Tire Pressure, Load Capacity, etc

[richard5933]

I've seen a few comments posted on other threads about tire pressure, load capacity, etc. which were concerning to me since I couldn't tell if they were based in science or just common practice. Rather than hijack the thread, I thought I'd start a new one.

I've learned a few things in my time driving, and especially driving and working on our coaches, but I'm no expert by any means. Some things I've seen posted elsewhere in the Forum got me thinking about how they would be interpreted by someone not too familiar with tire-related issues. Here's my take on a few of them. If you have facts to add, please feel free.

1) Air pressure in tires should be based on many factors, and usually not just by what's stamped into the sidewall of a tire. The vehicle manufacturer will have guidance, and each tire manufacturer publishes inflation charts to allow drivers to determine the proper pressure to use based on the load being carried. To calculate the proper air pressure in tires it's necessary to know the weight load being carried, the tire's inflation chart, the rim's max air pressure rating, as well as the vehicle manufacturer's recommendations and limitations for inflation.

2) Knowing each axle end weight is the ideal way to set tire pressure, but be sure that the tires on both ends of each axle carry the same pressure. That means if one side of your coach is heavier at the steer axle, you base the tire pressure in both front tires on the heavier side. Since the weight being carried on each axle is usually different, the air pressures used might be different for each axle.

3) Upgrading to a tire with a higher load rating will not by itself increase load capacity. Load capacity is a combination of many factors, and usually switching to a tire with a higher load rating won't increase capacity unless a higher air pressure is also used.

4) Increasing tire pressure above what's needed to safely carry the load (including a few PSI extra for a buffer or temp fluctuations during the day) can adversely affect handling and braking.

5) Just because the sidewall of the tire says the tire can be filled to 130 PSI doesn't mean that your vehicle will be safe with that air pressure in the tires.

6) Under inflated tires are dangerous. So are tires used to carry more than their rated capacity. Both at the same time is a disaster in the making.

7) Changing to a tire size different than what the manufacturer used can create an unsafe situation. Always best to check these things with experts before making changes.

8) Tires can 'age out' before they 'wear out' and be unsafe to use even though they have lots of tread left.

9) If you're buying a used rig, don't assume the tires are the correct size/type/load capacity for the vehicle. Our first coach had multiple sizes on it when we bought it, and only some of them were correct for the vehicle. Check the tires carefully before driving.

10) Tire air pressure should be checked with the tire cold (ie, not driven on for the day yet). Within normal circumstances, there is no need to adjust the inflation charts to account for the ambient air temps. If the tire has not been driven on for the day it should be filled according to the calculations made following the inflation charts.

For those wanting much more information, here is one site I've found with lots to read. http://www.rvtiresafety.net/

I'd love to see what other tips & safety advice others have related to tires & wheels, especially with an eye towards members who may be new to the game or not up to speed yet on tires. If I've got something wrong, please let me know.

[hermanmullins]

Richard,

Thanks for starting the new Thread. It should generate lots of good information on tire life and safety.

Herman

[manholt]

Richard.

Thanks for bringing the subject up.  We think about the tires and discuss same, all the time.  You reminded me about the rim, we seem to forget that the wheel size is equally as important as the tire!  Perhaps even more so, due to the weight we have added to the chassis!

[richard5933]

A whole other chapter could be added about rims/wheels. The should be inspected regularly for apparent damage, whenever the wheel is off the vehicle, and especially when the tire is off the wheel.

Wheels can and do wear out, and damage can also be done be improper mounting techniques such as using a pneumatic gun to over tighten lug nuts. Aluminum wheels are of course more likely to be damaged from things like this, but even steel rims can be damaged. Add to that the occasional curb strike incident and it's not hard to understand.

The ridge inside the edge of the wheel which holds the rim can also wear down. I've recently read about someone replacing his aluminum rims for this reason.

Many people have no idea that wheels have a load rating and a maximum air pressure just like tires, and no matter what tire is installed it will never be safe if the wheel's rating is exceeded.

[wolfe10]

Excellent, Richard.

One minor comment:  The tire manufacturer's inflation tables establish the MINIMUM PSI  for a given weight.  Said another way, if weight is higher, one moves to the next (higher)  PSI column.

So, I would suggest that the 5-10% that many of us add to the "minimum PSI for a given weight" has merit.  Merit because it can compensate for colder temperatures (which by physics means lower pressure), for added weight (full tanks for example), and also for a a safety factor (better to not have a tire running at its bare minimum PSI).

[wayne77590]

Brett, I call it the "Fudge Factor." It keeps you from having to inflate/deflate all the time to maintain the pressure.

[ispjs]

Very good information posted on tires/rims.  When I think of safety on the road with my RVs my first thoughts are always about the tires.  Even while stationary for a few weeks I still monitor them with my TPMS on occasion, clean, and inspect them.

[tbutler]

I was a graduate assistant for a geology course at the University of Missouri, Rolla, for several summers.  One summer we needed some wood blocks for an activity.  I purchased a 2x4 but didn't have a saw so took it to the shop that supported the class needs at an engineering school.  I asked for blocks which were square, 3 1/2 x 3 1/2 inches.  The person I was talking to asked me what tolerance was allowed.  In an engineering school, it wouldn't be uncommon to have a requirement for a tolerance of +/- 1/10,000 inch.  For my needs, +/- an eighth inch was sufficient but it did make me think about accuracy and tolerance in making measurements.  In that light, I offer the following comments...

In talking about vehicle weight, don't overlook Brett's comment about fuel tanks.  One of the things that makes knowing vehicle weight on a motor home so important and difficult is that the weight is constantly changing.  Fuel, diesel or gas, and propane will change as a motor home travels down the road.  On an extended road trip, waste tanks will fill and the fresh water tank will slowly empty.  Point #4 implies a degree of accuracy that can't be achieved in a vehicle which is constantly changing weight and weight distribution.  When you have your vehicle weighed by RVSEF, their form has a place to indicate tank levels and instructions for calculating what the full tanks would add to the weight of the vehicle at the time of weighing.  While it would be unusual, you have to inflate your tires for full tanks which will result in slightly over--inflated tires most of the time.  The likelihood that your tire inflation is perfectly on-the-dot correct for your current weight is very low and the expectation that it could be kept perfectly correct for your weight is something that would be impossible.  +/- what?

Thinking about tire inflation, one major consideration has been missed.  What is the accuracy of your measuring instrument?  Has your tire gauge been certified for accuracy?  How old is it?  How long has it been since it was tested for accuracy.  I have had tire dealerships compare readings from my tire gauge with the main gauge for their shop.  I don't know what the requirements for testing their gauges but comparing different tire gauges shows a 10 pound difference between two identical tire gauges is not uncommon.  Your average tire gauge is not a precision instrument that you may think it is.  The same is to be said for tire pressure monitoring systems.  You will find that the sensors show differences from one to the next.  Two tires which show exactly the same pressure on my tire gauge show different pressures on the TPMS.  Taking a tire pressure reading itself will introduce small errors in the system.  Apply the gauge, you hear a brief hiss, release the gauge from the tire valve, another hiss, losing air pressure each time.  My TPMS sensors have to be screwed on, more hissing as I quickly screw them on the tire valve.  How much pressure is lost?  Is that the difference in the readings of the two TPMS sensors?  Bleeding the pressure from tires will quickly let you know that a lot of hissing goes on to lower the pressure a pound so I suspect it is a very minor difference.  +/- what?

When getting a starting air pressure for the beginning of the day, it is important that the tires not be sitting in the sunlight.  A tire sitting in sunlight will warm quickly and the pressure will read higher than a tire in the shade even if both were at the same pressure before sunrise.  Adjusting the pressure on a tire which has the sun shining on it can result in wildly different pressures between tires on one axle where one tire is in sunlight and the other in shade.  This is why I never sleep late on a day when we are driving.  Unless you are parked in a very shady location or inside a garage, there is only one way to get an accurate before driving air pressure reading from your tires, get up before the sun hits them.  +/- what?

[ispjs]

"When getting a starting air pressure for the beginning of the day, it is important that the tires not be sitting in the sunlight.  A tire sitting in sunlight will warm quickly and the pressure will read higher than a tire in the shade even if both were at the same pressure before sunrise.  Adjusting the pressure on a tire which has the sun shining on it can result in wildly different pressures between tires on one axle where one tire is in sunlight and the other in shade.  This is why I never sleep late on a day when we are driving.  Unless you are parked in a very shady location or inside a garage, there is only one way to get an accurate before driving air pressure reading from your tires, get up before the sun hits them.  +/- what? "

Tom,  I think you are generally spot on with your post, especially the above part.  It is easy to see how the sun effects psi even when rolling down the highway if the sun is only hitting one side of the coach.  Just utilizing a TPMS over the years has educated me further about tires behavior.

I don't really think manual checking with a gauge, or installing a TPMS sensor actually lets out enough air on these large MH tires to make any difference.  IMHO you would have to sit there and manually check a 22.5 tires 30 or 40 times to let out 1 psi of air.