Tire pressure??

[weebl]

Originally Posted By: CapriRacer
A P206/60R16 has a load carrying capacity of 1356# at 35 psi. A P225/60R16 has a load carrying capacity of 1609# at 35 psi - BUT - at 26 psi that same tire can hold 1378# - virtually the same as a tire 2 sizes smaller.

So you have to be careful thinking that the pressure specification is low unless you consider the size of the vehicle and the size of the tire.


Hmmmm. Does tire pressure not have an effect on sidewall stiffness, though? I wasn't thinking so much in terms of cargo capacity as I was with the increased rollover risk. Or in your example, the two tires with the same capacity, but at different PSI, are the sidewalls just as "firm" in the same application?

 

[CapriRacer]

Originally Posted By: weebl
Hmmmm. Does tire pressure not have an effect on sidewall stiffness, though? I wasn't thinking so much in terms of cargo capacity as I was with the increased rollover risk. Or in your example, the two tires with the same capacity, but at different PSI, are the sidewalls just as "firm" in the same application?


Actually, high pressure increases the risk of a rollover, which is why the Ford Explorer spec'd that low pressure that everyone points to. Rememeber the reference to Bobby Ore? He uses very high pressures to get his vehicles to do tricks - and getting the vehicle on 2 wheels is helped by high inflation pressure. So I find it kind of funny when folks point him out as an advocate of high inflation pressure.

No, sidewall stiffness is a function of inflation pressure, which is why having high pressures decreases the ride quality so much.

 

[Volvo_ST1]

Originally Posted By: CapriRacer
Rememeber the reference to Bobby Ore? He uses very high pressures to get his vehicles to do tricks - and getting the vehicle on 2 wheels is helped by high inflation pressure. So I find it kind of funny when folks point him out as an advocate of high inflation pressure.


Well, we do have a few stunt drivers on board.

 

[AstroTurf]

Originally Posted By: CapriRacer
Originally Posted By: weebl
.......But as someone else stated, the 26 psi recommended for this heavy SUV seems rather low, along the lines of the Explorer/Firestone debacle.


I realize that sounds low, but let me give you an example of why you should ignore your "sense".

A P206/60R16 has a load carrying capacity of 1356# at 35 psi. A P225/60R16 has a load carrying capacity of 1609# at 35 psi - BUT - at 26 psi that same tire can hold 1378# - virtually the same as a tire 2 sizes smaller.

So you have to be careful thinking that the pressure specification is low unless you consider the size of the vehicle and the size of the tire.
Originally Posted By: CapriRacer
Originally Posted By: weebl
Hmmmm. Does tire pressure not have an effect on sidewall stiffness, though? I wasn't thinking so much in terms of cargo capacity as I was with the increased rollover risk. Or in your example, the two tires with the same capacity, but at different PSI, are the sidewalls just as "firm" in the same application?


Actually, high pressure increases the risk of a rollover, which is why the Ford Explorer spec'd that low pressure that everyone points to. Rememeber the reference to Bobby Ore? He uses very high pressures to get his vehicles to do tricks - and getting the vehicle on 2 wheels is helped by high inflation pressure. So I find it kind of funny when folks point him out as an advocate of high inflation pressure.

No, sidewall stiffness is a function of inflation pressure, which is why having high pressures decreases the ride quality so much.


Ding Ding Ding Ding Ding...

Thank You Capri Racer,

I have been searching for this answer, pouring over your web site, and many others. And, BAM, It hit me.

All P Metric tires rated load is achieved at 35 psi, regardless of the max pressure listed on the tires sidewall.

Let me say that again so everyone GETS IT:

All P Metric tires rated load is achieved at 35 psi, regardless of the max pressure listed on the tires sidewall.

You Sir, Are truly a diamond waiting to be mined in all of this muck and mire of misinformation.

Thank You Thank You Thank You, Jim

 

[AstroTurf]

Originally Posted By: weebl
I believe it reinforces that there is a lot to consider in fine-tuning pressure based on desired performance characteristics. A huge point of contention that I see is regarding the sidewall stamped maximum pressure. Those who have no regard to it; those who feel the stamped maximums are arbitrary, based on an interpretation of FMVSS109/FMVSS139, and that a perceived highest allowable for a given size/type of tire is closer to the true maximum; and those who feel that the tire engineers derived the maximum that is stamped based on a host of test data and should therefore not be exceeded at all.

A rule that I have always followed for myself is that the vehicle placard is a recommendation, and should be a minimum, the tire sidewall is the maximum, and in most cases on North American or Asian makes, I will typically go 2-3 psi above placard, without exceeding sidewall, and on European makes, typically use the higher end of the recommended placard range, again without exceeding sidewall. I have never had a compelling reason to explore beyond a known established saftey boundary (i.e. sidewall maximum).

But as someone else stated, the 26 psi recommended for this heavy SUV seems rather low, along the lines of the Explorer/Firestone debacle.


Just an FYI to all who might like to know...

The codes or test reports that weebl has listed, and I have highlighted in his quote can be found here: http://www.nhtsa.gov/
You must simply enter the code or report number into the search box and Wah Lah... Bob's yer Uncle.

Hope it helps, Jim

 

[kr_bitog]

Originally Posted By: weebl


Hmmmm. Does tire pressure not have an effect on sidewall stiffness, though? I wasn't thinking so much in terms of cargo capacity as I was with the increased rollover risk. Or in your example, the two tires with the same capacity, but at different PSI, are the sidewalls just as "firm" in the same application?


Yes, the tyre pressure also effect the sidewall firmness, but we also need to know the car and tyre application. Most people here discuss inflation with assumption to use on highway, where for SUV sometimes the car application is for mixed on and off-road driving. Therefore for SUV/4WD manufacturer typically design the car to use upsize tyre with lower pressure on placard to enhance offroad capability. That is why choosing tyre beside paying attention on size and load index, one should also pay attention on the tyre characteristic to match with car usage where it is intended to be used.

 

[IndyIan]

Originally Posted By: CapriRacer
Originally Posted By: weebl
.......But as someone else stated, the 26 psi recommended for this heavy SUV seems rather low, along the lines of the Explorer/Firestone debacle.


I realize that sounds low, but let me give you an example of why you should ignore your "sense".

A P206/60R16 has a load carrying capacity of 1356# at 35 psi. A P225/60R16 has a load carrying capacity of 1609# at 35 psi - BUT - at 26 psi that same tire can hold 1378# - virtually the same as a tire 2 sizes smaller.

So you have to be careful thinking that the pressure specification is low unless you consider the size of the vehicle and the size of the tire.

But isn't the consequences of letting the 26 psi tire drop 6 or 8 psi much greater than letting the 35 psi tire drop the same pressure? Plus the GVWR of the 26 psi tire vehicle is given at 35 psi? So the uninformed person has there tires set at 26 psi 4 months ago and then loads up the SUV for summer vacations on 19 psi tires and blows them out at 75mph... Seems almost predictable. Obviously people should know better but many don't even know that they should know.

 

[kr_bitog]

Originally Posted By: Indylan
But isn't the consequences of letting the 26 psi tire drop 6 or 8 psi much greater than letting the 35 psi tire drop the same pressure? Plus the GVWR of the 26 psi tire vehicle is given at 35 psi? So the uninformed person has there tires set at 26 psi 4 months ago and then loads up the SUV for summer vacations on 19 psi tires and blows them out at 75mph... Seems almost predictable. Obviously people should know better but many don't even know that they should know.

Well when multiple factors go wrong, it most likely the result still goes wrong especially when the driver is an ignorant.

That is exactly why they milk customer by introducing TPMS to add more warning and earn more money, which may not really be required for careful user that read and practice the owner's manual and aware that something is wrong with the car handling due to underinflation.

 

[HTSS_TR]

Originally Posted By: kr_bitog
That is exactly why they milk customer by introducing TPMS to add more warning and earn more money, which may not really be required for careful user that read and practice the owner's manual and aware that something is wrong with the car handling due to underinflation.

TPMS is mandated by Federal Government.

 

[kr_bitog]

Originally Posted By: HTSS_TR
Originally Posted By: kr_bitog
That is exactly why they milk customer by introducing TPMS to add more warning and earn more money, which may not really be required for careful user that read and practice the owner's manual and aware that something is wrong with the car handling due to underinflation.

TPMS is mandated by Federal Government.


Yes, it is mandated by Government because of Clinton's TREAD Act in response of Explorer/Firestone issue, that again is caused by ignorants who never check and pump the tyre regularly.

Some more information can be read here:
http://www.etv.com.au/TREAD_Act_Pressure.pdf

 

[CapriRacer]

There have been many threads where the issue of regularly checking one's oil level has come up - and with the same remarks as checking one's tire pressures.

Oil level sensors and TPMS's are basically the same technology. It just makes sense to have both given that we know folks do not do what they should do.

 

[AstroTurf]

Capri Racer,

That is a Great Point.

It does not matter that I check my family vehicles tire pressure every Sunday morning. If my wife is out and about on Wednesday and has an issue, she gets an early warning. Better to react to a light on the dash, than for her to be roadside on a flat tire or worse.

Thanks Again, Jim

 

[Ken2]

Quote:
Oil level sensors and TPMS's are basically the same technology. It just makes sense to have both given that we know folks do not do what they should do.

Way, way too many drivers think a car needs two things to run...gas in the tank and a key in the ignition switch. That's all. Everything else is a mystery to them, and they don't care enough to check it out. This includes all those who are honestly surprised when someone points out their bald tires, and who, while their car is being towed to the shop say, "yeah, it has been making a funny noise for a couple of months."

Quote:
A P206/60R16 has a load carrying capacity of 1356# at 35 psi. A P225/60R16 has a load carrying capacity of 1609# at 35 psi - BUT - at 26 psi that same tire can hold 1378# - virtually the same as a tire 2 sizes smaller.

So you have to be careful thinking that the pressure specification is low unless you consider the size of the vehicle and the size of the tire.
Yep, the air carries the load. The tire just holds the air. Ever see a tractor on a turf farm with its huge very low pressure tires--30" wide tire on a 26" wide rim? As an inexact analogy, think of the same amount of air (same number of air molecules) compressed to a higher pressure in a skinny tire or expanded to a lower pressure in a wide tire but carrying the same load.

 

[Shannow]

I'd differ on that last point.

A tyre is a structure, not just a bladder.

 

[CapriRacer]

Originally Posted By: Ken2
Yep, the air carries the load. The tire just holds the air.......


Ken2,

Thanks for the support, but in the interest of technical accuracy, I'm going to point to my web site where I analyze this:

http://www.barrystiretech.com/airortire.html

Bottomline: The tire is 100% holding up the vehicle, the air is merely a stiffening agent.

It's a minor point, but if you value accuracy like I do.......

 

[rpn453]

Originally Posted By: Astro14
While you're not racing the Nissan, here is how racers set up the cars. It's an informative article, as it talks about optimization, and reinforces what Astroturf and others have said: http://www.turnfast.com/tech_handling/handling_pressure

Note this part of the article:
(Note: use the car manufacturer's recommended tire pressure as the baseline. Many service and oil change stations with poorly trained techs will pump tires up to pressure stated on the tire side wall. Hello! That's the maximum setting, not the recommended setting. Watch out for those guys!)


To me, the most interesting part is right before that:

"So, if you're using street tires on the track, the generic, "start somewhere" point is 5 psig lower than the car manufacturer's recommended tire pressures."

No wonder I was noticing such an improvement in traction every time I dropped pressure after starting 8 psi over!

 

[AstroTurf]

Originally Posted By: rpn453
No wonder I was noticing such an improvement in traction every time I dropped pressure after starting 8 psi over!


Wow, 8 psi over...

You are on the verge of being a Hyper-Inflater. Is that what you normally run on the street?

Thanks, Jim

 

[rpn453]

Originally Posted By: AstroTurf
Wow, 8 psi over...

You are on the verge of being a Hyper-Inflater. Is that what you normally run on the street?

Thanks, Jim


No. I provided the details in your hyperinflation thread. Previously, I was under the impression that people typically went above the street pressure for track use.

 

[AstroTurf]

Originally Posted By: rpn453
Originally Posted By: AstroTurf
Wow, 8 psi over...

You are on the verge of being a Hyper-Inflater. Is that what you normally run on the street?

Thanks, Jim


No. I provided the details in your hyperinflation thread. Previously, I was under the impression that people typically went above the street pressure for track use.


Ah O.K.,

Ya had me worried there for a minute.

Thanks for the quick response, Jim