Correct Tire Pressure 3

[tomgranat]

I have OEM tires on my car with a 91 load index. The tire has a max inflation of 55psi. The car manufacturer only inflates to 44 psi. Can I assume that the load on the tire is (44/55)* rated load for a 91 load index?

I am getting new tires with same 91 load index however max inflation is 44 psi. Would I calculate the act load from the OEM tires and use this value to get the new inflation for the new tire (41 psi)?

Thanks for the help

 

[evelrod]

Your method will work. For maximum loads, I just reduce the sidewall rated cold inflation pressure by some ten percent on my trucks and motorhome. I have found that, at least on the cars I have bought recently, the inflation pressures given by the mfgr are pretty close to ideal, eg, Lincoln LS is 36psi.
For my race cars, the sidewall pressure is only a max value, "not to exceed", as I see it. I use a pyrometer and other tools to determine the pressure for each car at each track. Mostly the pressure is very near maximum hot on radial tires and considerably lower on bias/crossply tires.

Personally, I prefer finding a brand tire that I can live with and stick with it on replacement...gives me a better method to guage performance and wear patterns. JMO.

Rod

 

[NormPeterson]

See the following FAQ

http://www.eng-tips.com/faqs.cfm?fid=1148

Note the 35 psi mentioned near the bottom.


Norm

 

[CapriRacer]

Tom,

You've got this all screwed up and so far, you didn't get any help.

What is written on the sidewall of the tire is NOT always related to the load capacity of the tire. What is there is governed by regulations and the regulations are a bit vague.

Rather than go into a lengthy discussion, I'm going to cut to the chase:

What year, make, and model are we talking about?

Also, what was the original tire size and pressure? It will be on the placard. The placard is usually located on a doorpost or in the glovebox.

It's unclear from your post, but are you considering a different size? If so what size are you considering?

 

[tomgranat]

CapriRacer,

It is a 2004 VW Passat GLS sedan. The original tires are 195/65/15 91H; tire pressures 44 rear, 36 front. The original tires (Michelin) have max pressure of 55psi.

I am going to replace the originals with the same size tire, 195/65/15 91H but the new tires have a max pressure of 44 psi.

From what I have read, the load rating (91 in this case) is rated at the max tire pressure. With these 2 tires having the same load rating but at different pressures, won't that change the tire pressure that VW suggests for the car?

 

[CapriRacer]

Tom,

Ah, German Engineering!

For some reason, the German vehicle manufacturers list on their placard more than one inflation pressure and the conditions where each is appropriate. Most folks find this confusing.

Since you are using the same size, the placard inflation pressure is appropriate.

And, no, what you heard is wrong! The load rating of a tire does not necessarily occur at the maximum inflation pressure - typically it occurs at a lower pressure - commonly referred to as the "rated pressure".

If you want me to explain this, be forewarned, it will be lengthy!!

 

[jadcock]

Capri, I would like an explanation...not to question your knowledge, but to benefit from it. The max load and max pressure are printed right next to each other on many tires, which probably leads to the common perception (which I also held) that the max load DOES occur at the max pressure. But indeed, after reading your post I went out and looked at my tires and sure enough, it does not explicitly state that the max load occurs at the max pressure. I just assumed it did.

As an aside, wow, I've never seen an OE pressure as high as 44 psi! The highest I've seen is 36 psi, which I thought was high for an OE recommendation (typically around 30 or 32, at least for most American cars).

Thanks,
Jason

 

[CapriRacer]

OK, here goes:

There are several tire standardizing organizations in the world. In the US, it is the Tire and Rim Association (TRA). In Europe, it's the European Tire and Rim Technical Organization (ETRTO). In Japan, it's the Japanese Automotive Tire Manufacturers Association.(JATMA). There are others but those are the major players.

These organizations not only set the standard dimensions for a tire size, they also set the load curve - the relationship between inflation pressure and maximum load carrying capacity. While each of these organizations has a different "formula" that is used for the load curve, they consult with each other and not only are the dimensions very close, but the load curves are too. The key point here is that there is some slight differences, and you can see the differences when you look up the load carrying capacities of tires with the same "size".

At this point I should mention that these organizations - and the standards that they set - are voluntary. A tire manufacturer does not have to follow the standards. But it just doesn't make sense not to, as all the vehicle manufacturers use the standards to determine what tire size and what inflation pressure to use on their vehicles and this information is listed on a label commonly called the vehicle placard.

Part of the load curve standard is a point at which the load curve stops. For TRA passenger car Standard Load tires (that's a lot of qualifiers, isn't it?), it is 35 psi. The equivalent ETRTO and JATMA based tires use 36 psi.

However, all the standardizing organizations agree that standard load passenger car tires can (and should) use higher inflation pressures for certain circumstances - like high speed driving. So ALL the standards say 44 psi or 51 psi is an optional maximum pressure. - with no increase in load and in some circumstances a reduction in load.

Please note: ETRTO and JATMA use metric units, and I'm only discussing this in English equivalents.

Now we come to the source of the problem. What is written on the sidewall of a tire is perscribed by a government regulation. It has been many years since I actually read the regulation (and I can't seem to find it now that I want to quote it.), but it says something like "maximum load capacity and inflation pressure."

Some think the regulation means the maximum load and the corresponding inflation pressure. This would look on the sidewall like: Maximum Load XXXX at YY. Michelin seems to be the only one with this interpretation.

Pretty much everyone else thinks the regulation means the maximum load and the maximum inflation pressure. This would look on the sidewall like: Maximum Load XXX. Maximum pressure ZZ.

For passenger car Standard Load tires, the maximum pressure listed on the sidewall will always be 44 psi or 51 psi, even though the pressure where the maximum load occurs is 35 or 36 psi. (with Michelin being the lone exception)

So in some respects, the 44 psi VW lists on its placard makes sense. They are using the maximum inflation pressure available, and I'll bet the circumstances that VW says this pressure is appropriate are fully loaded and Autobahn driving.

For those of you unfamiliar with German cars - and their placards - they list several pressures and the circumstances where they are appropriate. This is not the practice for US vehicle manufacturers - they list only one pressure.

Hope this helps.

 

[jadcock]

Fascinating. Is 44 psi the "standard" maximum inflation pressure for passenger car all season tires? The reason I ask is because our Grand Caravan specifies a recommended tire pressure of 36 psi front and rear. When we bought tires for it last year, we had no problem finding tires that would allow that pressure, and in fact the ones we bought bear a maximum pressure of 44 psi. But SOME tires out there prescribe a maximum pressure of 35 psi. Does this mean those tires don't necessarily adhere to the TRA? I wonder why, since most tires do seem to have a max pressure of 44 psi, some manufacturers would specifically limit some tires to only 35 psi. Does that necessarily mean it's a "weaker" tire in some regards?

 

[CapriRacer]

First, I'll bet the tires on your Caravan did not have the letter "P" in front of the size. This means the tires were either ETRTO or JATMA based standard. TRA tires start with the letter "P".

Second, 44 psi is common for Standard Load (as opposed to Extra Load) passenger all season tires, but there isn't a "standard".

I'll also bet the tires that you found with 35 psi on the sidewall were all Michelins.

And, no, if the tires have 35 psi written on the sidewall, they DO adhere to TRA. "They" have chosen to read the regulation in a certain way. But, and this is the confusing part, the tires are NOT weaker just because they say 35 psi. The use of 44 psi is still allowable for certain conditions and that doesn't change whether the sidewalls say 35, 36, or 44 psi.




don't say 44 psi on the sidewall. This whole thing about what is written on the sidewall creates a lot of confusion

 

[jadcock]

The Grand Caravan specifies two different OE tires; one is P and one is not. My OE tire was a Goodyear Integrity, in size P215/70R15. Had the van been equipped with the 16" wheels, it specified Bridgestone Turanza EL42 in size 215/65R16, without the P.

Some of the tires in my size (P215/70R15) that only showed a 35 psi were indeed Michelins (Symmetry). Having said that, the model of Michelin tires that we did purchase for it (Agility) do show 44 psi as the max pressure, and do bear "P215/70R15 97S" on the sidewall (although the OE tire was actually a 98S tire).

The second part of your response definitely is news to me -- even though the tire may be molded with 35 psi as the max pressure, a higher pressure may still be allowable?

 

[CapriRacer]

"....The second part of your response definitely is news to me -- even though the tire may be molded with 35 psi as the max pressure, a higher pressure may still be allowable?...."

Yes!!!

Usually the sidewall says "Max Load XXXX at YY psi." So it doesn't say that YY is the maximum pressure you can use.

But even if it did, you can use up to 44 psi with the blessing of the standards organizations.

But from the point of view of the tire manufacturer, the increase in pressure is a bit problematic, but nevertheless, there are circumstances where 44 psi would be appropriate.

 

[jadcock]

Huh! Interesting.

Thanks again for your informative responses.

 

[damnaged]

hi guys,
i'm looking on an explanation for tire load to ground pressure relation and find this thread really interesting. If somebody care to help ease my wonderings, please do assist to answer the below.

lets consider 44psi is the standard pressure and adjust the tyre pressure to that. If i am to load my car with very heavy loads, will the pressure of the tyre be affected or changed? say from 44psi will go up to 46psi for instance? is it like that?

or is it if i load too much, more than the "max load at 44psi" than the tyre will simply burst, but without affecting the tyre inflation pressure. please somebody explain to me.

Thanks.

 

[CapriRacer]

Some time back, someone asked a similar question, so I walked into our vehicle lab and on the lift was a Buick LeSabre. I measured the front tire pressures (no sense doing the rears - they are lightly loaded), lowered the car to the ground, and found the pressure grew 0.5 psi (You have to have a special gauge to get it down to the nearest 0.1 psi.) - in other words, not enough to worry about.

Now that was an empty vehicle, but I wouldn't expect a fully loaded one to be much more.

Now as far as what pressure is written on the sidewall:

NO!!! EMPHATICALLY, NO!!!! The tire will not burst if you exceed the value written on the sidewall. (At least not from the pressure itself.) Static burst pressures for passenger car tires is well over 100 psi.

The value written on the sidewall is 1) a usage pressure and 2) all tire pressures are expressed as cold pressures, even when they don't say so. (with the exception of when we are talking about pressure buildup, and it's obvious when we do so!)

I will say that increasing a tire's pressure increases the risk that an impacting object will cause the tire to rupture, but the difference between 30 psi and 35 psi is far exceeded by the potential objects (debris) littering our roads. In other words, worry more about the objects than the pressure.

But if you only consider loading the tire in excess of the load written on the sidewall, then what you will have is an overdeflected tire. Overdeflection is a term we tire engineers use because from the tires perspective, we can't tell the difference between overloaded and underinflated (with the small range of values we normally deal with.)

Overloading a tire causes heat buildup in service and that leads to a faster chemical breakdown of the materials - and eventually a failure - and for steel belted radial tires that means a belt leaving belt separation (what is incorrectly called a "tread separation")

I'm sure this is just the first round of questions, so what's next?

 

[evelrod]

Okay, CapriRacer. Here is one from a MiniRacer.
Some years back, after racing for over forty years on bias/crossply tires, mostly cantilevered-low angled sidewall construction, we switched to a radial race tire. The switch ment some rather radical changes in suspension settings, etc.---Question: What are the TP/tire load changes when rim width is considered...e.g., rim specs for a tire are x" and you install the tire on a x+1 (x+2,etc.) rim?

From our experience, tire pres can be somewhat lower than max and tire temps seem to be a bit lower overall. Also, tread appears to wear more evenly. Load capacity changes are beyond our track test ability to determine. Not normally a problem in racing, but I see some pretty stragne combinations on the street.

Rod

 

[Fabrico]

I just happen to race a good distance every day in one of those Buick LaSabre's. The track is called LA Freeway and speed is about 128.7475 KPH. I run the fronts right at sidewall maximum (44psi) and the rears about 3 lbs less, same in the rest of the cars and SUV. It maximizes gas mileage, does'nt do anything to the ride, helps postpone the ineveitable wearing on the outsides of the tread. Never had a problem, but I usually drop it 3-4lbs F&R in the winter months. Driven over a million miles in my personal vehicles and it works for me.

 

[CapriRacer]

"....Okay, CapriRacer. Here is one from a MiniRacer....."

First, from a tire's perspective, racing is a peculiar activity and experience on the track doesn't translate well to knowledge about what happens on the street.

"....---Question: What are the TP/tire load changes when rim width is considered...e.g., rim specs for a tire are x" and you install the tire on a x+1 (x+2,etc.) rim?...."

Load capacity / tire pressure is based on deflection, and for practical purposes, changing the rim width within the allowable range doesn't significantly affect the way a tire deflects - so tire pressure recommendations (based on load capacity) don't change.

"....From our experience, tire pres can be somewhat lower than max and tire temps seem to be a bit lower overall...."

Going wider in rim width will cause the tire to "arch" less, which should result in the tire temps calling for more pressure. The fact that you are getting the opposite result leads me to believe that the tire is being stiffened up considerably by the change in rim width, and the result is less deflection = less heat build, less pressure buildup.

Remember, you are only using tire temperatures as a guide to getting an even footprint for maximum contact - and therefore maximum cornering ability.

Food for thought: When you measure footprint temperatures, you combine straightline temperature generation with cornering temperature generation. Since you are only concerned about cornering, shouldn't you factor in the effect straightline temperature generation has on your final answer - which I would think would result in less pressure. I never had the opportunity to test this out.

".....Also, tread appears to wear more evenly...."

And on a racecar, why is this important?

Most tire wear occurs in cornering,(but not all) and racecars are almost 100% about MAX cornering, which is something most of us don't do on the street. So tire wear on racecars doesn't translate to tire wear on the street.

"....Load capacity changes are beyond our track test ability to determine...."

I agree with that.

"....Not normally a problem in racing, but I see some pretty strange combinations on the street...."

There is a phenomenon called "tire stretching" where the tire is mounted on rims wider than the range published by the tire manufacturers. This is done strictly for looks.

For practical purposes, the sidewall is taken out of the picture, and the vehicle might as well be riding on solid rubber. (Maybe that's next?) I've heard of a couple of cases where the tire bead pops off during cornering - result is a ruined rim.

 

[NormPeterson]

Load capacity / tire pressure is based on deflection, and for practical purposes, changing the rim width within the allowable range doesn't significantly affect the way a tire deflects - so tire pressure recommendations (based on load capacity) don't change.

Can I infer from the above that a crude estimate of a tire spring rate curve could be made (given that you know something about the deflection at one load and pressure)? I should think that something simple that's still better than a flat "1500 lb/in" or "250 N/mm" exists.


Wouldn't cantilevered construction tires demand slightly higher pressures than more conventional constructions, such that the conventional tire would want lower pressures relative to the cantilevereds?

I must be frequenting the "wrong" other automotive forums. Nearly every time that I've seen questions about tire:wheel width combinations outside the approved range, it's the other way around, with tires too wide for the wheel (example: 245/40 on a 7" wide wheel).

FWIW, the tires on one of my cars are mounted on wheels half an inch above the listed range for that size (225/60 on 8.5", the rest is a long story), but I'm pretty careful about maintaining inflation pressure partly for the reason above regarding flattening the tread shape as seen in cross-section and partly to minimize flex. Needless to say, ride comfort is of rather low priority, though it isn't all that bad. An unexpected benefit was to discover that the car remained much more stable laterally/in yaw when one of the rear tires went flat (road hazard sidewall cut) than when flat rear tire situations have occurred with more conventional tire:wheel width fitments.



Norm

 

[evelrod]

Thanks, CR.

".....Also, tread appears to wear more evenly...."

And on a racecar, why is this important?

It's only 'important' as an indicator of overall performance as that performance relates to suspension settings. Also, 'even tread wear' on a race tire is NOT the same thing as you would see with 'even tread wear' on a street tire...as a matter of fact, it would be considered rather abnormal in apperance.

Norm, as to the TP variations, our use of cantilevered race tires...The last set of 9.5-20-13 Goodyears were mounted on 7" rims on a 2065# (gross wt.) race car (Lotus-Cortina SCCA GT-4, in 1991) front pressure ~22# cold, 26# hot and, rear ~20# cold and 26# hot---average but varried from track to track (tire temps were ~175f on a cloudy 70 degree day). The Yokahama A008R on the same car at the same event (with proper camber adjustments) TP was 34 front and 30 rear cold---41 and 36 hot average with tire temps 165f+ (both sets of tires used on 7" American/Libre wheels at RIR).

My current Mini Cooper (1389# dry wt.) is using 165-70-13 Yokohama A032R(s) mounted in 6"x10" Minilites at 30psi front, 25psi rear, cold. Hot generally goes to 35 front and 28 rear. The original OEM recommendations are for 4.5" to 5" rims at somewhat higher pressures than I use.

Rod