Motorhome weight and tire pressure

[Gordon Dewald]

Originally Posted by Kid Gloves

Do you have a number in mind, either in pounds or percent, for which you believe it would be safe to exceed the load limits published by a tire manufacturer?

While I don’t believe that you are suggesting that anyone exceed a published load limit for a tire, you are implying that a tire is designed to support more than the published load limit.

You’ve suggested that “there are numbers added for safety” yet our primary source of guidance is the load and inflation table provided by the tire manufacturer. Have any of your engineer friends discussed RV tire design factors with you?

I think RV have the most stable weights of any vehicles. Once you get all of your ancillary items aboard, and we have lots, the weight does not change much. I weighed with full water and fuel tanks (empty holding) so the weight will go down. Addition or consumption of food will not change it much. When a coach weighs 40,000 lbs trying to adjust for 100 or 200 lbs on an axle may be too fine.

As an operator you should be aware when you make a serious weight change, reweigh as necessary and adjust tire pressure accordingly. I live near a scale and have weighed numerous times and found the coach weight does not change enough to change the tire inflation.

To answer your first question I would not exceed the manufacturers guide. I doubt that I do. I loaded the coach, did corner weights, used the highest weight on each axle and inflated the axle to the recommended pressure. I note in the tire chart the next increment is roughly 300 lbs per tire for our tires on the front axle, similar for the rear. So I am comfortable since I 'rounded up to the higher increment it will be unlikely we will add enough weight to trigger an inflation increase.

2nd question. You are correct that I am not suggesting nor recommending that anyone exceed the load limit on a tire. My suggestion is as long as you are within the tire inflation chart it is unnecessary to add 5 or 10% to account for imponderables the user thinks the tire engineers did not consider.

3rd question. Any of the engineers I worked with rounded up when designing critical "furniture". Same as the tire engineers recommend going up to the next increment when the tire loads have been determined. Logic says that if they are recommending the user round up they would have "rounded up" when calculating the tables from the tire testing.

It is a great debate and everyone is free to do as they want regarding tire inflation. I look at the benefits of being properly inflated. The best contact surface for traction, best ride and even wear. My rationale is to use the tire manufacturers inflation because it is their recommendation.

[mlpeloquin]

Until all tire weights are weighed individually, the CAT scales are the next best thing. I add ten psi to the CAT scale weight from the load/pressure charts minimum psi required for the weight. When I did get my four point weight done, I was only five psi above the required psi. I like having five psi over the minimum required. When the air temp goes lower I don't need to adjust the tire air pressure. I do have a TPMS to monitor the tires air pressure.

[Kid Gloves]

Originally Posted by Gordon Dewald

3rd question. Any of the engineers I worked with rounded up when designing critical "furniture". Same as the tire engineers recommend going up to the next increment when the tire loads have been determined. Logic says that if they are recommending the user round up they would have "rounded up" when calculating the tables from the tire testing.

It is a great debate and everyone is free to do as they want regarding tire inflation. I look at the benefits of being properly inflated. The best contact surface for traction, best ride and even wear. My rationale is to use the tire manufacturers inflation because it is their recommendation.

Thanks for the response. I agree about using the charts.

What I disagree with is assuming that something is stronger than what we’re told. If we’re going to follow the manufacturers recommendation on pressure, we should also follow their data on the limits of the tire. If we use logic to conclude that the tire can actually support more weight than is indicated in the chart, couldn’t we also use logic to conclude that the tire can support the weight in the chart with less pressure? Manufacturer literature indicates that a tire is considered flat, if it is 20% below the recommended chart pressure.

It’s a dangerous path.

[Crasher]

Corner weights are a must for any coach with air suspension. Mine is a good example. When at the FMCA show in Gillette, WY in 2013, I had a four corner weigh. The FR weighed 8100# and the FL weighed 7200#. The tires were rated for 7840# each @120 psi. I drove the first 2000+ miles at the 110 psi on the placard next to the driver. That pressure put the FR tire 10 psi under the sidewall max. The 8100# was about 800# more than the tire rating at placard pressure. I could tell from the TPMS that the right tire increased to a higher pressure faster than the left and remained at a higher level all day. I wondered why at the time. After the four corner weigh, I knew. The problem was corrected when the ride height valves were checked. The left rear valve was set 1/4" too high causing the LR to ride above specs which forced the FR to carry more weight. Once corrected, both steer tires were within 100# of each other and they now run the same pressure after heated up on the road. Without the all position weights, I never would have known the problem till the FR tire failed from underinflation for the load. My experience might be unusual, however, everyone with air suspension should get position weights and adjust ride heights as needed.

[bentjm]

Here is the placard from my 2005 Newmar Mountain Aire 4032 DP.

As you can see, the chassis was built in May 2004. Maybe the placards are different on newer models, but this one lists 120 PSI all around. My placard basically recommends the max pressure on the original tires.

If I ran 120 PSI all around on the TOYO 144s I have on there now, it would support 15,120 lbs on the front axle and 26,860 on the rear axle.

My actual axle weights running heavy - full fuel, full water tank, full everything - are 13,000 front and 19,500 rear.

Based on the TOYO RV pressure tables, I run 105 PSI front and 85 PSI rear. That gives me 13,630 lbs front and 20,380 rear.

Would anyone here actually run 120 PSI all around on my coach?

[nehog]

OK this is the correct way to set your tire pressure!

1. Go weigh your RV. CAT scales are good. Most commercial recycling/refuse centers have truck scales and will weight your right for a small fee. CAT has a mobile app to make weighing easy and relatively fast.
2. Record the weight on each axle.
3. For axles with two tires divide the weight by two. For axles with four tires (dual rear wheel) divide the weight by four. This gives the per-tire weight.
4. Go to the tire manufacturer's web site and download the weight to tire pressure chart for your tires. Make sure you get the correct chart--look carefully at the specifications!
5. Take the weight from step 3, and look up the weight to pressure value.
6. Inflate to that pressure. I often go about 5% more because sometimes things get heavier. Just never exceed the maximum inflation value!

Remember: an overinflated (based on the weight/pressure from the chart) tire may give a harsh ride, but is much safer than an underinflated tire!

I strongly recommend a TPMS to monitor your tire pressures. For example, check all tires in the morning before you start driving, and make sure none are low.

[Kid Gloves]

Originally Posted by nehog

OK this is the correct way to set your tire pressure!

1. Go weigh your RV. CAT scales are good. Most commercial recycling/refuse centers have truck scales and will weight your right for a small fee. CAT has a mobile app to make weighing easy and relatively fast.
2. Record the weight on each axle.
3. For axles with two tires divide the weight by two. For axles with four tires (dual rear wheel) divide the weight by four. This gives the per-tire weight.
4. Go to the tire manufacturer's web site and download the weight to tire pressure chart for your tires. Make sure you get the correct chart--look carefully at the specifications!
5. Take the weight from step 3, and look up the weight to pressure value.
6. Inflate to that pressure. I often go about 5% more because sometimes things get heavier. Just never exceed the maximum inflation value!

Remember: an overinflated (based on the weight/pressure from the chart) tire may give a harsh ride, but is much safer than an underinflated tire!

I strongly recommend a TPMS to monitor your tire pressures. For example, check all tires in the morning before you start driving, and make sure none are low.

Your method does not take in to consideration the variation in weight from one axle end to the other. You’ve assumed that the total axle weight is distributed equally to both ends. That simply doesn’t happen. Differences of 5-10% are not uncommon. This is the reason that it is typically recommended to divide the total axle weight by the number of tires and the add 5-10% to that number. Consult the chart using the higher number and inflate all tires on that axle to the recommended pressure. Some folks add another 5-10% to that.

Alternatively, if the scale happens to be in an area where the coach can be driven onto the scale with only one side of the axle on the scale, corner weights can be obtained. The ground needs to be level to get accurate readings. I did this for my own amusement last week in a passenger car at a free public scale.

[Kid Gloves]

Originally Posted by bentjm

Here is the placard from my 2005 Newmar Mountain Aire 4032 DP.

As you can see, the chassis was built in May 2004. Maybe the placards are different on newer models, but this one lists 120 PSI all around. My placard basically recommends the max pressure on the original tires.

If I ran 120 PSI all around on the TOYO 144s I have on there now, it would support 15,120 lbs on the front axle and 26,860 on the rear axle.

My actual axle weights running heavy - full fuel, full water tank, full everything - are 13,000 front and 19,500 rear.

Based on the TOYO RV pressure tables, I run 105 PSI front and 85 PSI rear. That gives me 13,630 lbs front and 20,380 rear.

Would anyone here actually run 120 PSI all around on my coach?

I would not. This question almost begs for its own thread.

[bentjm]

Originally Posted by Kid Gloves

I would not. This question almost begs for its own thread.

My point is that there are hundreds of posts every week about the poor quality control of the RV manufacturers, but somehow the tire pressure placard supposed to be sacrosanct.

My placard shows that not to be true.

Weigh your coach, make sure you are not over the limits of your axles, and set your tire pressures based upon the tire pressure tables from the manufacturer of your tires.

[Gordon Dewald]

Originally Posted by Kid Gloves

Thanks for the response. I agree about using the charts.

What I disagree with is assuming that something is stronger than what we’re told. If we’re going to follow the manufacturers recommendation on pressure, we should also follow their data on the limits of the tire. If we use logic to conclude that the tire can actually support more weight than is indicated in the chart, couldn’t we also use logic to conclude that the tire can support the weight in the chart with less pressure? Manufacturer literature indicates that a tire is considered flat, if it is 20% below the recommended chart pressure.

It’s a dangerous path.

Seems to be a bit of a communication gap here. Not suggesting that you can apply more weight than you are inflated for. Just that you inflate the tire to the weight you have using the tire inflation tables prepared for the tire. Since we are supposed to fully load the unit prior to getting the weights no extra tire pressure is required.

I think what may have taken us off topic is suggesting that the tire engineers have rounded up during their determining the tables from their testing data. Sorry if that is the case. I believe they round up the pressure during their calculations when the calculations do not come out evenly.

Adding a few extra pounds air pressure when inflating your tire for whatever rationale will not hurt. As long as you DO NOT exceed the maximum cold pressure noted on the side of the tire. It may affect the tire performance (contact patch) or wear but since tires will age out long before they wear out on most coaches it is a personal choice.

[jadatis]

About that last.
Quote
Adding a few extra pounds air pressure when inflating your tire for whatever rationale will not hurt. As long as you DO NOT exceed the maximum cold pressure noted on the side of the tire. It may affect the tire performance (contact patch) or wear but since tires will age out long before they wear out on most coaches it is a personal choice.
End quote

If you are using a tire to its maxload , it is even saver to use higher pressure then written on sidewall. So DO EXCEED THE PRESSURE ON SIDEWAL.

Left away maximum because its the reference-pressure on sidewall. Exept for P-tires.
P-tires give maximum cold pressure only on sidewall of between 44psi and 51 psi, and an ocacional XL 60 psi.
Referencepressure though is standard load 35 or 36 psi, and XL% reinforced/ extraload 41 or 42psi.
LT and trucktires only give reference pressure on sidewall, mostly with AT in front of it, but sometimes maxpress or something behind it, and that gives the confusion.

There are old pressure loadcapacity lists higher maxload is given for higher pressure upto 40% more then referencepressure when standing still.
ST was given 10psi higher for driving 75mph instead of 65mph.

In Europe Continental group gives often on C- tires( = LT) and the referencepressure without AT in front of it direct behind the service descriptions and the " maximum inflation pressure " of 10 psi more further on sidewall.

Both are cold pressure ( when temperature of air inside the tire is practically the same as outside the tire)


So this not being allowed to go above the reference-pressure, often called maxpress, is a truth made by the internet, if you write it often enaugh, it becomes truth.
Even I believed it , that they changed the policy at a certain moment, but now I state, that as long someone cant produce an official document that says, from this date we dont allow it anymore, it is allowed to a certain amount .

So dont be afraid to fill upto 10 psi higher if needed, the tire wont blow from it, if it is not already damaged by using to low pressure.