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Old 07-03-2023, 04:15 PM   #1
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Tuning Tire Pressures

Tire Pressure is perhaps one of the least understood topics, even for long time RV'rs.

The tire itself is a spring attached to an axle connected to via damped spring [air or metal or ???] to a massive weight with an individual resonance all its own.

When asked what pressure they run, RV'rs respond with everything from 'what I was shop said' to 'sticker' to 'maximum' values. Some run the same pressure on single fronts as on dual rears.

A little history:
Back in the 60's our garage sponsored a stocker. I got to hang out and ask too many questions. One thing I learned was tire pressure needed to be set for tires, track and conditions. Later I was crew chief on an AMA superbike team. We adjusted tire pressure for the same parameters.

According to the missus, I've always driven too hard and too fast. That being said, I almost never used the sticker PSI and adjusted per vehicle weight distribution to ±¼lb precision, varying with season.

Our first class B sticker had the same F&R pressure. On the class A, the 'sticker' value was the manufacturer's MAX. Clearly wrong. Chassis manual values were close but superseded by tire manufacturer model change.

As I wander RV parks, I look at tire footprints and find everything from bulgy sidewalls to mini-contact patches. Recently, I asked a club member whose tires were the latter, what pressure he was running. He replied 115, the sidewall MAX. I advised him to weigh his coach axles and set to 10 pounds rear and 15 pounds front above the tire manufacturer's load pressure chart. Next time I saw him, he said "I can't believe how much better the coach rides and handles now!" [34' DP]

Which brings us to resonance. Any complex system with multiple compliances is going to have one or more sweet spots and a wide range of shillyshalliers, wanderers or boneshakers.

Being retired and in no hurry, we take US & state highways and drive truck speed on the Interstates. Each surface and speed will have an optimum pressure for every individual coach depending on length, load, suspension and tires.

So how to tune?
  1. WEIGH the coach on each corner. Adjust loading if necessary to ensure load range and balance. Reweigh if necessary.
  2. Download tire manufacturer load sheet for the exact tires on your rig.
  3. Set pressure cold. Rear to 5 PSI above and front to 10 PSI above mfg load range.

Go for a drive of at least 30 minutes and note how the coach rides and handles.

Read current hot pressure taking care not to deflate.
Increase pressure 5 psi from current [hot] reading.

Repeat same drive and note how the coach rides, handles and sounds. Pay attention for front / rear 'coherence': The front and rear feel as one when passed by a big rig or in a side wind.

Repeat above once or twice more.

Decide on the most comfortable and best handling pressures. Adjust cold pressure accordingly. Further adjustment can pay dividends. We run +10 PSI rear / +15 PSI front. [33' DP]

Measure frequently and adjusted seasonally.
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Old 07-03-2023, 04:19 PM   #2
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Oh My........Tuning tire's now.........this is a first........
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Old 07-03-2023, 04:25 PM   #3
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It is a very well understood subject.

Debated ad nauseum.

It is quite simple.

The tire manufacturers know in intimate detail what their tires can and cannot be expected to do.

Weigh your coach, follow the tire manufacturer's recommendations.

Well documented, well engineered, well understood.

Not so well followed.
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Old 07-04-2023, 04:23 AM   #4
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My tirepressure story began end 2007, when I got hold of the official used formula to calculate pressure for a load on tire and speed used.

Only by theoretical reasoning and gathering information using internet, I came to about the same reserves as you give here.

Difference is that I go from a percentage, and not a fixed psi. That way it can be used from normal car to heavy truck.
Then further experimenting is mostly not needed.

Also studied some side subject, that you might think interesting.
Then mail me at my hotmail.com adres with username jadatis ( combine yourselfes, spamm machines cant this way)
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Old 07-04-2023, 07:43 AM   #5
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Quote:
Originally Posted by palehorse89 View Post
Oh My........Tuning tire's now.........this is a first........
Quote:
Originally Posted by bentjm View Post
It is a very well understood subject.

Debated ad nauseum.

It is quite simple.

The tire manufacturers know in intimate detail what their tires can and cannot be expected to do.

Weigh your coach, follow the tire manufacturer's recommendations.

Well documented, well engineered, well understood.

Not so well followed.

My exact thoughts.
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Old 07-04-2023, 01:13 PM   #6
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Quote:
Originally Posted by palehorse89 View Post
Oh My........Tuning tire's now.........this is a first........
Tire manufacturers are concerned that their tires support the load.

They don't test all possible road, load, suspension and chassis dimension combinations. Each has an input to chassis dynamics.

Old timers will remember the Ford [?] commercial where they drove a pickup truck along a railway track. The wheels bounced like crazy and the body was steady as a rock.

Change the vehicle, speed, tie spacing / height and the body will bounce. At another speed, the whole truck would buck like a bronc.

An RV chassis has a resonance, the springs with shock damping another and the tires a third dependent on air pressure. At some combination there will be a minimum and the coach will be smoother and quieter.

At the tire manufacturer recommended pressure, our coach wallows, is more susceptible to cross winds and big rig buffeting. Higher than determined sweet spot and the rig jars and rattles. Interstate expansion joints are a nightmare requiring either too slow or too fast to compensate.

Quote:
Originally Posted by bentjm View Post
Weigh your coach, follow the tire manufacturer's recommendations.

Well documented, well engineered, well understood.

Not so well followed.
It may very well be that the recommended pressure is ideal but you won't know until you experiment.
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Old 07-04-2023, 01:44 PM   #7
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Originally Posted by IEales
It may very well be that the recommended pressure is ideal but you won't know until you experiment.
True, however I'd wager that for many of us "Good enough is good enough". We do not need the tires to stick to the track because we don't drive on a track. We slow down for turns. We drive around 60 MPH or so.

As long as I'm not overloading the tires, good enough is good enough for me.

Ray
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Old 07-04-2023, 02:28 PM   #8
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I didn't know I needed a math and science degree to set the tire PSI on my RV every trip. Then judge the feeling on ever changing road surfaces.

Then when I arrive at my destination I need a degree in electrical distrbution. I then get some home made testing equipment and run from site to site finding the right reading. Then on to connecting all kinds of electrical devices to protect the ultra sensitive electronic devices that seem to work fine on the same power in my house.

Once the proper power source is found, I then need to do a geological study on the site, to confirm that I can level my rig but still leave the proper weight on the tires. Maybe do some structural engineering to design and build supports under contact points to the ground.

Next is the quandary about how I should connect both the water and waste hoses. To much pressure, to little pressure, gray tank valve open/ closed, black tank valve position.

By the time I've got it right, it's Sunday and time to leave.
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Old 07-04-2023, 02:35 PM   #9
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We rarely drive over 55 except on the Interstate and no longer drift the corners

Sadly, I r enginerr and [according to the missus] never stop thinking...

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Old 07-04-2023, 10:52 PM   #10
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Quote:
Originally Posted by IEales View Post
Tire Pressure is perhaps one of the least understood topics, even for long time RV'rs.

The tire itself is a spring attached to an axle connected to via damped spring [air or metal or ???] to a massive weight with an individual resonance all its own.

When asked what pressure they run, RV'rs respond with everything from 'what I was shop said' to 'sticker' to 'maximum' values. Some run the same pressure on single fronts as on dual rears.

A little history:
Back in the 60's our garage sponsored a stocker. I got to hang out and ask too many questions. One thing I learned was tire pressure needed to be set for tires, track and conditions. Later I was crew chief on an AMA superbike team. We adjusted tire pressure for the same parameters.

According to the missus, I've always driven too hard and too fast. That being said, I almost never used the sticker PSI and adjusted per vehicle weight distribution to ±¼lb precision, varying with season.

Our first class B sticker had the same F&R pressure. On the class A, the 'sticker' value was the manufacturer's MAX. Clearly wrong. Chassis manual values were close but superseded by tire manufacturer model change.

As I wander RV parks, I look at tire footprints and find everything from bulgy sidewalls to mini-contact patches. Recently, I asked a club member whose tires were the latter, what pressure he was running. He replied 115, the sidewall MAX. I advised him to weigh his coach axles and set to 10 pounds rear and 15 pounds front above the tire manufacturer's load pressure chart. Next time I saw him, he said "I can't believe how much better the coach rides and handles now!" [34' DP]

Which brings us to resonance. Any complex system with multiple compliances is going to have one or more sweet spots and a wide range of shillyshalliers, wanderers or boneshakers.

Being retired and in no hurry, we take US & state highways and drive truck speed on the Interstates. Each surface and speed will have an optimum pressure for every individual coach depending on length, load, suspension and tires.

So how to tune?
  1. WEIGH the coach on each corner. Adjust loading if necessary to ensure load range and balance. Reweigh if necessary.
  2. Download tire manufacturer load sheet for the exact tires on your rig.
  3. Set pressure cold. Rear to 5 PSI above and front to 10 PSI above mfg load range.

Go for a drive of at least 30 minutes and note how the coach rides and handles.

Read current hot pressure taking care not to deflate.
Increase pressure 5 psi from current [hot] reading.

Repeat same drive and note how the coach rides, handles and sounds. Pay attention for front / rear 'coherence': The front and rear feel as one when passed by a big rig or in a side wind.

Repeat above once or twice more.

Decide on the most comfortable and best handling pressures. Adjust cold pressure accordingly. Further adjustment can pay dividends. We run +10 PSI rear / +15 PSI front. [33' DP]

Measure frequently and adjusted seasonally.
Show me just one tire mfgr. that states to ignore the Federal Tire placard in every on-road vehicle.
When adhering to load/inflation charts you are operating your tires at 100% operating capacity, no reserve. Your 10%-15% above the charts is what Tireman9 recommends too, he is a retired tire design engineer. Auto and pickup tires are generally operating with approx. 35% reserve capacity.
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Old 07-05-2023, 08:46 AM   #11
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We rarely drive over 55 except on the Interstate and no longer drift the corners

Sadly, I r enginerr and [according to the missus] never stop thinking...

My uncle was a drafting engineer. No matter what the project, big or small, he made detailed drawings and calculated to the penny, what was needed to maximize efficiency, strength, and longevity. If constructing, twice the nails, electrical, next heavier gauge wiring, plumbing, schedule 80 instead of 40, etc,,,. Yep, I know the symptoms!
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Old 07-05-2023, 09:03 AM   #12
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Quote:
Originally Posted by IEales
Sadly, I r enginerr and [according to the missus] never stop thinking...

Isn't it "interesting" how the wives seem to think that's a problem?

Me, I'd rather over-think than under-think. One way costs some time and the other way can cost money, time, and life and limb. Of course, I was a firefighter/paramedic so I got to deal first-hand with people's under-thinking, also known as "Hold my beer!", so I have been influenced by my life's experience. And my genetics.

Ray
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Old 07-05-2023, 10:39 AM   #13
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Quote:
Originally Posted by Ray,IN View Post
Show me just one tire mfgr. that states to ignore the Federal Tire placard in every on-road vehicle.
When adhering to load/inflation charts you are operating your tires at 100% operating capacity, no reserve. Your 10%-15% above the charts is what Tireman9 recommends too, he is a retired tire design engineer. Auto and pickup tires are generally operating with approx. 35% reserve capacity.
If the tire companies agreed with the tire sticker they wouldn't publish a chart of different tire inflation by weight and average speed.

My coach has a 24,000-pound rear axle and on a CAT scale weighs in at about 20,100 to 20,400 fully loaded and ready to camp. I was using the tire pressure on the tire placard of 120 PSI and I was getting uneven tire wear with the center of the tread wearing faster than the outside of the tire. The local tire company said the tire pressure in my tires was high for the load and need to decrease the pressure. The tire manufacture's chart said I should running between 85 and 90 PSI. I do like saftey margins and decided that I was going run a 100 PSI and only add air if the morning tire pressure was below 95 PSI. I getting even tire wear and I weight a couple of times a year. My front axles are 12,500 pounds and I'm running around 11,800 so I run my front tires at the placard PSI of 120 PSI.

I have been told and believe the tire placard is what you should run if the coach is at it's max weight
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Old 07-05-2023, 10:49 AM   #14
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Here we go again.

How many are OCD about the tire pressures on their daily drivers?
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