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Old 05-06-2009, 06:54 PM   #1 (permalink)
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Another take on hyper-inflation

Something got me to thinking while I was airing up my tires a bit today...

When I was racing my Ninja 250 around Deals Gap a few years ago, I was filling my tires to 30psi (which was recommended to me for good traction), and out of curiosity, I checked my tires immediately after a run, and naturally the pressure was higher, 36psi to be more exact.

Which got me to thinking again, today. The rated speed on the tire is related to heat, if I'm not mistaken, and if one fills the tire to sidewall, then proceeds to drive at the rated speed, the tire would naturally arrive at a higher pressure after some high-speed driving. I can only imagine that the tire designers planned for filling a cold tire at 60-70F which got to 120 or so in the afternoon, while traveling at the speed rating... (This of course is merely speculation).

The conclusion this lead me to, though, was that a safe over-inflation could be extrapolated by traveling at the top speed of the tire after having cold-inflated to the sidewall pressure, then experimenting to find the cold inflation pressure that would cause the tire to arrive at the maximum pressure at some lower speed, in my case a tire designed for 110mph that is only used at say, 40mph.

What say all you more learned than I?

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Old 05-06-2009, 07:54 PM   #2 (permalink)
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I had my tires inflated to the max of 40 psi when I drove across country. In the desert in summer driving 85+, the tires started smacking the minor expansion joints in the pavement and vibrating through the car as if they were made of stone. I slowed down and checked the pressure at the next stop and it took probably half a minute of bleeding the air before it would even register on my pressure gauge (cheap stick maxed at 50). So that might not answer any questions, but be careful testing in extreme conditions. I'm amazed they didn't blow out.
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Old 05-06-2009, 09:25 PM   #3 (permalink)
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Interesting. Your last paragraph makes sense to me. Still, tires behave very differently at high temps than at low temps - I wouldn't go overboard with extrapolations. Also, I don't own any cars capable of reaching the rated speed of their tires, nor do I have a road where I can drive that fast anyway.

You might do better to estimate or look up the temperature of the air in a tire driven at its speed rating, and at your new, personal, lower speed rating. You can apply the ideal gas approximation that tire pressure is proportional to absolute temperature (in Kelvins or Rankine). This is often oversimplified as one psi per ten Fahrenheits.
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Old 05-16-2009, 01:50 AM   #4 (permalink)
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The tires are going to be designed to withstand above and beyond by at least a factor of 1.2 of max sidewall, max temp and max speed(ambient air temp and then tire temp, because they will be drastically different). Given, they might not be able to get a safety factor of better than 1.2 in there so running it max sidewall on really scorcher days at high speed and you might end up upside down. I go to max sidewall and stay just under the speed limit. If you are at sidewall speeding is ok but you go over and the complicated equations for thin walled pressure members and tire elasticity with the thermal expansion of the tire belts. . .let's just say there exists the possibility of havoc. . .
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Old 05-16-2009, 11:18 AM   #5 (permalink)
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Quote:
Originally Posted by The Atomic Ass View Post
.........

The rated speed on the tire is related to heat, if I'm not mistaken, ........
You are indeed mistaken.

The speed rating of a tire is more related to the construction - specifically, the number (or absence) of circumferential fabric overlays over the belt package. Typically these are made of nylon, but other materials have been used. These overlays restrict the centrifugal growth of the tire, thereby increasing the speed before the tread will come flying off (Ref: See Top Gear's Rich Hammond's Jet Car Crash)

You can tell if these overlays (also called cap plies) are present in your tires by looking on the sidewall for the construction materials. A typical construction would say: Tread plies: 2 polyester + 2 steel + 2 nylon (The "2 nylon" being the overlays)

But since I am doing a post:

Quote:
Originally Posted by theunchosen View Post
The tires are going to be designed to withstand above and beyond by at least a factor of 1.2 of max sidewall, max temp and max speed(ambient air temp and then tire temp, because they will be drastically different). Given, they might not be able to get a safety factor of better than 1.2 in there so running it max sidewall on really scorcher days at high speed and you might end up upside down. I go to max sidewall and stay just under the speed limit. If you are at sidewall speeding is ok but you go over and the complicated equations for thin walled pressure members and tire elasticity with the thermal expansion of the tire belts. . .let's just say there exists the possibility of havoc. . .
I'm going to start in a different place:

Standard Load Passenger Car tires (the type 90% of us have!) have a range of inflation values between 26 and 35 psi. The maximum load carrying capacity at these pressures are defined by tire standardizing bodies such as The Tire and Rim Association (commonly referred to in the industry as TRA). The relationship between inflation pressure and maximum load carrying capacity for a given size are published in books that become the starting point for pretty much everything else that is done concerning tires.

For example, tire design usually takes the maximum usage inflation pressure (35 psi), and uses that as basis for the design. In order to account for fatique, a factor of 4 to 5 is used as a multiplier - and then additional factors are applied for the imprecision of theoretical analysis. - and then the factory can only have certain combinations of ply fabric, so the resulting value becomes a minimum.

Put a different way: If 35 psi is used as the basis, then a factor of 4 or 5 (for fatigue resistance) results in 140 psi to 175 psi - and these would be the minimum burst pressure if no additional design factors were applied. So typically quoted burst pressure values of "over 200 psi" are right in line with this.

What this means is that even if the sidewall says "51 psi max", the design pressure is 35 psi - and what is written on the sidewall as a maximum is a "permitted pressure" as indicated in the TRA yearbook.

So I am concerned when folks reference the sidewall maximum when discussing inflation pressure. I think they ought to be referencing 35 psi - and I also think the placard pressure becomes an important item to reference - as this goes to the size of the contact patch.

Last edited by CapriRacer; 05-16-2009 at 11:27 AM..
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Old 05-16-2009, 11:31 AM   #6 (permalink)
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Old racing trick. Use nitrogen in your tires. This will greatly reduce pressure build up.

Find a shop with a nitrogen system to do the initial file.

Then you have the option of going to your local welding supply to rent or buy your own tank for topping up.
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Old 05-16-2009, 11:32 AM   #7 (permalink)
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capri I would quote but its kinda long and then its tedious to do scrolling.

I'm not particularly worried about static burst pressure, I doubt anyone here will ever do that to a tire if they are paying any attention. The fatigue or pressure creep is more dangerous. I'm ok with inflating tires to the max pressure(not under max loading by any stretch of the imagination, either do one or the other not both) because of the safety factor put in place.

Going over that. It might be safe anecdotally, but is it worth it? How many of you have had a tire go away while driving? You get new rotors new axles, new rims new tires new brakes new paint and maybe more. If it happens in a corner above 45 and there are obstacles its likely you'll be in the hospital. When a tire goes you have 0 control after that.

Not worth it to go over max, I use a gauge I know reads high and fill to max sidewall(41) which is really like 38
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Old 05-17-2009, 07:19 AM   #8 (permalink)
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Quote:
Originally Posted by ZeroFuel View Post
Old racing trick. Use nitrogen in your tires. This will greatly reduce pressure build up.

Find a shop with a nitrogen system to do the initial file.

Then you have the option of going to your local welding supply to rent or buy your own tank for topping up.
Sorry, but air is 78% Nitrogen and even if it wasn't, the difference is gas doesn't really affect the pressure buildup. Pressure for all gases is governed by the same equation - Boyle's Law (also known as the ideal gas law):

PV = nRT, where

P = pressure (absolute)
V = volume of the gas
n = number of moles of gas
R = universal gas constant
T = temperature (absolute)

The reason racers use nitrogen is 2 fold:

1) Ordinary air contains water and when compressed it precipitates out. This is why it is recommended that compressors be drained regularly and why shop air lines, especially those used for painting, have a dryer close to the end of the line to remove any water that might make it into the line.

Water in a tire will go back and forth between water and liquid and this causes larger pressure swings. If you were to use "good" air (that is, dry), this is not an issue.

2) In many racing pits, you aren't allowed to have generators (fire potential), so it is common to use nitrogen tanks to power the impact wrenches. Since the teams already have this tank, it is common to use this to fill the tires.

For street tires, it's not so important to maintain the spring rate - and therefore the handling - so even the increased pressure build up due to the water going in and out of phase is not a problem.

Quote:
Originally Posted by theunchosen View Post
capri I would quote but its kinda long and then its tedious to do scrolling.

I'm not particularly worried about static burst pressure, I doubt anyone here will ever do that to a tire if they are paying any attention. The fatigue or pressure creep is more dangerous. I'm ok with inflating tires to the max pressure(not under max loading by any stretch of the imagination, either do one or the other not both) because of the safety factor put in place.

Going over that. It might be safe anecdotally, but is it worth it? How many of you have had a tire go away while driving? You get new rotors new axles, new rims new tires new brakes new paint and maybe more. If it happens in a corner above 45 and there are obstacles its likely you'll be in the hospital. When a tire goes you have 0 control after that.

Not worth it to go over max, I use a gauge I know reads high and fill to max sidewall(41) which is really like 38
Perhaps it wasn't clear, but I am with you regarding not inflating a tire over the max listed on the sidewall.

The point I was trying to make was that many folks say something along the lines of:

Quote:
"........I'm only using 50% more than the max - 75 psi .....",
when, in fact, this is actually TWICE the design pressure.

Last edited by CapriRacer; 05-17-2009 at 07:27 AM..
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Old 05-17-2009, 10:53 AM   #9 (permalink)
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Ah ok, I understand now for the max pressure issue.

As for the nitrogen, it can make some difference to run strictly nitrogen in the tires. Intuitively it doesn't make alot of sense to say that changing the composition from 80% to 100% makes a difference but it does. . . a little.

Its not going to be huge, but the reason it will influence it is nitrogen is more compressible than oxygen by a good deal. Ideal gas doesn't really hold even close to accurate for atmospheric air because the pressure reduced for atmospheric air is much higher than 14.7 psi, 1 atmo. Nitrogen on the other hand has very lovely properties because its pressure and temperature criticals put its compressibility at air temp and pressure almost right on the money to use the ideal gas equation.

Oxygen starts trying to react with its neighbors in air especially when it gets compressed or heated and it causes the volume to become less isentropic.

At tire pressure nitrogen is not dead on for a compressibility of 1, but its not very far off still. Its mostly a function of temperature not pressure. That said the only difference its going to make is if you get the tires toasty to start with the air tires won't dissipate heat as quickly as the nitrogen ones, they also will have a greater pressure at a given temperature. For ultime tire performance nitrogen would be preferred to keep pressure impact loading on the tire down and decrease the fatigue stress on the rubber.
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Old 05-17-2009, 01:53 PM   #10 (permalink)
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capriracer you are wrong if you don't think nitrogen makes a difference in tire pressure & the effect it can have on a race car.

With the sensitive setups on today's race cars a .5lb is huge on the handling characteristics.

As for moisture in the tires, changes in humidity affect tire performance two ways. First, the density of humid air fluctuates more with temperature than that of dry air, so removing humidity can keep your tire pressure more consistent, especially when the temperature climbs over 100 degrees Fahrenheit.

The point of this thread is how can you get the tires to their max without a failure, nitrogen will help.

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