Ford CMP axle ratios?

[Grant Bowker]

Based on the Instruction Book, Ford Special Pattern Vehicles Built for the British War Department, Third Edition (8th printing, not dated) The axle ratios are:
F8 4.11
F15 6.67
F15A 6.5
F30 to F60 (all variants) and FAT and rear engine 7.16

[Hanno Spoelstra]

Thank you, Grant!

So the 10.50-16 and the 11.00-20 tyres have (approx.) the same rolling diameter?

Hanno

[Grant Bowker]

As always when discussing CMP Tires, there are variations of actual dimensions by maker within the same nominal size.
Based on the AEDB Design Record, 10.50-16 tire diameter ranged from 37.4 to 38.2 inches and 10.50-20 ranged from 41.3 to 42.3 inches. All diameters given are for tires that are inflated on rims but not mounted on vehicles as the reduction from "free" diameter to "loaded radius" varies depending on the weight (vehicle plus load) the tire is carrying.

[Hanno Spoelstra]

So the difference in nominal size is roughly 4 inches. I guess this could be less with the larger tyres being compressed under a higher load. Apparently this difference was not worth to introduce yet another axle ratio.

[Howard]

Hanno,
I take it from what you are saying, that you are assuming that all of the different trucks had the same target revs at a given speed?
I would have thought it perfectly reasonable to expect the heavier trucks to run at higher revs, making use of the available power in the upper rev range, versus the smaller that would be less unhappy chugging along at lower revs.
(Boy, this stuff is difficult to put into words, isn't it!?)
H

[Phil Waterman]

Hi

Thought for you to ponder, keeping ratios somewhat matched with ground speed has a certain logic. If you are driving in a convoy of similar vehicles you want the shifting points to be relatively similar.

Back in the 70s and 80s I spent a lot of time convoying with a US Duce and my CMP because of different axle ratios, transmission ratios and slightly different torque curves. No mater which one of us lead we would be forcing the other to shift down prematurely.

So I would suspect that in military uses having convoys made up of the same vehicles would mean that convoys would move more smoothly.

Cheers Phil

[Tony Smith]

Originally Posted by Howard

Hanno,
I would have thought it perfectly reasonable to expect the heavier trucks to run at higher revs, making use of the available power in the upper rev range, versus the smaller that would be less unhappy chugging along at lower revs.
(Boy, this stuff is difficult to put into words, isn't it!?)
H

Except that the resultant effect is the other way around to what you describe. If both the 30cwt and 60cwt have the same diff ratio (and I've dropped the "F", because both Ford and Chev have the same diff ratios in each weight class), then for a specific road speed the vehicle with the smaller 10.50-16 tyres will be running at higher engine revs to keep up with the vehicle with the larger 10.50-20 tyres.

While the Ford and Chev engines have different torque curves (which result in a different "Sweet spot" cruising speed), tyre differences of around 10% between the Medium and Large CMPs will see a mixed convoy experience the downshift issues Phil describes. If the 60cwt's rpms are approx 10% lower than the 30cwt (and carrying a heavier load), it will naturally need to shift down a gear sooner than the lighter truck. In fact, with the larger tyre size, it may actually be more efficient (for the convoy, not for fuel!) for the larger truck to be revving high in 3rd gear to keep up with the lighter truck chugging along in 4th.

Note that within each brand of CMP, all weight trucks have identical engines (and power and torque figures), and identical gearboxes (with the same ratios). The only variation for speed is diff ratio and tyre size

[Howard]

OK, I can see I'm going to have to get the abacus out.
But not tonight.
H