Tire pressure-33 lbs PSI or what?

[CapriRacer]

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Take the weight on the axle, divide by 2.
Take that number and divide it into the maximum load rating of the tire(on sidewall).
take that result and multiply it by the maximum tire pressure on the sidewall.

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Sorry, mdocod, but the maximum pressure listed on the sidewall of a tire may or may be be connected to the maximum load also listed there. This is a common misconception. The good news is that the method will yield a result that can carry more load than indicated by the method.

EXCEPT!!!

There isn't anything in the method to account for:

1) asymetrical loading (Differences from side to side)

2) Extra capacity - Good engineering design practice is to always overestimate the loads, then underutilize the component based on its capacity.

There are also several other errors in the posting.


In the example of the Rodeo:

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Here's an example of a rodeo:
I've figured with my tools and a passenger or 3 and a full tank of gas, I'm sitting at around 4500lbs on average, with around 2100lbs in the rear, and about 2400lbs in the front.(aprox) My door-panel sticker says 245/70-16 tires, 29PSI.

example: BF AT KO 245/70-R16, load range "D", 65PSI max, 2535lbs max.
front: 2400/2=1200, 1200/2535=0.473, 0.475x65=~31PSI
rear: 2100/2=1050, 1050/2535=0.414, 0.414x65=~27PSI
31+27=58(taking average of 2 here), 58/2=29PSI (the recommended PSI on my door), coincidence?

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Yes, it is a coincidence.

There is a mixing of P metric and LT metric tires in the example.

A P245/70R16 has a maximum load carrying capacity of 2094 # at 35 psi, but at 29 psi the capacity is 1940 #. Now since this is a "multi-purpose vehicle, truck, bus, or trailer" (quote from the Tire and Rim yearbook), these loads have to be reduced by 10%. So the rated load carrying capacity at 29 psi is really is 1763 #.

An LT245/70R16 - as stated - has a maximum load carrying capacity of 2535 at 65 psi, but at 29 psi, the capacity is 1428 # (my estimate - the chart doesn't go that low).

Bottomline: I think the methodology is faulty.

 

[mdocod]

the vehicle weighs under 4000 empty, so I adjusted for extra weight.

You are saying that at 29PSI, a 245/70-16 has a load capacity of 1763 pounds. I'd be interested in learning more about how to calculate this. Everything I have read suggests that tire pressure and load and volume (size) are all factors that contribute to the load capacity of a tire. I've also seen variations from one manufacture to the next on LT tires for load capacity in the same size and load range.

So, if my Isuzu only weighs ~4500? and is only rated for a maximum a little over 5000lbs, why would the manufacture recommend a pressure designed for a 7000lb load?

I think you know something further (beyond my understanding), I'd like to know more. If there is a BETTER way to calculate for pressure/load than I am using, then that would be great.

 

[CapriRacer]

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You are saying that at 29PSI, a 245/70-16 has a load capacity of 1763 pounds........

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Not exactly.

A P245/70R16 used in a light truck application has a maximum load carrying capacity of 1763 pounds at 29 psi.

By contrast an LT245/70R16 has a [b[maximum load carrying capacity of 1438 pounds at 29 psi.

I could go into why that is so, but it is a lengthy discussion point and I'll leave that for later.

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...... So, if my Isuzu only weighs ~4500? and is only rated for a maximum a little over 5000lbs, why would the manufacture recommend a pressure designed for a 7000lb load?.......

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Let me explain it this way:

A 1/4" SAE grade 5 bolt has a rated tensile breaking strength of just under 4000 pounds. In theory, you'd only need a single bolt to hold your vehicle up. But if you look under your vehicle, I'll bet you'll find dozens of bolts - of various "sizes" - and their total rated breaking strength is many times the weight of your vehicles. There are a couple of reasons for this:

1) Some of the bolts are subjected to stresses that include more that just the weight of the vehicle.

2) The rated breaking strength is just that…..a rating.

3) No one wants a bolt failure, so it is always good engineering practice to use a bolt much stronger than the calculations say is needed. Plus, as hard as we try, we engineers can't account for all the stresses in our calculations - the analysis is way too complicated - so we add…..ah…..Mmmmm…..Well, we used to call them "Safety Factors", but I don't know what the common terminology is now, but we tire engineers refer to "Reserve Capacity" when referring to load carrying capacity in excess of the weight of the vehicle.

So to put this in perspective - You Rodeo has a GVW of 4640 pounds. Needless to say, the weight is not evenly distributed when the vehicle is fully loaded. Plus it is always a good idea to have unused capacity. Plus one of the lessons from the Ford / Firestone situation a few years back was that folks don't pay attention to inflation pressure.

Another lesson was that probability of tire failures from - oh, let's call them "road hazards" (cuts, impacts, abrasion, and the like) - were greatly reduced by relatively minor increases in reserve load capacity.

The bottomline is that your vehicle ought to have much larger tires than what the weight of the vehicle would ordinarily call for.

 

[Doc8404]

how is it the LT has a lower carrying load?

 

[sprintman]

I just go to the local tire dealer every three/four months and add Nitrogen if required which is rare.

 

[CapriRacer]

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how is it the LT has a lower carrying load?

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This goes back to the days when there was a clear division between tires for passenger cars and tires for trucks.

Tires for passenger cars were designed for softer ride and so allowed lots of deflection in the calculation. These tires were relatively small - 13, 14, and 15".

Tires for trucks are designed for low cost and ride quality is not an issue - and the tires were relatively large and the division was clear - pretty much all 16" for small truck and 20" (and larger) for big trucks.

The way tires for trucks worked is that to achieve a low unit cost, a fairly rigid tire was required, so you got a tire that had good wear properties, but required the rubber to withstand a high amount of load. Rubber tends to develop a compression set, especially if the load is high - and that is a problem for truck tires. So the compounds are designed with this in mind and they aren't very tolerant of high deflection.

Fast forward to today where the cars are heavier and the trucks (also known as SUV's) are smaller, and this creates an overlap in these two types of tires, and, of course, the loading per inflation becomes quite obvious and is a consequence of the history behind the development.

So you may ask, is it OK to use the LT in place of the P - and vice versa?

My answer is "No, it is not OK" Lots of folks may have done this and experienced no problems, but there are some situations where there have been some tire failures related to this, and while rare, I can't see any real technical advantage to interchanging the 2.

 

[mdocod]

awesome. learn something every day...

I guess I never thought of tires and the loads they deal with the way I think of bolts. But I do like the analogy. That works nicely...

I assume the LT is de-rated MORE because it is expected that a tire on a light truck is more apt to be hit with larger shock loads, (offroading, etc)... stiffer suspension probably also plays a role here... just thinking out loud...

I run LT285/75-16s load range D tires (BF AT KO) on my rodeo... I keep pressure in the high 20s most of the time. Seems to work fine. (It's a lot bigger tire than stock)

 

[Squishy]

Isn't it industry standard to take "Max Load" measurements at 35 PSI for SL tires and 41 PSI for XL tires? It might be different for LT tires, but I thought this was the case for P-metric radials.

 

[CapriRacer]

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I assume the LT is de-rated MORE because it is expected that a tire on a light truck is more apt to be hit with larger shock loads, (offroading, etc)... stiffer suspension probably also plays a role here... just thinking out loud...

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What is really going on is that the standards for these 2 types of tires come from different places.

Passenger car tires have been "low pressure" since about the 1930's. But tires for small trucks come from a "high pressure" arena (and by "small trucks" I mean pickups - even what is normally a "large" pickup is small compared to an 18 wheeler.)

This difference affects how the tire structure works, in particular the LT tires use much less material per load carrying capacity. This was very important in the days when people who operated small trucks were concerned about operating costs, rather than the vehicle's ride quality.

Those days are gone, but we have to live with consequences of that history. The alternative is to re-invent this type of tire, and that makes little sense given its limited application. (I guess what I am trying to say here is that I expect the US market to evetually move away from SUV's and pickups and become more like the rest of the world where these vehicles are "utility vehicles".)

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Isn't it industry standard to take "Max Load" measurements at 35 PSI for SL tires and 41 PSI for XL tires? It might be different for LT tires, but I thought this was the case for P-metric radials.

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I would phrase it this way: The rating pressure for P metric Standard Load tires is 35 psi. For hard metrics it is 2.5 bar (about 36 psi). Some folks call these "Euro metrics", but the Japanese tire sizing nomenclature looks the same.

For Extra Load tires, P metric tires are rated at 41 psi and hard metrics at 2.9 bar (about 42 psi).

LT metrics are rated by Load Range: Load Range C = 50 psi, Load Range D = 65 psi, Load Range E = 80 psi. (Up to 305 section width).

Just so we are on the same page: The load carrying capacities are standardized and each tire manufacturer is following one of those "tire standards". So the load carrying capacity for a P205/65R15 SL is the same regardless of who manufactures the tire. However, it is common for folks to omit the "P" in the sizing and there are 2 other "tire standards" out there that don't use the "P", and they don't agree as to what the load carrying formula is supposed to be, so they get slightly different answers. The results are very close, but they are different.

This means that if you were to survey tires of a given "size", you'd come up with 3 different load carrying capacities, and might erroneously think there is a difference between the strength of these tires. But if you were to look carefully, you'd notice that the load curves are very close in the vicinity of the rating pressure. For practical purposes, this means the tires are interchangeable. Put another way - if your car came with P205/65R15 SL's, any 205/65R15 will work.

 

[Doc8404]

well, my 2001 F150 SuperCrew came with LT265/70R17 and I'm now running P265/70R17 Coopers, made for a truck.
recommended by Cooper...along with the multitude of P-metric tires recommended by TireRack (and others) regardless of the fact that they would have made 25%+ more on the sale of an LT.

 

[CapriRacer]

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.....well, my 2001 F150 SuperCrew came with LT265/70R17 and I'm now running P265/70R17 Coopers, made for a truck.


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Sorry, but according to Tire Guides, a 2001 F-150 Super Crew Cab came with P255/70R16's or P275/60R17's for 4 X 2's, or P255/70R16 or P265/70R17's for 4 X 4's. That is the factory fitment - which doesn't mean the dealer didn't change it.

For fairness, I should mention that Tire Guides has been known to be wrong, but I don't think it is in this case.

 

[Doc8404]

OK, thanks...so why wouldn't it come with LT's?
because it's a half ton?

 

[CapriRacer]

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OK, thanks...so why wouldn't it come with LT's?
because it's a half ton?

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The general reason why 1/2 ton pickups use P metric tires is that using the low pressure results in a better ride.

3/4 and 1 ton pickups have too much load for a P metric.

 

[smokey1]

should try to keep tires within 2 lbs. over the vehicles recommended settings . So if 30 lbs. is stated ( usually in glove or near drivers door or frame ), 32 p.s.i. be fine and check at least once a month . If too much , make ride stiffer , could cause blowout , etc. . Too little would hurt fuel economy , steering of auto , etc..