Lowering tire pressures

[Midniteoyl]

Originally Posted by mmcl26554

I would like some advice on the pressures I calculated for my HR.
The previous owner had the home weighted and left the numbers which are:
Total weight: 18668, Front axle: 5100 Rear axles: 13568.
I had to make some assumptions that the weight on both axle sides was equal and for the rear since there was one set of duals and one single tag axle I had to divide the total rear axle weight by 6 tires. This is what I came up with:
Each front tire is predicted to carry 2550 pounds and the chart recommends 65 psi for 2625 pounds weight
Each rear tire (6) was predicted to carry 2261 pounds of weight for a recommended tire pressure of 55 pounds for 2335 pounds of weight.
So this would give me pressures of 65 for the front and 55 for the rear.
This seems low but there are 6 wheels supporting the weight in the rear.
I did find a tag behind the drivers seat which recommended 80 pounds, but at that pressure it handles poorly, requires very frequent steering correction. Last year I had all tires at 70 pounds and it handled much better. Sure it would be nice to weigh each axle end but this is not available to me at present. So what are the thoughts of using the calculated pressures of 65 front and 55 rear?
Michael

Personally? I would add 5 psi to those numbers and run 70/60 psi. Most people do this anyways for safety and you do not know exactly how the coach was configured for weighing.

[Midniteoyl]

Originally Posted by mmcl26554

I looked again at the tag behind the drivers seat and it not only gives the GVW but also the weights for each axle. It shows the axle with dual wheels as carrying more weight than the tag or front axle, which is understandable. I'm going to work out a percentage for each axle and use that to predict the weight and inflation pressure for each wheel. Then I did find a truck stop in my city which will weigh it including corner weights for $9.50. So I can verify my "arithmetic". I'll report back here my results and we can see how close one can get when predicting wheel weights and inflation pressures. A couple things I do know already is that max inflation is to high for the weight of my home as at that pressure handling was poor and strangely gas mileage was down 1 mpg. That was unexpected. I did notice when I went to lower the pressure I could see the contact pattern on the tag axle tire face was not all the way out to the edge of the tire so it most definitely was over inflated. I didn't see this pattern on the other tires. My feeling is that correct inflation pressure IS the only pressure which will give the best safety, handling and wear pattern of a tire. Over inflation is as bad as under inflation when considering all parameters. I remember I once owned a 1970 Rambler Ambassador station wagon. It came from the factory with a larger than usual tire and because of that the factory recommended tire pressure for the front was 15 pounds! Now if you inflated them to 32 pounds (max inflation pressure) the foot print of the tires became so small that handling was really bad. I mean it was very difficult to control. It was always a battle with "Pump Jockeys and Tire Monkeys" to get them to put the correct pressure until they would finally would look at the tire pressure sticker. Tire pressures were one of the big problems with the Corvair, if you put more than 15 pounds in the front tires it gave them really spooky handling to the point of being "Unsafe at any speed". In fact it was that situation which brought the auto industry to actually publish tire inflation pressures and post them in a visible location.

And thats the problem with tags, especially older ones: you do not know how much 'force' they are exerting on the road unless you have it weighed separately (ideal). They can sometimes be adjusted depending on the chassis, but you can also adjust the weight around the 'pivot' of the dually axle. More front to back as needed to increase/decrease the tag axle load. This also will effect your front axle weight as well and can be used to lighten the front end enough so as to require less tire pressure.

[Bigd2]

All the info you received here is right on. Basically, weigh your coach loaded to determine the correct weight, on each side if possible. If one side is "slightly" heaver than the other use the same air pressure of that side for the other side of the same axle, front and rear. Mine are 105 psi on the rear duals and 102 psi on both fronts. As mentioned above I also recommend a TPMS that will give you PSI and Temperature readings, and a set of Dually Valves.

During our last trip to the UP we were running with the outside temperature averaging around 90 degrees. Even starting out after staying overnight the PSI was up 3 or 4 psi. After traveling 50 miles or so the PSI increased up to 110 to 115 PSI but the temp remained around 95 to 105 degrees

[wa8yxm]

Originally Posted by Fossilhog

I've been told that for awhile now but no where near me to get it done.

I hear this from time to time, And I have a question for you.

Is there a Flat Spot near your home (A spot that is level) If so. then

RV Safety & Education Foundation

and follow the link which last time I looked was in the band just below the top banner.

They come to you, So that "No place near me" is ... Not true, it's as near as your telephone or keyboard.

[mmcl26554]

I got my motor home weighed today:
Front axle: 4500
Rear Dual axle: 8100
Tag axle: 4500
Total: 17100

Fresh water tank was full, 50 gallons of fuel on board (tank holds 70).

According to the tire pressure chart I should have 55 pounds in all tires.
Seems low but that's what the chart says!
Michael

[Midniteoyl]

Originally Posted by mmcl26554

I got my motor home weighed today:
Front axle: 4500
Rear Dual axle: 8100
Tag axle: 4500
Total: 17100

Fresh water tank was full, 50 gallons of fuel on board (tank holds 70).

According to the tire pressure chart I should have 55 pounds in all tires.
Seems low but that's what the chart says!
Michael

Whats the minimum pressure? 55 psi seems low as a minimum unless its 16" tires? But even then, 2300 lbs requires 65 psi....? I'm 15k lbs on 2 axles and run 70psi w/ 19.5" tires, which is the minimum pressure for my size tire.

[mmcl26554]

I should have posted the tire size: 235 /85R16 and the chart can be found on Google by searching for MICHELIN INFLATION CHARTS FOR RV. There you will see the recommended pressure for my weight is 55 pounds. They are at 65 now so I guess I will let 10 pounds out.
Michael

[Midniteoyl]

If 65psi works, then leave it. Most put in 5-10 psi extra 'just in case'..

[steelheadbluesman]

I'm with Jim.... 55 psi seems awfully low. If 65 feels, looks and works right, I would consider keeping it. Or maybe adjust to 60 psi.... Low pressures can cause a lot of problems with a heavy rig. I'm curious, is there a psi rating on the tire sidewall, or on a plaque inside the rig? If so, what is it? If you haven't read this entire thread, I think you might look at #5 and #10 (and some others that say the same sort of things.)

[mmcl26554]

The factory placard has the weight higher by 900 pounds and recommends the highest pressure 80 pounds. But I would think the actual weight and the charts put out by the tire manufactures is what you should use. I tried the 80 pounds and it's handling was poor, requiring an lot of steering correction, I was always moving the wheel. The last trip I took with 70 pounds was much better. I'm going to try 55 on a test drive of 5 miles and see what happens. But, it seem that most recommend going with the recommended pressure for the weight as from the charts. I have read most of the posts in this thread.
Michael















Michael

[Tony Lee]

My experience includes lots of driving on severely washboarded roads - thousands of miles, with one recent stretch being about 3000 miles without a break.

Some of it has been with the MC8 MH conversion (17 tons tag axle pusher) and a lot in an OKA light truck 4WD 5 1/2 ton.

In both vehicles, my practice is whenever I get off the bitumen and conditions are likely to be significantly worse, I lower the tyre pressure to 80% of highway values AND - most importantly - reduce the top speed to 80% of highway speeds (which in Australia is speed limited to 100kmph (62mph) for heavy vehicles). So that means a top speed of 50mph at that reduced pressure
If conditions get really bad, the pressures go down to 60% and the top speed becomes 60% also.
If you do a bit of digging at Michelin you will find tyre specifications that recommend those sort of pressure and speed reductions under those sort of road conditions.

Current trip just finished involved one continuous stretch across the middle of Australia that was corrugations form start to finish, interspersed with rocky sections and sand and it was all done at 60% pressure and 60% maximum speed and we had no damage at all because the lower speed protected the tyres a couple of ways - firstly there was no heat build up due to additional flexing of the tyre walls (and this is what causes low pressure damage at high speeds), secondly, the SEVERE washboarding was partly absorbed by the tyres and this made the ride far more comfortable and kept the shock absorbers cool, and third, the reduced pressure AND slow speed means the tyres can "ooze" themselves over sharp rocks instead of being damaged.
Recently there was a factory team of Mercedes off-road vehicles showed the world they could conquer the Canning Stock Route (part of the trek I referred to above) and be the first factory team to do so. Well, they didn't make it because of stupidity. Fast driving meant that all (five???) vehicles - but not he military-spec unit - ground to a halt half way with exploded shock absorbers and they were all stuck there until replacements were flown across the country and then into this very remote area. Standard advice for this track is to lower preesures and slow down to the conditions. They failed, and my 20-year old vehicle got through with the only failure being a small screw that fell out of the TV mount.

Now I don't take the BigRig quite that far off road, but it has done a lot of miles on even worse-washboarded roads and the procedure is always the same - lower tyre pressures AND slow down.

There is a roadhouse - called the Pink Roadhouse at Oodnadatta in South Australia and for years the owners have given advice to travellers in all sorts of vehicles in that remote area and that is the advice - air down and slow down.

The pressure on the tyre is that to carry the maximum load AT MAXIMUM RATED SPEED - and there lies the get-out clause. Slow down and you can safely air down without wrecking your tyres.

Pink Roadhouse advice is at http://www.pinkroadhouse.com.au/Pink...essure-pdf.pdf

The principles apply to larger vehicles and the advice is regarded as being exactly right for off-bitumen roads

[mmcl26554]

Very interesting Tony!

[Mr_D]

Originally Posted by Tony Lee

If conditions get really bad, the pressures go down to 60% and the top speed becomes 60% also.

The Rubber Manufacturers Association specifically says that a tire run at 20% or lower of minimum is to be considered as "run flat" and interior damage has possibly occurred.
From Michelins RV Tire Guide:

According to guidelines put out by the Rubber Manufacturers Association (RMA), any tire that has been run at less than 80% of recommended air pressure for the load it is carrying should be inspected for possible damage.

[Tony Lee]

Of course they say that as a general rule because they know that people will take no notice of speed restrictions. The term "Highway Speed" gets used a lot, but I'm not using highway speeds in these conditions. Can't use highway speeds even if I wanted to. My info comes from tyre specifications issued by Michelin France and the same principles are widely used by outback travellers who want to get to their destination in one piece. Those who know better and overload or travel fast or have tight tyres nearly always pay the price and the recovery firms and repairers do well as a result. The OP was specifically referring to "I drive on some really rough roads at times." and the assumption there is that he will be travelling at low speeds anyway. Mind you, if you lower the pressure on bad roads, you either need to pump them up as soon as you get to the highway OR continue to run at the lower speed until you can re-inflate them.
Mind you, some of the US interstates have had me thinking I ought to stop and lower the pressures a bit. Our outback roads are heaps worse and we do take our bigrigs on them without significant problems.

If you accept that down to 80% is "OK", isn't that the figure I gave in the previous post as a pressure suitable for off-bitumen. AND of course at lower speeds, NOT 75mph.

Other considerations - if your RV weight/tyre rating combo is such that you DO need to run at maximum sidewall pressure, then of course it might not be wise to lower pressures more than 10% and in fact, it might not be wise to travel off-interstates at all because tou are using up all your leeway. Give yourself more safety margin by fitting tyres of a higher load rating.

My tyres are part of the suspension and they can't do their job properly if they are unnecessarily pumped up to maximum sidewall pressure + hot day pressure increase. Same reasoning has me running the Airstream at pressures derived from the load tables and I don't add 5 or 10 pounds just in case. I like to be able to read the panel instruments when I hit the pavement joints.

Of course ifr there is only 10 miles of bad road, there is little point reducing pressures because of the effort involved and you just drive to conditions by driving even slower than you could if the tyres were down a little.. I'm talking about hundreds and thousands of miles, not a couple.