Full time 6x6 question

[SP5]

In the "real" halftracks, you use the transfer case to disengage the front axle when you are on a hard surface, the front end is engaged only for sand/snow/ice/slippery conditions. There is no provisions for the two driveshafts to turn at diff speeds while the front end is engaged.
The pitch diameter of the drive sprocket and the diameter of the front tires will determine the ratios needed. Just for a simple example; Lets say that the front tires are 36in. diameter, and the sprocket has a pitch diameter of 18in.
In that case, a easy solution would be to have a 6:17 ratio in the front end, and a 3:08 ratio in the rear end.
The biggest hassle will be to use front and rear ends that will allow you get the ring and pinion sizes you need.
Is'nt a Deuce axle around 6:17??,
Remember, the original half track was not a super heavy truck, the GVW was less than 20,000lbs, you really don't need 5ton parts for one,, and you're sure not going to be rolling along at 60mph., more like 40>45 flat out.

 

[Sikorsky502]

I've thought about putting Oman axletech T600 transfer case. Does anyone have experience with this? Their website says it has torque split, like on the MTV that suprman was saying.

 

[Sikorsky502]

Now that I'm thinking about it, the MRAP axles have air lockers. Doesn't that give the ability to disconnect them from the power train? Meaning it's like the manual lockers on civie trucks, but done with air rather than physically turning the dial on the hub.

 

[SP5]

Now that I'm thinking about it, the MRAP axles have air lockers. Doesn't that give the ability to disconnect them from the power train? Meaning it's like the manual lockers on civie trucks, but done with air rather than physically turning the dial on the hub.

No, the air lockers do not disconnect the drive train, they serve to "lock" both of the wheels together by eliminating the differential action of the third menber.
You're not going to re-invent the wheel, you need to use an old-fashioned transfer case, one that has 2 wheel/4 wheel high/neutral/and 4 wheel low range, just like a common m37, or an old Ford/Chevy truck, and use diff ratios in the front and rear ends.

 

[Sikorsky502]

Ahh, ok. So then would a spray clutch type transfer case work? I'm trying to read up on the meritor transfer case that uses the all time 4x4 and it says it has torque propotioning. Any idea what the **** that is? Just can't be the first time this has come up. I mean even with tires all the way around a vehicle there has to be some way to keep from binding when there's differences in tire pressure.

 

[SP5]

You keep getting "torque bias" confused with "speed/rpm bias", and they are not the same thing.
In a M1078/LMTV, when you're going down the road in "normal/highway" mode, the output torque of the trans is being sent 70% to the rear, and 30% to the front, but both drive shafts are turning the same rpm, only the twisting effort/torque is different.
In this mode, the internal couplings/clutches allow for SMALL variations of speed, for SHORT periods of time, so as to allow you to turn the vehicle without binding, because in a turn, the wheels follow different tracks.
When you put the transfer in "cross country" mode, the torque is sent equally to both axles, but the rpm of the shafts are still the same, only now, being "locked" together, their is no allowance for "slip", it is assumed that the "slippery" terrain being driven upon will make-up the difference, so as not to bind-up the drive train.
In a halftrack, we have a whole diff ball game, in that the front and rear have to turn at diff speeds.
If you lose air and a tire goes down, that tire becomes "smaller" in diameter, and when that happens the pinion gears in the differential start turning to make-up the rpm diff.
As an aside, when you lose a track on a halftrack,, you're going nowhere.
This whole discussion really belongs in the "tracked" forum, where there are several members who have built/rebuilt/restored halftracks could explain all about sizes/ratios/tires/sprockets, etc.

 

[Sikorsky502]

Questions are posted on track section already. I posted this here to learn what a full time 6x6 vehicle does, if anything, if the tire diameter is different, like in the event of a flat tire or less air pressure in the front from the back. My mixing up of torque bias vs rpm bias is why I'm asking the questions. I don't own an MTV truck nor have I had to worry about this issue before, hence my questions. So I'm assuming the MTV has no provisions to deal with this issue and there's no such thing as RPM bias on a transfer case? And why would a Halftrack not move without a track? It has front wheel drive.

 

[rosco]

What happens if the front tires are flat or have lower air pressure from the rear? This would change the tire diameter and cause binding in the system. Is this a problem on the trucks or does some component allow for this difference?[/QUOTE]

Nothing happens as long as the tire is not slipping/skidding. Its the circumference that is importance, not the height from the ground!

 

[SP5]

Yeah, I was being somewhat faceitious, in a halftrack, when you lose a track, all the torque/motive force being sent to the sprockets is lost, just like a 2 wheel drive car with one wheel on ice, (there ain't no Posi in the sprocket drive rear end).
So, with only the torque to the front end left, you might, (on a hard surface) be able to drag the vehicle a ways, but on any softer surface the small contact patch of the front tires won't do much to overcome the drag of the remaining track, not to mention the bogies sinking down in the ground. Because the front end is an open differential, so you really only have one wheel drive at that point.
It will be pretty hard to move a 19,600 GVW halftrack, with a blown track, using a few square inches of front tire contact area on any surface.

 

[Sikorsky502]

You have no idea how frustrating this has, and is, for me. I'm definitely not an automotive guy. I know the basics, but when it comes to the mechanics of how it all works it's beyond my current knowledge base. You want to talk aerodynamics or anything aviation, I'm your guy. I just want an updated version of the Halftrack. That's all. It's pretty much the best of both worlds. It's just getting this part right that gives me heartburn. I also had no idea that with one track off that the other won't turn.

 

[tobyS]

The 809 is a full time 6x6. The spragg clutch does relieve the difference you are talking about as long as the back is trying to over-run the front, which it will with a 13/14 ratio if you are even close to being equal with your front and rear final drive ratios. Also it is divorced, so works with the auto. Put "Rockwell Transfer Case T-138" into ebay.

SP5 gave a good example of the ratio or you can do what I had planned and figure revolutions of the drives, front and rear in relation to a mile.

Using differentials that do not lock will cause what SP5 states...the one with least traction will spin. Use a locking rear for the rear axle. It puts a lot of stress on the drive-line, but you could have individual brakes like a tractor (which might help to slow a slipping side and shift torque to the non-slipping side.

 

[SP5]

I'll try to clarify some stuff.
In a halftrack, you have, lets say, to make it easy, a couple of tracks in the rear, that are about a foot wide, by about 4 feet long, or so, contacting the ground.
If the ground was level/flat the natural tendency of the vehicle would be to go in a straight line, even if it had no front wheels.
The front wheels are really for "forcing" the vehicle to turn, even when it does'nt want to.
The reason for having the ability to "drive" the front wheels is not so much to increase traction, we already have lots of that from the big track area, but in slippery road conditions we need the front wheels to have some "tractive effort" to still be able to force the vehicle to turn.
Drivers of halftracks will tell you, that even with the front end engaged, when there is snow/ice/mud, etc., that you got to plan your turns, because the vehicle still wants to go straight. Because there is no "contact" point like a tire, the tracks have to be forced to "skid" in any type of turn, weather dry ground or slippery ground.
With regards to air pressure in tires, while the circumference of a tire changes very little with pressure, the effective simi-diameter does change. The effective simi-diameter is the distance from the ground to the center of the axle. When a tire has less air it is forced to turn faster for the same speed than if it was fully inflated, even though its circumference has not changed.
On any driven axle, if you had one tire at 60lbs, and the other at 40lbs, the pinion gears would be rotating in the differential to make up the rpm diff between the tires.
On an unloaded M1078, for example, at the same pressure, the front tires look flatter than the rears,, they are, and they have a smaller rotating simi-diameter, as such, they actually are forced to turn faster,, they are being "pushed" by the rears, and because the drive shafts are being driven at the same speed, the front tires are forced to "scrub" a tiny bit to make-up the speed differential, and thats why, even if the vehicle never-ever turned a corner, the front tires would still wear out faster.

 

[tobyS]

Two things. With turning already hard, a locked rear differential, unless it locks on demand (air, electric), would make the vehicle want to go in a straight line, as SP5 says. Most the time that's fine but turns would be hard (like a snowmobile at low speed). There is an argument to be made for left and right braking for turning (or times when one rear slipping) and not locking into equal speed.

Second, while I don't know about power divider operation on MTV or LMTV, when it is locked in there is no difference, they turn equally when engaged per Suprman.

The T-138 will release the difference in the front and rear drive-shafts via the spragg clutch. If your object here is to find a transfer case that relieves the drive-line stress from slightly, but not excessive differences in final drive ratio, the T-138 would be a solution. It renders minor differences a non-issue... by design (13/14 ratio = 93%).

I don't think you really have much choice on the transfer case, unless you can get to within 1% or better of (drive-shafts) turning the same on dry pavement by ratio and sprocket size. The guys with deuces that have a spragg may have a smaller version than the 5 ton's T-138 Rockwell.

 

[scottmandu]

Second, while I don't know about power divider operation on MTV or LMTV, when it is locked in there is no difference, they turn equally when engaged per Suprman.

With the power divider unlocked toque sent to the rear from the transfer case is divided up between the two rear axles. With it locked there is a 50/50 split and they spin the same speed.

 

[1stDeuce]

FWIW, if you can get under 5% ratio difference front to rear, you'll be fine on softer surfaces, and there is no reason to have the front engaged on hard surfaces anyway. Any traditional T-case with 2wd/4wd/N/Low range will work fine at that point.

If you can't match the ratios that well, you have two t-case choices that are worth checking into...
Both the old NP203 and the HMMWV/Jeep/Dodge NP242 have "full time" modes that will allow the front and rear driveshafts to turn at different speeds. Both have a 50% torque split in "full time" mode, and lock the f/r speeds in "part time". You'd still want to keep the ratios fairly close to keep from buring out the t-case differential. At that point, you could lock the transfer case on very soft surfaces to force both ends to drive. If you go this route, I'd be sure the front outruns the rear, not the other way around. This should work better than having the tracks push the front axle, since it's most likely you'd already be spinning the front axle anyway if the going is that rough...
Both cases should be fine in a GVW under 20k application, and at lower speeds.
The 203 is available in a divorced configuration, from 70's era Ford F250/350's. It has Hi, Hi Lock, N, Low, Low Lock.
The HMMWV 242 has the same settings. (no 2wd)
The Jeep 242 has 2wd, Hi, Hi Lock, N, Lo Lock (No unlocked Low range, but 2wd...)
The Dodge (Durango/Dakota) version 242 has Hi, Hi Lock, N, Low Lock (No 2wd or Low unlocked...)

For something like you're proposing, I'd go 203 or HMMWV 242... No need for 2wd, and having an unlocked low range option is useful for turning...


Good luck!!

 

[coachgeo]

FWIW, if you can get under 5% ratio difference front to rear, you'll be fine on softer surfaces, and there is no reason to have the front engaged on hard surfaces anyway. Any traditional T-case with 2wd/4wd/N/Low range will work fine at that point.

If you can't match the ratios that well, you have two t-case choices that are worth checking into...
Both the old NP203 and the HMMWV/Jeep/Dodge NP242 have "full time" modes that will allow the front and rear driveshafts to turn at different speeds. Both have a 50% torque split in "full time" mode, and lock the f/r speeds in "part time". You'd still want to keep the ratios fairly close to keep from buring out the t-case differential. At that point, you could lock the transfer case on very soft surfaces to force both ends to drive. If you go this route, I'd be sure the front outruns the rear, not the other way around. This should work better than having the tracks push the front axle, since it's most likely you'd already be spinning the front axle anyway if the going is that rough...
Both cases should be fine in a GVW under 20k application, and at lower speeds.
The 203 is available in a divorced configuration, from 70's era Ford F250/350's. It has Hi, Hi Lock, N, Low, Low Lock.
The HMMWV 242 has the same settings. (no 2wd)
The Jeep 242 has 2wd, Hi, Hi Lock, N, Lo Lock (No unlocked Low range, but 2wd...)
The Dodge (Durango/Dakota) version 242 has Hi, Hi Lock, N, Low Lock (No 2wd or Low unlocked...)

For something like you're proposing, I'd go 203 or HMMWV 242... No need for 2wd, and having an unlocked low range option is useful for turning...


Good luck!!

if you go with any FMTV part on this sounds like tranny would not be one of them...... due to electronics of the OEM FMTV tranny. There is 99% odds you will throw so many transmission computer sensors into mass hysteria with a different Tcase installed it would just shut down the tranny into some odd limp or no go mode. And that is a huge iffff on could even figure out how to remove the AWD back end off the tranny and install one of the different Tcase mentioned.

 

[1stDeuce]

Oops, I missed where he was using the FMTV drive line... If that's the case, the dilemma is solved... In normal mode the tcase will allow front/rear speed difference, and when locked, the speeds will be forced to match. Just get the ratios close and it'll be fine... Or prevent it from ever locking if the ratios are too far apart...

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[coachgeo]

Oops, I missed where he was using the FMTV drive line... If that's the case, the dilemma is solved... In normal mode the tcase will allow front/rear speed difference, and when locked, the speeds will be forced to match. Just get the ratios close and it'll be fine... Or prevent it from ever locking if the ratios are too far apart...

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Not sure that he is.. or for all we know.. he has abandoned this project. This thread is couple years old now it appears.

 

[1stDeuce]

Dang it... Sucked into an old thread....

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