RX-8 friend

Just a warning to owners. If your tire pressure monitor goes off when the car is cold, you need to add air to your tires. Tires loose air over time, and loose it faster when it's cold. Don't ignore the warning because it stops when you drive a short distance. Tire pressure must be maintained when the tire is at ambient temp., even if that is -25 (brrr).

An easy way to get the right amount of air in the tires, is to pump them up on the way home, to a few PSI over the spec. Next morning check the pressure and let the extra air out to set it to spec. Always check tire pressure BEFORE you drive on them. They will heat up and the pressure will appear normal after you drive a few blocks. There may still not be enough air in them.

idle0ne

great post, thanks for this information it's cold here 2/3 of the year

six_shooter

Tires do not normally loose any "significant" amount of air over over time whether it's hot or cold unless you have a puncture.

Air pressure is dependant on ambient temperature. It is perfectly normal for air pressure to read lower when the temperature is colder, and for the air pressure to read higher when it's hotter. Meanwhile, the actual volume of air in the tire remains the same regardless of temperature or the pressure, so don't go radically changing the tire pressure. Follow the owner manual, and check the tire when the tires are cool.

Gord96BRG

I would strongly disagree with the recommendation to not change the pressure. The volume of air doesn't matter at all - it's the pressure that is important. As you say, follow the owners manual and set the pressure when tires are cold - they don't say "recommended pressure at ambient temp 70F is 32psi, at ambient 0F is 25psi", they just say "cold press is 32 psi". When ambient temps fall significantly - you have to increase the tire pressure!

Regards,
Gordon

six_shooter

No, air volume is what matters, problem is you can't measure air volume so easily, so they have you measure pressure instead as an indirect measurment. Volume is what does the work, you reduce the volume you have less air, even if the pressure is the same.

Gord96BRG

Volume, in the case of a slightly-flexible tire carcass, is controlled by the tire carcass. Unweighted, the volume of a mounted tire is nearly/essentially the same whether it's got 0 psig inside or 50 psig inside. The mass of air inside the tire varies, but that's because the pressure is different, not the volume. Put it on the car and weight it, and the 0 psig air is contributing nothing to supporting the weight of the vehicle (and the volume will be reduced due to the sidewall collapse).

Say you start out with 40psi cold (which is really ambient) in the tires at 100F in the desert. Now cool that car down to -40F, and the tire pressure drops to 10psi cold/ambient (for illustrative purposes, I don't know exactly what the actual pressure drop for that temperature change would be). Are you really suggesting that driving around with 10psi in the tires will provide any protection of the rims/tires from damage and will provide proper and safe ride/handling characteristics?

Regards,
Gordon

RX-8 friend

It's the pressure at ambient temperature you have to keep at a set value. To do this you have to add air when it gets cold, and perhaps release some when it warms up. The tire is designed to operate properly with a certain pressure inside. The amount (number of molecules) of air has nothing to do with this (other than physics). The tire needs the pressure to support it and the weight of the car.

The air will expand when it warms and raise the pressure. This has been allowed for in the tire design. They are designed to be inflated to specified pressure before you drive on them. Once you drive a short distance, the tire warms up a bit and the pressure rises. Do not release air from a warm tire because it looks too high. Wait until it cools to ambient and check it and set it to spec. then.

I don't know about where you live, but here in Edmonton, I have to add air to my tires usually twice every winter and at least once every summer. If I forget, by spring when it warms up they will drop from 30 PSI to mid to low 20s. There are no leaks. This has happened to all the vehicles I have owned. It's more of a problem with performance tires (such as the stock RX-8 tires) because they have a low volume of air, so loosing a small amount affects the pressure more than my SUV tires, which have a large volume.

If you run the tire with low pressure, it will flex more and run hotter. It will also wear the sides of the tread faster. If you run the tire with very low pressure, it will run hot enough to weaken, and will blow out.

If you run the tire with high pressure, it will wear the centre of the tread and will affect handling and braking. If you run the tire with very high pressure it will exceed its' design strength and blow out.

Neither situation is very much fun. Just check the pressure at least once a month. The digital tire pressure gauges have been found to be very accurate, if you want a good gauge.

fast??

why the hell did mazda come up with this tire b.s
this has to be the most dumbest thing they did

Gord96BRG

You mean the Tire Pressure Monitoring System? Mazda didn't come up with it, the US Government did - it's required. RX-8s sold in Canada, for example, do not have the TPMS system fitted.

Now apologise to Mazda!

Regards,
Gordon

fast??

mazda still shitty though

Reeko

GREAT, ANOTHER ONE OF THOSE NEW a$$h0LES.
4 Posts from fast?? and nothing but complaints.
Get lost kid.

fast??

who is an a$$hole?
i am just telling you guys the facts

maxwell72764

Hey fast$$. Is there any reason for you to be here or can you not find anywhere you ARE wanted?

Has anyone had experiance with 'Fix-a-flat' vs the pressure sensor?

Reeko

Facts?
I Have seen nothing but complaints from you, and those were all opinions, not facts.

Does the above quote sound like a fact?

fast??

dont hate on my rx8
mine is spotless

justkn

Funny, that just happened to me on a cold morning...all were 27 lbs...hmmm

Aratinga

BACK on topic....

The VOLUME of a tire is essentially constant; although it's not as rigid as, say, a steel scuba tank, you shouldn't see an unweighted tire visibly swell when it's inflated.

The PRESSURE inside an uninflated tire is NOT "zero"! It's one atmosphere, or 14.7 psi... the same as outside the tire. There is still air in there, y'know... it's just that the air inside is at the same pressure as the air outside.

When you inflate a tire, what you're actually doing is creating a pressure differential between what's inside the tire and what's outside. We measure this DIFFERENCE in pressure (in the backwards non-metric USA, anyway) in Pounds per Square Inch (psi). So when you inflate your tire to "28 psi", you're actually making the pressure inside the tire 28 psi greater than whatever the atmospheric pressure is (usually about 14.7 psi at sea level).

Gas pressure varies with gas temperature. The cooler the gas, the less pressure it exerts. (Kinetic molecular theory, anybody? Essentially, the gas molecules are moving more slowly, thus striking the walls of the tire less frequently and with less force.) That's why your tire pressure measures lower when the tire is cold, and increases when the tire warms up as a result of being driven. The AMOUNT (i.e., number of molecules) of air in the tire does not change, and should not change assuming your tires aren't leaking.

Tires are designed to run within a specified pressure range. If you allow that pressure differential between inside the tire and outside to get too small, your tire will "look low" and become overheated as you drive, and will wear prematurely. If the pressure differential between inside and outside the tire disappears, you have a flat tire.

There will be a quiz tomorrow. Fast?? gets an automatic F.

DaveT

Expanding on Aratinga's comments, the following Laws of Chemistry apply to what's going on:

Boyle's Law - Volume varies inversely with pressure when temperature remains constant.

Charles' Law - Volume at constant pressure varies proportionally with temperature.

Or you can apply the Physical Chemistry "Generalized Gas Law" (which is what Aratinga has done) which is PV=NRT and accounts for all the components and the make-up of the gas in the number of moles, different elements in the gas make-up, universal gas constants of those elements, and all at temperature in degrees Kelvin.

What this means is that when it gets cold you have to add pressure to your tires to keep them at recommended inflaton. Trust us on this.

There will be a quiz on this tomorrow.

9-K Rever

So basicly and essentialy, the TPMS system is extreamly senstive and just becuase it goes off does not mean any of your tires are flat. My alignment was off on my 8 and it caused my TPMS to go off. Got it aligned and wala no more light and beeb.

I but I do notice on the mornings that are in the 20's the TPMS will beep.

I have come to the conclusion long ago that people are not used to haveing a standard TPMS in their cars and this causes them to be overly aware about their RX8's TPMS.

My only issue is becuas the system cries wolf so much, I can not trust it. I am thinking about pulling the fuse.

DaveT

What I found with mine was that the dealer only inflated the tires to about 28 psi, and my dash light would come on every morning until I drove 2-3 miles and the tires started heating up, at which time the light would go off.
Checking the door jamb, owner's manual, and this forum, I found recommended cold pressure is 32 psi. I upped the pressure to 32 psi and the light has not come on since.

Killerking1964

My Tire pressure monitoring sys. light came on when one of the tires reached 27.5 psi. The other three were at 28 psi. Any others noticed when their's went off? Also, no leaks in tires but I DO notice that the pressure does decrease faster in cold weather. I check my pressure once a week.

Gord96BRG

It's not only for warning you when your tires are flat - it's for when the pressures have fallen outside the operating range. The system is NOT extremely sensitive, it's just sensitive enough to do it's job.

What's the first step when you get an alignment done? The tech's set the tire pressure. THAT is why your TPMS wasn't going off after the alignment.

If your TPMS is going off on cold mornings - that means the pressure in one or more tires is too low. It's that simple - you need to add air.

I have come to the conclusion that people are not used to checking their tire pressures nearly frequently enough, and thus they are not nearly aware enough of how tire pressures vary and how often they really need to be checking and adjusting their tire pressures. The RX-8 TPMS system just lets them know what they should be doing.

The system isn't crying wolf - it's trying to educate you! Listen to it and learn - you should be trusting it instead of ignoring it!

Regards,
Gordon

DaveT

Amen, Gordon.
Ever notice how many cars on the road have the outsides of their tire tread worn while the center looks brand new? Probably due to never checking pressure, then complaining that tires don't last.

RX-8 friend

From Gordon:
"I have come to the conclusion that people are not used to checking their tire pressures nearly frequently enough, and thus they are not nearly aware enough of how tire pressures vary and how often they really need to be checking and adjusting their tire pressures. The RX-8 TPMS system just lets them know what they should be doing."

This is why I started this thread! Come on people, how hard is it to just check the pressure? Don't ignore the system. It's right and you're lazy!

rx8cited

It's really hard ....... if you don't have a good air pressure gauage or don't understand that you have to check the tires when they are at cold ambient temperature (vs the hottest part of the day or after driving on them) or don't compensate for them not being at ambient temperature - other than that it's a piece of cake.

My light has never come on in 3500 miles with either of my two sets of sensors (winter and summer wheels). I wish there was an easy way to test the sensors without over/underinflating the tires. At least people who had their light come on know their system work . Idea: I could do the over/underinflation sensor test prior to having my tires replaced - since at that point it won't matter to damage or wear them a bit more.

Also, keep in mind that summer tires require 32 psi cold, while snow tires require 34 psi cold.

One last thing while we're talking tires - if you've never thought about getting/repairing a flat tire on your RX-8, check out this thread
Better way to fix flats? from the Wheels, Tires and Suspension forum.

regards,
rx8cited