RV Tire Safety: Weighing an RV – 4-corner or CAT scale?

11

with RV tire expert Roger Marble

I was reading a thread about scale weights on one of the RV forums. Tireman9 here, an official card-carrying member of the “Weight Police.”

Seriously, some comments:
Regarding the original poster’s numbers – I would really be interested in learning the individual numbers from anyone who has done “4-corner,” be it Escapees, RVSEF, on a closed state scale, state police portable or homemade scale.

The primary reason for 4-corner is not to get concerned about a 250# (5%) difference in individual tire position weight, but to identify the outliers with 1,000# difference side to side, which I am told is not that unusual.

I have posted several times both in my RV Tire blog and on various RV Forums that until you get confirmation with side-to-side weights, I suggest you assume a 53/47% side-to-side weight split. I have received scale numbers from a few folks and they were +/- 1% from the 53% figure for the heavy end of the axle. I have had 4-corner done 3 times (RVSEF and state scale) and my Class-C runs at about 51.5% to 53.1% heavy for the heavy end of my 2 axles. I have also done a couple CAT scale readings just to ensure no significant “weight creep.”

Regarding doing 4-corner on CAT. This is against CAT corporate policy as they say loading the platform way off-center can affect the scale accuracy and calibration. If you look around you will see that many CAT scales now have a guard rail near the platforms to prevent off-center weights.

While I haven’t looked at the Escapee scales, I have compared the RVSEF scales with some state police scales and they appeared to be the same units.

The whole objective of getting tire weights is to avoid overloading your tires. No, your tires aren’t likely to “blow out” if you have an extra 1% on one side vs. the other, but if you discover you are at or above the load limit of your tires by 10% then you are really “consuming” the tire life faster than you might expect. This can result in a belt separation if you also push the speed rating for your tires in RV service (75 mph max).

Most folks never have belt separations on their cars but we all know that many RV owners have tire problems. One of the main reasons for this difference is that most cars are running around with 25% to 35% reserve load and are driving 20 to 40 mph below the tire speed rating for regular auto usage. RVs, on the other hand, are running with 5% or -10% reserve load and from 15 below to 15 mph above the speed rating for their tires when in RV service.

To address the calibration issues both with scales and pressure gauges, my gauge tests find between 8% and 15% are off by more than 5 psi with a few reading about 10 psi high, which means that those owners were running their tires significantly underinflated.

All of this is why I suggest adding 10% to the minimum inflation required to support the load when you consult the tables after you use the heavy end of a 4-corner weight or the 53% figure from a CAT scale.

 

Read more from Roger Marble on his blog at RVtiresafety.net or on RVtravel.com.

##RVT924

 

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Matt Johnson
9 months ago

Can we get a list of places that do individual wheel weighing? I would really like to have that done on our full time road home.

Roger Marble
9 months ago
Reply to  Matt Johnson

There are a couple of RV organizations that offer the service RVSEF and Escapees. Otherwise, you are going to have to look around. I suggest you review THESE posts
http://www.rvtiresafety.net/search?q=corner
from my blog as there are a number of options depending on where you travel.

J anne
9 months ago

To make use of the info in the 3rd comment paragraph I need definition of terms . I have stopped at truck scales and gotten overall wt and back and front axles with total on class B around 9k and back usually about 600 to 700 higher than front as I recall.i run per mfc 50psi frt and 80psi back tires. In a just guessing I try to distribute side wts but just guessing. I have no idea how to do what you described or what heavy end of axle since they are front and back! My max is 9600 and well below that always. E.g.dont carry water, one person etc chevy 3500 extended.i would appreciate some clarifications and reaction to info I just laboriously type on the tiny smartphone keyboard. Never go to rally’s and such where they are set up to do tests. Thanks

Roger Marble
9 months ago
Reply to  J anne

RE Side to side load split. I think that Class-B is less of a concern than in the larger Class-C and Class-A. In your case you can compare your “sticker” GAWR” and as long as your truck scale axle weight in no more than 85-90% of the GAWR you should be good to go with the sticker inflation.

ALAN CANDRA
9 months ago

If there is a weight difference side to side should the air pressures be the same with the heavy side in both tires or each side pressure set for that tires load?

Roger Marble
9 months ago
Reply to  ALAN CANDRA

Yes Alan, All tires on any one axle should be inflated to the same pressure i.e. the pressure from the load tables would be the minimum and I suggest adding 10% to that number. So even if the table indicated 65 psi for the RR and 75 for the LR I would go with 75 +10% or 82 for all tires on that axle.

Charles
9 months ago

Given that many of us don’t have access to a weigh event (Escapees or others) and are relegated to using a CAT scale. How do we use front and rear axle weights to our advantage? I wonder if you could touch on this Roger.

I know it is far from ideal but it would be helpful to be at least a bit compliant with our tire pressures. On my Newmar Northern Star I have been running 110 PSI (cold) in the front tires, and 100 PSI (cold in the rear tires).

Even after multiple years on the road full time I don’t know for sure that these pressures are correct. I only know that the RV “behaves” better at these pressures.

Thanks for your excellent write up.

Roger Marble
9 months ago
Reply to  Charles

Charles. For RV use we don’t worry about Front/Rear split. Treat each axle by itself.
So let’s assume your Front CAT scale reading was 10,000 53% is 5,300# Use that number to consult the Load tables and let’s say your tire size indicates 90 psi as capable of supporting 5,400 So add 10% we get 99 psi cold inflation for the front.

If your rear, which has duals, shows 18,000#, 53% is 9540# so each tire supports 4770# on the heavy end. Consulting the Load tables for your size we find your size is rated for 4650# @ 85 psi when used as a dual. But that is not enough to support 4770 so we ALWAYS go up. so now we have you at 90 psi minimum +10% gives us 99 psi. In the above, the pressures turned out the same but different loads or different size tires will indicate alternate inflations.

The above numbers are made-up as I don’t have the tables in front of me and I don’t know your actual tire size but I think you get the idea. You can always review other examples of tire inflation on my Blog http://www.rvtiresafety.net. My direct email is posted under my picture on the right side of the blog.

Ray. Williams
9 months ago

As a decades long rver , lv noticed new , expensive toy haulers , and others , sell with junk , marginal tires ! Why? Isn’t life and property worth more ? 2 of our dear friends went to Mexico in a 2.5year old , delux toy hauler . Toy hauler still there (totaled), because of cheap factory tires . PS. Even with expensive Mexican insurance , they had to cough up an extra 17,000.00 !☺

Roger Marble
9 months ago
Reply to  Ray. Williams

Ray, I don’t know the details of the accident or tires involved. While it is generally true that RVs do not have much Reserve Load (o% up till 2017 then 10%) it is also true that over half of the RVs on the road are overloading their tires either from overloading the RV or underinflating the tires or a combination.
It is also true that the RV market, especially for trailers, seems to be strongly price driven. When buying an RV you always have the option of getting better tires or walking away from the deal.

Chuck
9 months ago
Reply to  Ray. Williams

Better dealers, if you can find one, will swap Chinese tires for better ones at no cost. if they won’t, move on. I recently canceled a contract for a higher end 5th wheel that had marginal axles and poor 15″ tires, as they would not replace the tires. And don’t even mention drum brakes vs disc!