15" rim vs. 16" rim ?

[rodsterinfl]

I am interpreting the word "temp." to mean temperature, but I think you mean "pressure".

And a pressure buildup from 50 psi to 60 psi is way too much. That's 20%, and you don't want to see anything more than 10%. You need to use more pressure - and I would suggest that 65 psi is where you want to be.

Oh, and the load charts are MINIMUMS, not recommendations.

Yes you are correct on my meaning of temp; however, what I was not clear on is that I increased the pressure to 60psi myself not that it increased because of temperature. My tire minder indicates that now, at 60 psi, on my 3000 mile trip, that psi works for my load based on the temperature monitoring. Before when I ran 50psi I observed greater temp swings upward. With 60 psi I get about 3-4 psi increase and temp increase of about 10-15 degrees.

[rodsterinfl]

So it sounds like we can risk the damage to our wheel wells when a 15" GYM blows or switch and take the chance of shaking the trailer apart with 16" higher pressure LT tires. This is fun.

LOL, that is why I believe it is important to match load to air pressure. When I got the same tires on my truck as the trailer- E Rated after having D rated, my truck rode like a tank. I adjusted the air pressure and it was much better. I still had great load capability too.

[Tireman9]

Originally Posted by rodsterinfl

Yes you are correct on my meaning of temp; however, what I was not clear on is that I increased the pressure to 60psi myself not that it increased because of temperature. My tire minder indicates that now, at 60 psi, on my 3000 mile trip, that psi works for my load based on the temperature monitoring. Before when I ran 50psi I observed greater temp swings upward. With 60 psi I get about 3-4 psi increase and temp increase of about 10-15 degrees.

Temperature variation is not a recognized good way to learn proper inflation pressure. One major variable is the fact that in the tire industry we use internal contained air temperature as only an approximation while needle probes into the shoulder area of the tire is the accepted method of learning the critical tire temperature.
Internal contained air temperature can be affected by numerous external items such as brakes and bearings wheel material and design.
Additionally aftermarket TPMS temperature readings are further degraded bu the cooling affect of the air flowing over the TPM sensor and the valve body itself.

The proper way to learn the Minimum CIP is by using a calibrated scale and consulting Load Inflation tables. Then to avoid chasing your tail around whenever the ambient temperature changes it is recommended that you add 10% to the minimum inflation to establish a CIP you should use when inflating your tires. This way you only need to consider adding pressure when the pressure drops down to near the minimum needed to carry the load.

[rodsterinfl]

The proper way to learn the Minimum CIP is by using a calibrated scale and consulting Load Inflation tables. Then to avoid chasing your tail around whenever the ambient temperature changes it is recommended that you add 10% to the minimum inflation to establish a CIP you should use when inflating your tires. This way you only need to consider adding pressure when the pressure drops down to near the minimum needed to carry the load.

I agree with you on the use of the load inflation chart. I use that; however, a gentleman came up with a way to use a TPM readings to fine tune psi after the chart pressure recommendation is used. I believe like you that there is more to it than just using maximum pressure.

[Tireman9]

Originally Posted by rodsterinfl

I agree with you on the use of the load inflation chart. I use that; however, a gentleman came up with a way to use a TPM readings to fine tune psi after the chart pressure recommendation is used. I believe like you that there is more to it than just using maximum pressure.

Now we may have "a failure to communicate".
Please re-read my prior comment on the problems with using temperature readings, especially the ones provided by a TPM system.

Using the pressure associated with the maximum load as molded on the tire sidewall does have a solid technical background as I have covered in my post of Interply Shear on multi-axle trailers.

It is my position that multi axle trailers need to learn the actual tire loading to confirm they have a reasonable safety margin of capacity over actual loading (15 to 20%). This would also apply to motorized vehicles.
BUT the suspension design on multi axle trailers also need to consider their unique Interply Shear (Google TR Tire Interply Shear) to address the belt separation issues that may occur, so for this application increasing the inflation is the best method available to owners to lower the shear forces.

Non-tire design Engineers come up with numerous theories and ideas but so far I have not seen any of these theories match the accumulated knowledge of 100+ years of tire engineering used in the tire industry where the only meaningful temperature is of the rubber at the belt edges as is done on race tires. Lacking the ability to properly obtain that number the published load & inflation tables are the best tool to use to learn the minimum cold inflation.

[TheCabin]

I agree that the non-tire engineers like myself sometime find the tire engineering approach confusing. I also find the amount of time, study and analysis that goes into designing a tire to be overwhelming. It is no wonder that new ST designs are not being built with such a market. Just out of curiosity I asked a tire engineer to point me in the direction of books on the subject, here are a couple free ones:

I believe that this (Mechanics of Pneumatic Tires, ed Clark) is the classic, but dated:
https://books.google.com/books?id=zc...page&q&f=false

and this is an update (from nhtsa.gov):
https://www.google.com/url?sa=t&rct=...,d.cWw&cad=rja

So if you are having trouble sleeping during your end of the year vacation, these may help.

[Al and Missy]

Tireman:
I have read many of your posts and visited your website, but I'm still confused. In #145, you said:
"It is my position that multi axle trailers need to learn the actual tire loading to confirm they have a reasonable safety margin of capacity over actual loading (15 to 20%). This would also apply to motorized vehicles.
BUT the suspension design on multi axle trailers also need to consider their unique Interply Shear (Google TR Tire Interply Shear) to address the belt separation issues that may occur, so for this application increasing the inflation is the best method available to owners to lower the shear forces."

The first sentence seems to say that 15-20% over actual tire load is a good solution for multi-axle trailers.

The last sentence suggests "...increasing the inflation.... but doesn't say over what or to what. I'm confused in the light of your web site which, I think, says the best recommendation for multi-axle trailers is to run the maximum pressure listed on the sidewall.

In my particular case running the LTX 225/75/16 MS2s, rated for 2680# at 80 psi on my 25' Safari with a GVW of 6300#, 80 psi seems like overkill to the nth. Start with 6300# and subtract off 850# for tongue weight. The tires are carrying 5450#, about 51% of the tires' rating. I run them at 75 psi because that is the pressure at which they exhibit the same load carrying capacity as the original ST225/75-15 tires at 65 psi as recommended and fitted by Airstream.

Let's assume that I have done a perfect job of distributing the weight in my trailer and the load on each tire is 1363#. I'll be overly cautious and add 20% for the various factors you mentioned - that has each tire at 1635#. The Michelin inflation tables say that the XPS Rib (not the same tire, but the same load carrying capacity at 80psi) is rated for 1650# at 40 psi.

Understanding the desire to limit shock and vibration to the trailer at 75-80 psi, what would be your opinion at running them closer to 40 psi? Do I really need to run 75-80 psi?



Thanks,

Al

[Tireman9]

G

Originally Posted by Al and Missy

Tireman:
I have read many of your posts and visited your website, but I'm still confused. In #145, you said:
"It is my position that multi axle trailers need to learn the actual tire loading to confirm they have a reasonable safety margin of capacity over actual loading (15 to 20%). This would also apply to motorized vehicles.
BUT the suspension design on multi axle trailers also need to consider their unique Interply Shear (Google TR Tire Interply Shear) to address the belt separation issues that may occur, so for this application increasing the inflation is the best method available to owners to lower the shear forces."

The first sentence seems to say that 15-20% over actual tire load is a good solution for multi-axle trailers.

The last sentence suggests "...increasing the inflation.... but doesn't say over what or to what. I'm confused in the light of your web site which, I think, says the best recommendation for multi-axle trailers is to run the maximum pressure listed on the sidewall.

In my particular case running the LTX 225/75/16 MS2s, rated for 2680# at 80 psi on my 25' Safari with a GVW of 6300#, 80 psi seems like overkill to the nth. Start with 6300# and subtract off 850# for tongue weight. The tires are carrying 5450#, about 51% of the tires' rating. I run them at 75 psi because that is the pressure at which they exhibit the same load carrying capacity as the original ST225/75-15 tires at 65 psi as recommended and fitted by Airstream.

Let's assume that I have done a perfect job of distributing the weight in my trailer and the load on each tire is 1363#. I'll be overly cautious and add 20% for the various factors you mentioned - that has each tire at 1635#. The Michelin inflation tables say that the XPS Rib (not the same tire, but the same load carrying capacity at 80psi) is rated for 1650# at 40 psi.

Understanding the desire to limit shock and vibration to the trailer at 75-80 psi, what would be your opinion at running them closer to 40 psi? Do I really need to run 75-80 psi?



Thanks,

Al

Ya tire stuff can be confusing but it doesn't have to be.

We have two tasks. 1. To ensure a reasonable load capacity and 2 to lower Interply Shear.

My suggestion on increased inflation assumes no change in tire size but lets address Interply Shear in more detail. The book referenced in post #146 is reasonable but you can also do a Google search on "Interply Shear" for information for non-tire engineers.
Basically it is the internal force trying to tear the belt off the carcass. This force is very high in multi axle trailers as tires are "dragged" around corners when turning. One way to lower this shear force is to increase tire pressure.
Usually this means that inflating tires to the pressure on their sidewall will result is the lowest shear force for that trailer system. If we were only worrying about long tire life I would go with more air pressure to maybe even 25% load margin.

Many RVs come with OE tires that have less than 5% margin even if fully inflated. This suggests that going up in load capacity is desirable. If you know the actual loads on your tires (scale weight) then it is suggested to increase load capacity to give you at least a 15% margin on the heaviest loaded tire on an axle and running all tires on an axle at the same inflation.

Since Motorized vehicles do not have the same Interply Shear problem as trailers they can use the measures load and the Load Inflation charts to learn their goal cold tire inflation. This can not be done on trailers because of the suspension design.

In your situation it appears that the tires have significantly more load capacity than needed which assumes your load estimates are correct. You have confirmed the individual axle loads with real time scale readings haven't you? Your tires are larger and have increased Load Range.

Your LT225/75R16 LR-E may be a bit of overkill. Your 6,300# GVWR sounds very light. Are you sure that number is what is on your vehicle sticker / plate?

[markdoane]

I've run my tires at 95% max load (Marathon LR D) and 85% max load (Maxxis LR E) for several years with no problems.
Having a single axle trailer with no interply shear has some advantages.

[Al and Missy]

Originally Posted by Tireman9

G

Ya tire stuff can be confusing but it doesn't have to be.

...

One way to lower this shear force is to increase tire pressure.
Usually this means that inflating tires to the pressure on their sidewall will result is the lowest shear force for that trailer system. If we were only worrying about long tire life I would go with more air pressure to maybe even 25% load margin.

Many RVs come with OE tires that have less than 5% margin even if fully inflated. This suggests that going up in load capacity is desirable. If you know the actual loads on your tires (scale weight) then it is suggested to increase load capacity to give you at least a 15% margin on the heaviest loaded tire on an axle and running all tires on an axle at the same inflation.

Since Motorized vehicles do not have the same Interply Shear problem as trailers they can use the measures load and the Load Inflation charts to learn their goal cold tire inflation. This can not be done on trailers because of the suspension design.

In your situation it appears that the tires have significantly more load capacity than needed which assumes your load estimates are correct. You have confirmed the individual axle loads with real time scale readings haven't you? Your tires are larger and have increased Load Range.

Your LT225/75R16 LR-E may be a bit of overkill. Your 6,300# GVWR sounds very light. Are you sure that number is what is on your vehicle sticker / plate?

I went to the 16" tires because I didn't want to replace the aged out Marathons that were on my trailer when I bought it with more ST tires. The OE tires (based on the dated code on the spare) were ST225/75-15s. Inflated to 65psi they had a load capacity of 2540# giving almost 40% margin over the 6300# specified for the trailer on Airstream's specification sheet and the data plate, and the tires don't even carry the full 6300#.

As I saw it, my choices were the P235/75-15 LTX with a de-rated capacity of 1985# or the LT225/75-16s like Airstream is now fitting on some trailers. Several tire manufacturers' policies I read (and I thought I found a similar FMVSS reference but can't find it now) stated that replacement tires should have the same load capacity as the OE tires, so that seemed to say I shouldn't use the P225s. I chose the 16s.

I have weighed my trailer three times, twice on a set of scales at a local moving company and once on a set of CAT scales that are quite a drive away. I am pretty confident in my measured (combined) axle loads and calculated tongue weight. I do plan to get a weighing at Smartweigh or other facility that can give me per wheel weights, perhaps in January.

My question is still, assuming that my load is perfectly balanced, will the margins I have support running at less than 80 psi to minimize shock to the trailer? In other words, while I understand that running at 80 psi will minimize the potential for interply shear, given the high margins available for my lighter trailer and the fact that I will likely replace the tires well before they wear out can I settle for something less than minimizing the potential?

If the 24% higher loads some have attributed to interply shear are added to 10% L/R imbalance and 5% F/R imbalance, it looks like I could run at 50 psi. That might provide a substantially better ride for the trailer.

Thanks,

Al

[Tireman9]

Originally Posted by markdoane

I've run my tires at 95% max load (Marathon LR D) and 85% max load (Maxxis LR E) for several years with no problems.
Having a single axle trailer with no interply shear has some advantages.

All radials have interply shear. It's just that on multi axle trailers it can be 24% higher than what is seen on tow vehicles. Have not run the simulation on single axle trailers but believe it would be lower on that type of TT than on multi axle.
95% of max load is higher than I would suggest is desirable.

[Tireman9]

Originally Posted by Al and Missy

I went to the 16" tires because I didn't want to replace the aged out Marathons that were on my trailer when I bought it with more ST tires. The OE tires (based on the dated code on the spare) were ST225/75-15s. Inflated to 65psi they had a load capacity of 2540# giving almost 40% margin over the 6300# specified for the trailer on Airstream's specification sheet and the data plate, and the tires don't even carry the full 6300#.

As I saw it, my choices were the P235/75-15 LTX with a de-rated capacity of 1985# or the LT225/75-16s like Airstream is now fitting on some trailers. Several tire manufacturers' policies I read (and I thought I found a similar FMVSS reference but can't find it now) stated that replacement tires should have the same load capacity as the OE tires, so that seemed to say I shouldn't use the P225s. I chose the 16s.

I have weighed my trailer three times, twice on a set of scales at a local moving company and once on a set of CAT scales that are quite a drive away. I am pretty confident in my measured (combined) axle loads and calculated tongue weight. I do plan to get a weighing at Smartweigh or other facility that can give me per wheel weights, perhaps in January.

My question is still, assuming that my load is perfectly balanced, will the margins I have support running at less than 80 psi to minimize shock to the trailer? In other words, while I understand that running at 80 psi will minimize the potential for interply shear, given the high margins available for my lighter trailer and the fact that I will likely replace the tires well before they wear out can I settle for something less than minimizing the potential?

If the 24% higher loads some have attributed to interply shear are added to 10% L/R imbalance and 5% F/R imbalance, it looks like I could run at 50 psi. That might provide a substantially better ride for the trailer.

Thanks,

Al

Life is a series of trade offs. I just feel that people should know the facts so they can make informed decisions. There is no way for me to balance tire durability estimates with trailer construction features.

[markdoane]

Originally Posted by Tireman9

All radials have interply shear. It's just that on multi axle trailers it can be 24% higher than what is seen on tow vehicles. Have not run the simulation on single axle trailers but believe it would be lower on that type of TT than on multi axle.
95% of max load is higher than I would suggest is desirable.

Yeah, that's why I upgraded to LR E.


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[kcbwil]

Driving 1500-2000 miles last few weeks from Anacortes WA to Mesa AZ via Wasington state ferry, funky windy roads, heavy rain, gusty winds, construction road repairs, potholes and expressway trash our new rims and tires performed well. As you can see, we did have to replace one tire in Riverside, CA but not as a consequence of failure due to manufacture. The tires were inflated to 75psi, no problems, we're happy campers.

[rmkrum]

Ouch! I hate it when that happens, and it can't be repaired. When we built one of our homes in the desert we picked up 5 nails on my wife's car from visiting the site. Got one just like that on one tire. Mumbled under my breath a lot on that, as it was a shiny new tire! 😢


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