PSI

Forgot to touch on the fact that tires have many duties

Air spring to hold up weight (the down arrow below is weight and the
other arrows hold up and keep the tires shape)

Provide directional control

Provide acceleration media

Provide deceleration media

Provide suspension dampening

PIS is *NOT* just to hold up weight, but to also keep the tires form during
all of the above duties

Back on over stressing a wheel rim. Made this image years ago for anther forum
showing how the air/PSI inside pushes in all directions of the inside of a tire
and the wheel itself



The 'moment' or 'lever arm' is where the tire bead meets the wheel rim and the
force from PSI is outwards.

The rim holds back the tire bead

All of the above duties of a tire also constantly transfers all of the forces
to the wheel rim

PS...being a technologist most all of my life...see EVERYTHING from
the technology side of things...then the designer side kicks in to
see what they designed to and how it can be better...if possible

PPS...PSI...pounds per square inch adds up in a hurry

How many square inches are there in a tire bead/rim area?

Say an inch of contact 360*.

So a 18" dia:

Circumference = pi x Diameter = pi x 18 = 56.52" Area

Times PSI

So a 50 PSI tire on a 18" wheel will have 56.52 x 50 = 2,826 pounds of
force on the wheel rim/bead

So a 60 PSI tire on a 18" wheel will have 56.52 x 60 =3,391 lbs of force
on the wheel rim/bead

So a 'small' change in PSI can be a large force to deal with

{edit}...dumb...figured a solid circle area...not just the rim..fixed now

Gosh, Ben, you are so technical at the moment!!!

Really, I'm just wondering if anybody in the "real world" has ever had problems by replacing a C tire with a D tire on a wheel stamped for 50 psi. I have been running my Marathon 205/75R15 C's at 55 psi for the last 4-1/2 yrs with no problems. Really!! Marathons!!! And the same with our last trailer for 6 yrs (Marathons again!!!).

I just started thinking aboot this upon first seeing this thread and after talking with my son (Ford Service Manager) who mentioned that a lot of Ford PU owners who have steel wheels have been damaging wheels when they go from "P" tires to "LT". Mostly (for his shop) commercial trucks, and obviously a great change to tire pressure!! Door sticker psi for a P, to 70-80 (or more!!) with an LT.

Going from 50 psi wheel spec up to say 55-58 with a D tire really shouldn't put the same kind of forces on the wheel as say the 65 psi max of a D.

Agin, just thinkin!!!!!

TUCQUALA wrote:
Ok, ready for the shootout!!!

Has anybody out there replaced C rated with D rated on orig wheels (rated for C)??? In other words installed a tire which has a max inflation pressure 10 to 15# above the wheel rating??

Just a wondering aboot moment!!!!

Moment as an engineering term?

If so, yes there is lever arm on all of the wheel types we talk about
here


tirerimbead
This image is of a bicycle wheel, but not much different to automotive
with a bead on the wheel. The 'moment' or 'lever arm' is the upright
from the horizontal portion of the wheel

The tire bead works to try and bend that rim bead. The more PSI, the
greater the force trying to bend that rim bead

I've personally worked on a cracked steel wheel when working as a tire
monkey through college. He had been to another tire shop and they just
put a new tire on...leaked and then put a tube in a tubless tire, so
it went flat from the tube rubbing inside of a tubeless tire

It was a wheel with a 'P' class rating and they put in a higher class
tire with it's higher PSI requirements

How much will it take to crack a rim? No idea, as with the flexing
and the weight involved as factors...must say it depends...





These are crack rim beads. Don't know what from, but typically from
over pressure (PSI). Few know that all wheels flex when rolling and
the above moment (lever arm) is in a continual flexing to fatigue the
metal (both steel and alloy)

rimcrack

rimbeadcrack

beadcrack

wheelbeadcrack

wheelrimcrack

PS...put the wording underneath my images as a shorthand to find them
next time and not have to log into my web hosting site...yeah...lazy

bdpreece wrote:
None really. You need to go to a scale and get loaded weight then go to tire manufacturers charts for correct PSI.

EXACTLY! Then add 5psi per GY Tech Support

TUCQUALA wrote:
Ok, ready for the shootout!!!

Has anybody out there replaced C rated with D rated on orig wheels (rated for C)??? In other words installed a tire which has a max inflation pressure 10 to 15# above the wheel rating??

Just a wondering aboot moment!!!!

I replaced C with E rated and a size larger tire on the same rims. No tire problems for the 6 years we used the trailer.

Replaced D with E rated on current trailer. Same size tire and same rims. Season one, no tire problems.

Rims have been weight rated for several years, no mention of PSI.

It sounds like the tires have been upgraded from LRC to LRD and you want to run them at 65 psi as per the sidewall, not the placard psi.

From Roger Marble's RVtiresafety.com blog:

Trailers: You should set the Cold Inflation to the pressure on the tire sidewall. If you look at the sticker on the side of your trailer you should find the tire size, type, Load Range and pressure recommendation from the manufacturer. In almost all cases the recommended inflation is the inflation on the sidewall of the tires.

Basically running a tire when low by 20% or more, can damage the body cord.

It is well documented that as a group, RV tires are more likely to be overloaded and or underinflated than any other type of tire. Somewhere he says about 1/2 of all RVs.

Roger also highly recommends going to a scale to find the weight on each tire to ensure none are overloaded. You can't raise pressure to increase a tire's load rating though.

For GY Marathon tires only, you can increase the pressure by 10 psi to 75 psi and be able to run at max. 75 mph but only if your rims and valves can handle 75 psi. GY tech. bulletin

PSI is not a difficult concept to work with. Use it to your advantage......or not.

"This bulletin provides important information to help your customers obtain the best performance from “Special Trailer” tires. Please review the following important points with your trailer tire customers. Special Trailer (“ST”) Tires: Goodyear Marathon trailer tires are widely used in a variety of towable trailer applications and are designed and branded as “ST” (Special Trailer) tires. Industry standards dictate that tires with the ST designation are speed rated at 65 MPH (104 km/h) under normal inflation and load conditions. Based on these industry standards, if tires with the ST designation are used at speeds between 66 and 75 mph (106 km/h and 121 km/h), it is necessary to increase the cold inflation pressure by 10 psi (69 kPa) above the recommended pressure for the rated maximum load."

smkettner wrote:

2oldman wrote:
Google: over-inflated tires

5 psi is not going to cause the wild examples shown.

I would (and do) run 65 psi cold on my 50 psi trailer just fine.
(tires rated 65)

Wild not likely, this is what happens when you don't run the correct tire pressure for the load.
Yes, running over inflated might run cooler, but I would rather have full tire contact with the road than run a few degrees cooler.
I run LT tires, so I am not worried about a few degrees.

TUCQUALA wrote:
Ok, ready for the shootout!!!

Has anybody out there replaced C rated with D rated on orig wheels (rated for C)??? In other words installed a tire which has a max inflation pressure 10 to 15# above the wheel rating??

Just a wondering aboot moment!!!!

I replaced the D rated tires on my old 1 ton with E rated tires, using the original wheels.
I have D rated tires on the original wheels on my Ram 1500 that had P tires on it from the factory.
No problems in either case.

2oldman wrote:
Google: over-inflated tires

5 psi is not going to cause the wild examples shown.

I would (and do) run 65 psi cold on my 50 psi trailer just fine.
(tires rated 65)