CCKW Tire Pressure

[pfarber]

Read a post by someone who 'knows' all about CCKWS.

His 'tip' was to 'Ensure your tires are properly inflated to 55psi'.

WRONG. GO NOT PASS GO, GO NOT COLLECT $200.

Every tire has a pressure rating and load capacity molded into the sidewall. Make sure you inflate the tire to the MANUFACTURERS recommended PSI.

For example, the tires on my CCKW say I need 95PSI to get a rate 2250 load capability from it. Any less and you heat the tire up and increase the possibility of a blow out or tire failure.

If you buy tires from Universal Tire, their 7.5x20 NDTS say:

SKU: U77504
Construction: Tube Type 10 Ply Nylon
Load Capacity: 3140 @ 75 psi
Overall Diameter: 36.90"
Size: 750-20
Tread Width: 6.80"

http://www.universaltire.com/cart.php?t ... ory_id=342

If you follow 'the expert' your are running with a FLAT. DOT rules state that any tire under inflated by 20psi or more is considered FLAT.

All that knowledge and yet oh-so-wrong.

[pfarber]

Seems that I have be questioned about my tire pressure post. I have posted a bit of info on g503.com but will summarize here:

55PSI is in no way correct for modern 7.50x20.

The LOAD RATING on the tire (and the manufacturer) means 'if you want X lbs per tire inflate to X psi'. Somehow people are seeing MAX PSI and getting confused. LOAD RATING is NOT MAX PSI. It can be, but generally it is not.

Of the 2 major resellers, and the 2 major suppliers, NONE have posted 55psi as a working pressure.

There is a TON of info on the net about tire pressure.. most simply say 'read the manufacturers label and go with that'. That is not gonna work for CCKW owners because the tires of 60+ years ago are in no way comparably to today's materials and construction.

There is LITTLE HARM in running your tires at the LOAD RATING. Nothing bad will happen, they will not burst or wear poorly. Burst pressure for a tire is 100's of PSI.

There is a HUGE SAFETY ISSUE with running tires UNDER INFLATED. The sidewalls heat up and the rubber separates and all your air goes buh-bye.

Even if you know that a tire can run 3500lbs @ 90PSI doing the math and figuring what load per tire you need is not really going to work... tires are not that linear in their load capacity.

A loaded CCKW can, on pavement, reach 21,000 (10,000+ vehicle and 10,000 load capacity PER THE TM). Not that you do that regularly... but having your tires inflated to the RATED LOAD LIMIT will mean the you are safe and can support it. Guessing (or using an outdated TM spec) means that you will be under inflated and most likely pop a tire.

Simply put we don't know what the proper tire pressure is for modern 7.50x20s for a CCKW. But we do know that under inflating is bad.

I do fear that the CCKW owners will start posting 'I run at 55psi fine'.. OK, how far do you drive? Do you check your tire temp? How fast are you going? Most likely the answers are 'not far', 'No', 'Not fast'.. ie they don't know.

[pfarber]

To emphasize my point.. read the following and note how many times the phrase 'under inflated' is used to describe tire accidents:

I got these from pages at: http://www.retread.org/Rubber/

Statistics compiled by the U.S. Department of Transportation show that nearly all tires involved in any tire related accidents are underinflated or bald. Properly maintained tires, whether new or retreaded, do not cause accidents.

As with passenger car tires, most problems with truck tires can be traced back to poor maintenance (primarily underinflation) and abuse (overloading).

It is important to note that most of the rubber on the road comes from truck tires and is caused mainly by underinflation, overloading, and tire abuse.

The exploding tire shown in the photo of the truck was NEVER retreaded. The tire was not damaged in any way, but it was deliberately run with an insufficient amount of air pressure, as a result generating excessive heat which caused the tire to come apart. It took less than 50 miles to destruct.

Photo 1 shows the original tread on a passenger car tire with part of the tread missing. The tread separated (came off) from the belt package of the tire. Evidence that this tire was run underinflated is the excessive wear on
the outside edges of the tread area.

This is closely followed by problems caused by underinflation, overloading, mismatching of tires on dual wheel positions and other improper maintenance and inspection procedures,

Just 20% underinflation will reduce tire mileage by 16%, increase irregular wear (because the tire footprint is distorted) and cause extra heat build-up due to excessive sidewall deflection. Heat induced tire degradation reduces retreadability and makes the tire more prone to punctures (hot tires "suck up" puncturing objects).

See a pattern?