So I'm building a what if based on the M37- gear ratios

[Ferroequinologist]

Hello all,
So I have an M37 rolling chassis, that I finally figured out what to do with lol. I had planned to let the cat out of the bag once I really started building it, but I need engineering help now, and you'd need to know what I was planning in order to help. I am going to build a halfa halftrack using the drivetrain and frame of an M37.

I'm doing all the hard planning and parts collecting now, and everything is coming together except the issue of gear ratios-

Here is the problem, and I'd like your opinions on how to possibly solve it.

So-
Front axle stock 5.83 gears, 38.2in tires (36.672 loaded rolling diameter). Stock transmission, stock NP200 transfer case. Now, here's the hard part- I need 3.18 gears on the rear axle to drive the tracks, according to the calculations I have done with the sprocket size etc. Sprocket pitch will be approx. 20in. I know a normal halftrack has 6.8 gears in the front axle, and 4.44 in the rear, so that they play together in 4x4. I also want my creation to function correctly in 4x4. I can modify the sprocket size to get it right if I find a close enough gear solution. Halftrack sprockets are just under 23in , so I'd like to stay closer to 18-20in to better proportion the drivetrain.

Does anyone have any suggestions? So far I am thinking of trying to find a brownie box to get the rear gear high enough, I know they make 4.89 and now even 4.30 chunks for M37 axles. Or even using it to lower the gear ratio in the front axle when engaged to 7.88, in some ways this would be better because I wouldn't need as much torque to start the truck on the 4.30 rear gears. (going for a long block flathead for power, for the added torque.)

Really would like to stick with the M37 axles, as a 'what if' built by Dodge shortly after the Korean war, is what I am going for.

 

[G744]

You might think about a t-case from a Dodge 880 full-time 4WD...they have an internal diff.

DG

 

[gringeltaube]

................. Really would like to stick with the M37 axles, as a 'what if' built by Dodge shortly after the Korean war, is what I am going for.

Interesting project! Now, what if...... you went with a D60 rear axle with 35-spline shafts, instead? Not only would it be stronger than stock; it also allows a ratio as low as 3.33:1 - which is just 5% slower than your (correctly) calculated 3.18:1 - but could easily be compensated with a 21in sprocket, instead of the 20".
An alternative - if you wanted to go with a smaller, 19.3in sprocket would be a D61 with 3.07:1 gears.
Guess such a swap would spare you the expenses for a brownie.

You might think about a t-case from a Dodge 880 full-time 4WD...they have an internal diff.
DG

A TC with internal diff would definitely help. But he still needs to get both axle ratios close enough so that he could use it in the locked position, as well.

 

[Ferroequinologist]

Interesting project! Now, what if...... you went with a D60 rear axle with 35-spline shafts, instead? Not only would it be stronger than stock; it also allows a ratio as low as 3.33:1 - which is just 5% slower than your (correctly) calculated 3.18:1 - but could easily be compensated with a 21in sprocket, instead of the 20".
An alternative - if you wanted to go with a smaller, 19.3in sprocket would be a D61 with 3.07:1 gears.
Guess such a swap would spare you the expenses for a brownie.


A TC with internal diff would definitely help. But he still needs to get both axle ratios close enough so that he could use it in the locked position, as well.

I could see going with a more modern D61 or D60 for the sprocket drive axle, as it will be hidden up under the frame. But I really was shooting for keeping all the hubs, drums, etc as stock M37 parts. Also because I already have a mountain of them. If I could modify a newer axle to use a M37 style hub... Hmmmm... I do have my 16x54in lathe up and going... I like the idea of the D61 with 3.07 gears, and a slightly smaller sprocket. I might have to narrow the axle anyway, so building the ends might not be that big of an issue.

Then the question is, how hard are D61's with 3.07 gears to find?

 

[Ferroequinologist]

Ok after looking around, the fronts are D61's, and rears are D70U-2a's. And a lot of guys on Pirate4x4 say they can't give them away lol. Used in the 88-92 Dodge pickups with the non OD automatics. So might not be too hard to find. Maybe I can modify the axle housing with ends and spindles off a M37 axle, and I would have to have axle shafts either cut down with a new flange or custom shafts made.

Hmmmm...

 

[gringeltaube]

Ok after looking around, the fronts are D61's, and rears are D70U-2a's

Look more....: D61 rear axles do exist; Ford 3/4ton of the 80s had them and most were 3.5" tubes (which is what the D70s have).

............... Maybe I can modify the axle housing with ends and spindles off a M37 axle...

Sure thing; that's what I would do. It shouldn't be too difficult to weld a #5/8" flange to the end of the tube and then machine it to be able to bolt-on a M37 front axle spindle. But you'll need a lathe with a distance between centers of >78" and a swing over bed of at least 24 inches.

 

[Ferroequinologist]

Look more....: D61 rear axles do exist; Ford 3/4ton of the 80s had them and most were 3.5" tubes (which is what the D70s have).

Sure thing; that's what I would do. It shouldn't be too difficult to weld a #5/8" flange to the end of the tube and then machine it to be able to bolt-on a M37 front axle spindle. But you'll need a lathe with a distance between centers of >78" and a swing over bed of at least 24 inches.

Ok, I'll keep looking

I can get a spare M37 front axle for the spindles, if not they are available. And I have several sets of backing plates. I was going to use the spindles and hubs for the rear tensioner as well, make everything 'standard'. Yeah, I don't have a lathe that big, might have to farm out that work.

Thanks Gerhard! I'm sure I will need to pick your brain more.

 

[gringeltaube]

Hey, always happy to lend a hand... I'm a big fan of such "builds" - and not buying stuff but making it myself...!

Yeah, I don't have a lathe that big, might have to farm out that work.

OK, when the moment comes I'll show you a trick to do this without actually turning the whole thing.

 

[Ferroequinologist]

OK, when the moment comes I'll show you a trick to do this without actually turning the whole thing.

Sounds good I'll take you up on it!