Gearing and a top speed again

[Vladislav]

...Got the gearbox apart.

It's overdrive two-stick unit called TRD-37 with 0.78 top hole.

Don't know why it's not called TRD-370, probably in 1945 it was a different way.

When I pulled out the top cover I saw most the gears were pretty rotted, and that was more important, all the high gears.

Anyway I took all the stuff apart, cleaned, washed, all the bolts and nuts were sent to halvanic lab and they already came back now. Body parts were prepared to sand-blasting, some covers/levers/rods/pins were done and painted waiting assembly.

By the way I had a trip to The Netherland and found all the gears I needed to change. So now all looks like Ok and I must be happy. But...

The truck is 10 tonn twin screw NR-14 model of 1945. It has (will have) 131 bhp 519 c.i. Mack Lanova diesel under the hood.

It has 9.02 rears and 0.78 overdrive so with 11.00-24 tyres my Dutch friend can run the same unit about 40 mph at 2000 revs.

He told me he could reach 44 mph if go 2100 wich is limited. For shure I don't too like such the things. He also.

The first thing comes to everybodie's mind is to swap the rears.

For example, if I want to get about 55 I need 55/40=1.375, 9.02/1.375=6.56 or higher.

Looking the net for a couple of years I figured out it's not easy to find high-speed carriers off L-model wich seem to me the same attachment. Dutch experience gave me the fact a top gear set off a B-model carrier can be used in my housings (carriers marked CR32/CR33). Looking further it seems to me I also could be able to fit an upper set off CDR-92/93 but with a short design divider. I don't like this way because too want to keep everything outside stock. So there might be different ways around that.

Back to the gearbox. It might have some potential of increasing the top speed also.

Want to repeat I don't plan to use anything with a different body. Keep stock look.

Nevertheless an overdrive ratio can be increased. Figuring out a gearbox design I learned a constant mesh gear set must be changed to increase an OD, a OD gear sets itself are made about the same size and don't allow to play much.

Continue to looking over the books and BMT-wiki I got the mind that the inside of my TRD-37 might be combined with gears off TR(D)-720 or maybe some other Mack-built units I hadn't info of.

So OD might be increased up to 0.65 (combining high-speed constant mesh off TRD-720 and high-speed OD set off TR-37).

It could make the top speed of 40mph x0.78/0.65 = 48mph. Better but not quiet enough.

If increase it a bit with 7.32 rears I found (expensive and heavy so twice expensive when shipped overseas) it will turn out 48mph x9.02/7.32 = 59 mph wich is very good.

Although there might be different way.

Most overdrive units have big steps between gears. The original TRD-37 has 1.92 third (hi) and 1.00 fourth (hi) so needed to shift down compaund every time from 3 to 4. If use a direct top gearbox (gear set in my body) it makes shifts easier and (I hope) don't required splitting. Next, old direct-top boxes has "normal" shift pattern i.e. left-up - left-down - right-up - right-down when OD's shifts left-up - left-down than right-DOWN - right-UP. Maybe it's not a trouble but I absolutely don't used to do it.

It's easier to find a direct-top box for parts although it will required much more high rears - about 5.5.

All that shure has me scratching my head for awile although I want you guys to let me know one easy thing:

Maybe I do a stupid thing combining all those figures because that 131 bhp engine can't get such a quick moving to the vehicle weighted 12 tonns ???

[Freightrain]

Shot blast the gears and run it. Don't think the extra "oil pockets" in the gear teeth will hurt anything. It's called extra lubrication LOL!!

Unless the surface rust has gotten under the case hardening and will allow premature wear? You plan on lots of driving? Though in HI gear, the front box is running direct through(the cluster is not under load) so those gears are not really too much of an issue.

The hard part of going fast with a 9.x rear gear is the fact I don't know how well it will put up with running it at "X" rpm faster then normal trying to get yourself faster with more OD in the transmission.


Don't know, but is the TD37 the small box, kind of like the baby quad for the early gassers? Is the TRD720 the small box also? I'm thinking(no books to look at) it is the big box thus nothing would interchange?

[Vladislav]

Surface rust covered all the faces of many teeth nearby, there were not the holes in the surface but total rust-out.

I was worrieing about overdrive because both the constant mesh and the top-speed set work.

And I'm not as worry of a break down but a noise.

Revs is definitely the way to go although I was told about conrods came out on this type of engine.

No, TRD-37 is the big box, the same design as TRD-72.

TRD-67 looks less in size and arranged different.