluminum rim to match the resonance frequencies, solved Fermats Theorem, introduced the Bouguer gravity at my location using Plancks constant, the Coriolis effect, Chandlers wobble, global warming, string theory, the psychic hotline, tide corrections, barometric pressure, humidity, and my lat and long, performed error analysis, and the result was, wait, I think I left something out, Heisenbergs Uncertainty, will get back,
Jack
i just hope to god you put the moon phase into accont...i didnt see it in you calculations but most of us just make the finding in our head and adjust each calculation to that factor....and yes we do forget sometimes...and we have to start all over again...doing it myself i came up to the same end result that you did...and i did incorperate the phase factor so i guess your good to go...sorry i should hav waited untill i spoke up.....sorry about that.......hey are you going to that convention next week?...gona be a blast....
 

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nitrogen

Seems like filling tires on motorcycles with nitrogen is the way to go, :boom: has there been are discussion about this? Is anyone doing this?
 

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Way back when I raced and tires were carved from stone, we were told you wanted a 7 psi rise between cold and warmed up tires.

What was magical about 7 psi? :dunno:

Who was the guru that came up with it? :dunno:

Did it really work? :dunno:

It did mean we were inflating our tires about 34 to 36 psi most of the time and I can't say that I ever could attribute any slide from the tire being at fault (except for a Michelin manufacturing flaw that had side walls ripping open and they made good on for several people). I still use the rule of thumb, but add a couple of PSI to try and get a little more life out of the tires.
 

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mikedard said:
Seems like filling tires on motorcycles with nitrogen is the way to go, :boom: has there been are discussion about this? Is anyone doing this?
Only people with way too much free money and no sense. :wtf:

(Hint: look at the percentage of nitrogen in normal free air - ~78%)
 

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Can any members recommend something to be fitted to a 98 K1200RS to monitor tire pressure?

A previous post mentioned Smartire but having visited the website it states that they are no longer producing products in the motorcycle market.

Many thanks
 
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