Why 4 corner TP measurement?

[jwmaustin]

It seems most everyone weighs with full fresh tank, full fuel tank and empty black/gray. Why don't we collect all the weights that are coming from those who do the four corner measurements (including model number and other factors) and just see how much difference it is. If a given model is off from the factory then we need to know. If most recommended tire pressure settings are about the same then we prove a point. Data would minimize the number of opinions.

Who is up for that?

[hydro45640]

How many times have any of you saw an over the road trucker weigh the corners of their load and adjust the tire pressure.
I set my pressure at 85# on the rear and 80# on the steering. Never had any problems in ten years.

[DGundy]

Exactly! 🚍🤠

[Gary RVRoamer]

I'm always amazed at how this fairly simple matter quickly devolves into a nearly religious debate. 4-corner weighing is a Good Thing, especially as a one-time thing to determine if an RV does in fact have off-balance issues, but hardly a life or death matter. If 4-corner weighing is not convenient, use a rule-of-thumb estimate to cover the probability that one end of each axle is heavier than the other and then get on with life.

Tireman9 suggest allowing for a 3% heavy end (53% of actual axle weight), but for the lighter duty axles, say under 8000 lbs, I like 5% better. 3% isn't much total lbs when we talk about 4000-5000 lb front axles found on typical Class C's (E450 chassis). Take your pick (and he is a tire pro, not me).

If you can't get to a scale soon, use the axle rating (GAWR) and assume one end of that is heavy by 3% or more. The psi shown on the tire federal placard is probably based on the GAWR but assuming a 50/50 distribution.

[Crasher]

Originally Posted by hydro45640

How many times have any of you saw an over the road trucker weigh the corners of their load and adjust the tire pressure.
I set my pressure at 85# on the rear and 80# on the steering. Never had any problems in ten years.

Over the road trucks have axles with ratings that exceed any legal load they will carry. They inflate their tires to max which exceeds any weight the axles will carry. They are not interested in ride comfort and because they are towing a loaded trailer, unlike motorhomes, do not have to be concerned about handling characteristics. You have chosen 85 and 80 psi and got lucky. It works for you. Do like the truckers do and inflate your tires to the max on the sidewall and see how it handles. Better yet, if you were to do a four position weight, you might find that you could lower the psi for an even better ride than you have had for 10 years. Four position weighing is a proven way to carry the proper tire pressure, even if some don't agree.

[Mr_D]

I just saw the title of the thread and was trying to figure out why you were measuring TP (toilet paper)!

My owners manual is different than most:

Tire Industry Changes
The tire industry, as a whole, has changed its traditional stance on adjusting cold tire inflation pressure for RV tires installed on recreational vehicles and busses. Previously, tire manufacturers supported a policy where tire inflation could be adjusted according to the actual loaded weight of the vehicle. Now, the major tire manufacturers recommend that medium duty truck tires be maintained at the pressure that corresponds to the Gross Axle Weight Rating for the axle to which they are mounted. To make this recommendation uniform across the industry, tire manufacturers strongly urge the consumer to keep all tires inflated to the pressures recorded on the Federal Tire Label.

Tire Inflation
Country Coach recommends that the cold tire inflation pressures should at all times be maintained at the inflation pressure(s) recorded on the Federal Tire Label. There are no acceptable circumstances where tire inflation pressure(s) should be reduced below that pressure recorded on the Federal Tire Label.

[Murf2u]

Originally Posted by Crasher

Over the road trucks have axles with ratings that exceed any legal load they will carry. They inflate their tires to max which exceeds any weight the axles will carry. They are not interested in ride comfort and because they are towing a loaded trailer, unlike motorhomes, do not have to be concerned about handling characteristics.

You have chosen 85 and 80 psi and got lucky. It works for you.

Four position weighing is a proven way to carry the proper tire pressure, even if some don't agree.

Set your tires as precisely as you like ...........

Then go for a drive N/S for an hour or two on a warm sunny day, pull over and check your tire pressures again.

Then come back here and tell us how much a couple of PSI in one tire over another makes.

BTW, a 3% difference on an axle carrying even 10,000 pounds (likely way more than your front axle) is 300 pounds.

Do you adjust your tire pressures based on whether or not someone is in the passenger seat?

Religious indeed.........

[Crasher]

Originally Posted by Murf2u

Set your tires as precisely as you like ...........

Then go for a drive N/S for an hour or two on a warm sunny day, pull over and check your tire pressures again.

Then come back here and tell us how much a couple of PSI in one tire over another makes.

BTW, a 3% difference on an axle carrying even 10,000 pounds (likely way more than your front axle) is 300 pounds.

Do you adjust your tire pressures based on whether or not someone is in the passenger seat?

Religious indeed.........

I'm not sure what you are trying to say, but you seem to feel a four position weight is unnecessary and a waste of time. Maybe for you it is. When I did it, I discovered that one front tire was 1000# heavier than the other and was over it's load rating, however the total axle weight was below it's 15,680# rating. After correcting the setting of a rear ride height valve, both front tire are within 100#'s of each other and both are below their rating. Without a four position weight, I would have continued to run a front tire 300# over it's max rating. That might not be important to the guy that just randomly picks a pressure for his tires and hits the road, but it seemed important to me. Some might call that Religious, but I call it being better informed for safer travel.

[Ray,IN]

Originally Posted by Mr_D

I just saw the title of the thread and was trying to figure out why you were measuring TP (toilet paper)!

My owners manual is different than most:

The major tire mfgrs. publish a load/inflation chart to show the minimum pressure for the corresponding load. Then they state that tire pressure should never be below the federal tire placard numbers. I'd rather not be operating with tires running at their maximum capacity or having only 10% reserve.

Michelin, Goodyear and Bridgestone all have that stated somewhere on their load/inflation charts.
I too set pressure to federal tire placard numbers. Some are concerned about obtaining a softer ride, Goodyear says that is dangerous and ineffective.

[Wench]

Just got weighed in St. Cloud by the pros of RVSEF (RVSEF - RV Weighing & Safety Education) and are able to lower the front by 20 lbs and the rears by 25 lbs. Looking forward to a softer and safer ride!

[Murf2u]

Originally Posted by Crasher

I'm not sure what you are trying to say, but you seem to feel a four position weight is unnecessary and a waste of time.


Some might call that Religious, but I call it being better informed for safer travel.

What I'm saying is that a lot of people seem to think weighing their rig and reducing tire pressures below that specified on the placard, often reciting the 'better ride' idea, is the best way to do it.

They are misinformed. Running tires below that specified on the Federally required placard is potentially dangerous and could lead to other issues and liabilities. In the case of a puncture or other slow leak it would result in a tire reaching a dangerously deflated pressure sooner than if it would have if run at a higher pressure.

As stated previously, no tire manufacturers condone running pressures lower than specified.

From the Toyo tires website (bolding is theirs not mine);

"Reducing Inflation pressure – Vehicle Certification Label:

Under no circumstances should the tire inflation pressure be reduced below that stated on the vehicle’s tire information (certification) placard to achieve improvements in ride comfort. If you do not know where the tire information placard is, contact your vehicle manufacturer for its location and tire inflation recommendation. For RVs, the certification label is usually placed on the wall or bulkhead to the left of the driver’s position.

[Crasher]

The tire manufacturers print a load chart stating the pressures needed to support a given load. Testing has shown what the correct pressure should be for a given load. That correct pressure gives the best comfort, control and tread wear. I've used that method on all of my vehicles for over 50 years with great success. The pressures listed on the vehicle placard assumes axles are loaded to their max. Some coaches are at maximum rating and should be run at the stated pressure. Many coaches especially those with tags and below their maximum axle ratings and can safely be run at lower pressures according to the tire manufacturer's load chart. That's what I have done with great success and will continue to do it in the future. I add 5-10% to account for minor changes in load and air temperature. It was the six position weights that allowed me to set the pressures to the recommended safe levels.

[Crabby Mike]

Originally Posted by Murf2u

As stated previously, no tire manufacturers condone running pressures lower than specified.

Hmmm. Here's what Michelin says, in their RV Tires Brochure:

"To determine proper inflation pressure, Michelin recommends weighing each wheel position of the recreational vehicle individually. Weighing each axle end individually will give a clear indication of how the weight of the recreational vehicle is distributed across the axle. If there is a difference in the weight found from one side to the other, adjust personal cargo within the vehicle in order to redistribute the load more evenly across the axle."

I've read that brochure (which includes weight and pressure tables for Michelin's various RV tire lines) front to back, and the only mentions I see of the vehicle placard concern the weight ratings (GAWRs, GVWR, and GCWR), which are discussed right before the quoted language above, and a caution to replace tires only with the same tire size as the original equipment. Nowhere does Michelin say that you should use the pressures shown on the placard for those weight ratings. Which is a good thing, because the pressures on my placard are the same as the sidewall numbers (120 psi), which are in turn well above the pressures that would be required (per Michelin's weight and pressure tables) for even the maximum weights allowed under GAWRs for my coach.

Elsewhere in the same brochure, Michelin says (bolding is mine):

"Overinflation, on the other hand, will reduce the tire’s contact area with the road surface, which reduces traction, braking ability, and handling. A tire that is overinflated for the weight it is carrying is more prone to a harsh ride, uneven tire wear, and impact damage."

After getting a 4-corner weighing (during a visit to the factory service center), I used Michelin's weight and pressure tables for my tires (XZA2), and determined the calculated pressures to be 95 front and 88 rear. I bump those by a fudge factor of 10 lbs, and use 105 front and 98 rear. The ride is good, and I feel plenty safe, because I am following the tire manufacturer's recommendations, and adding a safety margin of my own. And yes, I also use a TPMS.

No religious prosletyzing here; I'm just explaining what I choose to do, and why. Everyone else should do what they're comfortable with too.

[Gary RVRoamer]

All this business about using the tire placard info is driven by legal liability concerns rather than engineering. Ever since the Ford Explorer-Firestone tire inflation debacle, all tire manufacturers are scared of any liability for improper inflation and insists that the vehicle manufacturer determine the recommended inflation. And since the federal tire placard also requires that the vehicle manufacturer make a recommendation, those go hand in hand. But since the vehicle manufacturer has no clue how you will actually load the vehicle, he can only base the recommendation on the max rated load of the axle.

They still publish inflation tables because thats what vehicle manufacturers need to make their recommendation, but no tire maker is going to recommend that you, the tire owner, do that. Too much chance you will screw it up and balme them.