"4-Corner" or CAT scale ?

Was reading a thread about scale weights on one of the RV forums.

OK Tireman9 here, an official card carrying member of the "Weight Police"

Seriously, some comments.

RE 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 that 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".

RE 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 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 "blowout" 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.

Are sidewall cracks the cause of tire failure?

Read an RV forum post about sidewall cracks. From the picture, it appeared the tire had what I would call Ozone or UV cosmetic cracking. The owner was concerned about the tire durability as he had suffered a couple "Blowouts" previously.  I responded....

"Not being able to inspect the "Blowout" tires or even see some pictures, I have no idea for the reason for the tire failures as "blowout" is not specific enough to suggest a possible cause. They might have been run for a few miles while losing air. They might have suffered impact damage 10 or 100 or 1,000 miles prior to the ultimate failure. They might have been run overloaded or underinflated or over speed recommended by tire manufacturer or tire industry engineering guidelines for thousands of miles.

Yes, tire failures can be expensive if the failure is only discovered by operating the tires at speed and not during the regular detailed close inspection by a "qualified specialist" as outlined in the Michelin Tech bulletin on RV/Motorhome tire inspection.

It is also true that sidewall cracks, in my professional experience, are not in themselves the root cause of tire failure. Just as a person developing a temperature of 101F or 102F is not the cause of an illness but is just an external symptom. Tire sidewall cracks are an indication of extensive tire age with the cracks developing due to time, temperature, flexing and exposure to Ozone and UV. All of which are detrimental to tire life.

"Zipper" sidewall failures of steel body ply are the result of fatigue from operating for miles when significantly underinflated or overloaded or a combination of those. Sidewall crack inspection guidelines published on pg 8 of THIS guide, suggest a maximum depth of about 2/32" on large radial tires (22.5" sizes) which usually have rubber thickness closer or thicker than 0.10".

Tires "Fail" for two basic and different reasons:
1. Sidewall Flex failure from low inflation/High load
2. Belt separation from long term rubber degradation due to excess heat and age

I covered these in two separate posts in 2012  and again with a slightly different focus in 2014.

While I can conceive of someone running tires for many miles with sidewall cracks that get deeper than 2/32", those cracks would need to penetrate deeper and get completely through the sidewall rubber to the depth of the steel and then water would need to be introduced and enough time pass to allow a significant portion and number of steel cords rust and be weakened before a "Zipper" like failure could occur.

The above would, in most cases, take many months of operation and would require that no or improper or incomplete inspection take place before a tire would suffer a catastrophic failure.

So the bottom line is two-fold.

1. Ensure you are not overloading your tires and that they are ALWAYS inflated to what is needed  for your application by running a TPMS

2. Starting at 5 years (3 years in trailer application) from the DOT manufacture date molded on the tire sidewall and annually or every 2,000 miles whichever comes first, have them inspected.  I have covered the topic of inspection previously.

##RVT923

Emergency equipment - Change a tire?

Was following a thread on the use of the special "tire lifts" similar to those seen here.

 One person discovered that the ramp they had purchased did not work on their model RV.


This points out the importance of testing and confirming that your "Emergency" equipment will actually do what you expect on your RV.


It doesn't make any difference if it is a ramp or folding ladder or an "X" wrench to get the lug nuts off a wheel if you get a flat, or some electric meter or other test instruments.
It's ALWAYS better to confirm you know how to use the tool and to confirm you have the strength if needed or the necessary knowledge to actually use the emergency device.

I learned this the hard way with a "bottle" jack that was too tall to fit under the axle of my Class-C RV.



I remember when I first started to drive my dad told me that before he would lend me his keys, I had to use the factory "lug nut bar" to loosen the nuts. Then figure out how to assemble the "bumper jack". Get the spare out of the trunk, remove the tire from the axle then put everything back where it belonged. I probably spent an hour working on it the first time but I did learn how to do every step correctly. ( he let me know if I was doing something wrong a\or in an unsafe manner ).

Should ST tires be outlawed?

A post on an RV forum caught my eye
"In 2014, when I bought my RV Trailer, I immediately went to Discount Tire for 6 Michelin XPS Rib truck tires. I don't baby them, I barely pay any attention to them, they've never been at 80 psi, and they have been wearing fine for 5 years. Been all over the US with no issues. Yet I see new trailers coming into the dealership where I work with blown tires just coming from Indiana. Chinese bombs need to be outlawed."

It's unlikely they will be outlawed given the price sensitivity in the RV market. With so many folks buying when the quoted price is just a monthly amount that is so low they will always be "underwater" on their purchase. The RV industry seems to only focus on making the sale and as long as it feels it can only make the sale by offering the lowest possible price the industry will fight tooth and nail to not have to spend an extra $100 on a set of tires that can provide better durability.

In 2000 after the Ford Explorer recall both Passenger and LT type tires were forced to meet newer, tougher quality & durability standards as required by the "TREAD Act" but I believe that because of pressure from the RV industry, ST type tire requirements were excluded from the new requirements as complying would have increased the cost to the RV company a few bucks and they simply didn't feel they could stay in business if they had to increase their prices.

So we are stuck with tires built to 1970 quality and durability levels.

IMO Until or unless the RV community in large numbers, demands an improvement in the tire quality with an update & upgrade in the performance standards required by DOT there will only be improvements on a small number of models that offer ST tires with more Reserve Load or offer actual LT tires. How many RV owners have made the minimal effort of filing a complaint on a tire failure to NHTSA? Or written a letter to the  Administrator of NHTSA?     U.S. Department of Transportation,    1200 New Jersey Avenue SE.  Washington, DC 20590

When was the last time you heard someone not ask about buying tires at the lowest price? Or walking away from a sale if the dealer didn't provide better tires OE?  or the dealer didn't offer a multi-year warranty on tires that came on the RV?

A review of some comments on this forum shows the truth in what I am saying. The new Goodyear Endurance seems to be providing a significant improvement in tire durability for ST type tires but we see several people stating they are not willing to pay the price.

If you are only willing to pay Harbor Freight prices for your tools why would you expect SK, Milwaukee, Proto, or MATCO quality?

From day one ST type tires were introduced as a low-cost option to higher cost Light Truck type tires when comparing Pounds load capacity per dollar cost. We also had the 55 mph National Speed limit so offering tires with a 65 mph max wasn't a deal breaker

Spare tire - mis-matched size What to do?

Class-A owner said:
"I use to advocate carrying a spare in my 2005 Monaco Diplomat. On that coach, it used a short 22.5 tire and it fit in my pass thru bay, unmounted. I could stuff a lot inside the tire and didn't lose much storage. Fast forward to my current RV and the 315's won't fit in a bay.

So......a lot of people say to just carry a spare, including a trailer owner with a rear mounted spare. It's nice to say and actually nice to carry a spare if you have room, but if you don't have the space, you need another plan. That plan is to have a roadside service plan and be willing to accept an odd sized or used tire to get you back on the road. Wait until you're in a major town on a weekday to resolve the issue with a new or correct size."

My reply:

While using an "odd" size on the front might get you moving (slower) mismatched duals will mean the larger OD tire is carrying more than design load.

I would limit the speed to 50 or less and be prepared to scrap BOTH tires as the internal structural damage is impossible to "inspect" for.

If you have two different size tires on your RV and can only carry one I would suggest the rear. Hopefully, you have confirmed your actual load on each tire position and have some margin available on the fronts. If you are running at 100% front capacity (bad idea) then the use of a smaller "rear" tire as a temporary front means you really need to limit both speed and distance you operate in that situation.

Overload does not always mean tire belts will overheat and a warning is received from your TPMS. The steel sidewall ply can fatigue from over deflection and you will have no warning and not know it till the sidewall fails catastrophically. 

All the above brings up the question of why did you have a tire failure in the first place?. Have you read THIS post.

A properly programmed TPMS should provide an early warning as soon as you lose 15% from your "set" pressure (slow leak) or 3 psi from your hot pressure within a couple of minutes (fast leak).  If you are also having a tire professional do a close and thorough inspection every year starting at 5 years for Class-A and Class-C or at 3 years for any trailer, it is unlikely for you to develop a belt separation without some advance warning.

Taking precautions is always better than trying to "fix" the tire after you have a flat.

NOTE. The above in no way should be taken as a suggestion that driving on an overloaded or under-inflated tire or mismatched duals is a safe thing to do. I am just offering suggestions for possible responses in an EMERGENCY situation where you feel you MUST move from your current location to a safer one.