Drive Line Ratios. Why did they do it?

[motto]

A question that has puzzled me for years that nobody has really been able to answer has to do with the choice of CCKW transfer case and gearbox. I am completely baffled why an overdrive gearbox (.799:1) was coupled to an under drive transfer case (1.16:1 Banjo,1.155:1 Split) when one cancels the other out. There is absolutely no gain for all those extra gears, shafts and bearings. Imagine the savings that would have been made over the 800,000 or so 2-1/2 ton 6x6 trucks produced by GMC, Studebaker and Reo and it was known early in the war that this class of truck would be needed in huge numbers.

One example of the poor choice of driveline components is that of the five speed overdrive Clark which is not a particularly good example of gearbox design. The ratios are not well spaced with a big jump from 3rd to 4th then a little hop from 4th to 5th and the overdrive area is the weakest and most troublesome part of the box. It wasn't that there were no other boxes to choose from. More suitable four speed gearboxes were in production that were simpler, cheaper and more robust. That used in the White M3A1 Scout Car for one.

There was also a straight through transfer case put into production that will bolt straight into the Banjo diff truck that would undoubtedly do the job as it used similar weight gears, shafts and bearings. This is the G506 unit. Similarly, Timken were producing a transfer case that could easily be adapted for use in the Split diff truck that was straight through in high range and once again, simpler, cheaper and probably more robust.

Some may argue that the bottom end performance may be compromised because these transfer case low range ratios (approx 2:1) are not as low as what was used (approx 2.6:1) but in my experience the GMC and Stude will always run out of traction before they run out of power unless grossly overloaded. In any event it could have been rectified by a slight redesign.

I suspect that the answer to the question is lost in the mists of time. The changes suggested could have been readily accomplished at any time during production as they are simply 'bolt on' and even a conversion kit produced to be used when a truck came in for rebuild. The savings would have been immense.

David

[pfarber]

The CCKW was never a purpose built military vehicle. It was adapted from a very civilian truck. The ACKWX in Army service still had chrome trim and glossy paint.

I don't know what civilian truck 'turned it' the ACKWX but it seems that the late 1938/39 GMC 9600 series trucks fit the bill. I would look to see what they had for a driveline... it may be as simple as 'because that's what they came with'.

Off the top of my head the only thing I can think of is that GMC did offer the 'hi torque' rear end. Not sure of the gear ratios, but at some point it appears that they did look into modifying the drive line ratios.

Also, when it came time to source a secondary drive line (the timken and corporation drive lines are different enough that they should have been separate truck models) they seemed to be 'happy enough' with the performance that they didn't look for any additional performance.

I'm sure that someone, somewhere has the original specs that the CCKW needed to obtain... in 1939/1940 the ACKWXs performance would have been average/low average compared to other trucks.

[pfarber]

Well this just makes no sense...

The ACKWX-353 entered service with one of 3 available engines: 228, 248, 256 making 30.45, 33.19 and 34.3 SAE hp, respectively.

But the final drive ratio is still 6.6-1 as on the later CCKWs with 270cu.in engines (still only making 34.35 SAE hp)

So it appears they were 'happy' with the 9600 series performance... as they didn't change a thing (does any one have an ACKWX/9600 parts book to get the transmission ratios??)

[motto]

It's said that hindsight is always 20x20 so it's easy to criticise decisions made under the pressures at the time, pressures that we have little or no knowledge of today. However, the components were available to make the CCKW a cheaper, simpler, more robust and more easily maintained truck with no loss of performance and one wonders why that wasn't done.

The more you look at it the less sense it makes. During the war the Canadians built their own version of the GMC 6x6, it was a COE truck and rated at 3 ton but used the same 270 engine, similar design rear bogie and utilised as many 'off the shelf' components as possible. The drive line used a straight through gearbox and transfer case. In the post war sales the Canadian trucks were always more sought after so always worth more than the CCKW. It was simply recognised as being a better truck by people who wanted value for money.

Having said that, I will admit that I own 2 CCKWs and have no desire to own a 6x6 CMP. The CCKW has the looks and the mystique and has earned its place in history.

David

[pfarber]

The ACKWX and CCKW also utilized many off the shelf components for the first 2.5 years. The ACKWX entered service in late 1940, and the drive line did not change much. Not sure what prompted the move to the GMC 270... the ACKWX and CCKWX were liked by most Lend Lease recipients (the UK seemed to not like them originally).

From the few books I have it does not look like the drive line changed at all.. other than the engine and tire sizes (the ACKWX came with 7.00x20s vs 7.50x20s on following models).

I think the answer lies in looking at the 9600 series trucks (which were offered in various wheelbases and cargo capacities) and seeing what, if anything past the bell housing changed. From the photos I have not muck appears to have.

As for 'post war value' I think that is not a valid metric. The Army needed a tactical cargo truck.... and it adopted the most readily available unit, then standardized its parts so it could maintain a fleet of well over half a million units with labor that was drafted. I can think of a lot of US equipment that was far from 'optimum' yet still made mass production/front line status. The biggest failure was the M4 Sherman tank. It was a horribly outclassed tank, under armored, under gunned, used gasoline vs diesel... yet we threw them at Tigers and Panthers like they they were confetti. The M3 Stewart likewise. After 42 there wasn't a front line German tank it could defeat. So it was a scout. There already were better scouts... the M8 and M20.

If you do go with 'post war value' even though the CCKW may not have been optimum, I can't think of anyone turning one down for the right price. There are several books showing CCKWs in just about every military and civilian livery up till the 1970's. Can the CMP make that claim other than collectors wanting one?