Tuning Tire Pressures

[Crasher]

The load inflation table will give the best and most uniform wear. I proved it to myself over 4 years and 84,000 miles with a set of Uniroyal tires on a 6400# Avalanche. It was a pain in the ???? to reweigh and adjust when the load changed or a big change in temperature, but it was just an anal thing I wanted to do and I had access to a scale at no charge. I rotated every 10,000 miles. As I recall the rear was @28 psi and the front was 30. At first, I was hesitant to run that low, but the Uniroyal table called for those numbers with the weight they were carrying. They still had legal tread at 84,000 miles and had uniform wear, but we had a long trip coming up and I was satisfied with my experiment. Since then, I adhere to the load inflation tables, but add a few psi for convenience when the temp drops. They might not be perfect, but it's good enough for me. I guess I just got lazy.

[IEales]

Originally Posted by Crasher

The load inflation table will give the best and most uniform wear. ...

Quite probably.

Most Class A drivers I've spoken to drive a fair bit above Load Table pressures and change tires due to age long before the odometer says we should...

[Crasher]

Originally Posted by IEales

Quite probably.

Most Class A drivers I've spoken to drive a fair bit above Load Table pressures and change tires due to age long before the odometer says we should...

[Quote: I adhere to the load inflation tables, but add a few psi for convenience when the temp changes ]

As I stated, I do the same, not for safety reasons, but for the convenience of not having to chase the pressure changes from OAT changes. I change out the MH tires at 5 years, as they still have value to sell to the seasonal vegetable truckers in my area.

[sibe]

WOW.... air pressure wars, oil wars.. shock wars....

Really interesting Talk..

Overall some common sense, some fact in the sense of 4 corner weight and tires correlate....
The Placard on your door is a good starting place BUT it is subject to time and the advances or changes in tires as years go by....
Lastly, the term Motorhome in this section of the forums.. can have many styles on the MH from the Van chassis to Med Truck to almost Bus/rig design..

Watching the TPMS and shifting load and or weighting every other trip is a bit over the top IMO..

Kick the tires, walk around the MH, bend a bit for alook-see, get in and drive her, enjoy her........

[Holland2017]

Weighed my coach, set tire pressures according to tire mfg recommendations.Then drive in 85-95 degree temperatures and a those tire pressures change so now what am I surpose to do? Just asking for friend.

[CWSWine]

Originally Posted by Holland2017

Weighed my coach, set tire pressures according to tire mfg recommendations.Then drive in 85-95 degree temperatures and a those tire pressures change so now what am I surpose to do? Just asking for friend.

You set them according to the tables at 100 and they increase to 20 PSI to 120 PSI. Set them at the max of 125 and they go up 20 PSI now you are at 145 PSI. I guess what you are comfortable with.

[IEales]

Originally Posted by Holland2017

Weighed my coach, set tire pressures according to tire mfg recommendations.Then drive in 85-95 degree temperatures and a those tire pressures change so now what am I surpose to do? Just asking for friend.

Tire pressures are normally set cold, which means indoors in the barn or outdoors before the sun hits them in the morning.

If I set them on the coast @ 60°F and it's 85°F in the mornings before the sun hits them, and I was going to be running for several days, I'd adjust.

Then ask myself, "What the heck am I doing here?"

Pressure is proportional to temperature: P2 = P1 * T2 / T1. T is Kelvin

68°F = 293.15°K, 86°F = 303.15°K

So 90PSI increases to 93PSI for temperature rise from 68°F ot 86°F.
93 = 90 * 303 / 293
3PSI = Not worth worrying about.

If I was running close to max sidewall pressure, I'd be more concerned, especially if I was a pedal to the metal type.

[CWSWine]

Originally Posted by Holland2017

Weighed my coach, set tire pressures according to tire mfg recommendations.Then drive in 85-95 degree temperatures and a those tire pressures change so now what am I surpose to do? Just asking for friend.

You set them according to the tables at 100 and they increase to 20 degrees to 120 PSI. Set them at the max of 125 and they go up 20 degrees now you are at 145 degrees. I guess what you are confortable with.

[Gr8bawana]

Originally Posted by IEales

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.

You've got way too much time on your hands. I can assure you that NOBODY is going to weigh each corner of their coach multiple times and test drive between weighing. Even if they had the means to do so.

[jadatis]

Originally Posted by IEales

Tire pressures are normally set cold, which means indoors in the barn or outdoors before the sun hits them in the morning.

If I set them on the coast @ 60°F and it's 85°F in the mornings before the sun hits them, and I was going to be running for several days, I'd adjust.

Then ask myself, "What the heck am I doing here?"

Pressure is proportional to temperature: P2 = P1 * T2 / T1. T is Kelvin

68°F = 293.15°K, 86°F = 303.15°K

So 90PSI increases to 93PSI for temperature rise from 68°F ot 86°F.
93 = 90 * 303 / 293
3PSI = Not worth worrying about.

If I was running close to max sidewall pressure, I'd be more concerned, especially if I was a pedal to the metal type.

If you want to play with the pressure/temperature relation, you can use my made calculator, in wich I also give it for wet gascompound in tire, in that I am first, I think.

If you want to try it, mail me at my hotmail. com adres with username jadatis.
Combine yourselfes, spamm machines cant this way.

[IEales]

Heat from underinflation is what destroys a tire. The sidewall delaminates and the pressure blows it out.

Burst pressure is well above what can be achieved by running a properly inflated undamaged tire.

Higher pressure reduces sidewall flex, so temperatures could rise less.

Recommended pressures are to put the maximum rubber on the road, ignoring comfort and the vehicle they support.

[NXR]

Originally Posted by IEales

Recommended pressures are to put the maximum rubber on the road, ignoring comfort and the vehicle they support.

Sort of. Comfort is not a consideration.

The load inflation charts are not a "recommended" pressure. They simply show how much pressure to put in a tire to assure the tire can safely support that particular weight.

The sticker inside is the minimum pressure that is needed in the tires installed by the final vehicle builder in order to safely support the Gross Axle Weight Rating of each axle, so that one does consider the vehicle.

Ray

[IEales]

Originally Posted by NXR

The sticker inside is the minimum pressure that is needed in the tires installed by the final vehicle builder in order to safely support the Gross Axle Weight Rating of each axle, so that one does consider the vehicle.

Mfg Sticker is 110PSI or ≈3000LB over Max axle load:
https://www.irv2.com/forums/attachme...1&d=1688912936

Tire Mfg recommended pressure chart inserted in the chassis manual.

Michellin is 27PSI LESS for MAX axle load:
https://www.irv2.com/forums/attachme...1&d=1688912936

Running 30PSI over required reduces the contact patch, wears the tire center prematurely and negatively affects handling and comfort.

Not all RTFM...

[jadatis]

Checked the Michelin list to be made with the official in Europe used formula for decades for all kind of tires, and since 2006 also for P-tires in US. So not bad calculation.
Done first in kg and kPa ( 100 kPa is 1 bar= 14.5 psi) and rounded, then kPa conversed to psi and rounded, and kg conversed to lbs and rounded.

When I make a list for psi and lbs, I use directly those and only round at the end.