Ford CMP axle ratios?

[Hanno Spoelstra]

Guys,

I don't have my references at hand. So can anyone on here help me with the axle ratios for the F30 and F60L with 10.50-16 tyres?

Are they the same as on the F60L with 11.00-20 tyres, or is there a difference?

Thanks,
Hanno

[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

[Hanno Spoelstra]

All good input, thanks!

If Howard takes out the abacus I'd be interested to see the differences at the WW2 convoy speed of 25 mph.

Hanno

[David Dunlop]

Not sure if this will highjack the thread or not, but I think there is a strong enough relationship it will work.

It makes sense different weight classifications of CMPs would be geared differently in such a way as to make convoy movements of mixed vehicle fleets travel together at a consistent road speed.

Would you not also expect differing fuel consumption rates across the CMP weight classifications? If one did not also take that into consideration, planners would run the very real risk of vehicles running out of fuel randomly along the supply routes. It would be far better to ensure each differing weight classification carried enough fuel that all could expect attaining the same range when they all start out with full fuel tanks. Loaded weights and gear ratios would all play into fuel capacities, would they not?

David

[Hanno Spoelstra]

Not having my abacus on hand, I used an online calculator (http://www.advanced-ev.com/Calculators/TireSize/) to calculate the following:

Both trucks traveling at convoy speed of 25 mph:

F30 with 7.16 diff ratio and 37" diameter (10.50-16) tyre: engine runs at 1626 rpm

F60L/FGT with 7.16 diff ratio and 41" diameter (10.50-20) tyre: engine runs at 1467 rpm

Pretty much the same? A heavier (loaded) truck would of course use more fuel.

Still some homework to be done. Grant, does the AEDB list 9.00-16 tyre diameters so we can compute F15 and F15A engine rpm @ 25 mph?

[Tony Smith]

Originally Posted by Hanno Spoelstra

Not having my abacus on hand, I used an online calculator (http://www.advanced-ev.com/Calculators/TireSize/) to calculate the following:

Both trucks traveling at convoy speed of 25 mph:

F30 with 7.16 diff ratio and 37" diameter (10.50-16) tyre: engine runs at 1626 rpm

F60L/FGT with 7.16 diff ratio and 41" diameter (10.50-20) tyre: engine runs at 1467 rpm

Pretty much the same?

Not even close. The 10% difference in revs results in a loss of nearly 20% of the power. My copy of the Ford Service bulletins is missing the Power/torque curve for the 1939-on 239ci 95hp engine ( ), but I have attached the pic of the 85hp to demonstrate.

At 1467rpm, the 85hp engine is producing about 38hp and about 151ft.lb of torque. At 1626rpm, those figures jump to 45hp and 155ft.lb.

I have found a chart online for the 95hp which seems to show corresponding figures of 42hp /160ft.lb at 1467rpm and 53hp/164ft.lb at 1626rpm. Clearly the 60cwt will be struggling carrying twice the load.

Using your linked calculator, if the 60cwt is in 3rd gear (1.69:1 x 7.16 = 12.1), at 25mph the engine will be turning 2479rpm. At these revs, the 95hp will be producing 81hp and 161ft.lb., or nearly 60% more power than the 30cwt in 4th gear.

The Ford V8 loves to rev, and that's how the 60cwt will be capable of carrying twice the load at the same road speed as the 30cwt. The Chev engines however, have a reputation of producing more power and torque lower in the rev range, and their comparative power outputs at certain revs WILL be different to the Ford. This trait of the Chev engine will see them shifting up a gear sooner than the Ford V8, which prefers to rev out fully in a gear before shifting up. Interestingly, the Ford 226 Straight6 in the 2GT trucks (has a very similar torque curve to the Chev 216, and would be a more compatible truck in Convoy with the Chev).

[Grant Bowker]

Originally Posted by Hanno Spoelstra

Grant, does the AEDB list 9.00-16 tyre diameters so we can compute F15 and F15A engine rpm @ 25 mph?

More tire sizes:
9.00-16, between 34.8 and 35.7 inches
8.25-10 27.8
9.00-13 30.7 to 31.5
9.25-16 33.3 to33.4
14.00-20 47.5 to 47.7

All of the above are from the table for "Regular Pneumatic - Cross Country Tread Design". There are separate tables for "Regular Pneumatic - Highway Tread Design" and "Run Flat - Cross Country Tread Design". Technically, a more accurate road speed to engine speed relationship would be obtained by using "loaded radius" rather than the quoted "free diameter" however the difference between the values will vary with vehicle weight/load, inflation pressure, tire wear etc.

[Hanno Spoelstra]

Thanks Grant for the tyre dimensions.

Tony, I did not realise the power curve of the Ford V8 was that steep!

[Tony Smith]

Originally Posted by David Dunlop

It makes sense different weight classifications of CMPs would be geared differently in such a way as to make convoy movements of mixed vehicle fleets travel together at a consistent road speed.

Would you not also expect differing fuel consumption rates across the CMP weight classifications? If one did not also take that into consideration, planners would run the very real risk of vehicles running out of fuel randomly along the supply routes. It would be far better to ensure each differing weight classification carried enough fuel that all could expect attaining the same range when they all start out with full fuel tanks. Loaded weights and gear ratios would all play into fuel capacities, would they not?

David

Absolutely, and even within the class of 3 ton trucks, fuel usage would vary between Ford and Chev, and between say a 4x4 CMP and a 4x2 Standard truck with narrow tyres. The logistics planners would have a lot to consider when allocating fuel for a particular convoy from A to B.

But even though a 60cwt is in a lower gear and revving higher than a 30cwt, it will still be a more economical way of moving a 3 ton load than providing 2 30cwt trucks. A 60cwt revving at 2400rpm will be using about 60% more fuel than the 30cwt revving at 1600rpm, which is a saving really on moving 100% more load.

Originally Posted by Tony Smith

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).

..... And the 15cwt, 30cwt and 60cwt have all standardised on the same fuel capacity of 2x 12 Gal fuel tanks, with the FATs having 2x 20Gal.

With the additional consumption of the 60cwt travelling in convoy with smaller CMPs, the design brief should have considered the 20 Gal tanks to give an equivalent range between refills.

[Tony Smith]

Originally Posted by Hanno Spoelstra

Tony, I did not realise the power curve of the Ford V8 was that steep!

I would like to see an official GM chart of the 216 power and torque curve should any Chev enthusiasts have one available to compare?

All the online sources I can find mainly deal with the later 216/235/261 on post-war fuel and compression ratios, and how to "Hot-rod" them.

[Phil Waterman]

Hi Tony

I've searched in the past and this is what I found.

http://canadianmilitarypattern.com/2...ifications.htm

Basically as the engine developed they revved higher and produced their power at higher RPM. One of the reasons the 261 is such a nice replacement for the 216.

Will try and find the original source and save the data to post directly.

Cheers Phil