Just Weighed RV, Need Review Of Process/Assumptions

[John USA]

Hi all.

I have a 34' Rexhall, with tag axle. I weighed it yesterday, would appreciate some feedback if I'm on the right track.

I filled the fresh water tank full, and filled the fuel tank prior to going to the scales. Me and the wife up front. Unfortunately, no supplies loaded in, yet.

I downloaded a worksheet (I think it was Bridgestone) with blanks to fill in scale weight, and weight from owner manual.

Below are the results. The GAWR, GVWR,, etc, came from the door plate.

The tag axle has an adjustable air suspension. I did a total of six different weighings in order to ascertain the load shift at different air pressures.

GAWR Front: 6000
Scale Weight Front: 5060 (tag suspension at 45 psi)
Scale Weight Front: 5060 (tag suspension at 40 psi)
Scale Weight Front: 5040 (tag suspension at 35 psi)
The results from this test seems to show that reducing tag axle air bag suspension has a negligible effect on front axle loading. This series of weighings had only the front tires on the scales.


GVWR-17,000, + 3,000 tag
Scale-16840. (Does this mean I can carry an additional 3,160 lbs before I'm overloaded?)

This weighing was the entire vehicle.

GAWR Rear-11000
Scale Weight-11,500 (this is weighing both rear axles, front axle off of scale)

GAWR Tag-3000
Scale Weight-3300

This series of weighings was tag axle only on the scale, front and drive axle off the scale.

At this point, I took 3 readings again, tag axle only, and adjusted the tag suspension the same as I did when I measured the front axle.

At 3300 lbs scale weight, I had 45psi in the air bags.
Reducing the air bags to 40 psi brought the tag weight down to 3160.
Reducing the air bags to 35 psi brought the tag weight down to 3020.

I know from the front axle series of measurements, the rear to front weight transfer by adjusting the tag suspension is statistically insignificant. So, I'm assuming that by reducing the tag axle weight, I was re-distributing the load it was carrying to the drive axle.

It seems to me that I should have no more than 35 psi in the tag air suspension.

If both rear axles scale weight was 11,500, do I deduct 3020 tag weight, and have a net drive axle weight of 8,480?

Does this mean I can carry 940 lbs more up front, and 2250 lbs more in the back?

Have I got it right?

Thanks.

[tireman9]

Normally I would let Brett Wolf handle this one as I know he knows a lot more than I do on Tag axle set-up.

But he is on the road and asked me to jump in.

First off you get points for making an effort to learn the actual weights on your axles. Too often people just look at the specs and guess for the rest.

It does look like the tag is not shifting much weight to or from your front which i find a bit surprising. BUT those are the numbers.

Are you sure your GVWR is 17,000 + 3,000 I normally expect the GVWR to be less than the sum of the individual GAWRatings but lets go with that number for now.

Based on the scale readings on the tag it looks like 35 psi or a little less is where you should be as you should not exceed the tag GAWR at all.

8480 on rear seems like the correct number so if you can place the load over rear axle then you should be OK

One thing to consider is the side to side balance on each axle. You seldom see 50/50 with 47/53 being more likely so this brings in the question of potential overload of tires as you approach 100% of each GAWR.

[John USA]

Thanks for the feedback, Tireman, I truly appreciate it. Trying to be as safe as I can.

I'm loading up half a dozen family members for a road trip. Estimate an additional 1600 lbs of load.

Estimate 900 lbs of human cargo, over and above pilot and co-pilot. (Pilot/copilot already factored in the original weighing.)

The 900 lbs breaks down to (estimate) 200 lbs of kids to be hanging out in rear bedroom watching videos, with remaining 700 lbs (four adults) mid cabin.

Estimating 300 lbs of luggage. I'll put that in rear most basement compartments.

Estimating 200 lbs of miscellaneous: kitchen/bathroom supplies, tools, oils/coolant, compressor. I'll put that just forward of the rear axle.

So, now, the question is tire pressures.

Working with 8480 at the rear axle. Loading 700 lbs pretty much around the rear axle (kids, luggage, miscellaneous). That brings the rear up to 9180. Assume another 350 (half of the additional adult load), and rear axle weight is now up to 9450.

The max load of my tires is 2778@80psi. (dually)

Do I divide the 9450 by 4? That would make each dually tire load 2362.5. Add 10% for safety, that is 2598.75. From the inflation tables, it looks like 65psi for LT 235/85/R16E.

Does that sound right?

[tireman9]

Quick post as I have errand to run.

16" tires? I am surprised that there is a 34' RV with tag on LT tires. What is the OE tire size & Load Range? What does your tire placard say for size & tire inflation.

Your tires need to be rated to carry at least the full GAWR or 3000# + on front and 2650+ on rears (dual load rating) and I would be surprised if you want different sizes.

We are getting a bit off topic on this if we get into too much tire talk.

Will check back in a few hours. We may need to move the discussion.

[John USA]

LT 235/85/16 are the specified tire.

Referenced in both Ford Chassis Service Guide that came with the RV, and on the vehicle placard.

The specific tire I'm using is rated at: 3042#, 80psi single. 2778#, 80psi dual load rating.

[tireman9]

OK, Now that the server problems have been handled...

This really belongs in the Tires forum but lets see if we can handle it quickly.

Based on actual weights with slight rounding and adjustment

your Front Axle carries 5,100 vs GAWR of 6,000

Drive 9,500 vs GAWR 11,000

Tag 3,300 vs GAWR 3000

BUT we don't know the axle side to side split. I like to assume a 47/53 to 45/55% split and we need to carry the heavier side so lets look at the range

IF 53%

Front 2,703 (single loading) 70 min

Drive 2,518 (dual loading) 75 min

Tag 1,750 (single loading) 40 psi min

If 55%

Front 2,805 for 75 psi min

Drive 2,613 for 75 psi

Tag 1,815 for 40 psi

Also I suggest a +10 % above the Minimum cold inflation for your AM set pressure so you don't have to worry about the pressure change due to temperature variation.

We also need to limit the CSP ( Cold Set Pressure) to no more than the wheel rating which in this case I believe is 80 psi

So we end up with a CSP

Front 77 to 80 psi

Drive 80 psi

Tag 44 psi

To make things easy I would go with 80 for Front & Drive and 45 on Tag as I like to round to 5 psi.

Now When you re-distribute your loading and get the pressure on the tag suspension resolved and get the actual corner loading you may need to tweak the inflation on the TAG.

Hope this helps

Sorry to sidetrack this "Weight" thread into a Tire Inflation thread.

[rondi]

Do remember--the further back you place anything in your rig, be it in the basement or under the bed, etc--you are really adding more weight to your rear axle/tires than just the weight of stuff you put in. It is like the opposite of a lever. There is a formula you can use to calculate it. You have plenty of available weight on the front axle--try and place the heaviest stuff up front, and the lighter stuff in back. If your fresh water tank is in the rear, like most rigs, consider hauling 1/2 tank or even less. Don't haul your sodas, etc--wait till you get to your destination to buy them. You're already at your limit of 80psi on your LT tires--so be careful and once you are on the road--stop at a Flying J or Loves and get another weigh with everybody on board and in the position they travel in.

hth, Ron

[John USA]

I wanted to thank everyone for the helpful advice. Unfortunately, I did not see it until now. I departed for our 2400 mile trip on July 4, and I intended to unplug from the internet, and focus on vacation.

Regarding the 45 psi tag tire recommendation, it would have been interesting to "feel" the effect, relative to the rest of the suspension, ie, softer in the back, maybe? Would like to hear from others who have experience with tag. The rear seemed a little rough, and I'm wondering if a lower pressure in the tag tires might have softened it out...it sounds like I was running too high psi unnecessarily.

Based on my interpretation of the manufacturer tire tables, I set all tires at 75psi, and set off down the road. Got there and back in one piece, no blowouts, and 7.5 mpg.

Oh, one more note, I did read (and I wish I could remember where, and cite the source) that both the tag and rear axle tires should be the same psi. This was the first I had heard of lowering the pressure in the tag tires, but it makes sense.