Thursday, August 13, 2015

Tires what a rotten trick

To continue to my comparison of RV's and Ships I wanted to bring up tires.

RV's have them on their wheels and use them to keep them going down the road safely and comfortably. Ships have them hanging down their sides on ropes and use them to cushion the blow when the Captain screws up and smacks sideways into the pier.

Where's the connection? Well RV tires are big, they don't get a great deal of mileage on them so they don't usually get worn out. Instead they sit in the sunshine and ozone, which chemically deteriorates the structure of the tire. If you see an ad for a rig that says "Tires good - slight ozone checking" what it is really saying is "Tires rotted out just waiting for the worst moment so they can blow out". Rotted out RV tires make good bumpers for ships but not much else. 

Sound scary? Well it is.

We all know what tires are. They are the black things under the wheels that wear out every couple of years. Or they are on a car, where the annual mileage is around 20,000 miles and they get replaced every 5 or so years due to wear. On a car they are fairly lightly loaded and get inflated to about 30 psi.
On an RV the tires might only see 1500 miles a year so the tread looks great. Surely that ozone cracking on the sidewall can't be bad? Well it is, RV tires are only good for about 5-7 years depending on how they've been treated and then they need replacing due to age. The RV weighs around 20,000 lbs loaded and ready to go. That's 10 TONS. Properly inflated they can have 80 to 120 psi in them which means when one deflates it EXPLODES.

Ever see those huge chunks of tire on the Interstates? 

Exploded tire.

Most of the ragged bits of tire are from truck tires, but RV tires are basically truck tires too. There is a date code on tires, check them even when buying new tires as the dealer might slip a set of tires on that have been sitting around for 2 years and now you have tires that are only only good for another 3 years. And RV tires are EXPENSIVE.


The date code on my new tires
This means they were made in the 8th week of 2015

Hard to read but this is the manufacturer's label giving the
MAXIMUM load at the tire's MAXIMUM pressure
Most manufacturer's websites can give you the appropriator pressure at your SPECIFIC load

Given the dangers of a tire exploding on a 10 ton projectile travelling at 60 mph down a steep twisty road with a mountain on one side and a 1000' drop on the other, it should make all of us want to take good care of the tires!!

How to do that? Well you may have seen RV's with white covers over the tires. That's not a fashion statement, they reflect the UV rays away from the body of the tires and prolongs the life of them.




How often do you check the tire pressures on your car? Many people rarely do unless they start hearing a grinding noise going around corners when the rim drags on the pavement. That's really bad for tires.

Under inflation is one of the most common causes of tire failure. Under inflation causes damage because when properly inflated the air in the tire keeps the tire the right shape and gives cushioning to the tire structure when it hits a bump. Low pressure lets the tire deflect more and roll around on the rim during cornering. All that twisting builds up heat in the structure of the tire that it was never designed to take (Unless you have run flat tires. But I don't believe they make RV tires like that). 
And that's just on your car. 

The big tires on an RV if under inflated also affect the steering and upset the balance of the rig due to one corner being lower than the others. Where there are dual rear wheels, if one tire has low pressure then all the load goes onto the other tire, drastically overloading it.

If you want a lecture (like this one) on tire care - talk to a motorcyclist. They are very focused on tire care because they have only 2 of them between their butts and the road, and if only 1 of those 2 tires suddenly deflates they are immediately out of control.

So get a good tire pressure gauge that will read up to 150 psi. You probably aren't going to find one in the local auto store so you'll need to look in an RV store or truck stop. Then you're going to need some way of inflating the tires to the required pressure. A little auto tire pump that plugs into the cigarette lighter isn't going to hack it. Look for one that is capable of 150 psi.
Gas stations may not have air supplies and even if they do it isn't likely they will have one that will go up to 150 psi. Be prepared, get a good compressor.


 Our compressor stowed away

RV's have more wheels than cars, at the back they generally have dual tires. One wheel sits inside the other on the same axle. This allows a greater load range on the axle. Some RV's have 2 rear axles. This is called a "Tag axle" and the second one isn't driven, it just takes some of the weight off the driven axle and allows an even greater load range. For dual wheels some method of reaching the inside valve is needed for checking pressure and inflation. Some people install valve extenders. Ours came that way but then one rear tire started losing pressure constantly. 

We were in for new front tires (at $450 a piece!!) so I asked them to see if they could find the leak in the rear. It was the valve extender. A nice looking gadget but not a good idea. The tire dealer showed me that truck tires (basically the same as RV tires) don't use the extenders they use a "Push/Pull" tire gauge and inflater. The valve stem on it's own is much more reliable. When we bought this rig the previous owner told us they'd had a tire blow out on the rear due to a leaky valve extender.


The guilty party - a valve extender

The valve without the extender
Harder to get to but much more reliable.


All the tires on all the axles have to be kept properly inflated so look for all the valve stems. I make it a habit to check the tire pressures and the air pressure in the rear suspension air bags the day before we leave home and once a week when on the road. That gives me plenty of time to do it and to correct problems if I find them. I carry spare valve cores and a tool to replace them as (apart from a puncture) they are the second most likely reason for a slowly deflating tire. (In my opinion based on my experience.)


A heavy duty tire pressure gauge
this one reads up to 180 psi and has both a "Pull" and "Push" feature on the end


An electronic tire pressure gauge
Accurate, light and small
Great for air bags and car tires

2 different "Push/Pull" inflators

Vave core tool to tighten and replace cores if needed

A valve core seals inside the valve stem and keeps the air in the tire.

So what tire pressure is the tire supposed to be inflated to? There's a Max Load and Max Pressure label molded into the tires. Is that it?
No it's not. As is says that's the max pressure allowable at the maximum load. If you inflate the tires to that pressure you will have a hard ride, the tire will wear out in the center before the sides and the tire will overheat, meaning you risk that dreaded explosion again. 
Around the drivers door or the drivers seat there will be a data plate that gives the maximum axle weights and Gross Vehicle Weight Rating (GVWR). Right on that plate will be the recommended tire pressures. On my rig it's 80 psi. Some rigs it's as high as 120 psi. Check your own data plate and use those pressures as a starting point. Get the weighed fully loaded. Weigh each axle independently and consult the tire manufacturer's website for the appropriate pressure at your specific loading.

If you are just getting used to a vehicle it's a good idea to make a note of the pressure when you check it, before you add any air. That way you get to know if one tire looses air faster than another and you can watch that one, check for a nail in the tire or consider replacing the valve core, valve extender or having the tire remounted if the the loss of pressure is excessive.

Why do tire lose pressure at all? Well nothing is perfect and a valve core may not seal perfectly, there may have been dirt on the rim when the tire was mounted, the tire extender may be causing a leak and air molecules can slowly find their way thru the structure of the tire. Another thing that may make tires appear to lose pressure is temperature.

Hands up all those who know and live by Boyle's Law.

Nobody but us engineers huh? Mr Boyle who the Law (as in Law of Physics) is named for did, some experiments with gas sealed in a container. He found out that if the container filled with a gas stays the same size and the temperature changes then the pressure in the container changes in relation to the temperature. There is more to it but that's the bit that we are concerned with. What it tells us that if the tire is hot when you set the tire pressures then when the tire cools down the pressure will be less than if we had set it when cold.

Tires heat up when we drive down the road due to friction between the tire and the road and the flexing of the tire as if hits bumps and goes round corners etc. The recommended tire pressures take this into account and will still give a comfortable ride. 
(In a situation where you have a partial deflation on a warm tire for some reason and you have to add air to one tire when hot, you can take readings on the other hot tires and inflate the low tire to that pressure just to get you somewhere where you can let the tires cool overnight.).

If you set the tire pressures in the snow up a mountain somewhere then drive down into Death Valley then the tire pressure will be different than recommended next morning due to the temperature and altitude difference. These variations aren't drastic enough to make you stop driving every time the temperature changes but if you are going out into the desert you might want to check the tires the first day after you get there.

My goodness you are sorry you started reading this right??

So what about filling the tires with nitrogen like the racers do in NASCAR?? In my opinion (and opinions are just like butts, everyone has one) nitrogen is just a way to part you from your money. The stuff that causes air to expand when heated is the moisture in it. Nitrogen can have less moisture in it if DRY nitrogen is used.

I know from testing instrumentation systems on ships that DRY nitrogen is nice stuff, it's very stable and doesn't corrode delicate systems. I also know from bitter experience that all nitrogen isn't DRY NITROGEN and will lead to mysterious expansion/contraction and corrosion in delicate systems. Now just as there is such a thing as dry nitrogen there is also such a thing as DRY AIR. Industrial air compressors can be equipped with dryers that take most of the moisture out of the air. 

If you would like 72% pure nitrogen in your tire FOR FREE. Just fill them with air which is 72% nitrogen naturally. 
There is some spin on nitrogen atoms being larger that air atoms so they don't leak thru the tire wall as fast.

Humbug (in my opinion) you'll never be able to measure the difference with your tire gauge.

The reason race teams use DRY nitrogen is consistency and availability. They can go to any industrial gas supply dealer around the world and get exactly the same product. If they use compressed air it varies in moisture content based on variations in the humidity wherever the compressor is located. The moisture content dictates the amount of expansion due to temperature change and in a race car the tire temperature can change rapidly during heavy braking and cornering. Race cars are set up on the ragged edge of performance and repeat-ability in all things affecting the braking and cornering. It is essential to getting the car tuned to that ragged edge as quickly as possible. Eliminating moisture from the tires allows them to do that.

So where does that leave us, apart from confused?? Well unless you need to consistently drive your 10 ton RV on the ragged edge of its cornering ability forget about nitrogen in the tires. Go out and buy a race car. If you want to drive your RV in safety and comfort (and without the distraction of the tires exploding now and then) you should fill them to the recommended tire pressure and check them regularly. 

And give your old tires to a friendly Ship's Captain so he can hang them down the side of his ship and stop him scratching the paint when he hits the pier.




2 comments:

  1. Well put Brian! Being a MCist I'm religious about all the tire pressures on my vehicles. Gonna go out and check my dualie now.

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  2. Wow! I'm impressed! And it wasn't a boring read either. Thanks, I have learned something and that is always a good thing. (from a fellow Shanty Shaker who just found your blog)

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