Lowering tire pressures

[Mr_D]

Originally Posted by koda59

Mr D I have Toyo tires that are 245/70/19.5.

TOYO is one manufacturer that DOES put the maximum pressure on the tire sidewall, that's why I asked what you have!

[mgtr]

Almost 20 years ago I had a left front tire blow on the interstate at 70 MPH. I was fortunate, I had a steering stabilizer, so I stayed true down the lane. I was lucky to find a Michelin truck tire dealer nearby. The biggest problem was that the metal wires in the tires ripped out electric, hydraulic, and fixed the fiberglass really well!
So, after that, I have been careful about tire pressures -- and better five pounds too much than five pounds too little.

[Dr4Film]

Originally Posted by freebirdbus

I fully agree Mr. D. I have used that method for what seems like forever. But things sometimes change. I had always read that tire damage can start at 20% loss of air pressure. But now I have read here a much higher air loss is need to damage a tire. That's why I asked, to see if that had changed. If damage does not start until 70% loss of maintenance air pressure, could the tire be run at a lower pressure over rough roads to improve the ride?

The tires on any axle need to be inflated to the recommended psi based on two specific piece's of information. The corner weights of that specific axle and the manufactures tire weight chart for your specific tire brand, model and size. The heavier side of each axle should be the weight used for calculating the proper psi from the tire chart.

When it comes to understanding under-inflation of tires, I use what the manufacture states but I also would rather rely on the setting of my Doran TPMS which is programmed to alert me at 12.5% and 25% below baseline tire pressure levels. If the alert ever gets to the 25% level, anyone using a TPMS should have done something way back when the alert was at 12.5% versus waiting until the tire is considered FLAT. And it will also alert me when the level increases 25% above baseline tire pressure level.

A tire is considered flat and should NOT be driven on when the tire pressure is 20-25% below it's normal level as far as I am concerned. And here is what I copied from the RV Goodyear Tire web site since a large number of tires supplied on new RV's come from Goodyear: Tires that have 20% or more of their recommended inflation pressure should be considered flat.

Here are two useful web sites for people to read that supports the 20% figure. I have also included a PDF file from Michelin because many RV use both Goodyear and Michelin tires

http://www.goodyearrvtires.com/pdfs/tire-care-guide.pdf

Tire Safety Article

Originally Posted by Kazoo Tom

Since I have not yet had a chance to weigh our rig, I run my tires at the tire manufacturer's recommendation for the maximum axle weight rating. The ride can be a bit harsh on bumpy roads, but I feel we are safer this way. When I finally get around to getting it weighed we will adjust the pressures accordingly.

I would hesitate to lower the tire pressures and risk a tire being over loaded. I'd rather endure a harsher ride then suffer tire damage for being under inflated.

You are using the exact method I would recommend to everyone until they are able to have their coach weighed preferably corner weights or at the least axle weights. As you say, the ride may be rough but the tires will NOT be under-inflated.

Originally Posted by freebirdbus

My original question was if the threshold where a tire develops damage from driving with less than the recommended maintenance pressure has changed from 80% to less than that. Anyone know?

I would suggest looking up the manufacture of your tires and see what they recommend.

Dr4Film ----- Richard

[Fossilhog]

Originally Posted by athuddriver

UMMMM....get your rig weighed!

Don't go by looks because they can deceive. Get 4 corners if at all possible and follow the tire charts. If you can't get 4 corners then divide the axle weight by 2 and then add about 5-10% for the probability that one corner of an axle weighs more than the other and use that with the tire pressure chart.

Here's a twist. Having aired up my RV and utility trailer using one guage everything looked a little low. So...I got 2 other guages that read exactly the same and find my original guage reading 10 lbs high, ergo I only have ten less in each tire than I thought!

[Sky_Boss]

Originally Posted by Fossilhog

Here's a twist. Having aired up my RV and utility trailer using one guage everything looked a little low. So...I got 2 other guages that read exactly the same and find my original guage reading 10 lbs high, ergo I only have ten less in each tire than I thought!

I hear THAT! Wish I had tossed the following out.

I did go through about 6 tire pressure gauges until I found the one that seemed to be consistently correct and easy to read too. I had one that was off about 8 PSI. I have since semi-validated my TPMS as a cross check. My TPMS is +/- 2 PSI of my hand gauge which is the system's tolerance. The fact they read the same helped further validate I picked the correct hand held gauge.

[Dr4Film]

Here is the ONLY gauge I will use to air up all of my tires.

Accutire MS-5515B Truck and RV Digital Tire Gauge with LED Light and Bleed Button : Amazon.com : Automotive

It pays to use a good one that is calibrated and you can always trust.

Dr4Film ----- Richard

[Mr_D]

Originally Posted by Fossilhog

Here's a twist. Having aired up my RV and utility trailer using one guage everything looked a little low. So...I got 2 other guages that read exactly the same and find my original guage reading 10 lbs high, ergo I only have ten less in each tire than I thought!

I bought a certified gauge at NAPA, I never use it for anything but checking other gauges. I use an electronic gauge for normal checking.

[koda59]

Thank you Mr _D
Now I know I am only half crazy.

[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

[Dr4Film]

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

If corner weights are not available, then I would go with the recommended tire/wheel manufacturer's maximum allowed psi for all tires. When I had a smaller rig with 19 inch tires and a tag, I carried 80 lbs. psi in all tires as I never had the rig weighed because it sat most of the time in one spot.

Dr4Film ----- Richard

[akadeadeye]

Corner weights can be off as much as 10% from the "average" across the axle weights. Best to weigh the corners, but if that is not available, then averages will have to do.

Get the corners weighed at the earliest opportunity.

Since you are using averages and you know they are incorrect so far as individual corner weights I would add 5 lbs. psi to your pressure numbers. 70 in the front and 60 in the rear, just to maintain a margin of safety. I would rather be 5 lbs. higher than I need to be rather than 5 lbs. lower.

The two main causes of blowouts are being overweight and under-inflated.

Don

[szy]

Originally Posted by Gary RVRoamer

The psi on the sidewall or the placard inside isn't "wrong", but it's probably more than needed. Perhaps much more, though that varies by coach and tire. Maybe it would be better to say those psi's are "sub-optimal" for most RVs?

But I feel it does give better or should I say best mileage possible when they are inflated to max cold pressure .

[bachler]

Which is right ;;; 20 responces 20 different Opions, And they all seem to be right;; Now for mine;;;; I strongly dissagree with haveing different air pressure in the Tires;;; The one with less air is in effect smaller and the wheel must turn more RPMS to keep up with the More pressured one. The less pressure in a tire will make the tire run hot; This is bad; I have 275 X 22.5 green X line, I have allways run 100 lbs in all 6 tires , We had 2 blow outs with the XRV at Reccommended Pressure. It rides a Bit rough But is vary stable. And a Bonus We set at 10 MPG 38' Ultimate Cat 330 with a tow car..

[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.