New Tires For Big Blue

[Nothing Special]

I don't disagree with anything Jim is saying regarding driving on roads.  But load range E tires are about the only thing you can find in a lot of the bigger mud tires, and that's what a lot of serious 'wheelers are running around 4 psi on slow speed trails.  I'm not saying they'll get 80,000 miles from those same tires if they air them up properly for street use, I'm sure the 'wheeling is hurting the life significantly.  But a lot of them do street-drive the tires too.  And looking at the tires of a lot of YouTubers, it doesn't look like they typically need to replace them long before the tread is worn out.  Really low pressure at trail speeds does not seem to be damaging the carcass significantly.  And the tires really don't build up any heat because they are adding it so slowly that it can dissipate rather than make the tire get hot.

Also I don't think 80 psi is the right base pressure to use for the rear tires.  True, that is what Ford recommends for my '97 F-250HD.  But that's for all use, including loading it to its 8800 lb GVWR.  Take 2200 lbs off the rear end and does it still need that much pressure?  Ford doesn't seem to think so because they recommend only 55 psi for the load range E front tires, even if it is at the GVWR.  The rear end weighs less than the front empty, so there's no need for 80 psi as a baseline in an empty truck.

That said, I think Gary's 35 psi might be on the low side.  I've run 40 psi in my rear tires before, but I find I'm more comfortable running them at 50 psi.

And no, I don't have any personal experience running load range E tires at really low pressures.  I did have 235/85-16 load range E BFG M/Ts on my CJ-5 when I had that.  I ran them at 30 psi on the street and 15 psi on the trails.  They were too stiff at 15, but that was as low as I dared go to avoid losing a bead (I probably could've gone a lot lower).  I put about 20K miles on those tires on the Jeep over about 10 years, then moved them to my '95 F-150 where they saw another 30K miles over another 3 years.  They were nearly bald with no cracks when I replaced them (one of them was still the spare on my Bronco about 10 years later).

[ArdWrknTrk]

Bob, I'm not saying go by the door sticker.
I'm saying that the links I posted above are talking about the tire pressure recommended by the manufacturer (on the side of the tire)
55 psi for a load range E tire is right at the 35% reduction those links call out (and also what I run in front)

I had a tire shop send me out the door at 35 psi once and I near crapped myself by the time I got home.
This truck felt like it was swaying around like a dingy.

Yes, choices are very limited in 16", much like 16.5 were twenty years ago.
I wish there were options with lighter carcasses.

[Gary Lewis]

I agree with both of you.  And disagree as well.

First, I don't think it is fair to say that the tire manufacturer is recommending 80 psi for all loads.  I read the 80 psi to be at 3750 pounds.

In fact, Blue's tires say "Max load 2535 pounds at 51 psi".  So the manufacturer isn't saying to always run them at 51 psi, but that when you load them up to 2535 pounds you need to put 51 psi in them.  And the sticker on the door says to run them at 35 psi, so apparently Ford has factored in the weight of the truck and determined that 35 psi is what is needed.  (By the way, I disagree with that as that doesn't account for heavy loads or trailers, so I put 50 psi in them when we pulled the 25' Sea Ray to Lake Powell and back.)

On the other hand, I've probably been underinflating the tires at 35 psi on Big Blue for highway use, so do plan to increase that.  But I do expect to air down quite a bit for rough going.  How much remains to be seen as 15 psi seemed a bit low to me when we were in CO last fall.  So that's going to take some experimentation.  That 460/D60/dual batteries/winch combo seems to be a bit heavy up front.  But when we load up for overlanding we'll also have some weight in the rear.  We shall see.  

[ArdWrknTrk]

I certainly don't think that pickup rear tires need to be inflated to 80 if the vehicle is not maxed out.
I have about #1,000 in the bed on a normal day, but if I'm going to abuse the helpers with a load of wet sand or a couple of one ton pallets I will make sure to air up.

Then again I would say that Big Blue is severely overloaded in the front if it weren't for the fact that his suspension is heavily updated.

I'm not sure where you see 35 psi recommended on the door tag.
Maybe there are BIG differences between '85 & '87?  

But I would like to link this:. https://www.yournexttire.com/load-inflation-chart/

Which (in part) says this. "Example: At 80psi, a 265/75R16 10ply tire will carry 3,085 pounds of load.
 
At 60 psi on a 10ply tire, the tire's load carrying capacity is reduced to 2,314 pounds per tire.
 
If the customer runs 65 psi in a 10 ply tire, his capacity is now able to carry 2,507 per tire.
 
Tire Weight / Tire Pressure = Load Capacity Pounds per PSI
 
Here is this formula using a 265/75R16 E with 60 psi in the tire;
 
3,085 / 80 psi = 38.57 pounds of carrying capacity per PSI.
 
38.57lb X 60psi = 2,314 load carrying capacity
 
So, to wrap this up, a 265/75R16 6ply tire inflated to 50psi, will not only hold more carrying capacity (2,470lb) than a 10ply at 60psi (2,314lb), it will also ride better."  (Emphasis mine)

38.57 x 15psi = 578.55 pounds per tire!
This is DEFINITELY exceeded in the front of Big Blue even if he is not bouncing over some potholed trail.

38.57 x 35psi = 1349.95 lbs per tire.
Which x 4 = 5,399.8 lbs (evenly divided)
IDK how much Big Blue weighs with two people, full tanks and some camping supplies.
While perhaps acceptable it is certainly not optimal and (again) the front tires are definitely overloaded at 35..

[Rusty_S85]

Gary Lewis wrote

I do understand what you are saying.  Running a tire rated for a max of 80 psi at 35 psi daily is hard on it, and then airing it down to 15 psi is REALLY bad.  But since I want a 33" tire on a 16" rim and the only ones appear to be E rated, I'm kinda stuck.

So perhaps I need to increase the pressure I'm running on the highway a bit, although I still want the tire to be running flat on the pavement.  And maybe not go down to 15 on the trails.

The takethetruck site said:

General off-road driving on rough dirt roads, two-track, and mild trails - a 25% reduction or 10 PSI is a good place to start

Traversing slickrock or more challenging trails with loose terrain, a 30-35% pressure reduction may be in order

Very soft terrain conditions such as sand or mud may benefit from up to a 50% pressure reduction

So for the trails Bret and I are going on in New Mexico, which are typically forest service roads, maybe drop to 30 psi if we are running 40 on the highway.  And then on the really loose or rocky stuff down to 20.  And finally, if we get the truck into really soft stuff like sand or mud we could go down to 15.  But we wouldn't plan on going over maybe 20 MPH at anything south of 30 psi.

Does that sound like a reasonable plan?

Have to remember the tire pressure on the side is the max pressure that tire is rated to.  Running the tire at say 32 psi with the rating on the tire of 70 psi will not cause an issue.  Too many confuse the pressure on the side wall to be what the tire was designed to operate at but its not.  You can run any pressure you want up to what is on the side wall.

So lets say you go to a 33" tall tire for example.  you could run the 80 psi the sidewall says or you could run what your trucks door tag says to run.  After all pressure is pressure.

Like my 31" KO2s, they say on the sidewall I think its 50 psi.  I run 35 psi front and rear like the door tag says.  It doesnt matter if I go with a taller tire or a higher rating, the pressure should not change as the pressure is designed for supporting the weight of the truck.  The only thing that will change is how much air it takes before you hit said pressure.  In my case 35 psi front and rear the tires sit nice and square and have a nice soft ride.  Filling to the pressure on the sidewall like the ignorant do at discount, the tires are so hard there is no give and you rely 100% on the suspension to dampen the road shock.  Like wise for a truck with no load in the back there isnt enough weight and it can turn your tire into a pizza cutter like it did in my case where only the very center of the tire touched the pavement.

For off road use, many will air down their tires down into the 5 - 10 psi range depending on the tire sidewall rating.  The point of airing down the tire is to get the tire to bulge out at the bottom which helps the shoulder come into contact and deform to the terrain for better traction.  In the case of KO2s, aired up properly the shoulders never touch the pavement, in an offroad situation you air it down now the tire sags and the shoulder knobs come into play for additional grab.  I dont know how they are able to give a percentage for how much to air your tire down cause if you have a tire that has a side wall rated considerably higher than the weight of your truck then that number goes out the window.  For example my old 31x10.50-15 discount branded Pathfinder A/T tires I had a tread strip come off and lost all air pressure.  The sidewalls were so stiff due to how much higher rated the tires were for weight that I was able to slow down from highway speed to 15 mph and drove another 9 miles to the local discount to get the tire replaced.  Yes the tire was low with no air in it but the side walls were able to support the weight of the truck and only appear like it was low on air.  In that instance I would play with it and see what your tires need to get the tire to squat down to get the shoulders to become active at providing traction along with your tread.  It may not follow that percentage guide line of pressure reduction

[Nothing Special]

ArdWrknTrk wrote

....  38.57 x 15psi = 578.55 pounds per tire!
This is DEFINITELY exceeded in the front of Big Blue even if he is not bouncing over some potholed trail....

I would change that to "This is DEFINITELY exceeded in the front of Big Blue UNLESS he is bouncing over some potholed trail."  Keep in mind all of the inflation specs are for all-around driving, which includes 80 mph on the freeway.  A tire can deflect and hold a lot more than 38.57 lbs/psi, it's just that the flex to do that creates heat.  But if you can get rid of the heat so the tire doesn't get hot it's not going to cause problems.

Rusty_S85 wrote

....  So lets say you go to a 33" tall tire for example.  you could run the 80 psi the sidewall says or you could run what your trucks door tag says to run.  After all pressure is pressure.

Like my 31" KO2s, they say on the sidewall I think its 50 psi.  I run 35 psi front and rear like the door tag says.  It doesnt matter if I go with a taller tire or a higher rating, the pressure should not change as the pressure is designed for supporting the weight of the truck. ...

Keep in mind that the door tag also says what size and weight rating of tire you are supposed to use.  I don't think it's a safe assumption that the pressure needed wouldn't change if you put a significantly different tire on it.  As Jim points out, a load range E tire will likely support less weight at the same (low) pressure as a load range C.  So if the door sticker recommends a load range C and 35 psi that wouldn't apply with a load range E.

Or in a more extreme difference, going from a load range C to a small trailer tire.  Trailer tires can carry a lot of weight, but being smaller they need a lot more pressure to do it.  Now I'm not saying it's a good idea to run trailer tires on a truck.  Just pointing out that the tire spec can't be ignored when you're looking at the inflation spec.

[Gary Lewis]

Jim - That link gives me a 404, but I think I understand anyway.  However, I'm surprised that the load capacity of a tire vs air pressure is linear.  I woke up thinking about this just now, and wondered if the ratio might go down exponentially as the pressure decreases given the heat generated at lower pressures.

Anyway, the tires I'm looking at are rated at 3750 lbs @ 80 psi so the magic number is 46.875  lbs/psi.  And at 35 psi the capacity would be 1641/tire or 6563 for four of them.  That's probably about what Big Blue weighs by himself, but it'll be more with passengers and cargo.  And we both know that it isn't evenly balanced.

I should probably weigh the truck, front and rear, to figure out what the actual load is per axle.  But if it really is 6500 lbs for the truck by itself, and if the weight is distributed 60/40 then the front axle will be at 3900 lbs and the back at 2600.  That says that the front tires would need to at least be @ 42 lbs - assuming even distribution side to side.  So by the time the truck is loaded 50 psi might be a better minimum pressure.  In other words, you are right that 35 psi is too low.

As for the 35 psi recommended on the door tag, I confused you as I was talking about Blue, the 2015 F150, not Big Blue.  Sorry.  Big Blue's door tag says 44 psi for the front and 80 psi for the rear at max load.  Or, at least that's how I interpret it.

So, using the #'s on that let me check things out.  Tire Rack says a Falken Wildpeak W/T3W in 238/85R16 has a capacity of 3042 lbs @ 80 psi.  That's 38 lbs/psi, and at the recommended 44 psi each tire could support 1673 lbs, or 3346 for the two front tires.  But that's ~500 lbs less than the GAWR.  What did I miss?  

[Gary Lewis]

Bob - Thanks.  You said basically what I was going to say based on Rusty's comment.  And I think my response to Jim addresses some of that as well.  And I wanted to address the comments separately.

I do wish there was a better range of tires available for 16" wheels.  But I don't want to spend $750 buying new wheels just to get that better range of tires.  So for now I'm stuck running Load Range E tires on Big Blue.

This discussion has helped me understand tire pressures better, and I can easily see I've been running too little pressure.  But I am still confused as to why BB's door tag says to run 44 psi in the front since that appears to mean the tires aren't up to supporting the 3850 lb GAWR.  It seems to me like that linear lb/psi equation is suspect.

[85lebaront2]

Interesting, if you remember I had gotten 215/85R16 LR E tires on Darth simply because the tire store has plenty of them in stock since the school busses use that size. Mine aren't off road tires as I don't plan on overlanding, driving on sand would be more likely for me.

What I find interesting are, in order:
Front axle BB - 3850, Darth - 3900
Rear axle BB - 5922, Darth - 7400
Rims both show 16X6 K, no different designation for the style, BB's are pretty standard, Darth's are dually
Front springs BB - 7, Darth - J
Rear springs BB - K, Darth - K
Front tires BB - LT235/85R16E 44psi cold, Darth - LT215/85R16D 58psi cold
Rear tires BB - LT235/85R16E 80psi cold, Darth - LT215/85R15D 65psi cold Dual

Since the rear spring codes are the same, unless Ford changed the codes between 1985 and 1986, that would indicate the BB does indeed have 1 ton rear springs with the load difference being a factor of the axle rating and tires (single vs dual).

BB at 5922 lbs is 2961/tire, Darth at 7400 is only 1850/tire

On both if you add the axle ratings, the result exceeds the GVWR, go figure.

[Gary Lewis]

The springs didn't change between 85 and 86 as I've looked it up before.  So yes, both trucks came out with the same rear springs.  And since BB's tires were rated at 3042/ea the 5922 limit must be the axle - Sterling 10.25".

It is interesting that BB's recommended front tire inflation pressure was 44 psi on LT235/85R16E's and Darth's was 58 psi on LT215/85R16D's.  I can't look up the max load for the D-range tires as no one has them in that size.  But I'm guessing that dividing the max load rating by the max inflation pressure and multiplying by 3900 would bring it in around 58 psi.  In other words, with a lower max rating on the tires you needed a higher % of the max pressure to get to the expected load.  And that's just what Jim's been saying.

[Nothing Special]

If you want to geek out even more on tire load vs inflation pressure, here's a link to a Toyo tires site that has some interesting stuff.  That page just has links to other places, and I haven't explored it much.  But the first link on that page goes to a Load & Inflation Table Application Guide.  It has mostly tables rather than graphs, so it's hard to see trends quickly, but it does appear that load vs inflation pressure is pretty linear.

[Gary Lewis]

Me geek out????  Ok, I'll be baaaack...  

[Gary Lewis]

Basically a straight line.  

[Littlebeefy]

Do you use nitrogen in your tires, Gary? It seems like you should if you are this concerned about precision in tire pressure. Nitrogen maintains more stable pressure throughout operating temperature ranges and beyond. Of course then you'd have to keep a tank of nitrogen with you to reinflate from when you're ready to get back on the highway.

So nitrogen obvioulsy isn't the best option if you are going to constantly deflate and inflate your tires because of the overhead involved. BUT, if you keep your tires at constant pressure, then the nitrogen is worth the trouble. I have seen pressure jump on a tire filled with air by 10psi after asphalt driving. I've also seen that same tire change pressure by only 2psi when inflated with nitrogen. PV=nRT finally comes in handy.

[Gary Lewis]

No, I've not used nitrogen for the reason you mentioned - I air down and then back up frequently when we are on the trail.

But, to be fair, I've not heretofore been that concerned about pressures.  However, Jim pointed out that I've been underinflating the tires, so that's why the tire pressure question has become so cussed and discussed.

[ArdWrknTrk]

😈. Well here I am, poking the sleeping tiger again

Gary I think that GVWR's from the factory are cooked up to put your vehicle just at the 8,600# emissions cutoff, or in Bills case right at 10k where one pound heavier Darth would be a whole 'nother class of vehicle.

I picked up on the linear tire pressure formula and ran with it to some absurd extreme.    
I certainly shouldn't have taken their example number and applied it to your tires and fringe off-road use case.

Bill has the slightly narrower tires less likely to pinch a rock between the duallies, but that narrower sectional profile wants to roll over even more.
Steer axles are going to be affected by that, and by how much weight is behind them.
You need them to track.
I'll  bet that Blue's wider rims and different factory tires are why he has a lower recommended front tire pressure.

Tires are pretty complex.
Street tires sold to the general public operate in many dynamic environments and no one wants to be Firestone in the '90's
.

[Nothing Special]

Gary Lewis wrote

....  But I am still confused as to why BB's door tag says to run 44 psi in the front since that appears to mean the tires aren't up to supporting the 3850 lb GAWR....

Going back a bit to this, it's not clear to me that there is A weight rating for tires of a certain size with a certain weight rating.  That information I linked to above would say that all LT235/85-16 tires (they don't even specify load range) have a weight rating of 3042 lbs at 80 psi.  But do they?

Looking at the Falken web site the Wildpeak A/T3W in LT235/85-16 (load range E) has a load rating of 3042 lbs at 80 psi, so that fits.

Looking at the BFG website for the A/T KO2 or the Michelin website for the Agilis Cross-Climate (what I have on my truck), both say the LT235/85-16E has a weight rating of 1380 lbs at 80 psi.  That is so clearly wrong that I decided to see if treating that as 1380 kg worked.  Converting 1380 kg to lbs gives 3042, so maybe there is a given rating.

Sorry for interjecting my aimless musings here...

[Rusty_S85]

Nothing Special wrote

ArdWrknTrk wrote

....  38.57 x 15psi = 578.55 pounds per tire!
This is DEFINITELY exceeded in the front of Big Blue even if he is not bouncing over some potholed trail....

I would change that to "This is DEFINITELY exceeded in the front of Big Blue UNLESS he is bouncing over some potholed trail."  Keep in mind all of the inflation specs are for all-around driving, which includes 80 mph on the freeway.  A tire can deflect and hold a lot more than 38.57 lbs/psi, it's just that the flex to do that creates heat.  But if you can get rid of the heat so the tire doesn't get hot it's not going to cause problems.

Rusty_S85 wrote

....  So lets say you go to a 33" tall tire for example.  you could run the 80 psi the sidewall says or you could run what your trucks door tag says to run.  After all pressure is pressure.

Like my 31" KO2s, they say on the sidewall I think its 50 psi.  I run 35 psi front and rear like the door tag says.  It doesnt matter if I go with a taller tire or a higher rating, the pressure should not change as the pressure is designed for supporting the weight of the truck. ...

Keep in mind that the door tag also says what size and weight rating of tire you are supposed to use.  I don't think it's a safe assumption that the pressure needed wouldn't change if you put a significantly different tire on it.  As Jim points out, a load range E tire will likely support less weight at the same (low) pressure as a load range C.  So if the door sticker recommends a load range C and 35 psi that wouldn't apply with a load range E.

Or in a more extreme difference, going from a load range C to a small trailer tire.  Trailer tires can carry a lot of weight, but being smaller they need a lot more pressure to do it.  Now I'm not saying it's a good idea to run trailer tires on a truck.  Just pointing out that the tire spec can't be ignored when you're looking at the inflation spec.

Would you really put a tire rated at a lower weight rating than the OE tires?

I dont know of anyone that would do that.

As far as the tire spec goes, it still stands the pressure on the side of the tire is the max pressure the tire can safely operate at.  Thats not what you are supposed to run your tire pressure at.  Your best bet is to run it at what your door tag says cause it doesnt matter what size tire you run, x psi is still x psi, it doesnt suddenly become y psi because you have a different tire size.  Its not about ignoring tire specs its about not confusing tire specs, such as with the pressure on the sidewall, that is the max pressure a tire can safely handle, just like you have the load rating of the max load a tire can safely handle.  Like my KO2s on my truck, they are a C1 rated tire which is a 6 ply rating with max load carrying at 50 psi with a load index of 109 which is 2,271 lbs carrying capacity.  Thats 9,084 lbs for all four tires at 50 psi.  The math comes out as follows, 2,271 / 50 = 45.42.  At 35 psi which I run my tires at front and rear it is 1,589.70 which comes out to 6,358.80 lbs for all four tires.  My trucks GVW which is the weight rating of the entire truck including cargo is 5,252 and the front GAWR is 2500 lbs and the rear is 2912 lbs.  For two tires at 35 psi they are supporting 3,179.4 lbs which exceeds the weight rating for the front and for the rear.  That is why when they had my tires at the psi listed on the side wall my rear tires were pizza cutters.  There is literally no way I would ever be able to run 50 psi on these tires as it would result in severely overloading my truck which would squat the rear end to the ground.

as far as the gvw being fudged I cant speak for the larger trucks but I know the gvw on my truck is fairly close as I have driven my truck across scales before.  I contribute the weight difference from the sticker due to running oversized aftermarket sized tires with aftermarket rims and then the wood in the bed in the back was marine grade wood which was a denser wood than your standard plywood strips.

Thats why I always tell people to run what the door tag says.  Only way I wouldnt recommend that is if someone is being crazy and decided to run a tire that doesnt exceed the weight rating of the OE tires.

[ArdWrknTrk]

Bob,  I think what you are looking for is the tire's load index.

Certainly not that every tire of a given size and load range is going to be the same.
The load index on the sidewall is much more granular.

The load range (P, C, D, E, etc) says more about their construction and carcass stiffness.
While a rating like '10 ply' describes that, it doesn't actually reflect how many layers of canvas are built into the tire.
But canvas went out of use 70 years ago and tires 60 years ago were touting nylon, now more typically aramid (like Kevlar and Nomex)

And now we're back to American "individualism" and why we hold onto arcane systems of measurement.
Why do we buy tires in metric dimensions stacked on inch rim diameters?
Why are lug patterns now discussed in both inches and mm?

Your example just goes to point out the challenges of global trade and economy.

[ArdWrknTrk]

Load rating and load index are not the same. (see above)

Yes, I would, and MANY other people do too.
See that original quote where a D rated tire carries more weight at less inflation pressure than an E rated tire.

If you want a compliant ride you should get a tire with a more compliant carcass (within reason!)

Also consider that tire pressure is essentially an undamped spring that supports your entire vehicle.
The ideal gas law (shown above) explains the relationship between pressure, volume and heat.