Correct Tire Pressure in the Rear and Tag

[sheridany]

I went and got weighed yesterday filled up with fuel, water etc. The scale was not able to weigh the tag and the rear drive separately. We did a modified weigh process.

We weighed left side, right side, total tag and total rear drive.

From that I created a spreadsheet and input all the numbers along with the Michelin air pressure recommendations. I calculated the separate rear drive and tag weights using the differences between tag and rear to match the total weights we obtained for the rear and tag axle.

The good news is we are under weight but we have some side to side difference in the front I need to resolve, but looking at the tire pressure recommendation does not make sense at all for the rear and or the tag.

[FLJOE]

I agree, on a regular CAT commercial scale it is hard to weigh correctly. The best position I could find on the scale showed my duals as 22,020 and my tag as 2,860. I'm not about to keep 65 or 70 psi in those tag axle tires.

I will get a 4 corner weight at some point this spring while hooked up to my enclosed loaded trailer, then I will have the perfect information for inflating my tires.

[Canuck53]

I believe Michelin has a minimum tire pressure and it is not 65psi I have noticed people set them to 80-85. Someone with real knowledge will be by shortly.

[BillJinOR]

Tags rarely have enough weight on them to require much pressure per the charts. The big problem as explained to me by the commercial tire dealer is the side scrub forces are substantial on the sidewalls and sufficient air pressure to keep the tire “stable” is needed
I was told by the dealer 85 was the lowest he would go on the Toyos I had installed. He actually wanted to put in 100
Having a tire with too much pressure is a ride quality issue not a safety issue. Most tractor trailers run max psi as they have varying loads. Too much pressure for the weight makes a rougher ride but keeps the heat down
In the case of the tag I don’t think you would notice. I settled for 85.

[ghaynes754]

Normal condition for the rear. Chart says you could use 75#, the minimum. I always add 5$ so 80# would be a good target.

On the CAT scales you can sometimes position the tag over the crack between the scales. It might take two trips over the scales one with the front on the forward scale and another with the front off of the scale splitting scales with the tag.

Watch for an RV show that is offering weighing, Escapees's has weighing in a couple of their parks, NIRVC locations typically do weighing for free. All of these would do individual wheels.

[GaryKD]

Hi sheridany,
On almost all Michelin tires the minimum PSI is 85. The statement is for RV tires.

[cruzbill]

If your travels take you to Oregon, you can just pull into one of their truck scales (as long as ODOT isn't using it to weigh trucks!) and get axle weights. I was worried that my steer axle might be a bit heavy, so I pulled in to a scale just south of Tillamook on hwy 101. I adjusted the tag axle pressure (not tire pressure) and rechecked; 14k, right where I wanted. Didn't take 10 min. Tire tables for my tires say to run the tag at 85, I keep them at 95, same as drive axle, simpler for me to remember.

Tag air pressure will significantly change your steer axle loading, steer axle weight should be checked several times to make sure your tag is adjusted right. Just a couple psi on the tag axle pressure regulator can throw a lot of weight onto your steer axle. I took off 800# off my steers by adj the tag by a couple of psi.

I had a 4 corner weigh done and the paperwork says your GVW should be 22-38% on the steer axle, and 62-78% on the drive axle.

Happy travels!!!

["007"]

There are questions answered here and your Owners Guide will have a section on your tire inflation's.

[BBelanger]

Originally Posted by cruzbill

If your travels take you to Oregon, you can just pull into one of their truck scales (as long as ODOT isn't using it to weigh trucks!) and get axle weights. I was worried that my steer axle might be a bit heavy, so I pulled in to a scale just south of Tillamook on hwy 101. I adjusted the tag axle pressure (not tire pressure) and rechecked; 14k, right where I wanted. Didn't take 10 min. Tire tables for my tires say to run the tag at 85, I keep them at 95, same as drive axle, simpler for me to remember.

Tag air pressure will significantly change your steer axle loading, steer axle weight should be checked several times to make sure your tag is adjusted right. Just a couple psi on the tag axle pressure regulator can throw a lot of weight onto your steer axle. I took off 800# off my steers by adj the tag by a couple of psi.

I had a 4 corner weigh done and the paperwork says your GVW should be 22-38% on the steer axle, and 62-78% on the drive axle.

Happy travels!!!

Someone will have to explain how putting air in a rear tire puts weight on the front axle.

[BillJinOR]

Not the tire .. the rear AXLE tag pressure . Different things
The tag axle has adjustable pressure to the axle to push it down or up. As you push it down you increase the weight on it and transfer weight to the front axle.

[BBelanger]

Originally Posted by BillJinOR

Not the tire .. the rear AXLE tag pressure . Different things
The tag axle has adjustable pressure to the axle to push it down or up. As you push it down you increase the weight on it and transfer weight to the front axle.

So as you add pressure in the rear it subtracts weight?

[osuallen]

Originally Posted by BBelanger

So as you add pressure in the rear it subtracts weight?

No, it redistributes the weight. As air pressure is added to the tag axle, it begins to carry more weight and causes the front axle to also carry more weight, while the drive axle weight reduces by an equal amount (fulcrum effect).

[BBelanger]

Originally Posted by osuallen

No, it redistributes the weight. As air pressure is added to the tag axle, it begins to carry more weight and causes the front axle to also carry more weight, while the drive axle weight reduces by an equal amount (fulcrum effect).

Hopefully we can agree that the total weight of the coach does not change.

If you are weighing at two different points and the front weigh point increases then the rear weigh point must decrease. Right? Unless somehow raising the rear end of the coach makes it lighter. I’d buy that if we were weighing a tub of water, as the water weight would move forward.

I agree that you can shift the weight between the drive and tag axles out back but I can’t believe that any shifts out back effect the weight on the front axle 35 feet away.

That’s like saying you have a 100 pound kid on one side of the teeter totter and two kids 75 and 25 on the other side. Then changing the kids to 40 and 60 and you don’t balance anymore? Center of gravity and total weight are unchanged. In order to change the weight on the front axle with the same total weight, you need to change the center of gravity rearward. You change the CG by moving weights. You can add all the air pressure you want, you’re not changing the weight. The rear two axles are already carrying their load, shifting it between those two axles doesn’t decrease their load.

I don’t believe you can increase the weight in the front without decreasing the weight in the rear.

[osuallen]

Originally Posted by BBelanger

Hopefully we can agree that the total weight of the coach does not change.

If you are weighing at two different points and the front weigh point increases then the rear weigh point must decrease. Right? Unless somehow raising the rear end of the coach makes it lighter. I’d buy that if we were weighing a tub of water, as the water weight would move forward.

I agree that you can shift the weight between the drive and tag axles out back but I can’t believe that any shifts out back effect the weight on the front axle 35 feet away.

That’s like saying you have a 100 pound kid on one side of the teeter totter and two kids 75 and 25 on the other side. Then changing the kids to 40 and 60 and you don’t balance anymore? Center of gravity and total weight are unchanged. In order to change the weight on the front axle with the same total weight, you need to change the center of gravity rearward. You change the CG by moving weights. You can add all the air pressure you want, you’re not changing the weight. The rear two axles are already carrying their load, shifting it between those two axles doesn’t decrease their load.

I don’t believe you can increase the weight in the front without decreasing the weight in the rear.

In a fulcrum, as you suggest, you are decreasing the weight on one side by adding a lifting force, decreasing the weight on the one side.

Bill

Yes the total weight of the coach remains constant. As the tag begins to carry more weight, that does in fact change the center. For example on your 4327 you have a 288 inch wheel base between the drive and front axle. I do not know the distance of the drive to tag, but for discussion we will call it 40 inches. If the tag is carrying zero weight, the support points (wheel base) are 288 inches apart. The weight of the coach behind the drive axle tends to lift the front sort of like the kid and the teeter totter. for an extreme example, if the tag was over pressured such that the drive was no longer carrying any weight, the support points would be 328 inches apart. In this example the percentage of the total weight carried by the front increases because there is 1) less " front lifting" by the weight behind the rear support point because the rear support is now much further back, and 2) a greater percentage of the total weight is now closer to the front than the rear since the support points are further apart.

As the tag pressure changes, the effective "wheel base" (support points) will vary between 288 and 328 inches (although we know it would never hit 328 or something would be horribly wrong). the effective "support point" of the rear is proportional to the amount of weight the tag and the drive are carrying.

Hope this helps.