Air Pressure of Carlisle Tire & Tow Haul Mode of Silverado

[pkmesser]

The 4L60 is a light duty transmission. Heat will kill it, period. So, the tow/haul mode which keeps it in lower gears longer helps. My wife's cousin is a GM mechanic, he tells everyone in the family that has that transmission and is towing any weight at all to keep it out of overdrive, even though you might buy more gas. I have driven GM pickups for close to 40 years. The 4L80 and 6L80 will take a lot of severe service, and keep going. The 4L60 with the same conditions will generally fail shortly after 100K miles. Other than staying out of OD when towing, and having a good tranny cooler, the absolute best thing to do is to keep the fluid changed out. Even with several 2500 HD's, that saw very severe duty, I had a complete flush at about every 35 K miles. One of those, I turned over to my foremen at 229K miles, that was 3 years ago, and it is still good.(that one has the 6.0 engine) No way the 1500 with the 4L60 would last that long, we had several of them, and about 100K miles was it on the tranny.

[RVNeophytes2]

Originally Posted by wingnut60

"What works with trailers could be lethal, in driven vehicles."

Could you expand on this, particularly relative in the danger of driving a vehicle with tires inflated to maximum listed sidewall pressure?

Joe

In the nutshell, it's like me deciding to enter turbulence at twice the recommended speed. Bad for so many reasons; but, the short story is, the plane is not engineered for it.

Likewise, the manufacturer engineers your tire for a certain pressure at given weights, speeds, environmental conditions. We use their pressure values so all those other parameters are met.

The DOT says their stamped sidwall pressure is "the greatest amount of air pressure that should ever be put in the tire under normal driving conditions."

Now, I'll give the mike to the experts:

Improper inflation of tires affects gas mileage, tire wear and safety. To determine the proper inflation for your car's tires, consult the sticker located inside the glove box, on the driver's side door post or driver's side door, or inside the fuel filler door. This sticker will list the proper pounds per square inch of air pressure (PSI) for front and rear tires. Check your tire pressure with a pressure gauge when the tires are cold.

Poorer Handling
Over-inflated tires don't grip the road as well. While this may result in slightly better gas mileage, it also means less traction and poorer handling. Poor road conditions such as a wet or icy road magnify this problem, making it more likely that you'll have an accident if you're driving with over-inflated tires.

Poorer handling also translates to a rougher, less comfortable ride. When Popular Mechanics writer Ben Stewart over-inflated his tires for a gas mileage test, he reported the car's handling was compromised and the ride was rough and full of vibration. He also found no difference in gas mileage with the over-inflated tires.

Blowouts
Improper inflation, whether over-inflation or under-inflation, causes tires to overheat and increases the risk of blowout. A blown-out tire ruptures and can cause the driver to lose control of the vehicle. This could lead to a serious, even fatal accident.

Tire Wear
When tires are over-inflated, the tread wears out faster in the center of the tire. You'll need to replace the tires sooner, an expensive penalty for driving with over-inflated tires.

Over-inflated tires, being stiffer and more rigid, are also more susceptible to damage from hitting pot holes or striking curbs.

There have been many postings offering recommendations regarding increasing tire inflation to increase fuel economy. There have also been many questions about the hazards of over inflating, many inquiring about “blow out hazards.” In my opinion, most discussions on the topic of over inflation offered opinions and very little technical knowledge. I am going to offer a technical view of the topic. Increasing the blow out risk is a possibility, but the real hazards of over inflation are related to traction reduction and altered performance and handling characteristics. Traction and handling issues have a significant impact upon risk exposure.

The key to understanding the issue of inflation pressure is to understand the adhesion patch. The adhesion patch is the surface area, or patch, of tire in contact with the road. Think of being in a deep hole that is covered by plate glass and a tire is parked on the glass. The portion of the tire tread that is pressed out flat onto the glass (road surface) is the adhesion patch.

Imagine a trailer that carries a load sufficient to put a 1000 pound load onto that one tire on the glass. The tire is inflated to 30 psi. Now, 1000 lbs. divided by 30 pounds /sq. in. is equal to 33.33 sq. in. (the pounds cancel) of adhesion patch surface area. Now, double the load in the trailer. Does the tire pressure rise with increased load? No. The tire “squats” and increases the size of the adhesion patch. 2000 lbs. divided by 30 pounds /sq. in. is equal to 66.67 sq. in. The adhesion patch has doubled in size to support the doubling of the load.

The adhesion patch is also changed by a change in tire pressure. Again, assuming the 1000 pound load but increasing the pressure to 50 psi results in 1000 lbs. divided by 50 pounds /sq. in. is equal to 20 sq. in. That’s a 66.7% increase in pressure resulting in a 40% decrease in adhesion patch size. Just as important as the total surface area of the adhesion patch is the shape of the adhesion patch.

Inflation may be an undesirable consequence of our economic times. But when it comes to tires inflation is always a good thing, proper tire inflation, that is.

The amount of air required to properly inflate a tire depends on the size and type of tire, the vehicle application (size and weight), vehicle loading (normal or extra loading), and driving conditions. A tire that is properly inflated will provide safe driving, maximum traction, good handling and optimum tire life.

Increasing tire inflation pressure beyond the recommended amount will reduce rolling resistance, thereby improving fuel economy. But the trade-off is a harsher ride and increased risk of tire damage when encountering bumps.

Excessive tire pressure may distort the tread to the point where it bulges like a donut, reducing contact with the road and increasing wear in the center of the tread. Under no circumstances should a tire ever be inflated beyond the maximum rating as indicated on the sidewall.

An over-inflated tire makes a vehicle unstable. It also can cause a blow out, which is dangerous not only to the primary vehicle but to those around it.

[bry4663]

I'm new to all this and thank everyone for the information. I'm learning a ton. I have what is probably a stupid question: do I fill my tv tires prior attaching my fifth wheel to the recommended tow pressure or after? Does it matter? In reading the above it looks like the pressure wouldn't change but just the surface area. However, with the air bags the pressure changes dramatically before and after the load is placed. Thanks for any help.

[RustyJC]

Originally Posted by bry4663

I have what is probably a stupid question: do I fill my tv tires prior attaching my fifth wheel to the recommended tow pressure or after? Does it matter? In reading the above it looks like the pressure wouldn't change but just the surface area.

You're correct. Tire pressure is tire pressure - the volume inside the tire doesn't markedly change, just the profile. Therefore, it really doesn't matter whether you check it with or without the trailer attached.

Rusty

[bry4663]

Thanks Rusty. Appreciate the help.

[Ray,IN]

Over-inflation is not as large a problem as under-inflated tires. There are many reports verifying this:
NHTSA report
GAO report
are just two.
There are pages of results from any search engine you choose to use, further documenting the results of under-inflation, which BTW causes over 80% of all tire failures.
The "tire inflation/load charts" all contain the caution that the listed pressure is the minimum acceptable for the listed load; not necessarily the optimum.

[wingnut60]

RVNeo,
Wow, what a lot of tire info...I'll go along with the 'decreasing the pressure' for a given load, but balk at the 'increasing the pressure."
I submit the tire mfg designs the tire NOT TO DEFORM under the maximum sidewall pressure listed, therefore the contact area WILL NOT decrease as the pressure increases UNTIL the maximum pressure is exceeded--in OTHER WORDS, OVERINFLATED. Then, and only then, will it form a crown in the center and increase wear on the center portion.

I say, run an RV tire at maximum listed sidewall pressure for longest possible tread wear, and decreased temperature buildup.

Joe

[RVNeophytes2]

Originally Posted by wingnut60

I say, run an RV tire at maximum listed sidewall pressure for longest possible tread wear, and decreased temperature buildup.

Joe

What you're describing runs contrary to the engineers' data about the size of the contact patch, they say is very dangerous. Every pound of inflation over the specified value decreases the size of the contact patch and can lead to loss of control. While I'm not qualified to address the severity of this hazard in the TT community, it seems the lack of understanding and sheer conviction to unsafe practices that thrives in the MH side makes this our biggest single hazard.

I'll guess that TTs incur a smaller hazard, one that at least a single manufacturer deemed insignificant compared to the chance of underinflating. Chances are, you're okay setting the maximum and forgetting about it.

The risk comes from "crossover," eg motorhome or toad maintainers adapting this practice in their own vehicles, where precision is (perhaps) more critical. They read these posts too, and could suffer injury or worse from adopting a general rule, when it comes to tire pressure.

At the risk of repeating myself, I cannot stress enough that RV owners contact the tire manufacturer directly, rather than rely on information gained from discussions in this forum.

[FastEagle]

The special trailer tire (ST) is designed for the freewheeling trailer axles. More than 90% of the manufacturers that provide ST tires will only recommend full sidewall pressures for their full range of operating conditions. It’s what they have designed into the tire. It’s one of the reasons why the 65 MPH maximum speed requirement is so critical.

The RV trailer manufacturer cannot be wishey washey about their recommended tire pressures. Doing so could create a very expensive recall.

Industry standards support the manufacturers recommended tire pressures. There is oodles of information available on the internet including tire data manuals from almost all of the major tire manufacturers.

Type this into your browser’s search engine - RMA CHAPTER 4 RECREATIONAL VEHICLE APPLICATION - and give it a read. The RMA is a major contributor to the rules and regulations process for all DOT certified tires.

FastEagle

[wandering1]

Run the Carlisle tires at 65 psi cold just like it says on the sidewall.

[wingnut60]

RVNeo,

You are probably correct on MH vs TT tires.
I have gotten lost on this discussion sort of--what is the 'specified value' that you reference in your last post?

Joe

[M zoll]

you mean t/h switch on is for short haul,not down the highway or up and down hill's 65 or so? Thanks

[Ray,IN]

Originally Posted by M zoll

you mean t/h switch on is for short haul,not down the highway or up and down hill's 65 or so? Thanks

You may use tow/haul all the time, either towing or empty, it will not cause any harm to anything except fuel mileage when empty.This is because it holds the A/T in a lower gear for a longer period to avoid lugging the engine, or overheating the engine on long upgrades.There are a lot more modified conditions, but for all practicality they may be ignored.