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 > Looks like 30psi ought to be plenty good for towing

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lawrosa

Horry County

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Posted: 06/23/18 08:39pm Link  |  Quote  |  Print  |  Notify Moderator

mike-s wrote:

P275/55*20 is good for 2216 lbs per tire at 29 psi. So, about 4500 lb on the rear axle @30 psi. What's the RAWR? If less than about 4500 lbs, more pressure won't get you more capacity. And, I'll bet it's right around 4500 lbs - manufacturers spec tires and pressures to support the ratings they give. If the sidewall max pressure allows it, I might run an extra 5 psi when towing, but not more for treadlife.


I believe a 117 tire is rated @ 2833 lbs. Derate that 9% for p tires and you get 2578 lbs per tire.

And constant working load usually another 20% per the manufacturer's. So that equates 2062 lbs per tire.

Whats your axle rate? 4000 lbs? Ahhhh.. Yup

To the OP. I would run at 40psi. Heat from the weight will be the issue. Higher pressure less heat.

Ive never ran the door tags psi in any vehicle I have ever owned. Always higher.


Now just talking here...

And from what I know a tires speed rating is calculated at max side wall pressure because of heat. So a v rating ( your tire) is 149mph. At that speed you would want 50 psi in the tires.

overinflated tires are safer than underinflated ones.


Mike L ... N.J.

2006 Silverado ext cab long bed. 3:42 rear. LM7 5.3 motor. 300 hp 350 ft lbs torgue @ 4000 rpms
2018 coachmen Catalina sbx 261bh


HadEnough

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Posted: 06/23/18 08:44pm Link  |  Quote  |  Print  |  Notify Moderator

mike-s wrote:

HadEnough wrote:

It's simple. Ignore everything except the max pressure on the tire sidewall.

Put the load on and look at the tires. The entire part of the tire that makes contact with the road (the tread profile) should be making contact. If it's bulging at the sidewall and the tread is only making contact at the edge, you are underinflated for the load. If the center of the tread profile is making contact, but the edges aren't, you have too much pressure.

Tire inflation is only a matter of getting the tire shape correct. Put as much air in as you need to get the shape correct, without exceeding the maximum pressure shown on the tire sidewall.

See these pictures for a visualization.
Well, no, judging by eye is probably the worst thing you can do. Are you really claiming that you've ever seen a tire which is making contact only at the edges (excepting one with zero pressure), or that you've seen the edges not making contact without it being vastly overinflated? There's a wide range where you have full contact, but the pressure will be wrong.

Use the manufacturer's recommendations, followed by the load tables. To figure it out independently, you need to travel a while under load, then quickly and accurately measure the tread temperature across the width, like the racers do.


Well, yes.

I've seen a ton of tires worn only at the edges or only in the middle from under/over inflation.

The exact pressure inside the "wide range" of pressures where the tire makes full contact is not at all important. It's a personal preference. I like mine on the harder, more inflated side of this vast range of pressures. I don't mind a bumpy ride and prefer better fuel economy. Others like a softer ride and will go with a lower pressure.

As a rule, the pressure should be equal for all tires on a given axle though, so it's not all touchy feely. You still need a gauge to make sure you select the same pressure on both sides of the axles.

mike-s

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Posted: 06/23/18 09:39pm Link  |  Quote  |  Print  |  Notify Moderator

HadEnough wrote:

Well, yes.
Keep digging deeper, some day you'll get to China.
HadEnough wrote:

I've seen a ton of tires worn only at the edges or only in the middle from under/over inflation.
That's completely different than claiming one can tell the proper inflation by simply looking at how the tire tread contacts the ground. What's your point?
HadEnough wrote:


The exact pressure inside the "wide range" of pressures where the tire makes full contact is not at all important.
Please stop providing completely incorrect advice - it doesn't help others, and just makes you look bad.

Your advice is demonstrably wrong, as anyone can tell. Underinflate by 20 psi - no, the center tread doesn't lift off the ground. Overinflate by 20 - no, the edges don't lift. But either will cause uneven wear. It's the pressure across the tread which determines wear, and you can't tell that by simply looking at the tire.

owenssailor

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Posted: 06/23/18 09:54pm Link  |  Quote  |  Print  |  Notify Moderator

The door sticker on my 2017 Silverado says 32 front 38 rear. When towing I run 40 front and 44 rear (44 is max). I feel this gives less squirm and less heat buildup.

When not towing I run 3 PSI above door sticker on all my vehicles. I fine handling is better as is tire wear.


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chrispitude

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Posted: 06/24/18 04:25am Link  |  Quote  |  Print  |  Notify Moderator

BurbMan wrote:

It should be noted that the door sticker says that the tires will carry the GVWR at 30 psi. You shouldn't need to inflate past that unless you are exceeding the GVWR, right?


Per the manual, this is the minimum air pressure required for the tire's load rating to match the axle's load rating. Per the TRA tables, additional air pressure would add margin to the tire's capacity (but not the axle's, of course).


mike-s wrote:

Use the manufacturer's recommendations, followed by the load tables. To figure it out independently, you need to travel a while under load, then quickly and accurately measure the tread temperature across the width, like the racers do.


You remind me that I do have a FLIR IR camera... The challenge will be in doing highway travelling, then finding a place to safely come to a stop and immediately hop out to shoot the tires before the heat dissipates or equalizes. Still, there might be enough benefit (and knowledge) to be gained here to try it.


lawrosa wrote:

I believe a 117 tire is rated @ 2833 lbs. Derate that 9% for P tires and you get 2578 lbs per tire.

And constant working load usually another 20% per the manufacturer's. So that equates to 2062 lbs per tire.

Whats your axle rate? 4000 lbs? Ahhhh.. Yup

To the OP. I would run at 40psi. Heat from the weight will be the issue. Higher pressure less heat.

Ive never ran the door tags psi in any vehicle I have ever owned. Always higher.


Wow, that's some nice math-ing you did there! Coincidentally, I set the pressures to 40psi all around to bring the trailer home, exactly for the reasons you describe.


owenssailor wrote:

When not towing I run 3 PSI above door sticker on all my vehicles. I feel handling is better as is tire wear.


This is what I normally do too (when not towing). I am also careful to do it in the early morning, so that the sun hasn't asymmetrically warmed the tires and the temperature is nominally low.

chrispitude

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Posted: 06/24/18 04:32am Link  |  Quote  |  Print  |  Notify Moderator

Everyone, there has been some outstanding analysis and discussion on this thread. Thank you!!

So we know that 30psi is the minimum to meet the rating. I prefer higher pressure for (1) more tire (not axle) load capacity margin and (2) less sidewall squirm, but not so much as to get poor steering or abnormal tire wear.

The TRA table for P275/55R20 goes up to 35psi, but that's for a 111-rated tire. My 117-rated tire has a sidewall max rating of 50psi, which is clearly too high.

So I think where this puts me is somewhere around 35psi to 40psi. Any further speculation will be of the armchair variety, and only experimentation (and perhaps IR measurement??) will yield further insight.

But further speculation is still appreciated. [emoticon] It's easier to come up with experiments while sitting in my La-Z-Boy versus dealing with a TV/TT in highway traffic.

StirCrazy

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Posted: 06/24/18 06:41am Link  |  Quote  |  Print  |  Notify Moderator

E-mail the tire manufacture and ask for the load pressure chart for that exact tire. they all have them and will send it out if asked. then just stop at a weigh scale and see what the weight is when you are loaded for bear and use the chart to determine how much air you need in the tire.

Steve


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chrispitude

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Posted: 06/24/18 07:19am Link  |  Quote  |  Print  |  Notify Moderator

StirCrazy wrote:

E-mail the tire manufacture and ask for the load pressure chart for that exact tire. they all have them and will send it out if asked. then just stop at a weigh scale and see what the weight is when you are loaded for bear and use the chart to determine how much air you need in the tire.


Great idea! Message sent.

TurnThePage

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Posted: 06/24/18 10:43am Link  |  Quote  |  Print  |  Notify Moderator

I've been towing with several different light duty pickups and various ply tires over the last 20 years. My experience is that towing with anything below 40 PSI results in less control. I always max out the pressure on my P rated tires for towing. XL and LT rated tires are a little different.


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bikendan

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Posted: 06/24/18 11:32am Link  |  Quote  |  Print  |  Notify Moderator

As a former tireman, the majority of bad tires i saw, were due to UNDERinflation.

I always run my tires at max inflation when towing.


Dan- Firefighter">, Shawn- Musician/Entrepreneur">, Zoe- Faithful Golden Retriever(RIP">), 2014 Ford F150 Ecoboost, 2016 PrimeTime TracerAIR 255 w/Equalizer and Prodigy, and 5 Mtn. bikes and 2 Road bikes


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