tire load/ pressure

[mbaker]

Went down a rabbit hole after doing some reading as I have been looking at trailer tires and interested in the load/pressure combinations. For sake of discussion the old tire (175-80-13) load rating is adequate by almost 1000#, the new heavier tire isn't necessarily needed but seems to be a common option that wasn't available years ago and in some cases is less expensive now.

Current tire is load index 91 (1356) at 50psi
Optional tire is load index 97 (1609) at 65psi

Based on an article I was reading you could calculate the new tire psi needed to meet the old load requirement with some simple math.
1609 (new tire)/65psi= ~25#/psi
Work backwards into the old tire load and ~55 psi with the new tire would equal the load rating of the old tire. Is it that simple? In this case it isn't that much difference in max psi to get the capacity but perhaps not increase ride harshness as much and/or still have a little extra capacity if you do air them up more? I was always a 'max air in trailer tire' guy but at the same time didn't want to add harshness or jarring to the trailer if not necessary and on such small 13" tires.

More extreme in my mind would be P-metric vs LT truck tires.
P-metric 115 index (2650# reduced to 2409# for truck use) at 44psi = 55#/psi
LT 121 index (3300#) at 80psi = 41#/psi

To match the full P metric rating you would need to run 59# in the LT. At first this seemed backwards, the P-metric is carrying more load per psi of air than the LT making it seem like it is a more capable tire. Instead I think it means the LT is more durable because it is built heavier by design and the sidewall is carrying more load therefore it is carrying less load per psi of air so less stress overall or at least more evenly shared stress? At the same time the P-metric just isn't as stable when you add in other factors like sway, cornering under load etc?

In the past I think I felt (wrongly it appears) like if you ran LT in place of metrics at the same psi the load capacity would simply go up but that doesn't seem to be the case. LT for the same load would require more air but in turn that spreads the load better across the volume of air and the sidewall. Makes sense why the ride would be harsher if both the sidewall is stiffer and air psi is higher in the LT for the same load capacity, but that combines to get you a more stable longer lasting tire under that same load. I guess its also possible that at any given pressure the LT probably does technically carry more load but is now using the sidewall more and not sharing load across sidewall and air volume effectively shortening its life due to stress.

On the right track or off base?

 

[totegoat]

I run E rated trailer tires at max psi. Loads ride just fine.

 

[CapriRacer]

Went down a rabbit hole after doing some reading as I have been looking at trailer tires and interested in the load/pressure combinations. For sake of discussion the old tire (175-80-13) load rating is adequate by almost 1000#, the new heavier tire isn't necessarily needed but seems to be a common option that wasn't available years ago and in some cases is less expensive now.

Current tire is load index 91 (1356) at 50psi
Optional tire is load index 97 (1609) at 65psi

Based on an article I was reading you could calculate the new tire psi needed to meet the old load requirement with some simple math.
1609 (new tire)/65psi= ~25#/psi
Work backwards into the old tire load and ~55 psi with the new tire would equal the load rating of the old tire. Is it that simple? In this case it isn't that much difference in max psi to get the capacity but perhaps not increase ride harshness as much and/or still have a little extra capacity if you do air them up more? I was always a 'max air in trailer tire' guy but at the same time didn't want to add harshness or jarring to the trailer if not necessary and on such small 13" tires.

More extreme in my mind would be P-metric vs LT truck tires.
P-metric 115 index (2650# reduced to 2409# for truck use) at 44psi = 55#/psi
LT 121 index (3300#) at 80psi = 41#/psi

To match the full P metric rating you would need to run 59# in the LT. At first this seemed backwards, the P-metric is carrying more load per psi of air than the LT making it seem like it is a more capable tire. Instead I think it means the LT is more durable because it is built heavier by design and the sidewall is carrying more load therefore it is carrying less load per psi of air so less stress overall or at least more evenly shared stress? At the same time the P-metric just isn't as stable when you add in other factors like sway, cornering under load etc?

In the past I think I felt (wrongly it appears) like if you ran LT in place of metrics at the same psi the load capacity would simply go up but that doesn't seem to be the case. LT for the same load would require more air but in turn that spreads the load better across the volume of air and the sidewall. Makes sense why the ride would be harsher if both the sidewall is stiffer and air psi is higher in the LT for the same load capacity, but that combines to get you a more stable longer lasting tire under that same load. I guess its also possible that at any given pressure the LT probably does technically carry more load but is now using the sidewall more and not sharing load across sidewall and air volume effectively shortening its life due to stress.

On the right track or off base?

Mbaker,

You and I need to have a long conversation. Email me at [email protected]

 

[mbaker]

Email sent

 

[mdocod]

I would say it's pretty normal to see more pressure required at equal loads, in higher load range tires. I suspect this has to do with the fact that the sturdier tire sidewalls "hold" the edges of the tire tread in place more firmly, so more air pressure is required to balance the load to the center of the tread properly. The thicker/heavier tires also probably generate more heat at the same load/pressure, so need more starting pressure to achieve the same target heat rise for a given speed and load.

Everything is a tradeoff with tires. A heavier load range tire might buy some durability and capacity headroom, but at the cost of ride quality and extra weight.

 

[CapriRacer]

Looks like I need to step in and correct some misconceptions.

First, you can not calculate the load at a given pressure by a simple proportion. It doesn't work that way. That's why they have tire load tables.

Here's a link to my favorite: TirePressure.org

Go down and towards the bottom they have those tables. They call them "Tire Load Inflation Charts". Pick any one of them and you'll notice that none of them result in a proportion vs inflation pressure.

But to answer the OP's question: If I am reading this right, the question he is asking is: If I change Load Ranges, do I need to change inflation pressure?

The answer is No! At least not if you want to carry the same load. If you want to increase the load carrying capacity, then you'll need to use more pressure - and the load table tells you how much more load carrying capacity you'll get!

If you want a deeper dive, try these webpages:

Barry's Tire Tech: Tire Standardizing Organizations

Barry's Tite Tech: Tire Load Tables

I wrote these about 15 years ago, and I'll bet they need updating. I'll do that later as rewriting these things takes time.

Any questions?

 

[mdocod]

Looks like I need to step in and correct some misconceptions.

First, you can not calculate the load at a given pressure by a simple proportion. It doesn't work that way. That's why they have tire load tables.

Here's a link to my favorite: TirePressure.org

Go down and towards the bottom they have those tables. They call them "Tire Load Inflation Charts". Pick any one of them and you'll notice that none of them result in a proportion vs inflation pressure.

But to answer the OP's question: If I am reading this right, the question he is asking is: If I change Load Ranges, do I need to change inflation pressure?

The answer is No! At least not if you want to carry the same load. If you want to increase the load carrying capacity, then you'll need to use more pressure - and the load table tells you how much more load carrying capacity you'll get!

If you want a deeper dive, try these webpages:

Barry's Tire Tech: Tire Standardizing Organizations

Barry's Tite Tech: Tire Load Tables

I wrote these about 15 years ago, and I'll bet they need updating. I'll do that later as rewriting these things takes time.

Any questions?

I agree when talking about LT C/D/E/F etc load "ranges" when keeping the same tire size.. For those trailer tires, they are "on the same chart" so to speak so the pressure can stay the same....

However, when switching from SL to XL, or SL to LT, or XL to LT, this doesn't hold true. XL requires more air than SL for a given load, and LT requires even more air than either SL or XL for a given load.

OP asked about switching from a P-metric to an LT tire in one example. In this case, the tire pressure for a given load will likely change.

 

[mbaker]

Ok, it makes more sense now. That answers my question about the trailer tire ratings and now I know I need to look deeper for the P to LT switch for proper inflation. I felt like there had to be more than just simple math calculation involved but just goes to show how one bad link can lead you astray.

Makes me wonder how many people swap from metric to LT on their truck then run same pressure thinking they are getting a boost in capacity just because the tire is heavier?

 

[CapriRacer]

I agree when talking about LT C/D/E/F etc load "ranges" when keeping the same tire size.. For those trailer tires, they are "on the same chart" so to speak so the pressure can stay the same....

View attachment 186823

However, when switching from SL to XL, or SL to LT, or XL to LT, this doesn't hold true. XL requires more air than SL for a given load, and LT requires even more air than either SL or XL for a given load.

View attachment 186827

OP asked about switching from a P-metric to an LT tire in one example. In this case, the tire pressure for a given load will likely change.

mdocod,

Thanks for taking the next step in the explanation. Allow me to continue.

The differences between the load tables can be the result of various things.

For example, the difference between the LT metric load table and the one for ST tires is the result of the difference in service conditions. LT tires go on vehicles that have engines, steering axles, and driven axles. ST tires go on trailers that don't have those things. That difference puts more input into LT tires, so ST tires are rated for about 10% more load.

But the difference between P metric tires and LT metric tires is partially due to the fact that LT metric tires have to carry more load for a given size. That places more stress on the rubber and there is an interesting phenomenon called "cold flow" (aka compression set) where the rubber permanently deforms due to stress. Obviously, the more stress, the more the cold flow. So LT metric tires use a rubber that is compounded to do that less - and that means the rubber is more prone to cracking. To counter act that, the load on the tire is proportionally lower.

Then we have the difference between P metric tires and Euro-metric tires (sometimes called "hard metric" or just "metric"). There isn't really a difference in either service conditions or in the way the tires are made. The difference is due to the different ways each group describes how the tires work - that is the tire standardizing organizations. This difference is very small, but nitpickers will note the difference.

In the old days when the cost to ship tires was fairly expensive, each group could operate independently. But today, these differences become apparent. As I said, those differences don't mean a difference in the tires - they are made the same.