question about tire pressure...

[scooterlord]

well, i want to know what happens when you increase the pressure in a tire. does its volume increase? or it's mass? can someone help me out on this one? i would be glad to accept answers using mathematics...

i mean what happens? a friend of mine claims that the air mass increases (volume stays the same) and then pressure increases.

does the pressure give the tire it's form? or volume?

 

[Max Speed]

One thing I did find is to run the top listed tire pressure marked on the Busa tire and it lasts much longer.

 

[PACIFICBUSA]

hrmm...not too sure about that one. I think it gives it it's volume and a little bit of form.

 

[BusaFever]

If ur not sure the why post? Ya fuggin post whore.

 

[Raising Cane]

One thing I did find is to run the top listed tire pressure marked on the Busa tire and it lasts much longer.

I will never run the max or any where near it. Much better grip around 32 psi. But then not all of us hate chicken strips.

 

[PACIFICBUSA]

If ur not sure the why post?  Ya fuggin post whore.  

Hey!! Just tryin ta help....

 

[the grinch]

If ur not sure the why post?  Ya fuggin post whore.  

Hey!! Just tryin ta help....

yeah ya post whore!

 

[gcal]

32 is what i run

 

[stkr00]

Okay...you asked for it...

It has to do with the ideal gas law. Here's a link if you want to find out more.
Ideal Gas Law

Basically, it is a law that defines how a gas reacts when changes occur in either/all of the following: Pressure, Temperature, and Volume. The formula can be used to calculate changes in any variable from scenario (1) to scenario (2). If the volume remains constant, as in a tire, then the change in pressure will be proprtional to the change in temperature. If temp goes up, then so does pressure. and so on.

Important to note, that to perform these calculations, the temperature has to be converted to the Kelvin scale. If you try to use Fareinheight or Celcius the numbers will be way off.

PV PV
-- = --
T T
(1) (2)

For a motorcycle tire, the Volume is constant as long as you don't get any deformities in your tires such as bubbles/separated belts.

The pressure will change by adding/removing air. As air is added, the Mass is increasing. You are cramming more air molecules into the air space, and since volume remains the same, then pressure will increase. A molecule of air has an atomic weight(mass), so the air mass will change as you add/remove air. This will also change the unsprung weight of the bike, although it is negligible.

Temperature has a large effect on the pressure inside a fixed volume container. Tire manufacturers take this into account when developing the recommended pressures, and why they are always listed as "Cold" pressures. As the tire heats up, then the pressure increases. This also plays a part in tire failure during high speed runs and burnouts due to the heat build up. This raises pressure and also weakens the rubber compound.

Just to throw a wrench into this...Normal air is 21% Oxygen, 78% Nitrogen, and 1% Other. This air will not follow the ideal gas law exactly due to the partial pressures and characterististics of the individual molecules. Nitrogen on the other hand, will act exactly like the ideal gas law. This is why some high performance vehicle will use Nitrogen only in their tires. This includes aircraft tires, but has more to do with the sub-freezing temperatures seen at higher altitudes, than anything else, due to the moisture content in normal air. Moisture will condense out of the air and freeze into ice inside the tire.

I haven't looked at the whole Air vs. Nitrogen thing in a long time, so I may very well be all messed up on that, but the rest of it is correct.

Hope this is what you were looking for, because now my hands are tired.

Remember...you asked.

 

[scooterlord]

stkr, i think that thanking you just can't be enough! thanks for your thorough explanation on this one. well, i guess i am not very happy with the answer cause i thought that the volume also increased - aka i lost the bet :/ nm though, i am glad to learn new stuff...

damn, i want to kick myself, i was supposed to know this stuff, i finished school with 18.5/20 in both physics and chemistry (which is an absolute BITCH to get - 1.000 people get it out of 80.000). oh well, i guess i was to selfish to accept my friend's explanation which sounded very logic back then

oh, and btw, how can you people ride with pressure less than stated? damn, i mean, i am 41 psi and i can a feel a difference towards the worse. back when i had the cbr 600 i used to read the exact same thing in the forums (lower the pressure, lower the pressure, bla bla) so i did and almost killed myself. and yeah, i know that the tire has to get warm first. how does a 200km high speed trip sound like? anyway, haven't got the chance to check my hayabusa's tires yet (will do so today for the first time) cause i felt like something wasn't going right last night.

oh, final question. if you BURN the tire (not rolling burnouts, etc - just plain FULL Gas hehe) does it's pressure lower?

 

[stkr00]

No problem scooterlord. Physics is something that if you don't use it all the time, then you tend to forget some of it. It happens to all of us.

I went through the US Navy's Nuclear Power Program, and had to learn WAY too much of this crap, including Reactor Kenetics and Nuclear Physics. I don't remember much of those, but I've worked in and around steam plants for 20 years so the Thermodynamics, Fluid Dynamics, and Gas Laws have stayed with me quite well.

It's a curse I think.

Just remember...The volume is defined by the shape of the container, and unless the container has elastic properties, then it's volume will not change with the change in pressure. A balloon would be a good example of an elastic container. A SCUBA tank would be inelastic. I also SCUBA dive, so it's important to me.

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[WWJD]

I ride less than stated also to increase contact patch and speed up warm up time. Better sticky = more safer fun! For long range touring or super high speed runs yeah, full pressure. But for track and fun riding, underinflate. Feels like gummie bear wheels until you adapt to it... and you will. I don't like sliding. Winter 32ish, summer 38ish, touring 42. What was the question??

 

[Revlis]

I still disagree with running lowered pressures on the street. I ride hard, and have for alot of years. I have had buddies repeatedly extol the virtues of dropping 8-10 lbs out of their tires, but all I ever noticed on their bikes was crapped low speed handling and squirmy high-speed dodginess. A corner, straights, doesn’t seem to matter. I think with modern street radials there just may not be as much to gain as folks think, seems to me that all your doing is accelerating tire wear and OVER heating your rubber...

Tell you all what though, before you launch into your "Well I Run 8 lbs in my tire because it feels better" tirades and flames, I am going to find out for myself this weekend, I'll run back to back laps out in my playground and report back... It will still just be my opinion which frequently isn't worth a sh!t (As you all love to remind me)

but let me try it out on my busa and see what I think. I'll let you all know Sunday... Fair Nuff?

-My hypothesis is this, folks running street compound modern radials are doing nothing but increasing tire wear and overheating their rubber by dropping tire pressures much below the manufactures recommendation. That in fact, it's in your head, you see what the racers are doing on the strip or track and it sounds good. I cannot really do Back to back timed runs with a start and finish line, due to traffic and what not, but I can at least report back how things felt out there. No BS, I'll report back exactly how it felt, if it feels better lower then I will sound off, trust me... It'll just be my opinion FWIW but It'll be an honest one without pretense of "Boyracer Heroics" or "curmudgeonly old dude" Bullsh!t... Fair?

Now any recommended pressure for 230lb riding on warm surfaces?

 

[dezzy7]

Dang Stkr00 ... that was so detailed it was profound... do you have that in an English version

J/k

 

[Haywood]

Personally, I agree with Revlis... if the tire mfg and Suzuki say to run a specified pressure, I run with that... and I check it regularly. You can tell when a tire is under or over inflated I think... I don't need to add any more risk to the sport.

 

[EAKbusa]

Metzeler states in it's fitment chart the ideal PSI for each make and model (same as what the OEM recomends) then adds that for higher tread life you can go up to the maximum psi listed on the tire itself (usually 42). The standard for a Busa is 36 front and rear I believe.

So I would tend to believe Metzeler on this one, higher pressure lasts longer. As far as lower pressure giveing extra grip, perhaps. It at least makes sense that you would have a larger "footprint" but I would be concerned about added sidewall flex at lower pressures which would cause poor handling and poor feeedback at the bar.

 

[Ride_or_Fish]

Lower pressure will mean a greater contact patch, more rolling resistance, less mpg, less wheelspin, increased tire temperature. However, I personally think it's only worthwhile to run with lower pressure in the rear tire when at the drag strip, imho. Otherwise, I like the handling at 40+ psi and can see the difference in longevity of the tread. The bike just seems to be more stable for me.

 

[stkr00]

Dang Stkr00 ... that was so detailed it was profound... do you have that in an English version

J/k

Sometimes there's just no other way to explain this stuff.

What's really sad, is that after I typed all of that, I started browsing through the link I gave scooterlord. I started looking through all my old nuclear physics stuff remembering what a pain in the a$$ it all was. About an hour later I said...WTF am I doing?

My wife thinks I'm sick, but I love physics. You can explain almost anything with it, and see its applications in everyday life.

I never said I was normal.

 

[justintime2]

Dang Stkr00 ... that was so detailed it was profound... do you have that in an English version

J/k

Sometimes there's just no other way to explain this stuff.

What's really sad, is that after I typed all of that, I started browsing through the link I gave scooterlord.  I started looking through all my old nuclear physics stuff remembering what a pain in the a$$ it all was.  About an hour later I said...WTF am I doing?

My wife thinks I'm sick, but I love physics.  You can explain almost anything with it, and see its applications in everyday life.

I never said I was normal.

Great job in the thorough explanation. I think he could also claim he won the bet because it is rubber and steel belts after all and it will expand some when inflated to higher pressures.

 

[WWJD]

I ride lower PSI because of the way I ride. If I'm full PSI, I unlock my tires on almost every take off and I HATE spinning. Having been on the track and feeling the difference myself, I'm happy paying more in tire replacement and feeling stuck to the road. Did I mention I HATE tire spinning? I just like it when it takes off, forward, quickly.

Depending on how you ride REV, it may or may not make any difference to you. My tests have me lowering my PSI. HOWEVER, on the street, I don't go so low as to circumvent [much] of the street riding suspension from bumps and such. Again, I chose stick over treadlife.

EAK, they drop PSI on the race track all the time. Cooks your tires up nice and toasty, makes them stick, lets those guys do those 140 sweepers you see on TV. Far as control goes, feels very different, but cooool after you get used to it. I believe that is where most of this business started from. maybe...



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