wanted: Chev CMP chassis views

[Hanno Spoelstra]

Originally Posted by Tony Wheeler

As far as I can make out, the only SPECIFIED difference between C60S/F60S and C30/F30 is wheel size. This gets back to our earlier discussion concerning CMP load class rating, which appears to based solely on wheelbase and wheel size.

Tony,

Agree on the misnomer to describe CMP axles in terms of load rating, that is why I also use the "heavy" and "light" designations, by which I mean the same as your "large" and "small" designations. When I write "3-ton axle" I mean to say "the type of axle fitted to a truck with a 3-ton load rating".

Indeed the factory specifications do not specify a load rating for the axles themselves. But, next to wheel size they also list the differential ratio and number of spring leaves. It is this combination which was required to match the specified load rating for a certain type of truck. The C60S and C60L are thus fitted with 20" wheels, 7.16:1 differential ratio and 18 (12+6 aux.) spring leaves on the rear axle, which enables them to carry a 3-ton load.

So in the case of the subject vehicle, does the fact that it has "heavy"/"large" steering ends (the type with six bolts on top) means nothing to identify it? Or is it the case that an axle with "heavy"/"large" steering ends always has the 7.16:1 differential ratio?

One fact which helps to ID this truck is that it is fitted with 18 (12+6 aux.) spring leaves on the rear axle, which was fitted to the C60S and not the C30 which had only 12 spring leaves on the rear axle. As per factory spec it should thus be fitted with 20" wheels and not the 16" wheels it currently has.

H.

[Tony Wheeler]

Originally Posted by Hanno Spoelstra

So in the case of the subject vehicle, does the fact that it has "heavy"/"large" steering ends (the type with six bolts on top) means nothing to identify it?

In order to answer that question Hanno we'd need to know Chev practice with the C30, which can only be confirmed by checking wartime photographs and surviving examples. If you find the C30 retained small steering ends universally, just like the C15A, then the large steering ends would identify this vehicle as C60S. On the other hand, if the C30 steering ends were upgraded in later production, just like the C60L/S, they would not be definitive.

My own belief is that the C30 would have retained small steering ends, just like the C15A. Certainly this appears to be the case with the F30 chassis in Australian ambulance use. These were very late production chassis, and they differ greatly from late production F60S chassis supplied to Australia. Based on my own observations:

late production F60S: large steering ends, large steering box, auxiliary springs, reinforced chassis.
late production F30: small steering ends, small steering box, absent auxiliary springs, non-reinforced chassis.

Interestingly though I have seen ONE ambulance chassis which is late production F60S in every respect except absent auxiliary springs. For the moment I've dismissed this chassis as anomalous, perhaps reallocated from F60S production, however it may not be the only one.

On the question of diff ratio - normally this correlates with wheel size: 6/39 diff with 16" wheels; 6/43 diff with 20" wheels. On that basis the C30/F30 diff may seem anomalous, however the C30/F30 ran much bigger tyres than the 15A, so it's not a direct comparison. More importantly perhaps, the 2-speed transfer case would call for the more robust 6/43 diff.

Another identifying factor on the C30/F30 is absent brake booster, just like the C15A/F15A. It's worth noting here that the Ford workshop manual associates the booster directly with steering end size:

BRAKES - All Vehicles Having 6" Steering Ends.
Booster..........Yes

BRAKES - All Vehicles Having 4 7/8" Steering Ends.
Booster.........No

This strengthens my belief that the C30/F30 retained small steering ends universally, which also implies 2" front brakes. In other words the C30/F30 chassis was nothing more than a lengthened C15A/F15A chassis fitted with 2-speed transfer case, 6/43 diffs, and big tyres. My only query concerns auxiliary springs specified in the Ford manual - is it an error, or is the ambulance an exception...?

[David Dunlop]

Tony.

One of the standard comments one finds about the CMP production during the war, when compared with other vehicles manufactured, is the very high degree of standardization of parts, not only between the various weight categories, but between the two manufacturers. They kept it as simple as possible and didn't change things unless they really had to do so.

On that basis, your evaluation of the common components between the 15 and 30cwt classes makes a lot of sense. For the 60-cwt, certain things had to be more robust and were implemented accordingly.

Regarding the 60-cwt you ran across that lacked the auxiliary springs, perhaps they were simply removed from that particular vehicle, for whatever reason, but like you noted, one vehicle is interesting, a bunch of them would make a statement.

Cheers,
David

[Hanno Spoelstra]

Interesting discussion guys. Now, concentrating on Chevrolets for the moment as I am trying to ID a C60S or C30 on 16" wheels.
I just looked up some more facts in the Chevrolet MB-C2 maintenance manual and list my findings here:

Front axle:
6” steering ends (”steering arm is a separate part and attached to the lower part of the knuckle by six cap screws”): C60S, C60L, CGT
5” steering ends: C8A, C15A, C30, ”and on some special equipped C60L vehicles”

Brakes, front drum:
15” diameter, 3.5” width lining: C60S, C60L, CGT
14” diameter, 2” width lining: C8A, C15, C15A, C30

Brakes, rear drum:
15” diameter, 3.5” width lining: C15A, C30, C60S, C60L, CGT
14” diameter, 2” width lining: C8A, C15

Brakes, booster actuated:
C60S, C60L and CGT

Wheel sizes:
16” rims: C8A, C15, C15A, C30
20” rims: C60S, C60L, CGT

Differential gear ratio:
5.43 to 1: C8A
6.17 to 1: C15
6.5 to 1: C15A
7.16 to 1: C30, C60S, C60L, CGT

Springs:
As per factory specifications.

Winch:
Standard on CGT, supplied as optional equipment on C30, C60S, C60L.

Steering gear
Ratio is 23.6 to 1, with the exception of C60L with Gun Portee or Petrol Tank which has a ratio 30.6 to 1
C8A, C15 have mounting different from all other types

[Hanno Spoelstra]

Translating the above to identifying technical specs per vehicle type (limited to C30 and C60S as this the object of our discussion):

C30:

• Front axle with 5” steering ends
• Brakes, front drum: 14” diameter, 2” width lining
• Brakes, rear drum: 15” diameter, 3.5” width lining
• Wheel size: 16” rims
• Differential gear ratio: 7.16 to 1
• Rear spring leaves: 12

C60S:

• Front axle with 6” steering ends
• Brakes, front drum: 15” diameter, 3.5” width lining
• Brakes, rear drum: 15” diameter, 3.5” width lining
• Booster actuated brakes
• Wheel size: 20” rims
• Differential gear ratio: 7.16 to 1
• Number of rear spring leaves: 12 + 6 Aux.

So the Chevrolet I am trying to ID is a C60S, by virtue of having:

• 6” steering ends - see the picture with six cap screws
• 12 + 6 Aux. rear spring leaves - see the picture showing the auxiliary springs
• True, it is fitted with 16" rims, but it is possible to fit these on 15" diameter brake drums (C15, C30 have 16" rims on 15" diameter rear drum brakes)

Now, if I check if the subject vehicle has (had) a brake booster, I am sure this is a C60S rather than a C30.

Riddle solved!

[Tony Wheeler]

Originally Posted by Hanno Spoelstra

Translating the above to identifying technical specs per vehicle type

Excellent work Hanno. It occurs to me we've never seen the 15/30/60cwt differences specified like this before. They're always buried within tables containing other information. It's very useful to see them presented separately as you've done here.

It would appear the exact same differences apply to both Ford and Chev, although I wasn't aware of 14" & 15" brake drums. Evidently the Chev manual provides a lot more specs than the Ford manual.

All in all a very instructive discussion - I now have a much better understanding of the 30cwt CMP.