Michelin Tire PSI Table Starts Too High For My Load.

[Phil503]

My axle weights at Certified CAT scale are: Steer axle 13140 lb, Drive axle 14160 lb, Tag axle 8540 lb,

I have not been able so far to get individual weights. Tire size is 305/7022.5 LRL ARV all around. The Steer axle is on the Michelin chart, 6570/ tire= (95-100 psi) and I am using 105 as safety factor until individual weight can be done. But the chart for the tag (8540/2=4270) is below the Michelin chart which starts at 5375 lb, @ 75 psi. The chart for the Drive axle dual (13140/2=6570) is below the Michelin chart which starts at 9530 lb @ 75 psi. What is the recommended inflation psi for these tires?

[andyshane]

Man, did we ever talk this to death in the other forum...

Here, too. We have some great tire expertise at FMCA, they'll be joining us shortly.

The consensus is to err high when deciding which psi to use, for a host of reasons. I'll touch on a couple, our friends will add some more. While we should never get to the far right edge of the chart, remember not to inflate over that figure (also stamped on the sidewall). As a personal technique, on both RVs and my aircraft, I try to inflate once the outside/shop temp is about 65F. The important thing is, inflate them "cold" (before driving more than a mile, says Michelin). For safety, many manufacturers recommend using a locking chuck with the gauge mounted at the other end of a long hose (mine is five feet). I believe the consensus of most tire manufacturers is, if you have to put 20 psi or more in, the tire is considered a "run-flat" and must be inflated by a professional with the right tools (it can kill you, inflating it).

First, nobody knows your loading as well as you; but, we all run the chance of having a bit more weight on one side than the other. To that end, let's pick out a weight slightly higher than half of the scale readings to arrive at a worst-case axle-end weight. For example, if we toss in another 10% in the case of the steer axle, that brings you to 7,227 lbs, or 110psi. For those of us who haven't done axle-end weighing, establishing an assumed axle-end weight that includes a surplus for lateral imbalance is a good safety technique.

As we talked about recently here, some experts also suggest adding a bit of extra pressure for the forgetfulness factor: if your tires lose a bit of pressure between checks, you're covered. Whether you wish do to this or not, depends on your own maintenance habits. So, you might hear folks tell you to bring the steer tires up to 115.

Of course, surplus can come in two forms: pressure you add above and beyond the manufacturer's recommended value, and that which the table gives you by virtue of where you fall in the weight category.

Such would be the case with your tag: I'd accept the difference between 4,270 lbs and the table's 5,375 as all the margin you need, inflate them to 75 psi. Not lower, since the tire was not tested/certified at less than 75.

One thing I like to do: get a little IR scanner to gage the temp of your tires, at each safety check. If the temps go much over 120F, consider adding a little air. Scanning with sun on the tire might give a false high reading, base your action over several checks.

More guidance from Michelin is HERE.

[wolfe10]

The lowest PSI on the inflation chart for your tire is as low as you want the PSI, even if it weights less.

And cold PSI is NOT at a specific temperature-- it is before driving the tire at whatever ambient temperature you are in. So it could be 10 degrees or 100 degrees F.

[tireman9]

Hi Phil,

First off be happy your RV manufacturer provided you with tires that can easily carry your loaded weights. Too many owners discover they have been sold a unit with tires that are almost in overload as soon as they leave the dealership and fill the tank with gas/diesel.

Now to your specific question.

I agree with the info provided by both Andy and Brett. While my thought process is slightly different we end at essentially the same end point.

Start out no lower than the lowest inflation on the Michelin chart. The only exception to this would be if you get something IN WRITING from Michelin corporate (NOT THE LOCAL DEALER).

Since you don’t have actual individual tire loads of the fully loaded RV I suggest the following:

Assume a side to side variation of at least 5% so you calculate a 55/45 split. Using the 55% of total load I check the company tables to find the minimum inflation needed to carry that load. NOTE you are going up in the inflation in the table to the PSI that exceeds your calculated load.

For those pulling a multi axle trailer you also need to assume a 55/45 axle to axle load split before you do the 55/45 side to side split.

I then add at least 5% to the table inflation and again round up to the next 5 psi to allow for daily temperature variations and to have an easy to remember inflation number. This extra 5% means you are not chasing your tail and trying to calculate and adjust the pressure every day. Remember all tires will loose 1% to 2% air each month in addition to the variation due to temperature, elevation and minor variations due to barometric pressure. So unless you enjoy spending a half hour every day calculating and adjusting your pressure just add the extra 5% so you only have to add air every couple of months or so.

FYI If you are going to FMCA Int’l Family Reunion in Indianapolis check the schedule as we are reviewing the schedule and I think I will be presenting my Tire Basics for the RV Owner on Tuesday.

If you make it be sure to say Hi.

[Briarhopper]

Phil503,

The total description makes me curious how the axle weights compare to the GAWRs on the coach placard and if you know what the curb weight (unloaded vechicle weight) of the coach should be?? Are the tires the size specified on the vehicle placard?

[Phil503]

GAWR's are: Front-14,600; Drive-20,000; Tag-10,000. App. UVW 31,500. It appears the tires are sized for the Front (Steer) axle load and kept consistent size all around. The loads are well within limits for Drive and Tag axles and design with four slides and center tanks (except propane), refrigerator opposite side from kitchen, my guess is it is fairly balance. the only concern might be heavier on one side of the Front axle, but there seems to be adequate margin there and I am running with 110 psi in the front to allow for that. Thanks too all for the info. I emailed Michelin several days ago but have not heard back from them. Note of interest, this is a 2012 Newmar Dutch Star model 4020. Tire sizes are as specified on vehicle placard.

[Briarhopper]

On the original question, some say never to run less pressure than the vehicle placard which makes sense in a lot of applications. Now I'm curious if the placard pressure is the same or different for each axle. If it happens to be around 110psi for the front and lower for drive and tag, that might be your answer.

Now for the uncanny: Looking at the numbers, the steer and tag are both 1,460lbs below their GAWRs and the drive is 5,840 (1,460 x 4) below the GAWR. Perhaps you should play 1460 in the local pick 4.

[wolfe10]

On the original question, some say never to run less pressure than the vehicle placard which makes sense in a lot of applications. Now I'm curious if the placard pressure is the same or different for each axle.

Stuart,

The placard PSI is based on each axle being loaded to its GAWR (Gross Axle Weight Rating). It may be close, or (hopefully) will be off with your actual weight being under the rating to give you some safety margin.

[andyshane]

Stuart,

The placard PSI is based on each axle being loaded to its GAWR (Gross Axle Weight Rating). It may be close, or (hopefully) will be off with your actual weight being under the rating to give you some safety margin.

On my own rig, I have a huge gap between the rig's GAWR and our normal max loaded weight, ie all tanks full, all our gear, the four of us aboard.

Were I to use the placard weight, my ride would be needlessly harsh and the contact patch for my tires (surface area of tread against the pavement) would shrink. In the longer term, I'd begin to notice abnormal tread wear, losing the center/middle of each tire first.

If the gap between your operating weight and the placard is narrow, it might be best to inflate to the higher value, like we normally do in passenger vehicles. It's all situational, there's no right answer for everyone.

This is one of my favorite topics, as a lifelong safety officer, then author, now manager (that's what us airline swine do, really). Kudos to you guys for such a high level of expertise, near total absence of misinformation. That translates into a safer operation, more enjoyable RV'ing!

[Briarhopper]

Stuart,

The placard PSI is based on each axle being loaded to its GAWR (Gross Axle Weight Rating). It may be close, or (hopefully) will be off with your actual weight being under the rating to give you some safety margin.

My placard PSI is less than the PSI required for the GAWR. Interesting, don't you think. I suspect it has to do with handling and stability, but I do not know this for sure.

[wolfe10]

Stuart,

Would be very interested in the details.

What chassis?

What front and rear axle GAWR?

What tires (Manufacturer and size)?

What PSI on the GVWR Plaque?

Thanks.

[Briarhopper]

Brett,

Vehicle Label:

GAWR Front = 10410

GAWR Rear = 19000

255/80R22.5

PSI front and rear is 100

The original tires were likely Michelin XRV, but not positive.

[wolfe10]

Stuart,

From the Michelin RV tire guide IF the original tires were Michelin 255/80R22.5:

10,400 front calls for a little over 105-- 105 is good for 10,300.

19,000 rear calls for 110, good for 19,220.

So, you are correct. Wonder why Tiffin reduced those recommendations??? Any chance the tires were 275's?

[Briarhopper]

Original equipment would have been Michelins. The placard size is 255/80's so I assume the original tires were Michelin XRV 255/80R22.5

[deenad]

The only exception to this would be if you get something IN WRITING from Michelin corporate (NOT THE LOCAL DEALER).

On our 2002 DSDP we had a different situation: Front axle was overloaded when we filled the fuel and propane tanks even before we loaded anything and without us aboard.

Newmar had Spartan replace the front 12,000# axle with a 14,600# unit. Used the same 275/70 tires but put them on heavy duty steel wheels (we had steel wheels with liners). They then issued us a letter on Michelin letter head allowing the tire to be rated for it's capacity at 125 psi with only 120 psi in them. The wheels were only rated for 120 psi.

When I changed tires after about 8½ years I went with new, wider Alcoa wheels and 305/70's on the front axle. Now running 100 psi in front and 90 rear on the 275/70's there.

I had to go with the 305/70's as there was no clearance for the 295/80's or even 275/80's. Even with the 305/70's I have to be sure the air is fully up before I turn the steering wheel or they'll catch the fender flares.