CCKW TIRE SIZE

[joel gopan]

We have had some difference in the comments on the off road performance of the CCKW. It is all relative, as the 6X6 trucks were designed with a balance of tire size in proportion to the weight distribution of the different classes of vehicles. Note that as the Class of the vehicle gets heavier, the tire size also increases.
Examples are:

2 1/2 Ton 6X6 CCKW--------------------------------750X20 Tires
4 Ton Diamond T 6X6-------------------------------900X20
6 Ton 6X6 White/Corbitt Prime Mover-------------10.00X22
7 1/2 Ton 6X6 Mack Prime Mover------------------12.00X24
6 Ton 6X6 Heavy Wrecker (M1 Kenworth)---------11.00X20
12 Ton 6X6 Truck Tractor M26 Tank transporter--14.00X24

Its all relative, and these were designed to have equal performance ability in order for the trucks of all types and sizes to accomplish maximum movement of convoys in similar terrain. That is, if all trucks were loaded as designed, they would be able to perform similarly in like terrain. The CCKW was at the bottom of the food chain, and if loaded with only 5000lb of cargo, it would accomplish its mission as designed.

In the situations weere they could not, for whatever the reason negotiate the terrain, there were recovery vehicles with winches.
Tank Retriever and Wrecker Operators were not just everyday Truck Drivers, they were skilled at using all the equipment, and expedients necessary to accomplish the job. As for the ability to drive the CCKW and other vehicles effectively, the Army had Wheeled Vehicle Driver's Schools.When coupled with training, good discipline and leadership by the chain of command, army transportation was quite effective overall. The Army did a good job in WWII, don't you think???

[joel gopan]

I would like to inject a little human interest
Shorty Adams, USAF retired, is a WWII Vet and is well known for his colorful WWII stories involving his exploits as an ETO B-24 Bombardier, and cold War stories as a Radar Intercept Officer in F-89, F-94, F-101, and the F-15.
He started out in WWII in Basic Infantry training. Shorty is 5'5" and wears a size 6 boot, and commented that when loaded down with Steel Pot, Field Gear and the Rifle, his small feet would sink into the soft mud and he could not keep up with troops with larger feet.
It is all relative, small CCKW with small tires vs larger 6X6 with greater tire size.

[joel gopan]

Although this is not a Jeep Site, I would like to give a little history on Army Jeep tire size for those of you that also think Jeep,

The WWII MB/GPW 1/4 Ton Truck used 6.00X16 Tires with an axle ratio of 4.88:1, its' rated speed was 60 MPH @ 4000RPM.

The Korean War era M-38/M-38A1 1/4 Ton Truck used 7.00X16 Tires with an axle ratio of 5.83:1, its' rated speed was 60MPH @4000RPM.
The weight of the later Jeep was increased by a few hundred pounds and larger tires were used to improve the floatation.

Now you will be able to relate to the M-Series 2 1/2 ton M-34/M-35,and M-135/M-211 trucks that were heavier, but designed to carry the same 5000 lb off road. They adjusted the tire size, 900X20 with duals, 1100X20 with singles.

It is all relative, and the Army trucks for the 50s were really an improvement over their WWII brothers. Wish we could say that for the Army trucks of the 90s.

The Army taught me all this starting 45 1/2 years ago. Ain't you glad I learned(and remembered) all this stuff so that I could share it all with you?

[dr deuce]

Joel, you did not mention the 9.00 x 16 that the 3/4 ton dodges had.

That tire size was much more appropiate to the vehicle.

[joel gopan]

I considered it, but I wanted to stick to examples of the various 6X6 ten wheelers. The tires on them were in direct proportion to their Military load rating. We all understand that necessities of war caused them to be loaded in excess of off road rating.

The Dodge 3/4 Ton 4X4 and 1 1/2 ton 6x6 were indeed a well balanced vehicle with adequate footprint., but were not really a heavy 6X6. Those and the later M-37 were real mudders.

[dr deuce]

If you compare the Dodge WC-62/63 1.5 ton 6x6 with a Chevrolet 1.5 ton 4x4, I maintain that the Dodge will go places that the Chevrolet cannot even begin to think about and it is all because of the tire size.

[joel gopan]

Yes and no, but they Dodge 6X6 and Chevrolet are not a good comparison as they are "apples and oranges"

[dr deuce]

It is carrying capacity, 1.5 tons and the number of tires on the ground 6, and the tire size 7.50 vs 9.00. The fact that one has 6 wheel positions and one has 4 makes some difference, but even a 3/4 ton Dodge loaded with 3/4 ton would go places a Chevy would not loaded with 3/4 ton.

[joel gopan]

But we are comparing a different class of vehicles, the thread was about the 6X6 and the comparison of tire size to the class of vehicle. There is a reason why Dodge 3/4 Ton 4X4 WC went with the 900X16, and the upgrading of it to 6X6 with the same tire size was, most likely, to continue commonality of parts. To argue the point of whether the Chevy 4X4 would benefit from a tire such as the 900X16 on the Dodge would not be reality. There would be a problem fitting 16" wheels to it as they would not fit over the Chevrolet brake drums, necessitating a redesign of the Chevrolet 4X4, and upsetting the parts supply system, as the Chevrolet wheels and brakes were common to that other GM truck, the CCKW.
The WC progressed from the 1/2 Ton 4X4 which ended production in 1942, the 3/4 Ton 4X4 Dodge WC became Military Standard as the Armys choice of 3/4 Ton 4X4. The increase to 900X16 was a natural for that vehicle, and they had a winner. Chev 4X4 and CCKW are a different class truck with a different requirement, they are 30% heavier than the Dodge 4X4 and 6X6. Note that GM used a brake booster to assist in stopping the G-506 and G-508. Did you ever make a panic stop with a heavily loaded WC-they will make you check your shorts as they were not the best at stopping. We are straying from the 6X6 a bit.

But ,if you insist, let's look at the weight comparisons of empty WC vs empty Chev 4X4 and CCKW 6X6 without Winch.


3/4 Ton 4X4 5645 lb Chev 4X4 7545 lb

1 1/2 Ton Dodge 6X6 7545 lb CCKW 6X6 10300 lb


Again apples and oranges, the Dodge was a lightweight when compared to the Chevrolet and GMC by about 30%.

Note the difference in the cargo space of the Dodges when compared to the G-506 and G-508, they were designed that way on purpose.

In order to be really accurate, and it would be now impossible, we would have to develop statistics of the collector and his experience with his one or two vehicles, and compare it to the overall performance of the larger Military fleet operator of hundreds of thousands of these vehicles.

Vintage Video has interesting footage for sale of WWII trucks being tested in all types of terrain and weather. Seeing is believing. I remember these Army Training Films well as they were available as training for the Wheeled Vehicle Mechanics and Drivers in the early 60s.

Joel

Stay tuned.

[joel gopan]

Something else, there were Tire shortages, note the ETO photos of CCKWs without the outside dual on one rear axle.

I remember during the Korean War, hanging a tire on each handlegrip of my bike and traveling to the Firestone Store to trade them for Movie Tickets. Rubber was short then, too.

Joel

[dr deuce]

No need to redesign the Chev to 9.00 x 16, they had the 11.00 x 18 for the DUKW. It is the contact area and width of the tire, not the wheel diameter that matters in soft loose sand.

I have driven my Bomb truck with both 9.00 x 20 tires for 8 years and about 4 years now with the correct 7.50 x 20 tires, so I have seen and driven it both ways. There is no comparision of its ability in sand with the 9.00 x 20 tires. Night and day!

From my experience: The 6x6 will pull things that the 4x4 cannot and the in soft loose sand with the small 7.50 x 20 tires on either of them is asking to get stuck.

I believe that there have been a couple of articles in Army motors, one by Bryce Sunderlin stating that the tires were too small by one or two sizes on most WW2 trucks.

[joel gopan]

Like I said before, the fleet of hundreds of thousands has statistical value, one vehicle not tested to Army specs is not a fair comparison. There are other factors to consider also. Remember, the army, after rigorous testing, bought 562,000 CCKWs.

[dr deuce]

How many vehicles did the Army test (Aberdeen etc) in soft loose sand? From my experience at the Aberdeen vehicle test facility numerous times, I did not observe one test set up for sand, loose or otherwise. They have the hill climb, the water pit, the twister (my favorite) etc.

Remember your 7.00 x 20 agi tire for the 3 ton GMC model 4929? They up tired and up engined the CCKW from that experience. Remember the REO M-series deuce? How many years was it before they went away from the sprag-clutch xfer to the in/out (air shift) transfer like the WW2 CCKW?

Just because the Army/Government does something, even repeatedly does not make it right or good. Other factors come ito play; cost, political considerations, narrow-sightedness, etc.

Having had direct experience with both the CCKW (empty) and the Bomb truck (again empty) in loose sand with 7.50 x 20 tires and the Bomb Truck with 9.00 x 20s in the sand, well lets just chalk this up to agree to disagree.

[joel gopan]

Air Shift Transfers were not used in WWII, again the comparison between a 6350 lb Bomb Service 4X4 and a 10,000 Lb+ 6X6 CCKW is "apples and oranges". Aberdeen was not the only site that CCKW and other US Army vehicles were tested in WWII. Vintage Video will be good viewing in reference to CCKW Operations, as there are several good films offered. Seeing is believing.

[Degsy]

I think that the tyre size would have been a compromise based on many factors, cost and amount of materials needed as well as performance,
whilst a large footprint is desirable on soft sand or boggy ground a small
footprint works best on ice, snow and mud.

[weberwlk]

this tire issue is quite something. The british had single tires on there vehicle either 6x6 or 4x4. we,ll take the 10.50x20 tire. having single on and inline as they did and the dukw did was alot better off road. but the US had such a fleet of vehicles that the cost was prohibitive as many vehicles were used on roads even thou they were primitive. The US did use the 11-18 on the cckw and the dukw as this was alot better off road. Mainly used on specialized vehicles. But look at the difference in the amount of rubber put into each tire. plus if a tire was shot out the vehicle was not disabled.

even so I have heard vets testimony from the pacific and england that in the mud if you put the front duels on and in low range there wasn't too much that stopped them.

as with the dodge 1 1/2 ton seemed to replace the chevy. Its of road performance was alot better as it had better floatation on the ground and it had a lower probile for front line work. The dodge was terrible on hard roads and the extra axle made it under powered. Don't get me wrong , i like them all, but each vehicle was taylored to a specific use. If you want something that will do better off road gear up a Gmc or Chevy with 11-18s. but you find that there will be certain trade offs, such as hiway and engine power. your call.

[joel gopan]

I noted a mistake(mine), the actual rated top speed of the MB/GPW was 65MPH @4000 RPM, the higher gear ratio made up for the smaller 600X15 Tired. A well tuned M-38A1 will still have notable acceleration @ 60 MPH and leave the MB/GPW in it's dust.
Joel

[dr deuce]

My point with the air-shift vs the sprag was that the US Army 'corrected' a deficiency (sprag) that it had carried on for years after WW2.

The earlier M35's both gas and diesel, had a sprag xfer. The later M35s had the airshift.

[joel gopan]

It took them 18 Years to do it.
The Army has rules for abuse of the M-35 which leads to bad sprags, one is driver Training, the other is the UCMJ. The problem was there, but trained drivers helped keep the problem to a minimum.
It is hard to convince civilian collectors and firemen that they do not know how to properly drive an Army truck. There are a lot of precautions to learn about when driving the M-35 with sprag shift for front axles, but this is a CCKW site.