Over intlated tires - how much do you do?

[IsaacCarlson]

I run newer tires at max or max+. Older tires get the "properly inflated" treatment plus a few pounds. The tires on my trailer get about 40 lbs. They are not new and were run at about 30 lbs before I bought it. They have some cracking on the corners, so I choose to play it safe. The DOT on the tires says 174 so they might be from 04. My car tires are at 50 psi and the truck is normally at about 50.

[bluejoey]

I've had my 44 rated tires up to 75 in the front and 70 in the back a few times (by accident; I used a gas station gaugeless pump). Now I use my bike pump to check the psi, and I've gradually lowered them to 65 in the front and 60 in the back. I might let them drop to 55 front/back, just to be safe. According to the person I bought the car from, the tires were new a couple of years ago, and I can still see the threads on them. So the tires are in relatively good shape, but I don't want to have to replace them soon or deal with a blowout, so I'll probably lower the pressure more next time I check them.

[CapriRacer]

Originally Posted by IsaacCarlson

...... The DOT on the tires says 174 so they might be from 04.......

Sorry, but they are NOT 2004 tires, they are 1994 tires:


Barry's Tire Tech

But you should check to see what your trailers tires say fro maximum inflation pressure. If they are ST type tires, they are more like LT tires and operate at higher pressures.

Nevertheless, they are 16 years old, and I'd recommend you replace them.

Originally Posted by JacobAziza

The The International Tire and Rubber Association burst tests I linked to that found a minimum burst pressure of over 300psi was (mostly) of used tires, not new ones......

I hope you are aware that medium radial truck tires are inflated to over 100 psi - many are higher - but they were designed for that. As a tire engineer, it is not surpising they burst at higher pressures, but remember, a burst test is not the same as an operating a tire at that pressure.

Passenger car tires are designed to operate in the 26 to 35 psi range, with the 44 and 51 psi being a permissible inflation pressure for certain operating conditions. Truck tires ttoo can be inflated above the maximum pressure on the sidewall - but under certain operating conditons, and over-the-road is not one of them!

Originally Posted by JacobAziza

........The number is set not by the manufacturer or car maker, but by an independent organization that has permanent set standards based on tire size only......

You have to be very careful how you say that. If by "the "number" you mean inflation pressure, then you need to look at one of those standards. Here is a page from The Tire and Rim Association's 2005 Yearbook for passenger car tires.:



Notice that the inflation pressure range is 26 to 35 psi. Those are the conditions that a passenger car tire is designed to operate against. These are the conditions a tire manufacturer tests against. They do NOT test outside that range.

Originally Posted by JacobAziza

......It was set once, by law, and never altered (as far as I can gather from the DOT website) since 1948.........

Sorry, but the DOT adopts the existing tire standards whatever they might be - not the other way around.

- AND -

The modern DOT regulations date from 1968 with recent updates. But they reference back to the tire standards like the page above.

Originally Posted by JacobAziza

......I can't find any thing explaining what, if anything, the 'max' pressure ratings were originally based on - but it is clearly not anything to do with an individual tire's actual burst pressure, strength, or performance (since, as people here have pointed out, they aren't even tested at any higher pressure to see whether they preform better or last as long or are as strong).......

So here is a page that explains where those numbers come from:



What you are looking for is the last thing on the page.

I am going to stop there are let all that digest a bit as I am sure there are going to be questions.

[rmay635703]

Something that hasn't entered into this discussion is how much rolling resistance as a percentage of total resistance increases as speed decreases, obviously rolling resistance does not massively change with speed but aero does, this means that it becomes more important when you are hypermiling and driving slowly. What might only be a 2% fe improvement at 65mph could easily become 15% or more at low speeds.

My buick moving from 27mpg to 34mpg had some to do with speed but a lot to do with inflating my tires. I simply couldn't reach the 40mpg steady state without higher air pressure.

Cheers
Ryan

[JacobAziza]

Originally Posted by CapriRacer

I hope you are aware that medium radial truck tires are inflated to over 100 psi

I know the 400psi number was referring to truck tires. I also posted a source suggesting 200psi minimum burst pressure for passenger car tires. Either way about 4 times the listed maximum. And I understand that running conditions can increase pressure, so you would never want to get close to 200psi. What I am asking is what possible downside (other than a harsh ride) is there to running at the max listed on the sidewall.

Originally Posted by CapriRacer

Sorry, but the DOT adopts the existing tire standards whatever they might be - not the other way around.

Its hard to read legislative text, but the way I read this, it sure looks like TRA sets the max inflation standard, not DOT: Part 571.139: Standard No. 139; New pneumatic radial tires for light vehicles. - Federal Motor Carrier Safety Administration

Originally Posted by CapriRacer

The modern DOT regulations date from 1968 with recent updates. But they reference back to the tire standards like the page above.

So here is a page that explains where those numbers come from:
What you are looking for is the last thing on the page.

ok, 1968.

I don't think anything you wrote really contradicts what I wrote.
That page doesn't explain what those numbers are based on, it just lists them.
Where did the Tire and Rim association come up with them from in the first place?

I realize cars are only tested at the recommended pressure. Cars are also tested without grill blocks or underbelly panels or boattails or power steering deletes. The whole point of this website is that we as owners don't always agree with the manufacturers decision when it comes to the balance of comfort vs efficiency.
And what I am saying it that the doorjam recommended pressure is based on ride comfort.

Every source I can find says that low pressure causes a myriad of safety concerns, while there are no substantiated safety concerns of running at the listed maximum.
You say the max is meant "only for certain conditions" but where are you getting that information from? If running at max pressure was actually dangerous on anything but the track as you are implying, wouldn't we expect to see a warning in the tire manufacturers literature?

Like you said, they don't test the tires above the listed pressure (they do test them below, they are required to, to simulate real world conditions), so on what basis can anyone say that going above doesn't improve performance, safety, longevity, or efficiency?

Given that: Tire wear, braking, handling, hydroplaning, and potential for overheat induced tire damage and blow outs, all get worse the lower pressure is below the pressure listed on the tire sidewall, and all improve as it rises back toward it, it seems there should be some concrete and specific concern before choosing to inflate them to anything less.

[BamZipPow]

One of the things I found out with running max pressures on my LT tires were that when it came time to unmount them from the rim, the bead ended up tearing from the tires. I never had them unmounted from when I bought them brand new 3 years ago. Something to think about if you git a flat and they have to patch it...you might end up buying a new tire.

[redpoint5]

Originally Posted by JacobAziza

I don't remember who it was, or what the post topic was, but somebody on this forum once claimed that high tire pressure does not in fact adversely affect braking distance (on a smooth dry surface) as is commonly assumed.

I only assumed based on my experience with bike tires. Now that you mention it, I'm not surprised if braking distance is not negatively impacted.

Originally Posted by BamZipPow

One of the things I found out with running max pressures on my LT tires were that when it came time to unmount them from the rim, the bead ended up tearing from the tires. I never had them unmounted from when I bought them brand new 3 years ago. Something to think about if you git a flat and they have to patch it...you might end up buying a new tire.

I'm not buying a new tire if they destroy it while I am within the pressure ratings. I'd probably lower the pressure before I came in anyways, just to give them less to lecture me about.

[cfg83]

redpoint5 -

Originally Posted by redpoint5

...

I'm not buying a new tire if they destroy it while I am within the pressure ratings. I'd probably lower the pressure before I came in anyways, just to give them less to lecture me about.

That's what I do.

CarloSW2

[IsaacCarlson]

I have dismounted lots of tires that were run at 80-120 psi and never had any problems. I run my tires at decent pressure and have only noticed good results.

[brucepick]

I and my cars seem to be happy around 50 psi. I run that pressure regardless whether the tire is rated 44 or 51 psi.

I tried 45 psi on my Winterforce snows on the Volvo (those tires rated 35 psi, if I recall). That didn't work out; the ride really felt too hard. I dropped them to 38 psi and OK. I only drive it occasionally so even if they are better at 40-45 psi, 38 will work for me on those snows.