Tire Pressure; condensed information

[Christofoo]

Hello,

I noticed there are a dozen threads on tire pressure in the archives, and it's one of the most frequent google search hits on my personal website, so I've been trying to rewrite the most condensed and accurate page of information ever.

I would love it if some of the experts around here could give it a look, point out why my page is not needed, disagree with my conclusions, point me to better references, let me know if Wayne would prefer his real name or not to be quoted at all, suggestions, clarifications, editorials, etc.

http://cannonblast.net/christofoo/cartirepressure

(Sorry I didn't copy and paste the whole thing here, the formatting doesn't copy, I'm too lazy to reformat it.)

Thanks!
Chris

[CapriRacer]

First, under "Placard Pressure" you said ".....Placard pressure is not for a heavily loaded vehicle....."

The placard pressure INCLUDES the vehicle loaded to its max GVW.

".......Why do my cars specify 29-30 PSI on the placard, while my tires specify higher max pressures on the side-wall?...."

Beacsue the placard pressure not only covers the load at max GVW, but the vehicle manufacturer specified tires LARGER than needed so he could specify a lower pressure to get a better ride. There's also some handling advatages by using a larger tire.

"........I've also been able to find anecdotal tales of pothole damage with overinflated tires. I still feel like I need to see test data to prove the significance of pressure in the tire's tendency to take damage from potholes....."

Here's an old study, but the principles apply regradless:

http://www.barrystiretech.com/sae800087synopsis.html

Look under Dynamic Bruise Resistance.

And lastly, larger tires (meaning tires with larger load carrying capacity) have lower RR - not to mention what affect a larger diameter - but the affect is small. Advocating "narrower" is NOT supported by data. Here's some food for thought:

http://www.barrystiretech.com/rrandfe2.html

[phoebeisis]

Some posted data-on a small car-maybe it was a Metro or an Insight-
Data indicated around 45-50 psi was about it
45psi-50 psi is pretty close to sidewall max
Make it easy on yourself-and passengers-just use sidewall and call it a day
Wet weather braking is said to drop off a bit at really high pressures-I assume that is true-never bothered to hunt any data on it-not really into second guessing the OEM engineers much.
Charlie

[xcel]

Hi CapriRacer:

When driving the BMW 335d a few years back, the placard was a static 35 psi IIRC. The manual however had varying pressures depending on load from placard with one person to 49 psi all around with 3 individuals and 2 pieces of luggage in the trunk.

Wayne

[Christofoo]

CapriRacer (Barry, I believe),

Thank you for that link and for your pages - exactly what I was looking for. (Updated my page.) I may have more questions for you after I finish digesting them.

Charlie,

I remember that, this one if we're talking about the same thing:
http://ecomodder.com/forum/showthrea...tml#post265379
The problem is that a Geo Metro has a curb weight of only 1600 lbs, resistance is a function of Crr*weight. It's tempting to speculate that the sweet spot on a Civic would be 58 PSI vs 40 PSI on a Metro, being 1.4X heavier. Bottom line is I think this test should be duplicated in a heavier car.

Wayne,

Just looking at my Civic, I see about 1,000 lbs margin between max tire load and max GVW. Perhaps it depends on the car, CapriRacer appears to be correct in relation to the Civic.

[Christofoo]

CapriRacer,

My two questions for you:

When you discussed the tradeoffs between traction, tread life, and rolling resistance on your page, do you specifically mean dry traction? (As opposed to wet or snow traction.) The reason I'm after the distinction is that in some folks' views its okay to tow trailers, maybe doubling stopping distance, in the name of vacation. I think there's a fine line between an "expected", and might I say marginal, tradeoff in traction on dry pavement, vs "unexpected" loss of traction under adverse conditions. One is a real safety issue, the other one arguable with proper buffer maintenance.

Also I noticed a note on another page suggesting 35 PSI max tires can be overinflated in your view, can you elaborate?

Thanks,
Chris

[CapriRacer]

Originally Posted by Christofoo

CapriRacer,

My two questions for you:

When you discussed the tradeoffs between traction, tread life, and rolling resistance on your page, do you specifically mean dry traction? (As opposed to wet or snow traction.).....

Definitely dry traction - and to the extent that we are excluding the hydroplaning portion of wet traction, then wet traction is included in that triangle. Snow traction and hydroplaning are mostly about tread pattern so they don't really figure into that.

Originally Posted by Christofoo

.......The reason I'm after the distinction is that in some folks' views its okay to tow trailers, maybe doubling stopping distance, in the name of vacation. I think there's a fine line between an "expected", and might I say marginal, tradeoff in traction on dry pavement, vs "unexpected" loss of traction under adverse conditions. One is a real safety issue, the other one arguable with proper buffer maintenance.

Also I noticed a note on another page suggesting 35 PSI max tires can be overinflated in your view, can you elaborate?.......

Context would help at lot, here. Can you provide a link?

But in general, I worry about severe deveiations from the placard inflation pressure.

[lightfoot]

Originally Posted by CapriRacer

But in general, I worry about severe deveiations from the placard inflation pressure.

Is there any published hard data confirming that there is an issue in real-world use at substantially higher pressures than placard, either (a) at sidewall or (b) above sidewall?? We have a limited amount of data from folks here, including me with three vehicles, that running current radials at or above the sidewall pressure number is not a problem and in fact has some distinct benefits.

In terms of English, the "max load @ pressure" printed on the side of the tire does not say that the pressure number is a maximum, it simply states what the maximum load is at that pressure. But this is just another example of vague English usage. It doesn't stipulate that this is the maximum permissible pressure, especially at lower loads. Strictly speaking, all it says is what the max rated load is at that pressure, but the inference is that this is the max load rating at any pressure.

IOW, this rating is more about load than pressure, at least as it's written.

[CapriRacer]

Originally Posted by lightfoot

Is there any published hard data confirming that there is an issue in real-world use at substantially higher pressures than placard, either (a) at sidewall or (b) above sidewall?? We have a limited amount of data from folks here, including me with three vehicles, that running current radials at or above the sidewall pressure number is not a problem and in fact has some distinct benefits........

I was hoping someone else would answer this question - but here goes.

No, there is very little hard data on anything related to pressure above placard pressure on tires. It's pretty much all been anecdotal. That means you need to pay attenttion to what others are reporting. Here's what I have noticed

A lot of folks have reported center wear. There are reports of wet traction issues. Groove wander issues. While there have been reports of failures, it's unclear if the rate is different than the norm - seems to be, but far from clear.

Other issues have been reported, but those issues don't seem to be out of proportion to the rest of the poulation.

[Christofoo]

Originally Posted by CapriRacer

...
Context would help at lot, here. Can you provide a link?
...

Sorry, on this page:
http://www.barrystiretech.com/rrandfe.html
this sentence:
"But under no circumstances am I advocating the use of inflation pressures higher than what is written on the sidewall (with the exception of tires with 35 psi max. on the sidewall - and there's a long story there!)"

I'm ready for a long story.

Thanks.

Originally Posted by lightfoot

...
In terms of English, the "max load @ pressure" printed on the side of the tire does not say that the pressure number is a maximum, it simply states what the maximum load is at that pressure. But this is just another example of vague English usage. It doesn't stipulate that this is the maximum permissible pressure, especially at lower loads. Strictly speaking, all it says is what the max rated load is at that pressure, but the inference is that this is the max load rating at any pressure.
...

My tires say:
MAX LOAD 510 kg (1124 LBS)
AT 300 kPa (44 PSI) MAX PRESSURE

To me that translates as "44 PSI is the correct pressure for the max load of 1124 lbs, and 44 PSI is the max pressure".

Not all tires state this in the same way. Another set on my Corolla says:
MAX LOAD 435 kg (959 LBS)
MAX PRESS 240 kPa (35 PSI)

Chris