Save money at the fuel pump

Tire Pressure.
 Ya, we tire engineers are like a broken record when it comes to tire pressure.Tire Pressure Monitor System. If you don't have a TPMS then we tell you to check your pressure every morning you are traveling.

We tell you you need to monitor it all the time by using a
We also want you to get your RV weighed then to consult some confusing tables to learn how much pressure is needed for your motorhome.
To make things worse we expect you to have an accurate gauge but then we don't tell you how to check your gauge to be sure it is accurate, or even why its important to have an accurate gauge.

Is that what's bothering you bunkie??

Well this post is going to help answer your questions, solve the problem of how to be sure your gauge is sufficiently accurate and to top it off tell you that its possible that many of you will end up saving more money than the "fix" costs.

To start off lets take a look at accuracy of gauges many RV owners are using today.
In a previous post I showed the results of a series of gauge tests conducted at an RV Rally Sept 2012 on 11 gauges, we saw that there was a 45% failure rate. This was higher than the failure rate seen on a similar sample from 2011 when 11% failed. This year my test sample was 24 gauges and we had a 12.5% failure rate. Failure is defined as more than 5% off when compared to a certified digital gauge that reads to the nearest 0.5 psi. For all these tests the test pressure was between 77 and 95 psi.

According to sources there is a range of fuel economy savings possible from keeping your tires properly inflated because tires are only one of the contributors to fuel consumption. Also different tires have different affects on fuel economy based on a number of variables. Some of these include different rubber compounds, basic tire construction features, tread design and tread depth. Without getting technical we learn that the impact of lower inflation on mpg ranges from  0.05% to 0.3% per psi. Now you may say this isn't much but lets look at how this hits you in your pocketbook

Assume fuel is $4.00 a gal when you do a fill-up. Lets look at the range of savings at -2psi and -10psi from our goal of 100psi (our cold inflation to carry the load with a built in +5psi safety factor). With a 30 gal fill-up the range of "extra cost" for being low on inflation is between $0.12 to $3.60. If you do a 70 gal fill-up running low on air is costing you an extra $0.28 to $8.40 for that fill-up. Remember this is from just being 10 psi low on your tires. A number of the gauges we tested had the owners running from 9 to 18 psi low so they were wasting some real cash.

With this much potential loss from your pocket each fill-up you certainly do not want your pressure gauge to be giving you wrong readings. Having an accurate gauge is easy and based on the above calculations you might save enough in a few tanks of fuel to pay for what you need.

A quick search for "Digital tire gauges" at Amazon. shows you could spend well over $300 for a digital gauge but that really isn't necessary. You can get a good digital gauge for less than $10. This becomes your "Master Gauge" Then you get a second gauge for every day use. When the gauges are new you compare them and they should give the same reading. If you notice a sudden change in tire inflation pressure you can dig out your master gauge and confirm. You should also do a comparison at least a few times a year. The chances of both gauges going bad the same amount at the same time is vanishingly small.

Finally don't just throw your master gauge in the bottom of your tool box. I suggest you keep it in a safe place that offers some type for protection.

"Interply Shear" and other Techno Babble

I have been putting this topic off for quite some time as I was worried about overloading you with too much "Techno-babble" but I find myself having to constantly repeat this information in individual posts on various RV forums I monitor so figured it would be easier to do a complete post that I could direct people that want to understand why the loading of some trailer tires is much more complex than the average person thinks.

To make this less painful I will give the Bottom Line info first, so those not interested, can stop reading before I put them to sleep.

BOTTOM LINE
When a radial tire is loaded, the belts and body have to bend from a round shape to a flat shape in the area that contacts the road. In addition when you turn a corner the forces generated to move the RV sideways have to be transferred through the tire structure.

This causes additional bending of the belt and body structure. The more the bending the higher the stretching of the rubber. With enough stretch, microscopic cracks form and existing cracks get bigger. Eventually with enough cycles and enough force the cracks may grow and join up with the possibility of tire components separating which could lead to a tire failure. You can lower the stretching if you lower the bending and you can lower the bending if you increase the inflation.


So now on to the Engineer Speak and Techno Babble

If you own a multi-axle trailer these forces can be much higher than those seen on a tow vehicle, motorhome or car, where the tires are not close together but at the corners of the vehicle.
I found an excellent video that shows the results of these forces at Keystone RV. Watch the section from time 0:46 to 1:07 and note that the tires on one axle bend inboard while the others are forced outward.

Special consideration for multi-axle trailers. Warning, this gets technical.
When not driving in a straight line there are special side loads on multi-axle trailers because the tires are fighting each other because they are not "pointed" to the center of the radius of the turn. These loads cause interior structural tearing. Sometimes 24% higher loads than those seen in tires on non-trailer application. Initially tearing is at the microscopic level but with time and repeated cycles these forces grow which can lead to small cracks at the belt edges as seen here at the arrows.

 If not spotted these cracks continue to grow to almost the full width of the tread as seen below.

 If you are lucky you will see the bulge in the tread as seen here and now you know this tire has failed and MUST be removed AT ONCE as the separation can grow and  can cause a belt to come off the body of a tire.
You can lower these forces by either decreasing the load 24% on the tire (probably not something you want to do or may not be able to do) or you can increase the inflation to stiffen the structure and decrease the slip-angle. In this case you could increase the tire inflation from the minimum inflation needed for the static load to the inflation associated with the max tire load as molded on the tire sidewall. BUT you need to be sure you are not exceeding the max rating of the wheel.

So the best recommendation I can give to trailer owners is to run the inflation molded on the tire sidewall. For owners of a TV or motorhomes, I recommend you run the inflation needed to carry the actual measured tire load plus at least a 10% margin.


 

Should I jack up my RV?

I see this question come up each year when some people are parking their RV for the Winter. There is a quick and simple answer and a more involved answer,

Quick Answer:
I do not jack up my Class-C RV over the winter but I do inflate the tires to 80 psi (the pressure on the tire sidewall). I do cover the tires with white covers over the winter (and whenever I camp at a location for more than 1 day).

There are other things you can do such as keeping the tires out of contact with wet sand and dirt. I covered this with pictures in my post on “Winterizing” 

More involved answer:
Jacking up the RV and removing the tire is one of those things that in a perfect world would be easy to do but in reality it isn't as many RVs don't have good jacking points or are so heavy you need very HD jacks so it may just not be safe to do..
I do understand the theoretical damage done from long term parking in one spot but again theory and reality clash. Lots of actions can theoretically prevent microscopic problems but if you extend the life of a tire by 1 month if you were to jack it up every time you were planning to park for more than 3 months, would it be worth the effort? Probably not.

There are actions that are easy to do and relatively inexpensive that can provide real "bang for the buck". I have covered these in various posts on my blog but will touch the big ones here.

1. Your actual static load should not exceed 85% of the max load capacity on any individual tire for the inflation you run..
2. You need to weigh and learn the actual load, when fully loaded and not simply take the total axle load and divide by the number of tires. You will NOT get the correct tire load by doing the simple division. You can download a worksheet on how to do the math HERE Or HERE

3. Get a TPMS so you will get a warning when (not if) you start to loose air due to puncture or leaking valve
4. Cover your tires with WHITE tire covers if they will be in the sun for better part of the day. A couple of hours in full sun does about same damage as a full day in the shade in Phoenix.
5. Replace the snap-in valves or rubber parts of your bolt in tire valves whenever you buy a new tire.
6. Get and use a digital hand held gauge at least once a month, even if you have a TPMS. You will probably be adding 1 to 3 psi each month anyway to maintain the tire inflation. This will also serve as a check on the TPMS.
7 NEVER drive on a tire that has lost 20% or more of its air. Structural damage will be done. Such damage is cumulative and this damage does not repair itself. (See post on Potato Salad)

8 Do not believe everything you read on RV Forums. Ask the poster for their actual training in failed tire analysis. Simply having owned tires from company XYZ does not make that person a knowledgeable source.
9. If you have a multi axle trailer your cold inflation should be the inflation on the tire sidewall

10 You might put on your To-Do list to read the posts in my blog (including the ones where I later point out my errors), then you could subscribe so you get a notice when I do a new post. I do about 3 a month so you will not be overloaded.
11.If you have a multi-axle trailer, learn how to do a rotating tire inspection and do your tires at least once a year. Watch this VIDEO  read the background in this POST to see what a failed tire looks like before it comes apart.