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Old 07-11-2023, 10:09 AM   #57
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Quote:
Originally Posted by NXR View Post
The formula is a 2% change in pressure for each 10 degrees F change in temperature, up or down. 100 PSI tires would see a 6 PSI change, not 3.

Ray
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Correct 2% / 10°F. Brain fart going the wrong way in °C to °F
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Old 07-11-2023, 11:33 AM   #58
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Tires are designed to carry a given load at a given speed at a given temperature. The manufacturer's tables assume a speed and temperature, then recommend pressures based on varying the load. Since all of these factors are variable, finding the perfect pressure to run is not always the same answer.
The tire is designed to flex in a specific area on the sidewall of the tire. The best ride and handling are achieved when the flexing takes place in the designed area.
The pressure in a "cold" tire (one that has not been running down the road for several hours) should increase once it is on the road and has been working for 30 minutes or more. The increase should be no more than 10% of the starting pressure. ie: If the starting pressure is 90 psi, the "hot" pressure should be no more than 99 psi. If the pressure is more than 99 psi in this case, add 10 psi and continue to monitor it.
If the "hot" tire is still at 90 psi, it is overinflated. This is almost always the case when the maximum pressure listed on the sidewall of the tire is used instead of the recommended pressure for the load that the tire is actually carrying.
The nice feature of this tire pressure tuning technique is that it can be done at any time and you do not have to know the actual weight that the tire is carrying. For those who have a TPMS, it will let you observe this without having to stop and take the pressures.
May all your trips be smooth.
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Old 07-11-2023, 12:22 PM   #59
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Originally Posted by Donskiman View Post
I look at my TPMS in the morning before driving. Depending on outside temp sometimes a colder temp will put the PSI slightly below. I don't mess with adding 1-3 PSI. I look at what the PSI rises to during driving and it goes up to the same pressure regardless of not adding more when it's really cold.
Does the TPMS compensate for temperature? It could have a thermometer in each sensor and normalize to STP.

[Years ago, I hired a vehicle in the summer back east. We were staying high up in the mountains where it was quite cool at night. Every morning the built-in TMPS said the tires were low. Once the sun came up and warmed the air, no problem. I verified the pressures at a station a mile or so away and they were sticker values.]

Quote:
Originally Posted by AccessMaster View Post
Tires are designed to carry a given load at a given speed at a given temperature. The manufacturer's tables assume a speed and temperature
Never seen a temperature in a tire load table.


The original intent of the thread was to adjust pressure to your vehicle and the roads you drive.

Road surface, tires, suspension and coach mass all interact.

If I only drove the Autobahn in our rig, I'd definitely use different pressures than in the US. Once when driving across Germany, I came across a few MAX 125KMH signs. I asked my friend why as the road seemed fine. He stated "Bad surface." Once I stopped laughing, I said "Wait 'til you come to California!" He did & he didn't...
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Old 07-11-2023, 03:03 PM   #60
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Quote:
Originally Posted by IEales View Post
Does the TPMS compensate for temperature? It could have a thermometer in each sensor and normalize to STP.

[Years ago, I hired a vehicle in the summer back east. We were staying high up in the mountains where it was quite cool at night. Every morning the built-in TMPS said the tires were low. Once the sun came up and warmed the air, no problem. I verified the pressures at a station a mile or so away and they were sticker values.]
Yes, once the outside air warms the PSI rises so starting PSI on a cool morning is lower than on a warmer morning. It only differs by 1-3 PSI at most though. I really see no need to go through all the mental gymnastics when the method I'm using works just fine.

BTW, the person who mentioned the 10% rule hasn't ever worked for my tires regardless of what the starting PSI is. My inner dually's will increase from 85 up to 103-105 on a hot day and right around 100 on a cooler day. Outer tires increase from 85 up to 101-102 on a hot day, 98-99 on a cooler day. When I was using 100 PSI as a starting value they would increase to over 120 and the ride was very harsh. Perhaps these rule of thumb calculations don't always work for every brand of tire.

Not sure why Tiffin recommends 100 PSI all around when based on the axle weights and the tire manufacturer charts, this is overinflated.
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Old 07-11-2023, 04:03 PM   #61
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Originally Posted by Donskiman View Post
Yes, once the outside air warms the PSI rises so starting PSI on a cool morning is lower than on a warmer morning. It only differs by 1-3 PSI at most though. I really see no need to go through all the mental gymnastics when the method I'm using works just fine.

BTW, the person who mentioned the 10% rule hasn't ever worked for my tires regardless of what the starting PSI is. My inner dually's will increase from 85 up to 103-105 on a hot day and right around 100 on a cooler day. Outer tires increase from 85 up to 101-102 on a hot day, 98-99 on a cooler day. When I was using 100 PSI as a starting value they would increase to over 120 and the ride was very harsh. Perhaps these rule of thumb calculations don't always work for every brand of tire.

Not sure why Tiffin recommends 100 PSI all around when based on the axle weights and the tire manufacturer charts, this is overinflated.
I agree with those pressure increases. I would consistently see a 15% increase in pressure on a heavily loaded tire plus any change caused from increase in OAT. 120 psi on the steers @70 degrees could go to 144 psi @ 100 degree OAT. Less on cooler days with no sun. When I contacted Michelin with theses numbers, the tech said it was all normal and not to be concerned. I wasn't. I sold those tires @5 years with 50,000 miles.
I should add that the load inflation chart called for 115 psi and the sidewall max cold was 120 psi.
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Old 07-12-2023, 02:13 PM   #62
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Originally Posted by jadatis View Post
Depends on the cold filled pressure, so is for for instance 30 psi filled 3% /10 degr F, as rule of tumb.

I made a list for cold filled pressure at 70 degr F.
Then behind that the degrF change per 1 psi.
Is easyer to calculate with on the road, and you only have to remember for your cold filled pressure at 70 degr F. But if you use it for 60 degr F cold filled, it wont give dramatic differences.

100psi gives 4.5 degr F / psi.
30 degr F/ 4.5 degr F = 6,7 psi
And that is dry gascompound, no water in it.
So for 100 psi, 2% is a good rule of tumb.


Here it is,

70degrF./degrF/psi

20 psi/ 15,5F/psi
snip
177 psi/ 3 F/psi

Sorry jadatis but the Science of the "Gas Law" does not support your answer pf 1% per 10°F 2% for 10F is close enough unless you have a pressure gauge that reads to 0.5 psi accuracy. Please stop trying to re0invent the wheel with your theories on tire inflation.



So are you are ready to present your findings and collect your Nobel Prize and show that Robert Boyle, Jacques Charles, Joseph Louis Gay‑Lussac, Benoît Paul Émile, and Edme Mariotte, were all wrong.

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Old 07-12-2023, 02:24 PM   #63
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Originally Posted by MAXEPR View Post
If you really want to be OCD, you would stop every 100 miles and adjust for the weight, outside air temperature, and barometric pressure changes.
Tires have issues if they are over or under-inflated. But there is a fairly wide margin in the middle where they can operate safely and you wouldn't be able to see any wear difference.
And if you are going to operate like this I would have at least two sets of gauges that are calibrated monthly.



No you should NOT be adjusting pressure during your travel day unless you are stopped for more than 2 hours and have shielded the tires from direct sunlight.
If you set it in the morning when tires are at ambient your tires will respond according to your speed and load.


Unless of course if you set it in Death Valley and then want to drive around an indoor ice rink




People we are not talking Rocket Science.
Learn the weights
Consult the tables
Set your pressures with my suggestion of +10% on the PSI number to give you some cushion.




Unlike some on this thread I am an actual Tire Design Engineer and a court recognized Forensic Expert on tires in RV application. So please stop making this more difficult than it is for some to accept actual Science.
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Old 07-12-2023, 03:16 PM   #64
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Quote:
Originally Posted by jadatis View Post
Depends on the cold filled pressure, so is for for instance 30 psi filled 3% /10 degr F, as rule of tumb.

I made a list for cold filled pressure at 70 degr F.
Then behind that the degrF change per 1 psi.
Is easyer to calculate with on the road, and you only have to remember for your cold filled pressure at 70 degr F. But if you use it for 60 degr F cold filled, it wont give dramatic differences.

100psi gives 4.5 degr F / psi.
30 degr F/ 4.5 degr F = 6,7 psi
And that is dry gascompound, no water in it.
So for 100 psi, 2% is a good rule of tumb.


Here it is,

70degrF./degrF/psi

20 psi/ 15,5F/psi
21 psi/ 15F/psi
22 psi/ 14,5F/psi
23 psi/ 14 F/psi
24 psi/ 13,5F/psi
25 psi/ 13,5F/psi
26 psi/ 13 F/psi
27 psi/ 12,5 F/psi
28 psi/ 12,5 F/psi
29 psi/ 12 F/psi
30 psi/ 12 F/psi
31 psi/ 11,5 F/psi
32 psi/ 11,5 F/psi
33 psi/ 11 F/psi
34 psi/ 11 F/psi
35 psi/ 10,5 F/psi
36 psi/ 10,5 F/psi
37 psi/ 10 F/psi
39 psi/ 10 F/ps
40 psi/ 9,5 F/psi
42 psi/ 9,5 F/psi
43 psi/ 9 F/psi
45 psi/ 9 F/psi
46 psi/ 8,5 F/psi
49 psi/ 8,5 F/psi
50 psi/ 8 F/psi
53 psi/ 8 F/psi
54 psi/ 7,5 F/psi
58 psi/ 7,5 F/psi
59 psi/ 7 F/psi
63 psi/ 7 F/psi
64 psi/ 6,5 F/psi
70 psi/ 6,5 F/psi
71 psi/ 6 F/psi
77 psi/ 6 F/psi
78 psi/ 5,5 F/psi
86 psi/ 5,5 F/psi
87 psi/ 5 F/psi
96 psi/ 5 F/psi
97 psi/ 4,5 F/psi
109 psi/ 4,5 F/psi
110 psi/ 4 F/psi
126 psi/ 4 F/psi
127 psi/ 3,5 F/psi
148 psi/ 3,5 F/psi
149 psi/ 3 F/psi
177 psi/ 3 F/psi
Some understand my list wrong.
No percentage per 10 degr F.
But howmany degrees Fahrenheit is needed to change the tirepressure 1 psi.
This is easyer to calculate on the road, even when driving, and reading TMPS.
And you only have to remember your cold filled pressure.

So determine needed pressure, and fill that at an average ambiënt temperature of 70 degr F.
Then on the road only check, enaugh is by means of tmps, and if you see cold a pressure that would be, calculated back to 70 degr F, change nothing.

Then hot inside tire temperature I estimate on 140 degr F. max allowed wen outside 70 degr F.
And at 100 degr F outside, 160 degr F in tire.

Mayby I have to adyust these temperatures in time, when I learned from readings in real world. Also mayby in time it apears that cold pressure was determined by the tiremakers for 60 degr F cold filled.

And this is without external factors, like sunshine on tire, severe braking, hanging brake or bad wheelbearing, exaust pipe pointing on tire, and mayby I forgot one.

These warm temps can be calculated from pressure rising, and if you have external factors, you know at that moment.

Then never adyust pressure downward, only up.
This counts for warm and cold pressure.
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Old 07-12-2023, 05:26 PM   #65
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Unlike some on this thread I am an actual Tire Design Engineer and a court recognized Forensic Expert on tires in RV application. So please stop making this more difficult than it is for some to accept actual Science.
No one is denying science. It's also a scientific fact that everything has a natural resonance that can be damped or shifted and that temperature can affect the resonant point. Damping can reduce a resonance, but may only shift it to another frequency. When resonant systems are coupled, they can increase, decrease or create new resonances.

"No Highway in the Sky" is a film made the year I was born and I first saw it on TV about the time I was becoming interested in HiFi and sound in the late 50's. Jimmy Stewart, playing a boffin, hypothesizes the tail will fall of the Rutland Reindeer after so many hours due to metal fatigue from resonances.

Years later in various careers, damping resonances was often a priority. It still is today to prevent the Home Theatre from shaking the house down.

Perhaps you'd care to comment on the original intent that ride and handling can be altered by adjusting tire pressure to suit vehicle, suspension, load, speed and road surface?

AND I'm sure we'd all be interested in the forensics of tire failure as relates to Class A motorhomes.
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Old 07-13-2023, 03:22 AM   #66
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Shockbreakers ( as we call dampers for cars in dutch, now freely translates), do their work lineair, as far as I know.

Now tires also have a damping fuction., but I concluded that it is not lineair.

I concluded this of the rule of tumb that deflection is quadratical related to the treathsurface on the ground, and so the force /weigt on tire.
So. If weight 2 times as much, 2 times as much surface on the ground, but 2 x2 = 4 times as much deflection of tire
1,41 times as much weight /force on tire 2x as much deflection
If this now gives progressiv or degressiv damping, I dont know.

Was driving behind a agricultural combination, and they use low pressure, even on road.
Saw the tires deflect more only once when going over a traffic- bump(?), so this suspects a good damping.

Mayby something to do research on.
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Old 07-14-2023, 11:48 AM   #67
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No one is denying science. It's also a scientific fact that everything has a natural resonance that can be damped or shifted and that temperature can affect the resonant point. Damping can reduce a resonance, but may only shift it to another frequency. When resonant systems are coupled, they can increase, decrease or create new resonances.

"No Highway in the Sky" is a film made the year I was born and I first saw it on TV about the time I was becoming interested in HiFi and sound in the late 50's. Jimmy Stewart, playing a boffin, hypothesizes the tail will fall of the Rutland Reindeer after so many hours due to metal fatigue from resonances.

Years later in various careers, damping resonances was often a priority. It still is today to prevent the Home Theatre from shaking the house down.

Perhaps you'd care to comment on the original intent that ride and handling can be altered by adjusting tire pressure to suit vehicle, suspension, load, speed and road surface?

AND I'm sure we'd all be interested in the forensics of tire failure as relates to Class A motorhomes.







Ian
I think we have been talking at cross purposes.
For cars and to some point Light Trucks the pressure is evaluated extensively by Vehicle Dynamics Engineers that work for the car companies along with Engineers from the tire and Shock MFG. This process usually takes a couple of years as a change in tire construction to address one property such as fuel economy or noise will probably affect other NVH (Noise Vibration Harshness) which is a broad category that includes other features such as steering response, On Center Feel etc.


I my career I had a number of opportunities to ride along with the Dynamic Specialists and to feel, first hand the results of minute changes in the suspension. In one case we evaluated two different wheel designs. The dimensions for tire mounting were identical and we even ensured that the 4 tires were on the same position. The only difference was the design of the cast aluminum wheels.



Most cars are working with a range of inflation that results in a +20 to + 35% extra load capacity so they have significant latitude to "tune the components", including tire inflation to achieve the goal the auto company has for that vehicle.


RV companies, to my knowledge, do not do any tire evaluations either with Class A, B, or C motorhomes that have had the tire selected by the chassis MFG. Tow-able RVs are a completely different story.


For these vehicles RV assemblers for the most part select tires that meet the minimum DOT requirements for load capacity. This results in most applications requiring the max load capacity from the tire which in turn requires the highest inflation withing that Load Range tire. There is no allowance for "Tuning" the pressure as an increase in pressure will not result in any increase in Load Capacity and a decrease in inflation will not provide sufficient load capacity to meet Federal Safety Requirements.


What you covered in your original post is true. That changes in inflation pressure will change the bounce frequencies etc. But meeting Federal Safety Requirements is not optional so we end up with no room to lower tire inflation so your observations become theoretical but not applicable to the vast majority of RVs.


The inflation of some Motorhomes may allow a lowering of PSI BUT care must be taken to ensure the tires are never overloaded. Data from tens of thousands of motorhome weight measurements shows that the majority of motorhomes have one or more tire and or axle in overload so the reality is that most owners do not have the skill, or knowledge, or ability to "Tune" their tire inflation to get better ride or noise comfort without increasing the potential for tire failure which is certainly more important than ride comfort.
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Old 07-14-2023, 12:55 PM   #68
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@ Tireman. Great post.

Point 3 in the original:
Quote:
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  • Set pressure cold. Rear to 5 PSI above and front to 10 PSI above mfg load range.
Written better:
Set pressures w cold tires.
Inflate Rear to 5 PSI above measured loaded rear axle weight according to the TIRE manufacturers load sheet.
Inflate Front to 10 PSI above measured loaded front axle weight according to the TIRE manufacturers load sheet.

Thanks for participating.
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