Another question on "4 corner weights"

 On Tue, Nov 9, 2021 Andy  wrote:

Good afternoon Roger,

 I wanted to double check myself regarding tire position weights.  In reading your blogs you said for double-axle trailers the tire pressure should be the max cold pressure amount as stamped on the tire because of trailer sway and turning forces on those tires is different than on a motorhome.  Is it important to get individual tire position weights for trailers?

 As I just got a new 5th wheel trailer and I am getting ready to install a Tire Tracker tire pressure monitoring system I have a few questions:

1.      Do I need to get 4 corner weights (or in this case 8 tire position weights)? 

2.      Is load on each tire position important for trailers vs. motorhomes, or is axle weight sufficient?  (I can get overall and axle weights easy enough.  And, I just got an appointment for tire position weights with a SmartWeigh club in FL, if you think it is important.)    

3.      When I go to get the weights, should ALL holding tanks be full or only Fresh Water? (The SmartWeigh group literature states that they only want the fresh water tank full.)

 The 4 tires on my 5^th wheel trailer are: (And the spare, too.)

Carslie    CSL16   ST235/85R16

Load index 132/127

Speed Rating: M (81 MPH)

Max Load Single 4400 lbs. at 110 psi. cold

Max Load Dual 3860 lbs. at 110 psi. cold

DOT JETB 1821

 

The 6 tires on my Ford F450 DRW are:

Michelin

225-70R19.5

Max. Load Single 3970 lbs. 110 psi. cold

Max Load dual 3750 lbs. 110 psi. cold

Then they have LRG in an oval followed by 128/126N in another oval and then DOT B6 YB NFL X 0621 in various ovals

 

I don’t have weights on the trailer yet but the GAWR is 7,000 lbs.   GVWR is 17,000 lbs.

 Thank you very much,  Andy

I wrote:

The "4 corner weight" is usually talking about Class-A motorhomes and the data shows that some number of those units can be significantly (1,000# or more) unbalanced side to side on an axle,

Smaller RVs can probably get away with just learning the weight on each axle with the RV and TV loaded to its heaviest.

Then calculating 51% or 52% for each axle and then using that weight number to confirm you are not exceeding the load capacity of the tires.

For towables, including 5vers, your calculated heavy weight should be no more than 90% of the tire capacity when inflated per the Load & Inflation tables. I covered the reason for this 90% limit in  my blog post on Interply Shear and the RVIA (see that Gold sticker near the door on the trailer) has a requirement that tire capacity should be = 110% of GAWR.

The max load capacity for a tire is load in pounds shown on the tire.

Your Dry weight is not important when we are talking about tire loading. The GAWR is just a number for all the tires on any one axle. The tire on the Passenger side has no idea what the load on the driver side is so you can't average the axle weight to learn the max weight on the heavier loaded tire.

"Dual" loading is when 2 tires are mounted side by side on one end of an axle as we see on the rear of most Class-C motorhomes and you have on "Dually" Pick-up trucks like your F450. Dual does not apply to RV Trailers. LT tires do have different load capacities if mounted on the front (single) or mounted on the rear (dual).

Yes, you do not need both freshwater and holding tanks full. Think of your loading when you start a trip. Fresh water is normally full, so is propane, gas or diesel and food pantry but holding tanks are empty, don't forget your tool box. That would be what I would call "Heaviest expected weight".

This is when you get on a truck scale and get readings with just one axle on a scale pad. You may need a couple of readings depending on pad spacing and your axle locations. Get both trailer and truck weights as you should also confirm you are not overloading your TV tires either.

Example:  Suppose you have tires that say 3,900# @ 80 psi. Your GAWR on the certification label is 7,000#   RVIA requires the tires have a capacity of 110% of the  7,000 or 7,700 total or 3,850# each so you might think you are OK but if your RV axle is "out of balance" side to side by 1% or 3,885# on the heavy end you would have a tire in overload. Not a great deal but in overload assuming the scale is accurate +/- 1% and you have a 100% accurate pressure gauge.  What if your axle is unbalanced by 200# or 300#?  TV are normally more balanced so just take the axle load and divide by the number of tires on that axle.

BUT the RVIA 110% requirement went into effect in Nov 2018 so there are many RVs out there that only require that tires be capable of supporting 100% of GAWR so that 10% "cushion" is gone.

I strongly support the RVIA's 110% load capacity. In fact my Interply Shear data suggests that something closer to 125% would be desirable but the RV companies simply have not designed their vehicles with large enough wheel wells to allow that large of a tire even if the cost penalty was less than $200 an RV.

Back to your original question: You do not need to learn the individual loads on each tire position. BUT I do suggest you assume at least a 1% out of balance and apply that to the weight reading you can get at truck scales (just need to pay attention to where each axle is on which scale pad)

##RVT1027

Weight Creep

 I have covered the importance of knowing your actual RV weight, as that has a direct impact on your tire loading which affects your tire life.

The idea of "4 Corner Weight" is that you get on scales that can measure the load on each end of each axle because very few RVs have an exact 50/50 end to end load split for every axle.

 Now I know that finding a location that can measure each tire position is not easy. Large RV gatherings such as FMCA Conventions and Escapees meet-ups sometimes have vendors offering that service. HERE is some information from another tire engineer on where and how individual weights can be measured. Others have learned that their state scales as in Oregon and I believe Washington state are left "on" and they can many times get the weight of each  end of each axle on the scale so they can calculate the actual loading.

This is my RV on a scale in Oregon. The red circle is the weight sign.

Some folks have contacted their state police and found them more than willing to provide the service. HERE is a work sheet you can use when you get the scale readings..

One other consideration for every RV owner, even those that have not yet learned their corner weights. This is WEIGHT CREEP. This slow increase in overall weight occurs as we travel and add small items to our RV. This might be another tool or book or nick-knack. Individually they only may be a pound or two but over time the total weight can become significant.

Now, I am not saying you have to search out a company to re-do your 4-corner weights every year but you can get a handle on your weight creep with a quick visit to a regular truck scale where you can learn your individual axle weight. With that new information you can compare the total for each individual axle of your 4-corner weight with the new truck scale weight. Hopefully you will not see any significant weight gain that would require you to get new corner weights BUT you will at least know the facts and know if the extra "stuff" is adding up to a significant weight gain which suggests that you put the RV on a "diet".

 ##RVT967

Side to Side Motorhome weight balance

Got this question and thought that others may have similar problem.

Hi I have followed your postings on this forum and I own 2017 Newmar Ventana 4002. I am trying to understand and resolve an issue with my coach where I have a 1000 lb difference in weight on the front axle side to side. I have had the coach weighed on all 4 corners 3 times and get the same results. My understanding and I think it may have been in one of your post that the difference should never been more than 500 lbs side to side.

I am having a conversation with Newmar about this and pushing them for answers why as I am well under the GAVR and well under the GVWR. I have unloaded and moved the contents in the basement and inside the coach around hoping to make a difference and the reality is I don't have much to move.

I am looking for any studies or evidence that I can show Newmar that this is not safe if indeed my information is correct that side to side should not be more than 500 lbs. I am hoping you might know of something I can get my hands on or someone I can talk to.

Thank you for taking the time to read this PM.

Sincerely,  Newmar owner.

My reply:

You may have misunderstood the side to side weight comments.

It is suggested that people get "4 corner" weights rather than just axle weights, as it is known that some coaches are unbalanced side to side.

We do this because if you only get axle weights and simply divide by 2 and assume your side to side weights are even, you could end up with an overloaded or underinflated tire IF the sides are not close to even.

Not sure where the 500# figure comes from other than looking at the load increments in the tables for each 5 psi. You will see that with some large tires the load capacity can increase by a few hundred pounds with an increase of 5 psi.

I do not remember ever seeing a "should" not be more than 500# different statement

You have confirmed that your coach is not balanced side to side and it appears you have made a good effort to get more balanced but have not been able to because of the design / layout of your coach.

With that in mind, and knowing the load on the "heavy end", you need to consult the Load/Inflation tables and use the heavy end weight to learn your MINIMUM inflation for all the tires on that axle. I still recommend you add 10% to the table inflation number as long as you do not exceed the max inflation rating for the wheels. This 10% helps avoid TPM low-pressure warnings due to an occasional drop in Ambient air temperature 

I trust you have confirmed with your RV Mfg that the wheels are capable of supporting the heavy end load.

I hope this clarifies what you need to do and answers the "Why" we suggest you get the weights on each end of each axle.

##RVT942

"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.