Tire inflation advice needed

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Originally Posted By: HTSS_TR
If increase pressure decreases handling/performance then they would not recommend these pressures.


That is correct.

Increasing the tire pressure does a few things. It reduces the slip angle of the tire (reducing the slip angle increases transient response), which makes the car respond faster to steering input. Btw, moving to a lower profile tire does much the same thing, which is why high performance cars use lower profile tires.

It reduces sidewall flex and tread squirm. This reduces heat in the tire.

It reduces rolling friction. This increases gas mileage.

It can balance the tire to the speed for impacts. At low speeds a lower inflation pressure may be adequate, but at high speeds hitting a pothole with a low inflation pressure can damage tire, wheel, and suspension.

If you think of the tire and wheel, with the suspension, as a single system you can see how balancing tire pressures to speed actually can reduce suspension stress and wear.





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Originally Posted By: HTSS_TR
Mercedes and some high performance cars recommend different pressures depending on driving styles.


So did my Dodge spirit 4 cylinder automatic!
LOL.gif
 
Originally Posted By: crosseyedwx

In my opinion, go to 44psi.


Bad Advice all around. In fact 'life threatening' advice I'd say. Life threatening not just to the driver but to the drivers and their passengers around them!
If you fill to max on a cold tire what in your expert opinion do you think would be the tire pressure when the temperature has gone up ? and after the car has been running at highway speeds for just 10 mins ?
 
Originally Posted By: youdontwannaknow
Bad Advice all around. In fact 'life threatening' advice I'd say. Life threatening not just to the driver but to the drivers and their passengers around them!
If you fill to max on a cold tire what in your expert opinion do you think would be the tire pressure when the temperature has gone up ?


If the tire says 44 psi on the sidewall, it means 44 psi cold.

http://www.nhtsa.dot.gov/cars/rules/TireSafety/ridesonit/brochure.html


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The burst pressure of today's radial tire is well over 100psi. Even warm, a tire that is 44psi cold would increase to about 48psi per my past measurements. The tires will actually run cooler at higher pressure since there is less deflection in the sidewall.

If it was not safe, it would not be posted on the tire, plain and simple. I have better control of my vehicle, increasing safety to myself, the driver, and others on the road. I have run tires at or over max for the last 6 years, around 100,000 miles of driving on multiple vehicles, and have testimony of one who have gone over 100,000 miles on a set of OEM Michelins that still have 4-5/32 tread depth, among other stories.

If you run tires at door placard pressure on a passenger car or van, with 5 people and cargo, your tires are now dangerously underinflated, and that thought does not cross the mind of well over the majority of drivers.
 
Sorry, but while I've been hestitant to comment on some of the misconceptions being posted as fact, I've got to call this one out:

Originally Posted By: crosseyedwx
......

If you run tires at door placard pressure on a passenger car or van, with 5 people and cargo, your tires are now dangerously underinflated, and that thought does not cross the mind of well over the majority of drivers.


This is patently untrue.

All - and I even mean the infamous Explorer - have (had) enough inflation pressure specified to carrying the load as indicated on the placard.

We can argue about whether a few more (or a lot more) pressure would be a good idea, and whether the OEM's specify pressure that are biased towards a soft ride, but the one thing that can not be argued is whether or not the placard pressure results in underinflation from a load carrying capacity point of view. This would be an instant recall and the lawyers would make a mint off this. It has not happened!
 
So why did so many Exploders go tumbling? I think its that the GVWR is way to low for many vehicles based on average and expected use.
My buddies Echo hatch back had a load capacity of 680lbs I believe... Duh, so they expect you to inflate your tires more when you carry 4 average males... or just not carry 4 average people in the car?
I have no idea what the load capacity is on the placard of an Exploder but I'd bet having each seat full with 200lb guys is "overloading" before you add all their stuff.
Seems like a lawsuit is needed to get realistic pressures put on the door placards. Some Exploder pressures were 26 psi... For some reason Ford was pushing the limits of tire loads by specifying such low pressures. I assume it was to sell more trucks.
Ian
 
Originally Posted By: CapriRacer
This is patently untrue.

All - and I even mean the infamous Explorer - have (had) enough inflation pressure specified to carrying the load as indicated on the placard.


http://www-nrd.nhtsa.dot.gov/pdf/nrd-30/NCSA/RNotes/2005/809846.pdf

27% of the passenger cars in the US have one seriously underinflated tire.

57% of 15 passenger vans have at least one seriously underinflated tire, 29% of light trucks, and 54% of all other vans.

So, if your placard says 32 psi, and it drops to 24 psi, you're a statistic.

If your placard says 32 psi, and you inflate it to 40 psi, and it drops to 30 psi, you're not a statistic.

And that, in simple terms, is what the Explorer fiasco was all about.




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Pressure(s) on placard should be minimum. Running few PSI's above placard is okay for most cars and trucks, but never above max pressure on side wall.
 
Doesn't anybody ever READ what is on the sidewall?
It give a maximum pressure for a maximum weight. But the weight and it's relationship is never mentioned or considered.
Only the pressure - it's all people see.

Up to 5 lbs over the factory's recommendation is OK for general street use. I would never go lower, but some do .
 
Originally Posted By: mechtech2
Doesn't anybody ever READ what is on the sidewall?
It give a maximum pressure for a maximum weight. But the weight and it's relationship is never mentioned or considered.
Only the pressure - it's all people see.

Up to 5 lbs over the factory's recommendation is OK for general street use. I would never go lower, but some do .


Are you saying up to 5 psi over the door sticker psi or up to 5 over the sidewall psi ?
 
I ALWAYS read the maximum pressure as stated on the sidewall, and I usually inflate my tires to that pressure or perhaps (rarely) 1-2 PSI below, cold. Sometimes in the winter I'll run 'em a little bit lower but not much.
 
Originally Posted By: IndyIan
So why did so many Exploders go tumbling?

.....



Dr. Sanjay Govindjee of the University of California, Berkley, issued a report dated January 30, 2001, on the failures of the Firestone tires on the Ford Explorer. It is a very long and technically involved report.

Short version: The Firestone tires failed because of a flaw in the tread design that resulted in rubber insert between the belt becoming more compr4essed than intended. Tires from the Decautur, IL plant were more affected than tires from other plants for reasons not explained in the report.

Unfortunately, I can not find a link to the report, but did retain a copy for my records.

There used to be a link to an engineering analysis done by the National Highway and Traffic Safety Administration )NHTSA) on the Firestone tires on Ford Explorers.

http://www.nhtsa.dot.gov/hot/firestone/firestonereport.pdf

Unfortunately, this page has disappeared. However. I downloaded a hard copy some time ago.

It specifically highlights the same failure modes and causes as Dr. Govindee. Further. It specifically addresses the issue of inflation pressure vs GVW and fails to conclude that the tires would be underinflated at the placard conditions.

Some background

In this application (an SUV), a P235/75R15 SL has a load carrying capacity at 26 psi of 1594#. This means that the Ford Explorer GAWR’s (Gross Axle Weight Rating – the maximum axle load of a fully loaded vehicle) can be no greater than 3187# and the GVW (the maximum the vehicle is allowed to weigh fully load) can be not greater than 6374#.

According to Tire Guides than largest GVW of any 1998 Ford Explorer was 5560# - which is over 800 pounds less than the maximum load specification for the tires.

At the time of the recall (August, 2000), I located a 1998 Ford Explorer and recorded the placard information – which included the GAWR’s. Later, Firestone published (and posted on the internet) the presentation given to Congress by John Lampe, Firestone’s president at the time, which also had the GAWR information. Both of these verified that the tire’s pressure specification resulted in enough load carrying capacity for a fully loaded vehicle.

Unfortunately, I did not retain either of these. If someone has a 1998 Ford Explorer, I would appreciate it if they would forward a photograph of the placard so I can document the GAWR’s – as well as the tire size and pressure specification (which also comes into dispute from time to time).
Hopefully that explains why I can state with confidence that even the infamous Ford Explorer placard has (had) enough inflation pressure specified.

Originally Posted By: IndyIan

........

My buddies Echo hatch back had a load capacity of 680lbs I believe...

.......


Sorry, but Toyota Echo's came with P175/65R14's inflated to 32 psi which results in a load carrying capacity of 981#



Originally Posted By: Rolf


http://www-nrd.nhtsa.dot.gov/pdf/nrd-30/NCSA/RNotes/2005/809846.pdf

......

And that, in simple terms, is what the Explorer fiasco was all about.



So you are agreeing with me that there was enough pressure specified on the placard and the real problem was people not maintain that pressure?
 
Last edited:
Originally Posted By: CapriRacer
There used to be a link to an engineering analysis done by the National Highway and Traffic Safety Administration )NHTSA) on the Firestone tires on Ford Explorers.

http://www.nhtsa.dot.gov/hot/firestone/firestonereport.pdf

Unfortunately, this page has disappeared. However. I downloaded a hard copy some time ago.

It specifically highlights the same failure modes and causes as Dr. Govindee. Further. It specifically addresses the issue of inflation pressure vs GVW and fails to conclude that the tires would be underinflated at the placard conditions.


The spreadsheet data supporting the conclusions can be found at:

http://www.nhtsa.dot.gov/cars/rules/ruli...inal-NoVINs.xls

Since Ford and Firestone announced a voluntary recall of the P235/75R15 Wilderness AT tires made in North America on August 9, 2000, NHTSA's report was rendered moot.

It's worth pointing out that both Ford and Firestone vigorously maintain positions contra Dr. Govindjee's analysis in particulars (although at odds with each other), and have won some, lost some, and settled some in court.

My own impression is that had owners consistently maintained 30-32 psi inflation in the tires, the indicidence of tread separation would have been much much less.




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Rolf ....... My own impression is that had owners consistently maintained 30-32 psi inflation in the tires said:
Rolf,

Thanks for the link - most helpful.

I don't think anyone would disagree that more inflation pressure would be better.

I also think no one would disagree that a larger tire or a tire with a higher speed rating would also have been better.

But this doesn't change the fact that the placard pressure specified enough inflation pressure to carrying the max load of the vehicle - so it was not the cause of the failures - and the link you provided proves that!
Click to expand...
 
To the OP:

Try this:

What is the LOWEST expected ambient temperature for your region (that you would drive in)? Let's say 0 F.

Now, one morning, record the outside temp and adjust your tire pressure accordingly. If it is 40 outside, then adjust your tire pressures to 37 (30 recommended, +4 for temp changes, +3 for general increase).

This way, when the outside temp is 0 F, your tire pressure should read 33 psi. Your tires should never be underinflated at your coldest expected temp. From there, go up or down 1 PSI until you get the "feel" you like, without exceeding the sidewall max or the vehicle min. Obviously you will need to maintain pressure and will eventually have to bleed some when it gets hot. I know, I know, personal experience but... I have always done my tire pressures this way and have NEVER had an issue, whether it be blow out or impact damage (knocks on wood). Just my humble opinion.
 
I consider the door placard to be the safe minimum tire pressure.
I routinely run 35psi as my starting minimum in most vehicles. My one exception is the wide low profile tires, i.e. a P275/40R17 on my previous Camaro. Going with higher pressures warped the tire more than a 205mm wide tire and wore out the center much quicker.

Now Im considering running 45psi in my Michelin's to see if it changes anything thanks to the link above to uneven tire wear. Pretty interesting!
 
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