Tire pressure

[KK4YZ]

Guys, it just occurred to me that we’re dealing with relatively small changes and measuring pressure with devices that read out in whole numbers (coarse resolution).. at least in my case.
For my situation one of my tires went from an INDICATED 66psi to INDICATED 72psi, leading me on the surface to see this as a 6psi (or 9%) change. However,
if we take into account the uncertainty due to the measurement resolution alone, the TRUE temp rise can be anywhere from 5.02 deg to 6.98 deg actual, Or 7.5% to 10.65%.
I’m assuming the readout on the TST TPMS changes at near 1/2 deg point. This does not take into account the specified accuracy of the TPMS +/- 1.5 deg, I believe....just the readout resolution.

The moral of this story is to be careful of making decisions without considering the accuracy of the measurement.
Just my 2cents worth.
Jim

[dznf0g]

Originally Posted by KK4YZ

Guys, it just occurred to me that we’re dealing with relatively small changes and measuring pressure with devices that read out in whole numbers (coarse resolution).. at least in my case.
For my situation one of my tires went from an INDICATED 66psi to INDICATED 72psi, leading me on the surface to see this as a 6psi (or 9%) change. However,
if we take into account the uncertainty due to the measurement resolution alone, the TRUE temp rise can be anywhere from 5.02 deg to 6.98 deg actual, Or 7.5% to 10.65%.
I’m assuming the readout on the TST TPMS changes at near 1/2 deg point. This does not take into account the specified accuracy of the TPMS +/- 1.5 deg, I believe....just the readout resolution.

The moral of this story is to be careful of making decisions without considering the accuracy of the measurement.
Just my 2cents worth.
Jim

This is very true, when discussing temps. There are several ways to measure temps:

1) external stem mounted TPMS sensors. Issue: the air temp in the stem is substantially lower than the internal rubber temp at the hottest part of the tire (near the intersection of the tread and side wall, where the tire bulges and flexes as it rolls)

2) internal mounted TPMS sensors. Issue: while probably more accurate than stem mounted sensors, they still only measure air temperature (along with some conducted heat from direct contact with the wheel...which is actually a heat sink and is effected by heat from braking, bearing friction and cooling of air in the assembly)

3) IR thermometer. Issues: wide variation of temps depending upon where the operator points the thermometer. Again, most accurate, would be at the shoulder area between tread and sidewall, but still is affected by cooling of the exterior tire surface by air.

4) Temp needle probe thermometer. Issue: While this is highly accurate, it is destructive to the tire as the needle must be inserted into the rubber in order to get the temp of the area where high temps are destructive to the tire compounds and structure. Not practical (and probably or certainly not safe) for everyday use.

Temps by any of the first 3 methods are useful for reference purposes only and not definitive diagnoses.

[Tireman9]

Originally Posted by dznf0g

"I suspect that the 10% was for passenger car tires, not LT's - although it might be to account for side to side and front to rear weight variation."

Hmmm, I think you're right...that seems to sound familiar, so I am really inside my target adjusted load rating (2278# @ .85X2680#).

Yes the 10% is actually required for P type tires not LT type.

[Tireman9]

I have two posts I suggest people read
measurable vs Meaningful February 24, 2014

and
don't-get-your-shorts-in a bunch about inflation October 7, 2015

Sorry, I am not allowed to post hot links so you will have to do the search yourself.

[Tireman9]

Originally Posted by Mollysdad

Wow! I usually make people mad!


What temperatures do you see?
I just bought one of those laser digital thermometers, and I'd like to see what temp to expect.
I shot the truck tires this morning and they read 100 f.
Ambient temp about 80 f.

If ambient was 80 and your tire was 100 then the tire had been in sunlight.

You may want to check out my blog posts on tire covers where I show the reading for sun vs shade.

[KK4YZ]

So, Mollysdad, after all the input, what will yo go with: 65psi or80 psi?
Just curious
Jim

[TouringDan]

Originally Posted by CapriRacer

Dan,

Thanks for the additional info. Very helpful. Let's do the math.

A 7.00-15 had a load carrying capacity at 60 psi of 2040#

An ST225/75R15 requires 46 psi to carry the same load.

- BUT -

I like to see a 15% reserve capacity and they didn't do that back then, so I think the load we should be looking at is 2346# (2040 X 1.15) - and that means an ST225/75R15 needs 59 psi. So let's call that 60 psi.

So going backwards, how does that compare to what we know about the weight of the trailer?

OK, there are some things that don't make sense. The weights we just did the calculations on seem to indicate that there are only 2 tires on the vehicle, albeit a bit short on load carrying capacity - BUT - not only did you mention duals (Actually, they would be tandems!), but a quick internet search says that at least some 1966 Airstream Tradewinds were 2 axle trailers - but that seems like a lot of extra capacity.

So let me do the math based on what you gave me to see if we can get something reasonable out of that.

If the weight on the 4 tires is 4800#, and I assume a large amount of uneven weight distribution, the worst load a tire would be would be 1380# (4800# divided by 4 times 1.15) - and if I want a 15% reserve capacity, that's 1587# - and the only way this makes sense in light of the original tire specification is that a 7.00-15 at 45 psi (light load per the owners manual) carries 1710# - and that seems like a real stretch.

So I'm puzzled - by the original tire specification vs what is in the owners manual. - AND - that puts me in an awkward position.

So I'm going to recommend you do an experiment the next time you tow for a long distance.

1) Start with 60 psi cold. Measure right before you take off.

2) Then drive on the freeway for a half hour - stop and measure the tire pressures. You don't want the pressure buildup to be more than 5 psi (~7%). If you are below that, continue the test. If you are above that - STOP and add more air (say 5 psi), then continue the test.

3) Repeat step 2 until the pressures stabilize. I think that will take a total of an hour and a half.

4) How much pressure buildup did you get? If you got ~ 7% (taking into account any additional pressure you had to add), that's a good number. If you got below 3%, that probably means you could use less pressure to start with - but you may not want to do that unless the trailer is bouncing excessively.

CapriRacer

I will start with 60 psi on our next trip and check the pressure change as you have suggested. Presently my Airstream is down for repairs/improvements.

What I have learned from this thread is that you start with the pressure recommended by Airstream, if available, and then measure the pressure after traveling on the highway for 30 minutes. If you have started with the correct inflation pressure you expect this to increase by 7%. Adjust the inflation pressure until you get a 7% increase after traveling 30 minutes on the highway.

Maybe another way to come up with the initial trial inflation pressure is to look at the fraction of the inflation pressure recommended by Airstream (45 psi) divided by the max load inflation pressure (60psi). Take this ratio(.75) and multiply this by the max load inflation pressure for the tires you are now using (80psi in my case) to get the initial trial inflation pressure of 60 psi.

Thank you so much CapriRacer for taking the time to educate and help me.

Dan

[CapriRacer]

Originally Posted by TouringDan

CapriRacer

I will start with 60 psi on our next trip and check the pressure change as you have suggested. Presently my Airstream is down for repairs/improvements.
........

That's good.

Originally Posted by TouringDan

....... What I have learned from this thread is that you start with the pressure recommended by Airstream, if available, and then measure the pressure after traveling on the highway for 30 minutes. If you have started with the correct inflation pressure you expect this to increase by 7%. ........

Ah ....... No! It takes longer than 30 minutes for the pressure to stabilize - on the order of an hour and a half. The purpose of the first stop at 30 minutes is to make sure you don't reach dangerous pressures (and temperatures). You're just checking on things before the pressures reach their max.

Originally Posted by TouringDan

......... Adjust the inflation pressure until you get a 7% increase after traveling 30 minutes on the highway. ........

Again, the 7% is to the max value you are looking for - and it will take 1 1/2 hours to reach that. The 30 minute check is just to watch it as it builds.

Originally Posted by TouringDan

......... Maybe another way to come up with the initial trial inflation pressure ..........

What followed after this point was fundamentally bad tire logic. I truncated this to prevent people from reading it a second time and having it *learned*. I want them to forget it.

Originally Posted by TouringDan

........ Thank you so much CapriRacer for taking the time to educate and help me.

Dan

My pleasure.

[mbenson1234]

Great Thread. Does anybody use TST TPMS to monitor the tires?

Thanks,

Mike Benson
TST TPMS

[Adventure.AS]

Originally Posted by mbenson1234

Great Thread. Does anybody use TST TPMS to monitor the tires?

Thanks,

Mike Benson
TST TPMS

Yes, thanks. You were kind enough to provide five sensors for me - four road tires and the spare.

[TouringDan]

Originally Posted by CapriRacer

.

Again, the 7% is to the max value you are looking for - and it will take 1 1/2 hours to reach that. The 30 minute check is just to watch it as it builds.

.

CapriRacer

So I am going to start at 60 psi in search of the “correct “ tire pressure for my load. It sounds like I am looking for an increase in tire pressure of between 3% and 7% to settle in on the correct pressure. Is the ideal pressure increase 5% ?

I have learned that having too low tire pressure leads to heat and tire failure, so I certainly don’t want to go there. The other extreme seems to inflate to 80 psi. This seems to be what a lot of Airstreamers are doing. What pressure increase would you expect at 80 psi after 1.5 hours? Why not just go with 80 psi? I am not proposing this in my situation, just asking the question.

Thanks, Dan

[Tireman9]

Originally Posted by TouringDan

CapriRacer

So I am going to start at 60 psi in search of the “correct “ tire pressure for my load. It sounds like I am looking for an increase in tire pressure of between 3% and 7% to settle in on the correct pressure. Is the ideal pressure increase 5% ?

I have learned that having too low tire pressure leads to heat and tire failure, so I certainly don’t want to go there. The other extreme seems to inflate to 80 psi. This seems to be what a lot of Airstreamers are doing. What pressure increase would you expect at 80 psi after 1.5 hours? Why not just go with 80 psi? I am not proposing this in my situation, just asking the question.

Thanks, Dan

There is a reason to run higher pressure on multi-axle trailers. It is called Interply Shear. You can Google "Interply Shear tires" if you want to understand the Science behind one of the reasons for tires to suffer belt separations.

[Larry C]

After reading the last 7 pages of comments on tire pressure, and learning from the pros on the subject, perhaps there should be a new DOT requirement that anyone pulling an Airstream should have at least a Master's Degree in tire inflation.....

For whatever it's worth

Larry

[Tireman9]

Tire inflation is not difficult.
In trailer application, the tire load capacity based on the inflation should be at least 15% greater than the actual load measured on scale readings.

To further improve tire life in trailer application I suggest people with multi-axle trailers set the cold inflation to the inflation molded on the tire sidewall.

Maybe only a High School Diploma in tire inflation is needed.

[Mollysdad]

Originally Posted by Tireman9

If ambient was 80 and your tire was 100 then the tire had been in sunlight.

No, I had been driving and attributed the increase in temp to the compression and flexing of the tire.

Certainly not scientific, but in my rally trip of 1400 miles, the GY Endurance rode great, and didn't lose a smidgen of air. Best of all, they didn't blow out. (knock on wood).

Originally Posted by KK4YZ

So, Mollysdad, after all the input, what will yo go with: 65psi or80 psi?
Just curious
Jim

65

[Action]

Originally Posted by TouringDan

The other extreme seems to inflate to 80 psi. This seems to be what a lot of Airstreamers are doing. What pressure increase would you expect at 80 psi after 1.5 hours? Why not just go with 80 psi? I am not proposing this in my situation, just asking the question.

Thanks, Dan

Think Goldilocks!!!!

Too low pressure = tire wear on the outsides of the tire and over heating
Too high pressure = tire wear in the center of the tire and the tire has less contact with the road.

With in reason having the correct tire pressure in the middle of extremes is desired.

In the history of trailer and RV manufacturers, tires have been look at with cost cutting in mind. Install a tire that can cover the load. If it doesn't have much (or no) margin that is OK. So max inflation pressure is needed with that type of tire sizing.

Moving to a larger diameter wheel (if the tire/wheel assy fits) gets much more load capacity than the smaller diameter wheel.

Car and truck tires have so much extra load capacity it is almost never an issue. And max inflation is almost never required.

>>>>>>>>>>>>>Action

[CapriRacer]

Originally Posted by Action

......... Too low pressure = tire wear on the outsides of the tire and over heating. Too high pressure = tire wear in the center of the tire and the tire has less contact with the road. .........

I know this is about trailers and what I am about to write doesn't apply, but on motor vehicles, steer tires tend to wear on the shoulders and drive tires tend to wear in the centers. This sometimes fools people into thinking they have a pressure problem.

Plus, radial tires are not as prone to uneven wear as bias tires, and within the small range that can be used, changing tire pressure on a radial isn't going to change the wear enough to matter.

[tjdonahoe]

Originally Posted by CapriRacer

I know this is about trailers and what I am about to write doesn't apply, but on motor vehicles, steer tires tend to wear on the shoulders and drive tires tend to wear in the centers. This sometimes fools people into thinking they have a pressure problem.

Plus, radial tires are not as prone to uneven wear as bias tires, and within the small range that can be used, changing tire pressure on a radial isn't going to change the wear enough to matter.

Once the wear starts, it is hard to change it....

[TouringDan]

Originally Posted by Tireman9

Tire inflation is not difficult.

In trailer application, the tire load capacity based on the inflation should be at least 15% greater than the actual load measured on scale readings.



To further improve tire life in trailer application I suggest people with multi-axle trailers set the cold inflation to the inflation molded on the tire sidewall.



Maybe only a High School Diploma in tire inflation is needed.

Ok Tireman9 I have a scaled tire loading of only 1,000 lbs for my 66 Tradewind, what tire pressure do you recommend and why for my Goodyear Endurance tires, max tire pressure of 80 psi supports a load of 2,870 psi?

Thanks, Dan

[Jebeene]

Tireman, I just bought a 2016 Intnl Serenity 28.(our first!). The gal that had it ran the original Marathons, apparently at 35 psi the whole time she had it. Has at least 10k miles. She said don't worry, they are "ten year tires". I never heard of "ten year tires". I plan to replace them immediately. Thoughts? I don't think I am being too cautious.
Thanks for the great expert advice above!