Can X-Chocks damage trailer tires?

[danemayer]

Gravity (the weight of the trailer) is a minor player in the force vectors applied to a tire.

For discussion purposes let's assume a typical G-rated tire on our rigs (31.5" diameter, 6" tread width, 7.5" sidewall height) inflated to 100PSI.

The interior of the tire has a surface area of approximately 5,000 square inches. At 100 pounds per square inch, the internal forces applied are 500,000 pounds (1/2-million), precisely EVENLY applied to all interior surfaces of the tire. (Similar force is also applied to the rim, but I ignore them in this discussion because the rim is rigid.)

Now consider the (approximate) 2,500 pounds of weight applied to the exterior of the tire when bearing the weight of the trailer. This weight (force) amounts to a mere 1/2 of 1% (.005) of the force being exerted on the other side of the rubber!

But because the tire is not rigid, this weight does compress the tire, making the interior air volume slightly smaller (presuming the tire is strong enough not to stretch). As a result of the compression, the interior air pressure rises by .005PSI from 100PSI to 100.005PSI, and because air is an elastic gas, that pressure (force) is EVENLY APPLIED to the entire interior surface of the tire.

Hans,

Thanks for elaborating.

 

[tireman9]

After discovering a separating tread on one of our tires on the way back from the Las Vegas National HOC Rally last October, I've been wondering if this could be an issue.

Typically when we roll into an RV park or campground and I install the X-Chocks, after the tires cool off some I have to snug them up some.

Then a few hours later I have to snug them up again!

And generally the next morning I'll have to do one more snug-up of the X-Chocks.

I love the X-Chocks, but I now wonder if they can damage the tires at all?

Comments anyone?

View attachment 50688

Tire supports need to uniformly support the entire footprint (contact area). For example parking on a single 2x4 would be bad for the tire as the tire footprint probably measures closer to 6" x 8".

HERE is a post on Winterizing but it has good and bad examples of tire supports.

 

[danemayer]

Gravity (the weight of the trailer) is a minor player in the force vectors applied to a tire.

For discussion purposes let's assume a typical G-rated tire on our rigs (31.5" diameter, 6" tread width, 7.5" sidewall height) inflated to 100PSI.

The interior of the tire has a surface area of approximately 5,000 square inches. At 100 pounds per square inch, the internal forces applied are 500,000 pounds (1/2-million), precisely EVENLY applied to all interior surfaces of the tire. (Similar force is also applied to the rim, but I ignore them in this discussion because the rim is rigid.)

Now consider the (approximate) 2,500 pounds of weight applied to the exterior of the tire when bearing the weight of the trailer. This weight (force) amounts to a mere 1/2 of 1% (.005) of the force being exerted on the other side of the rubber!

But because the tire is not rigid, this weight does compress the tire, making the interior air volume slightly smaller (presuming the tire is strong enough not to stretch). As a result of the compression, the interior air pressure rises by .005PSI from 100PSI to 100.005PSI, and because air is an elastic gas, that pressure (force) is EVENLY APPLIED to the entire interior surface of the tire.

Hans,

I've been thinking about your explanation all morning. If as you say, there's 500,000 pounds of pressure evenly applied to all interior surfaces of the tire, then in this additive scenario, the valve would have to also have 500,000 pounds of pressure on it. Yet when I apply my tire gauge, it reads 100 psi, not 500,000. And when I let air out, I would think a stream of air with 500,000 pounds of pressure would blow a hole right through my hand, and then blow all the leaves in my yard well down the street.

And I've seen some people post about hoses that are attached to a pair of tires to equalize pressure so you only have to fill the outside tire on a dually pair. In your additive scenario, connecting the 2 tires would double the volume of air, just like getting a larger tire, and 500,000 pounds would then become 1,000,000 pounds. I probably shouldn't have cut physics class when they explained all this, but I'm thinking that there's some law of conservation of energy that would be violated.

But like I said, I cut class the day they covered all this, so maybe I'm wrong.

 

[JohnD]

Tire supports need to uniformly support the entire footprint (contact area). For example parking on a single 2x4 would be bad for the tire as the tire footprint probably measures closer to 6" x 8".

It is the small footprint of the X-Chocks on the tires that makes me wonder if the belts in the tires could be damaged by the chocks. *



As for long term parking on 2x4's or other leveling blocks, I was very excited when I discovered the Lynx Flat Tops for their leveling blocks.

I didn't like leaving the tires on the circular locking rings on the Lynx blocks for long periods.

I have four of the flat tops right now and will pick up another set of four next time I go to our dealership as they are the only place (other than online, of course) that I've ever seen them for sale (I got the Lynx Blocks at Walmart as Camping World doesn't carry them).



From now on I will wait a few hours after parking before I put the X-Chocks in to give the tires a chance to cool off so that I only have to tighten them up once.


* Footnote:

Again, I don't think this was the cause of my tire issue, but that was what got me thinking about the possibility of the chocks putting too much pressure in small spots on the center of the tread.

 

[dlw930]

Dan, the force on any given area on the tire is the pressure divided by the area. The total force on the entire tire is the pressure times the total area. The latter may be on the order of 500,000 lbs, as Hans suggests, although I'm not sure what significance this has except it may give one some appreciation of the force involved when a tire blows.

The force of the air exiting a valve stem is on the order of 22 lbs. That's 110 psi x .2 in^2 assuming a 1/4 diameter valve stem opening. Considering the valve stem core probably reduces the opening area further, the force of the exiting air may be even less.

We tend to sloppily use the term "pounds" to refer to both force and pressure. It's important to remember that pressure is actually force per unit area.


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[mlpeloquin]

The X-chocks obviously are not as wide as the tires, but they do not put anywhere near the pressure on the tires that the weight of the trailer does. They just need to be snug and not overly torqued to keep the trailer from moving. So if used properly, I cannot see how they could cause any problem with the tires.

 

[NYSUPstater]

Has anyone heard if someone else picked up the Roto-Chock patten and make them again?

 

[dlw930]

Has anyone heard if someone else picked up the Roto-Chock patten and make them again?

I don't think so. You can sometimes find a used set available on eBay or RV forums. It seems many folks that used them quit using them when they upgraded to rigs with 6 pt Level Up.


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