Tire Pressure Carrying Hawk on Tundra w E-Coopers

[klahanie]

Something about these high tire pressures doesn't make sense. Educate me. Here's my thoughts:

• Stock tires, placard pressure is 35 psi in our truck.
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• Then we add a heavy camper on the back. Front axle load barely changed, 2700 lb. Rear axle was way lower, but went way up to 3300 lbs.
• 
  
• OK, leave the front pressure at 35 psi. Up the rear pressure by the ratio 3300/2700 = 43 psi. (Stock tires rated at 51 psi max)
• 
  
• Now replace the stock tires with E-rated. Why would the pressure change to such big numbers? The load is really carried by the air. And the E-rated tires are probably stiffer (likely a second order effect), so maybe even lower the pressure.
• 
  

Or replace all those complicated steps with the chalk test...

rl, please consider this as comment rather than education...

I like using the TRA (Tire and Rim Assoc) tables. That's where I found the "max loads at a given psi" figures I referenced, post #4 (doc link within) and I use them for my own tires. Good thing is, TRA is the industry standard for Can/US and the commonly available mfrs are members.

Re your calculation, note that according to the tables the math is not always linear. eg for P245/75R16 max load at 32 is 2172#. increase the psi by 28% to 41 and the stated max load is 2469# - not the expected 2780#

I picked a P metric size because that's all I found listed for the Tacoma I looked up. I didn't notice any P metric size in the TRA tables showing a load at 51 psi but I know you mentioned "stock". Point is, if changing from P metric to LT it's good to reference the tables to ensure the new starting psi is adequate. This is covered in the Guide (same doc, see pg 11).

Note also that P metrics are derated by ~10% when used on a light truck.

In the Guide example given, P metric to LT meant going from 35 psi to 45 psi. So, again, good to watch the math. [ON EDIT so both the psi and the stiffness of the tire construction increases. Oh, well...]

Finally, when shopping for different tire sizes the tables are handy. I was able to find a size that is rated for ~10% more load at a given psi than the OEM size. So for a slightly larger size, that still fits on the OE rim, I can run the replacement tires at a ~10% lower psi. Which for me, is a very good thing.

As for the high pressures and making sense, I've already said my bit. What hasn't been mentioned is uneven tire wear tire service longevity - if dollars and cents matter.

 

[rubberlegs]

Hi klahanie,

I'm starting to get it. Your Toyo tire pressure link has a paragraph that explains one thing:

WARNING! Please note that size-for-size, LT-metric tires require higher air pressures to carry equivalent loads of P-metric tires and that any failure to adjust air pressure to achieve the vehicle’s load requirements will result in tire fatigue and eventual tire failure due to excessive heat build-up. Due to the higher PSI requirements of LT-metric tires they may not be suitable for replacing O.E. P-metric tires because of the ride harshness that results from higher PSI requirements.

I'm working through the tables to figure out the tire pressure in my situation, and thanks for pointing me in the right direction.

 

[rubberlegs]

After much study of my Tacoma situation, here's some conclusions about the existing tires.

Our stock tires are P265/70R16 Goodyear Wrangler All Terrain Adventure with Kevlar.
Placard pressure is 30 psi (cold) for all tires.

Weight of axles when truck bed is empty, 2 passengers, full fuel tank:
2700 lb front, 2000 lb rear.
Weight of axles for a long trip with camper, full tank:
2800 lb front, 3450 lb rear.
GAWR:
2910 lb front, 3280 lb rear.
GVWR:
5600 lb

So the front weight was barely affected. Rear is overloaded by 5%, vehicle overweight by 12%. (Let's not talk about the overweight vehicle!)

Can the existing tires handle it? Per something I found on trucktrend.com (which I can't find again), you should have a 20% safety margin for a constant working load. Also P tires should be derated by dividing the max load by 1.1 for light trucks.

In our situation, the tires have a 112 load index (2469 lb at 35 psi cold pressure). Derate to 2068 lbs (divide by 1.1). 35 psi is the max pressure in the tables. https://www.toyotires.com/media/2125/application_of_load_inflation_tables_20170203.pdf

Safety margin: 2068 / (3450/2) = 130%, so on pavement the tires are ok since more than 120%.

My conclusion for these tires is we should use 30 psi for the front tires, 35 psi for the rear tires (max recommended). 35 might be a bit low since the max pressure on the tires is 51 psi, and we are overloading them... still thinking about that. (I've been using 38 front, 40 rear, based on guessing but no science.)

The 30 psi for the P tires corresponds to 42 psi for LT tires in this case. 35 psi corresponds to 54 psi. So when we get LT tires we'll up the pressure 42 front, and something around 50-54 rear to get the same load carrying capacity. The ride should be a lot harsher.

 

[klahanie]

RL, sounds good. Math is fun !

First off, well done on limiting your loaded camper weight. 1550 is impressive (to this pig anyway !)

Interesting that the placard says 30psi for rear which, by my math works out to a max load equivalent to ~125% of the RGAWR. So your idea of + 20% "safety margin" seems to be shared. And I'd agree, there should be something...

One thing with pick ups, obviously the rear load can vary considerably. Full size PUs are famous for having a "worst case" placard with a psi ridiculous high for driving around empty. But, as mentioned earlier in this thread the inflation psi listed in the tables are for max load at a given psi, nothing saying you can't go higher psi for the same or even lower load, which is what the mfr often does on the placard.

also...

35 might be a bit low since the max pressure on the tires is 51 psi, and we are overloading them...
actually you're not. your loaded camper weight is still well under the max for the tire. just need to watch the psi, and you've got that covered.

The 30 psi for the P tires corresponds to 42 psi for LT tires in this case. 35 psi corresponds to 54 psi. So when we get LT tires we'll up the pressure 42 front, and something around 50-54 rear to get the same load carrying capacity.
if same size, just mind that different sizes have different ratings

The ride should be a lot harsher.
why I like to go as low a pressure as I am comfortable with

Now, would be interesting to compare your calculations to a chalk test, something I'll have to do one day

 

[rubberlegs]

Hi klahanie,

Yup, calculations can be fun. I spent a few years designing landing gear mechanisms, but never worked the tires. It's a pretty complex subject.

Weight was always a big deal working aircraft, and for my former passion of backpacking and current cycling. So when we got the Fleet I pushed my wife to keep the weight down. We've pulled out several things we don't use, like one of the propane tanks, the bed extension and pads, and try to keep the water tank as empty as we can. We didn't get a lot of options, sticking to just fridge, table, heater and cold water. Even the truck is a short cab to save 200 lb (with bonus of shorter which is better off road and parking). The short cab is difficult when guests join us though -- very squished back there! But we are short.

I spent a lot of time adding up weights while waiting for the camper, and convinced myself we'd be 200 lb over, worst case. On our first long trip we were 600 lb over. Oops.

Anyway, I agree on your 125% sizing for the tire pressure. Good point that I'd not considered.

On 35 psi, the tables stop there, so it's extrapolating going to higher pressures. But then the whole rig is extrapolated to overweight, so who'm I to talk? Maybe I'll stick with 38 psi rear, but lower front.

I convinced myself that the kevlar tires were good 'nuff for the small percentage of time we spend on the really rough roads. It's 1000 miles to southern CA where the playgrounds are, and maybe 50 miles on the bumpy stuff. Then another 1000 miles home. It all made great sense but we punctured past Cerro Gordo -- sidewall. Maybe E rated tire would also have punctured too but I'm leaning towards spending more bucks on those tires. Then again I prefer hiking in running shoes while many others think they need mountaineering boots... then again it's easier to see a nail while walking than looking over the gigantic hood on these modern "small" trucks.

We aired down to about 25 psi, but maybe should have been lower like everyone says. 25 is barely below 30. The bead shouldn't pop off. But I was incrementally lowering pressure to see how it worked. Can you tell we are newbies in "off road" travel?

 

[smlobx]

I have to thank Wallowa for starting this thread.

It is one of the most informative subjects I’ve read on WTW!

Maybe a sticky??

 

[rubberlegs]

I'm trying a new approach to decide on tire pressure for front and back for P and E tires.

I took the Toyo tables mentioned several times in this thread. For the two tires (stock P and C/D/E-rated) the numbers are pretty linear.

Then I tried to reverse-engineer Toyota's pressure for my Tacoma. The placard is 30 psi. With 2 passengers and full fuel, nothing in bed, the front axle weight is 2700 lb. Front GAWR is 2910.

What weight did they use to select the placard pressure? I figure Toyota must think typical use is an empty truck much of the time and heavier truck some of the time, so they pick 30 as an average. I decided to scale the tables so 2800 lbs was at 30 psi, and got a safety factor of 148% for the stock P tires.

I kinda like the 2800 lb for front, since GVWR is 5600 lb, so that leaves another 2800 lb in the rear (2900 for us), which is pretty close and maybe what Toyota was thinking for the rear axle. OK, I'm rationalizing now because the rear GAWR is 3280... but Toyota recommends the same pressure for all four tires. I'm not sure what they were thinking with that recommendation. They are probably using the KISS principle: keep it simple, stupid.

So the graph below scales the Toyo tables dividing by 148% for the stock tires -- resulting is 30 psi at 2800 lb. I reduced the safety factor by the 1.1 factor for the E-rated tires, or 135%. I think this is the proper way to compare apples with apples according to the Toyo document.

The tables stop at 35 psi for the P tires, but the tires max pressure is 51 psi. One could argue that going above 35 psi is ok if the tires are heavily loaded. So the graph extrapolates -- in my case at 3450 lbs rear axle, I need 44 psi. This seems kinda high so I'd pick something like 40 psi. So 30 front, 40 rear is today's estimate for the P tires, and 42/56 for E tires. Subject to change without notice.

I need to find chalk somewhere...

 

[klahanie]

RL,

I'll want to give future thought as to what constitutes a "safety factor"...

I'm not suggesting 125% of required load per se, not that that matters to me here - we are just kicking around ideas.

For the mfr tire placard (TIP) I imagine the vehicle mfr may want to ensure the tires are inflated sufficiently to cover the GAWRs. As referred to earlier in this thread, there have been deaths caused in part by under inflation (I don't recall all the details) but for safety I can see the mfr posting a max axle rating and a psi to match. If your load is lower, not their problem. Not sure they'd take an average.

Why the front and rear TIP pressures are the same on your vehicle, IDK. Maybe the front tire psi is for weight transfer when braking ? But the GAWRs are so close and 30 psi so low (to me) I'd prob stick to that for the front.

Re: "safety factor", I wonder... if a tire that having a max load rating higher than the intended application load, - say 50% higher eg 2250max, 1500 load - gives the same "safety factor" as having a tire inflated to a "table rating" to the same, in this eg 150% of the load.

The same tire might spec to both, but the psi would be higher in the second instance.

Is it not the stronger construction properties, for heavier load capacity and required to contain the higher air pressures of some LT tires, that we desire in an off highway tire. Are these not still present when the tire is inflated "only" to the psi required for the actual load.

Presumably we'd like a stronger sidewall and tread for improved puncture resistance. Would that be already inherent in a higher load replacement tire, without adding an "extra" inflation factor ?

At least some of the lighter GVWR trucks have the upgrade option. I know for some 3/4 and 1 tons the RGAWR is limited by the OEM tire's max load rating. Sometimes replacement, regular LT tires with a substantially greater max load rating are not available without going commercial or military.

I squeezed out ~10% more for my rig, but choose to drop the psi rather than pocket the higher load capacity. Still like to think I gained some toughness tho

One thing I didn't appreciate, first look, at the TRA spec for the P265/70R16, is how the max load increase drops off after 35psi. Maybe the TRA spec ends at 35. Goodyear states 2469 lbs at 51 psi. I note, according to the table (and table lists as Load Index 111, fwiw):

26 to 29, +3 psi, +111 lbs
32 to 35, +3psi, +77 lbs
35 to 51 (mfr max), + 16psi, + 66 lbs

= better you than me to calculate 35 - 51

 

[trikebubble]

I have 285/18" Nitto Ridge Grapplers on my Tundra. I run them at 55psi on road/highway. I'll lower that to around 35ish on forestry roads, and then down to 25-30 for more "extreme" off-roading.

 

[rubberlegs]

Since I'm retired and have too much time on my hands, I continue to surf the web trying to learn more. The more I look, the more misinformation I find. Some of it is almost entertaining! However, I did find one thing on another forum that was interesting, from Goodyear:


http://download1476.mediafire.com/v...vnt/Goodyear_Tire_Inflation___Load_Charts.pdf

Here's the interesting part of the document:

Is it not the stronger construction properties, for heavier load capacity and required to contain the higher air pressures of some LT tires, that we desire in an off highway tire. Are these not still present when the tire is inflated "only" to the psi required for the actual load.

Presumably we'd like a stronger sidewall and tread for improved puncture resistance. Would that be already inherent in a higher load replacement tire, without adding an "extra" inflation factor ?

Yeah, I'm still trying to wrap my head around LT tires needing higher pressure. Surely the construction is different. Maybe it's more than just adding plies. Maybe extra plies rub against each other, so you need more pressure to reduce the heat. But then heat kinda helps mitigate the problem by raising the pressure, self-correcting to some extent. Obviously not enough. Maybe the tires are thicker, which insulates and doesn't let the heat out. I can't find anything on line that explains it.

One thing I didn't appreciate, first look, at the TRA spec for the P265/70R16, is how the max load increase drops off after 35psi. Maybe the TRA spec ends at 35. Goodyear states 2469 lbs at 51 psi. I note, according to the table (and table lists as Load Index 111, fwiw):

My tires say 112, so Goodyear must be different than Toyo for some reason. So I used a nearby 112 row of numbers. It ain't quite the same, and in fact not a lot different than 111, but for some reason I rationalized one row over another. I'm weighing the truck at local rest stops that are probably off by more than that.

 

[klahanie]

I want to reconsider my first reply to the OP ...

To recap, I suggest 2 methods, or a blend there of, and the applied results to be monitored with a tire gauge [ON EDIT: tire tread depth gauge] :

1) match the vehicle placard values per the doc linked in this thread, as prev posted. Likely the placard psi posted for the OE size will calculate out to a higher value than the respective GAWR. This overage I'll call the load reserve. This seems to vary quite a bit for different vehicles. In rubberleg's case it works out to 141% FR / 125% RR (my calc from data given). For my truck it is 107% FR / 103% RR. The auto mfrs have their reasons for this - I don't pretend to know them all. (fwiw tho, I'm not certain that the construction and target application of a LT tire would motivate the same reserve allowance as a P metric might.)

If the vehicle axle weights are within the GAWRs then the equivalent mfr load reserve is included in the matched psi for the new size/type, per doc. If actual wt is higher, increase the look up wt by the load reserve % and find a new psi using the tables.

2) use your actual axle wts to find min required psi for your size, per the tables. This is minimum psi, can add more as desired, eg. perhaps 4 psi for highway. NOTE: after some reading I believe 35psi is the min suggested for LT tires.

Long story short - by my guess - this could put the OP at 35-49FR / 42-49RR. A broader and higher range than my first post implied. YMMV

Final Word. There lot's of info online. This comment I found interesting, proving me wrong once again. source: Nitto TB

"LT tires do not offer any benefits of being "heavy duty" when under-inflated."

 

[rubberlegs]

That last link you posted is really informative, thanks.

This one, however, indicates that if you drive slower, you can carry more load at low pressures. It seems to be all about temperature.

http://download1476....Load_Charts.pdf

I'm not sure how this applies off road yet. Anecdotal stories say that E rated tires are tougher off road, presumably because of more or thicker plies. This is at low speed and sometimes at pressures much, much less than the tables. Today at our FWC dealer, one of the techs said they go as low as 6 psi on snow (and don't seem to pop the bead). He recommended 12-15 psi front and 20 rear after eyeballing our truck. Instead of sidewall life, it seems to be about puncture resistance.

 

[Wallowa]

OP here. First I do appreciate and respect the opinions put forth in this thread.

To let you know why I have not been commenting on these posts is that I limit my internet time to one day a week, Friday. This is my only “social media” conduit and I find that the internet by and large is for me a time and energy killing event.

My original inquiry was for first hand experiences of those using “E” tires of the size and hopefully brand of tire in that post. Specifically what pressures for a comparable load off road do you use as a standard starting tire pressure with “E” rated tires.

Being a vintage person, old, my background leads me to trust more in experiential information that theoretical. In the protracted process of “higher education” degrees I too have engaged in mathematical gymnastics but still trust more in real world events and outcomes. Nothing is so simple that it can’t be made more complex. Try OODA to unravel knotty questions.

I will close with this advice; if possible and if you are inclined read the excellent books on off road preparations and techniques by Chris Scott or Tom Sheppard. I have found their information has been spot on and of great value for my off road adventures both on my dirt bikes and Tundra/Hawk.

FYI…higher tire pressures do help prevent, not eliminate, punctures for several reasons. And for my off road use and load the tire pressures listed on the vehicle door plate are NA.

Again thanks for all the input, it has been of value and appreciated. And of course I could be wrong.

[SIZE=11pt]Phil[/SIZE]

 

[Advmoto18]

OP here. First I do appreciate and respect the opinions put forth in this thread.

To let you know why I have not been commenting on these posts is that I limit my internet time to one day a week, Friday. This is my only “social media” conduit and I find that the internet by and large is for me a time and energy killing event.

My original inquiry was for first hand experiences of those using “E” tires of the size and hopefully brand of tire in that post. Specifically what pressures for a comparable load off road do you use as a standard starting tire pressure with “E” rated tires.

Being a vintage person, old, my background leads me to trust more in experiential information that theoretical. In the protracted process of “higher education” degrees I too have engaged in mathematical gymnastics but still trust more in real world events and outcomes. Nothing is so simple that it can’t be made more complex. Try OODA to unravel knotty questions.

I will close with this advice; if possible and if you are inclined read the excellent books on off road preparations and techniques by Chris Scott or Tom Sheppard. I have found their information has been spot on and of great value for my off road adventures both on my dirt bikes and Tundra/Hawk.

FYI…higher tire pressures do help prevent, not eliminate, punctures for several reasons. And for my off road use and load the tire pressures listed on the vehicle door plate are NA.

Again thanks for all the input, it has been of value and appreciated. And of course I could be wrong.

[SIZE=11pt]Phil[/SIZE]

Phil

Actually, for off-road, lower air pressure vastly reduces the potential for tread punctures on modern tires due to tread conforming to obstacles. Great article here. Now this is a jeep related article, but, the optimum psi principle remains the same for our rigs regardless.

 

[Wallowa]

Phil

Actually, for off-road, lower air pressure vastly reduces the potential for tread punctures on modern tires due to tread conforming to obstacles. Great article here. Now this is a jeep related article, but, the optimum psi principle remains the same for our rigs regardless.

No belly bumping from me...your opinion is valid for you...

But higher pressures resist punctures by rolling over the object that could puncture the tire while low pressure conforms to the object and focuses the point of contact into the rubber... and higher pressure = less side wall flexing, and flexing generates higher tire temps that softens rubber which in turn makes the tire more prone to penetration...speed and load make flexing and heat more critical.

Again many mall pundits....read Scott and Sheppard...both have decades of off road experiences under extreme conditions....again take away what you wish but the internet is full of experts, at least so they say...

Phil

 

[Advmoto18]

No belly bumping from me...your opinion is valid for you...

But higher pressures resist punctures by rolling over the object that could puncture the tire while low pressure conforms to the object and focuses the point of contact into the rubber... and higher pressure = less side wall flexing, and flexing generates higher tire temps that softens rubber which in turn makes the tire more prone to penetration...speed and load make flexing and heat more critical.

Again many mall pundits....read Scott and Sheppard...both have decades of off road experiences under extreme conditions....again take away what you wish but the internet is full of experts, at least so they say...

Phil

Check!
Certainly no belly bumping from me intended or desired!

I've been part of the 4 wheeling and off-road jeep/truck scene since 1971. Started out truck mud racing and finally got my fill jeep rock crawling in Uwharrie NF.

In all the decades, I have had 2 through the tread punctures as a result of rocks while off-road, both in the late 70s (that I can recall). I don't recall the tire brand but most likely a Cepek as it was the "hot" tire to own back then. I've damaged more than a few sidewalls but that was due to a bad line. All the while running very low psi in the mud and rock. And running about 15-20 psi during off-tarmac poker runs.

Back in the 70s and 80s so called off-road tires were very heavy, stiff, and did not tolerate mistakes by the driver. Modern, off-road radial tires are far more flexible and forgiving, and puncture resistant. Yet, they still remain susceptible to damage if the driver isn't paying attention.

Incidentally, I've had far more flats as a result of hitting unseen debris and/or through the tread punctures on highways, operating at highway speeds and psi then in any other type of encounter.

So I totally agree, each to his own! Find a psi that works for you given the terrain or road composition/condition.

Cheers!

 

[rubberlegs]

Great article here.

Another interesting article. With our truck (Tacoma) with front axle 2800, rear axle 3460 lbs:

Stock P tires: street pressure 35/43 psi, can go down to 12/15 psi for off road. (compare to 30/40 for another method).

LT tires: street pressure 42/52 psi, can go down to 15/18 psi rear (compare to 42/56).