New Tires For Big Blue

[Nothing Special]

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.

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]

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.

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

[Gary Lewis]

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

Basically a straight line.

[Littlebeefy]

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

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]

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.

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]

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?

😈. 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]

.... 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]

.... 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.

.... 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]

.... 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...

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]

.... 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.

.... 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.

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.