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Old 11-14-2010, 09:14 AM   #1
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New air pressure theory?

My friends in the race car business have always told me to weigh all four corners and then adjust the air AT EACH CORNER. I have always made sure the axle was the same on both sides. They insisted I was changing the "square inches of rubber on the road" and my motor home would not run straight if I had the wrong air pressure in one corner.
Well, now another friend said he talked to a Michelin person and they are now saying "it is okay to make the air pressure the same across the axle BUT you will get a better ride if all four corners are adjusted based on the weight in each corner." In other words if one corner is lighter (because the other side of the axle has a washing machine on it) and you put more air in side with less weight, you are decreasing the amount of tire touching the pavement. The result is the motor home will not steer straight. (This is what my race car guys are saying).
So, go ahead and beat me up I would love to know who I can contact at Michelin or Goodyear to verify their new theory. As I undestand it the Michelin guy said the studies from the recent two or three years are showing the best tire performance is to balance all four corners.
Any help or input would be appreciated.
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Old 11-14-2010, 10:08 AM   #2
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The rule has always been to set both tires on the same axle to the psi for the heaviest side. Goodyear's web site reflects this. I haven't checked Michelin's web site, but I'd bet it says the same thing.
I set my tire pressure this way and don't have any pull due to the weight difference.
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Old 11-14-2010, 11:47 AM   #3
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What you are saying makes sense to me. It's easy to picture the tread as it meets the road with the same psi's but carrying different loads. Not sure how it applies to
an RV? Or if makes any difference.

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Old 11-14-2010, 12:39 PM   #4
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So you are full of fuel, water, groceries, etc... Weigh each corner and adjust pressure. Drive 300 miles, burn fuel, camp, use water, distribute waste to tanks. How are you going to weigh it now for optimum tire pressure?

Why don't you ask your friend that talked to the Michelin person to give you his name so you can contact him? I think it would be corporate suicide to suggest this scheme.
(You did say to beat you up!)
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Old 11-14-2010, 01:03 PM   #5
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I will remain a same pressure on both sides of the same axle type of guy. I don't know anyone that is going to want to continuously micro manage their air pressures on a day to day basis.
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Old 11-14-2010, 01:21 PM   #6
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Here's a quote from the Michelin Truck Tire Data Book.....9th edition.....RV Tires section

"For RV use only, Michelin displays tire loads per axle
end in the load and inflation tables, as we recommend
weighing each axle end separately and using the heaviest
end weight to determine the axle's cold inflation tire pressure.
For control of your RV, it is critical the tire pressures
be the same across an axle, while NEVER exceeding the
maximum air pressure limit stamped on the wheels."
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Old 11-14-2010, 03:42 PM   #7
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Written recommendations/guidelines always override verbal ones.
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Old 11-15-2010, 12:12 AM   #8
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Dennis, from my experiments with tire pressure, you will not notice a difference in steering with 10psi difference in rear tire pressure at 60mph as I have accidentally driven under this condition.

I had a problem with my Michelin 255/80R22.5 XRV (G load rating). My dealer inflated all tires to the maximum pressure of 110psi. The ride was extremely hard and rough and almost unbearable. I called Michelin and they were very helpful but would not give me advice until I weighed each corner of my RV. Following their directions I had each corner of my RV weighed at a Grainary in the midwest. After adding 125lb of load to each corner to compensate for my tanks that were not quite full I got:

left right
rear dual 8160 8460
front single 3950 4300

I used the link below to find my correct tire pressures:

Michelin North America RV Load & Inflation Tables

from the chart I get

rear dual 90 psi for 8460lbs
front single 80 psi for 4300 lbs

These are actually the minimum cold tire pressures for the rated load. Since my maximum pressure is 110psi, I conclude that anything greater than the minimum but less than the max would be safe (avoid blowout). Following this (verified by Michelin), there is no problem in inflating the lighter corner tire(s) to the same pressure as the heavier corner.

I also notice that the tire pressure can increase by 10-12 deg when driving in warm weather.

I added 5 to 10 psi to the minimums to get:

rear dual 95 psi
front single 90 psi


The ride with these pressures was significantly improved over the 110psi.
My conclusion is that 20psi difference in pressure is a hugh difference in the ride and the 10 psi is definitely noticeable.
Hope this helps.
Pls confirm with Michelin directly to confirm your tire pressures, I found them very helpful after you know your corner weight.

hope this helps,

jim & debbie, 04 Journey, 34H
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Old 11-15-2010, 08:14 PM   #9
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Quote:
Originally Posted by DriVer View Post
I will remain a same pressure on both sides of the same axle type of guy. I don't know anyone that is going to want to continuously micro manage their air pressures on a day to day basis.
And then there's the early morning ambient temperature when you're checking the tire pressure for the day. Traveling in the mountains in the fall early morning temps may range from 30 degrees to 50-60 degrees (depending on how early) but the day time driving temps can be anywhere from 60-70 degrees those same days.

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Old 11-16-2010, 09:50 AM   #10
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Your friends logic may hold true for race cars but that doesn't translate to what should be done on the street. In the racing business traction is everything. In the RV business load carrying capacity, traction, longevity, and wear all have to be taken into consideration.

Most racing tires have to be hot to get the traction required. Heat is the enemy of normal street tires. Racers need the largest foot print possible so adjusting the tire pressure individually is necessary. Keep in mind however that during long distances races like the Indy 500 or NASCAR races they wear out and change tires several times.

You might get better traction with the tire pressures set individually but remember you're not going around the same radius corner with the same load every time you turn. Cornering changes the load and the forces applied. As mentioned earlier manufacturers make load and inflation tables for a reason. They do considerable research to be sure their product can handle a variety of loads and speeds while providing as much comfort, safety, and wear as possible.

Stick with the manufacturers recommendations.
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Old 11-16-2010, 10:21 AM   #11
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I see a lot of folks who say "Oh all the tires on an axle MUST be the same pressure.

I say: The folks at Michelin, who tell you that each wheel is set independently of the others... They know what they are talking about.. http://www.rvsafety.com
They told me the same thing when I had 'em weight my coach.

The important thing is that the distance from the center of the axle to the road be the same on both ends of the axle.. This will not happen if the tires are at the same pressure and the load is different If you run "Same pressure both sides" then you are correct ONLY if the load is perfectly balanced.

It is that simple.

For those who wish a graphic representation.. Go out and buy a couple of "Hoppity-hop" balls or other inflated exercise balls.

(Or better yet.. Just one will do)

Inflate both balls to exactly the same pressure,, Measure it.

Now. put a 250 pound man on one of them (or it) and measure the distance from his... Rear.. To the floor

Now put his 125 pound wife on the same ball and make the same measurement.. NOTICE how much higher she is sitting.

Imagine instead of the ball, it's 5,000 pounds of motor home on one corner and 6,000 pounds on the other side. Same thing, only not as much difference in height.

Adjust the pressure in HER ball. and her behind and his will be the same height.. Adjust the pressure on the tires, and both sides.. Same height... And both tires are then, and only then, in proper contact with the road (Assumes one of them was before)
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Old 11-16-2010, 11:02 AM   #12
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There's a poor mans trick to measuring tread contact. It's crude, but very effective.

Get a tire crayon from a parts store, air all your tires to max pressure and go to an empty parking lot.

Across the tread block of the tire draw a line a couple of inches tall all the way across from edge to edge. Then drive perfectly straight for a couple of hundred feet. Don't turn as that changes the angles for this test. Check your drawn line.

If worn off at the edges the tire is underinflated/overloaded. The sidewalls are putting more pressure down than the center of the tread. If worn off in the middle then the tire is overinflated. If worn off evenly then it's just about right since the pressure is consistent across the tire.

Since you're at max PSI then you should be worn in the middle. Drop five PSI at a time, repeat the test until you wear the line off evenly. However I won't recommend that you drop below factory chassis spec.

Do this loaded to the hilt since that's when under the most stress. You can do this fuel load only also if you want more than one set of numbers.

This dynamically measures the weight load at each corner.


YMMV, all normal disclaimers apply. This was taught to me by a truck/trailer tire salesman with 30+ years of experience and worked great on the way overloaded trucks we had coming through the shop.
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Old 11-16-2010, 01:11 PM   #13
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Quote:
Originally Posted by wa8yxm View Post
I see a lot of folks who say "Oh all the tires on an axle MUST be the same pressure.

I say: The folks at Michelin, who tell you that each wheel is set independently of the others... They know what they are talking about.. http://www.rvsafety.com
They told me the same thing when I had 'em weight my coach.
If that's what Michelin told you they're talking out of both sides of their mouth. At the bottom of the article you linked there is another link:


Here's a quote from that link Under the heading "Tire Safety"

Inflate all tires on the same axle to the same pressure. Although this may cause a difference in the tire patch (the area of tire tread in direct contact with the ground surface) if your coach is significantly heavier on one side, there are other considerations, such as spring rate, that dictate the same pressure. (The ultimate solution is to balance your coach so that each tire on each axle is carrying the same load.)

So which is it?
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Old 11-16-2010, 01:52 PM   #14
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So which is it? Good question.

In a round-about way, that's what the OP was asking. And what about those 5 extra psi everybody says they add. Don't follow the Michelin booklet then, do "they"?

So which is it?

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