CCKW Tyre Pressures

[Vulture]

CCKW Tyre Pressures

Guys, what pressure do you run the front and back tyres at on your CCKW when it is unloaded ? Spoken to several people, and had varying answers, which range from:

1. 50lb all round.

2. 50lb on the front 45 on the back.

3. 50lb on the front 40 on the back.

4. 40lb all round.

What’s the consensus here on the HMVF ?

Kind regards to all

Vulture

[deadline]

The answer is 55PSI. If you read the TM9 it does call out for another pressure.. but only for a very specific SN range that use BEAD LOCK RINGS (CCKW-353-236404-2 to 236520-2) and they are inflated to 40psi.

 

All references to 'standard' tires is 55psi.

 

Obligatory TM9 page:

http://www.tm9-801.com/searchTM9/tm9_pages/435.pdf

Read para ©.

 

If you read further into para © it will specifically mention bead lock tires. You don't have those.

 

CCKWs did not incorporate a bead lock ring aka the jeeps tire. But evidently a very small lot of CCKWs did.

 

Now there WERE other PSI guidelines for sand, mud and even when overloaded to 5 tons (1psi per additional 1,000lbs of cargo). But the book number for a CCKW on hard surfaced roads is 55psi.

Edited March 26, 2012 by deadline

[N.O.S.]

Deadline - running the trucks unladen, it is possible to get away with a slightly lower pressure than the recommended 55psi.

[deadline]

Lower PSI is a HUGE no no. Low PSI at road speeds allow the sidewalls to flex and heat up. Heat destroys the vulcanized rubber and you get a blow out.

 

The reason again, heat buildup and the rubber degrading and causing a blow out.

 

I can look it up for you, but you can do a simple google of 'underinflated tire'

 

I could cite an AM article... or any number of NHTSA reports... or X, Y and Z.

 

40PSI is TO LOW (a jeep is 35psi!) If you run at 40 PSI you have underinflated the tire by almost 30%!

 

And with tires (or tyres) cost over $260USD each I think that getting the most mileage out of them is also going to be a key factor.

 

An 'unladen' CCKW can still top out at 13,000lbs (5800+ kg)

 

A tiny jeep (2500lbs) can still blow out an under-inflated tire.

 

Its not the weight that get you... its the heat from the tire flex.

 

Ever wonder why all cars now have a tire pressure monitor system?

Edited March 27, 2012 by deadline

[N.O.S.]

Agreed Deadline, but doesn't lower load = lower sidewall flex?

 

Hey guys, help me out here, he's winning.....:-|

[Vulture]

Yeah, I'm aware of the what the TM says, BUT, having put 6 new six tyres on mine just over a hundred mile ago it is obvious, I mean really obvious that at 50psi on the rears, only about 60% of the tyre is making any meaningful contact with the road. I'll post a photo when I get home. The outer part of the tyre is still shinny with a glazed surface.

 

Surely the tyre pressure should be set at a figure where the width of the tread footprint has an equal pressure exerted over it, to ensure the best grip, and equal wear across the surface ? I mean is this not how the tyres on a CCKW are meant to operate ? Classic symptoms of overinflation are excessive wear in the tyre centre and conversly under inflation is excessive wear on the shoulders. Are we saying here that the correct running form for an unladed CCKW running on tarmac is to prematurely wear your tyres out ?

 

Kind regards to all

 

Vulture

Edited March 27, 2012 by Vulture
Spelling

[Vulture]

Deadline - running the trucks unladen, it is possible to get away with a slightly lower pressure than the recommended 55psi.

 

Hey N.O.S. come on now, front up, what are you running yours at ? :-)

 

Kind regards

 

Vulture

[Vulture]

Chewing on this further, looking in the manual it only gives the one pressure 55lb all round (page 370 of TM9-801). It makes no mention of whether the vehicle is loaded or unloaded. Therefore it could be assumed/construed to that avoid issues such as tyre-wall damage/failure the 55lb would have been set by definition to cover vehicle operations when fully loaded which on a 352b1 is 9,167lb + 5,000lb = 14,167lb.

 

Taking a step back for a moment, and looking in the manual which comes with your normal car, it normally gives pressures for the front and back (which are frequently different) and gives figures for normal operations, and also when fully heavily loaded. The latter always being higher than for the former. This thinking is further supported by the 1lb extra for each 1,000lb overload referred to earlier in this thread. If you follow the rationale that the 55lb is for fully loaded, then running the vehicle when it is 35% lighter (i.e. at 9,167lb) surely means by definition the tyres would have to be over-inflated ?

 

An interesting follow on question is what pressure should the front be set at ? Given the vehicle weight distribution it could be argued that the rear tyres are disproportionately affected by variations in the overall vehicle weight, although not exclusively so…

 

Kind regards to all

 

Vulture

Edited March 27, 2012 by Vulture
Sorting a formatting issue

[N.O.S.]

Who me? 45 psi front, 40 psi rear.

 

Having said that, at 35 psi the rears do not flex in the slightest. The fronts do need more than that though.

 

Reminds me, I must get my tyre kicking foot re-calibrated :-D

 

How you drive, where you drive, and how fast, are also factors.

 

(Found those canvas bow setscrews - more later)

[Degsy]

I would be very wary of dropping much pressure especially in the front tyres as they are very sensitive. It is usually characterised by a sudden and vicious shaking of the steering strong enough to rip the steering wheel out of your hands. The possible consequences of this are obvious. I always ran mine at 55psi and never had any untoward wear problems.

[N.O.S.]

Degsy - I had that jimmy shimmy once, quite frightening. ISTR when I checked the front pressures right after the incident one was down to 30/35psi.

[JOURNEYMAN]

Hi vulture,

on my 353 i run the front tyres at 45 psi and all the rear tyres at 40 psi. I have never had a problem with

shimmying , but i must add that all my tyres are 8.25 x 20 michelin road pattern tyres as fitted by the french

army. I assume that your tyres are bar grips ? I dont believe the tyres are under inflated at these pressures

and its fair to say that the truck is never loaded anywhere near its capacity . I suppose that tyre pressures

may be more critical depending on the type of tyre fitted / ply rating ?

Regards

steve

[deadline]

You cannot tell anything by the static visual appearance of a tire.

 

When a tire spins centripetal force extends the sidewalls out. Once the tire rotates and comes into contact with pavement you get flex (as its only the bottom part that supports any weight). That is the point the tire flexes and builds up heat.

 

If you run on hard surfaced roads NDTs are the ABSOLUTE WORST TIRE. They only get WORSE in rain.

 

You are not saving anything by running less than 55psi. If anything you are increasing fuel consumption, increasing heat buildup, lowering handling and generally making a bad decision.

 

How hot is the pavement in the summer time? Now add to that heat added by low air pressure.

 

A tire will handle up to 400PSI before it blows out. But heat of only 100C will make it go BOOM.

 

Read this.. its has pretty pictures:

 

http://www.irsst.qc.ca/media/documents/PubIRSST/R-590.pdf

 

[TABLE=width: 800]

[TR]

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[TR]

[TD=width: 100][/TD]

[TD][/TD]

[/TR]

[TR]

[TD][/TD]

[/TR]

[/TABLE]

[Degsy]

Degsy - I had that jimmy shimmy once, quite frightening. ISTR when I checked the front pressures right after the incident one was down to 30/35psi.

 

I had it on a friend's truck, scared the s--t out of me.:shake:

[Vulture]

Here's a photo of one of the rear tyres I was telling you about. All 4 on the rear are the same. You can clearly see the area getting worn by the road, and the element which isn't...

 

[N.O.S.]

That look quite respectable. Dropping rears by 5psi or would give slightly greater contact but it won't make a vast difference to the wear pattern - to achieve full contact you'd be looking at 25psi or less and that ain't a good idea as we seem to be agreed.

 

These bar tread tyres are funny - especially the bigger sizes. They rapidly wear down to around 50%, during which time you have fits about the cost of replacing them, but after that (when there is more contact on the road) the wear stabilises and they take forever to wear out the last 50%! Matadors are a classic example.

 

Let them bed in a bit before pannicking, but if you go on what others are doing with pressures you won't go far wrong.

[deadline]

NDTs are not made for hard surfaced roads. They are made for soft ground. Other than 'the look' running NDTs on pavement is an expensive way to scrub rubber off a tire.

 

No modern army runs NDTs any more.

 

If you want to be horrified by poor tread wear find a parking lot and do a 90 degree turn and look at the inside turn tires.. the scuffing is almost 90 degreess.... lowering the air pressure will not help that at all. In fact it makes it worse!

 

The NDT is molded WITH A CROWN. The center rib is there for a reason. Why? Its the only part of a tire that touches the road 100% of the time! As noted above you have to deflate the tire to less than 50% of manufacturers recommendation to get your side bars to touch.. and as many TMs will tell you that's only for use in sand, at low speed, for a limited time.

 

If you are truly want a 100% contact patch, switch the road tires.