[Frogfoot]

Okay, I'm having an issue with a new front tyre (BT021) that got me thinking back to uni physics and the Universal Gas Law!

I think I am experiencing a slow leak from the tyre, but its so slow and inconsistent I got to thinking about the ambient temperatures I'm operating in and how that might have affected the tyre measurements. Like everyone I know that we measure the tyre when its cold, but how cold is too cold, and how much affect will the ambient temperature have on my measurements.

Would a 10C difference in temperature equate to a 0.1PSI change, 1PSI, 10PSI etc??

I measure the tyre at the same time each morning and the only two variables are the air temp and the actual pressure in the tyre (though you should probably include atmospheric pressure??) the readings are all over the place for the front tyre, but reasonably steady on the back, hence the reason I think I have a leak. I have also had the valve changed, and the readings don't seem to have settled down.

But what about the bike weight? If I have the preload high on the rear and full or empty tank???

Am I thinking into this too much?

[y2kvfr]

aaaaahhhhhh!!!!!

Flashbacks to college. piv equal nert. Or rather PV = nRT.

Yes, temp will affect the pressure. But enough to actually feel it? I didn't sign up for the math.

[Greek71]

Boy, and I thought I cared about my tire pressure! Just go to an auto parts store and pick up a $1.00 schrader valve tightener tool, the core might be a little loose form always checking your tire pressure or if you reused the valve over multiple tire changes it could be worn and leaking very slowly. I doubt that 10 degrees would have any effect on your tire pressure.

[V4 Rosso]

Frogfoot, on Oct 10 2008, 01:06 PM, said:

Would a 10C difference in temperature equate to a 0.1PSI change, 1PSI, 10PSI etc??

I measure the tyre at the same time each morning and the only two variables are the air temp and the actual pressure in the tyre (though you should probably include atmospheric pressure??)

A 10K temp difference equates to about half a psi, depends on what the start temp was. That is too small to be bothered by it. But if you do, you should include atmospheric pressure variation into the eqation

But more important is where do you fill up the tires? At home, or do you have tot ride to the gas station? If you fill up the tires at a gas station the tempareture in the tires may have made the tire pressure go up by several psi already. Also do you allways fill up at the same gas station?

[Action]

The ideal gas law states that that pressue multiplied by volume and divided by the temperature of a gas is equal to a constant (PV/T=c). If you due the math it comes out to about 1psi for every 10F degrees. I talked to a tire rep last year and he said on the street during summer you'll see a 50F rise in tire temp, which he agreed was equal to about 5psi.

Action

This post has been edited by Action: 10 October 2008 - 08:26 AM

[Dutchy]

At sea level
At zero degrees Celcius
A cubic centimetre of air
consists of

45,000,000,000 time 1,000,000,000 molecules

so start counting tose that are in your tyre.....

[Frogfoot]

Thanks for the physics, I always remember it as PV/T=constant!
Answers to the other questions.

- I had used that valve over a whole bunch of tyre changes, but its been changed now.
- I check it at home, but have to ride about 500m to the nearest petrol station to top it up. I put in whatever I decided at home.

Thanks all, I'll keep monitoring it and see if the valve changed has fixed it.

[TimC]

Great topic. I was thinking of starting this discussion myself, so I'm glad to checked out the open threads first.

My front tire has felt very sluggish each morning this week. The morning temp has dropped to 30F-35F here most mornings. Once warmed up a bit the tire feels better, but it's definitely low when I start out.

So my question is what psi should my "cold" inflation be set to, considering I don't want it to 'balloon' to 50+ psi once the tire is warmed up? It seems wrong to set my cold inflation all the way to my normal 40-42 psi when I know the tire is going to warm up substantially after a few miles.

What's the general consensus on setting 'cold' inflation levels in freezing ambient temps? Should I up it a few psi or just deal with an underinflated/sluggish tire for 5-10 minutes each morning?

[tpierce(MP)]

Tim, i just set my tire pressure cold in a cold garage and run with that. I rode in today (30F out), got here, hop of the bike and check the temp of my tires with my hand (very scientific...). Even after 25 miles of mostly highway, they are only mildly warm. I wouldn't really worry about too much pressure. On warm or hot days, the tires get pretty darn warm, not so much in this cold weather.

[Tightwad]

I think the more important question is:

Where can I score a blue 6th gen in the States?

[hondalover]

I'm not sure if it is a regional chain or what, but Costco fills there tires with nitrogen. That is a gas that the pressure isn't affected by temperature the way air is affected. Hence the reason they use it in airplane tires. If it's good enough to stand-up to the harsh conditions of an aircraft's landing gear (0-150 mph in a nano-second without blowing), it will sure hold up to the stuff a motorcycle cycle can throw at it. Many motorcycle shops should have nitrogen to refill shocks, if they rebuild them inhouse.

[Adam30]

Another factor that doesnt have much to do with ambient temps is the makeup of our wheels. A few seminars Ive been to either by Honda and Dunlop advised that slow pressure drop is normal to an extent, due to the wheels being pourous, and very very slow air loss is the result.

Bottom line.....just do what I do and check/adjust your pressures before every ride. I find myself adding at least a little every couple weeks to keep them at the spec I use of 42/36.

[Macs06VFR]

Um, isn't our air about 78% nitrogen? Costco seems to be charging a lot for that last 22%...

[Adam30]

Yeah, we have a Costco near by here that opened last year, and they're using nitrogen too. We here have lost a few tire sales here and there due to them. Im thinking they're pushing the nitrogen thing as a selling point...even though the public really doesnt care I dont think....no one checks their air pressures anyways in car tires, lol. Independent service shops in this area havent really made much of an effort to get into the nitrogen thing, including myself here. And I can tell you that i havent had one customer yet ask about it. For places that sell tires and dont use nitrogen, most are finding its not worth the cost of the equipment to offer nitrogen when the public at this point simply doesnt seem to care. That and its yet another investiment in equipment for a product like tires that you're not making any money off of anyways...

[Tightwad]

I found this link very insightful on the issue of Nitrogen in tires:

http://home.comcast....r_tires_FAQ.htm

[jeremy556]

Adam30, on Oct 22 2008, 11:54 AM, said:

Another factor that doesnt have much to do with ambient temps is the makeup of our wheels. A few seminars Ive been to either by Honda and Dunlop advised that slow pressure drop is normal to an extent, due to the wheels being pourous, and very very slow air loss is the result.

Bottom line.....just do what I do and check/adjust your pressures before every ride. I find myself adding at least a little every couple weeks to keep them at the spec I use of 42/36.

Nitrogen is a gas and adheres to the ideal gas law just like nitrogen and oxygen in regular air.

The reason it is used in aircraft tires is because of risk of fire from compressed oxygen.

[jeremy556]

TimC, on Oct 22 2008, 11:02 AM, said:

It seems wrong to set my cold inflation all the way to my normal 40-42 psi when I know the tire is going to warm up substantially after a few miles.

42psi is the maximum rated pressure for the tire, it is not the pressure you should be running.

Factory recommended is 36 psi, IIRC.

[TimC]

jeremy556, on Oct 22 2008, 12:57 PM, said:

TimC, on Oct 22 2008, 11:02 AM, said:

It seems wrong to set my cold inflation all the way to my normal 40-42 psi when I know the tire is going to warm up substantially after a few miles.

42psi is the maximum rated pressure for the tire, it is not the pressure you should be running.

Factory recommended is 36 psi, IIRC.

My front Diablo sidewall says max cold pressure is 42 psi, but I don't normally run max pressure in the front.

So are we supposed to go by the tire's max psi, or the wheel (which like Jeremy said is 36 psi IIRC, but wouldn't that have been for the OEM tires)?

Man, I feel like an idiot asking these questions.

(BTW, thanks for the info, Travis!)

[tpierce(MP)]

The recommended tire inflation is what is on the sticker on the swingarm, 36F, 42R. That is what is recommended for all tires under all loading conditions. The 42psi on the tire is just there so that someone doesn't put tires with a max inflation of 38psi on a bike that recommends 42psi. Also, you can deviate from the recommended rating to see if you like the feeling more, just don't go above the max rated on the tire itself. I tend to run my car tires 2-3 psi high in the front for a bit better steering feel, but i keep my bike tires right on Honda's specs.