Axle interlock vs 8X8

[73m819]

In the HET ALL four axles are open, with standard driving the number 4 axle is the drive axle, flipping the traction control lever to engage engages the number 1 axle, putting the transfer in low automatically engages the number 1 axle, flipping the traction control lever to engage engages the number 2 and 3 axles.

 

[JDToumanian]

In the HET ALL four axles are open, with standard driving the number 4 axle is the drive axle, flipping the traction control lever to engage engages the number 1 axle, putting the transfer in low automatically engages the number 1 axle, flipping the traction control lever to engage engages the number 2 and 3 axles.

That's interesting... So basically the inter-axle diff lock is on the 4th axle instead of the 2nd?

 

[JDToumanian]

I am talking about getting power to all 4 axles in High range which cannot be done unless there is another axle interlock lever . Thus, transfer case in high range, dash lever in 8X8 and an over ride lever for axle interlock. Then all 4 axles will be powered in high range, not just low range. That is what I mean by installing a switch to over ride the axle interlock.

I see. Indeed the HEMTT can't do that. But I don't really see why you'd want it to... If you're on a surface where there is a risk of getting stuck, the transfer should probably be in low range as you'd be going relatively slow. Though if you wanted to do it anyway, I think it would be as simple as changing some plumbing and adding another air toggle. Maybe you already said that in your previous post...

Jon

 

[Castle Bravo]

Castle Bravo is completely correct in his explanation... I like his analogy of thinking of each pair of HEMTT axles as a separate 4x4 pickup truck. In high range, with the air toggle in neutral, the front pickup truck is in neutral and the rear pickup truck is in 2wd. Flip the toggle to lock and the front is still in neutral and the rear is in 4wd. Flip the toggle the other way to 8x8, and both pickup trucks are in 2wd. With the transfer case in low range and the toggle in neutral, both trucks are in 2wd. Flip it to 8x8 and it does nothing different... Both trucks are already in 2wd since 8x8 is automatic in low range. Flip it to lock in low range and both trucks are in 4wd, which is one wheel on each axle being driven. The HEMTT doesn't have true lockers where both wheels of any axle are driven at the same time.

Gunzy, I'm not sure you need to change anything... The HEMTT can already do what you describe.

Jon

I think a better analogy than the rear tandems being in 2WD would be that they are in all wheel drive (AWD). Examine a typical highway scenario - Transfer case in high range and traction control lever in neutral position - interaxle differential unlocked and no power to the front tandem axles. Driving down the highway with equal traction to all tires, all four rear tires are powered. The interaxle differential does not prioritize one axle over another in the same way that the axle differential does not prioritize one tire (left or right) over another. Nor does it prevent power from going to those tires. The interaxle differential IS a differential, not just a on/off switch for the #3 or #2 axle. Its not like the transfer case where power is on/off to the front axle.

Here's a logical (but not physical) diagram -

Transfer case high range -



Like I mentioned before, with equal traction at all the tires, in the 1st example, the 4 rear tires are powered. If you happened across some ice or mud or whatever, (Or jacked up one of the rear tires off the ground) the tire with the least traction would spin and the vehicle would not move. In the 2nd diagram, with equal traction, all 8 tires are powered. If there is loss of traction, the least amount of tires powered is one of the front four and one of the rear four. In the 3rd diagram, with equal traction, the 4 rear tires are powered. If there is loss of traction, the least amount of tires powered is one tire on axle #3 and one on axle #4.

Transfer case low range -



The front and rear tandems are powered whenever the transfer case is placed into low range. Putting the traction control switch into the 8x8 selection when the transfer case is in low range changes nothing. In the 1st and 2nd diagram here, with equal traction, all 8 tires are powered. If there is loss of traction, the least amount of tires powered is one of the front four and one of the rear four. In the 3rd diagram, with equal traction, all 8 tires are powered. If there is loss of traction here, the least amount of tires powered is one on each of the four axles. This is the maximum traction selection and is not possible in high range, as Gunzy says.

The interaxle differential is physically located in the #2 and #3 axles, but they don't have anything logically in the powertrain to do with the axles they are located in.

 

[Castle Bravo]

In the HET ALL four axles are open, with standard driving the number 4 axle is the drive axle, flipping the traction control lever to engage engages the number 1 axle, putting the transfer in low automatically engages the number 1 axle, flipping the traction control lever to engage engages the number 2 and 3 axles.

With the HET, it works like this -

Transfer high, switch off = high range, rear axles only powered, interaxle diffs unlocked
Transfer high, switch on = high range, all axles powered, interaxle diffs unlocked
Transfer low, switch off = low range, all axles powered, interaxle diffs unlocked
Transfer low, switch on = low range, all axles powered, interaxle diffs locked



In the 1st diagram above, the 6 rear tires are powered if there is equal traction. If there is loss of traction, only one of the rear six tires is powered, on any axle. In the 2nd diagram, all 8 tires are powered if there is equal traction. If there is a loss of traction, any one of the rear tires and one front axle tire are powered. In the 3rd diagram shows 8 tires powered if there is equal traction. When traction is lost, one tire on each axle is powered. There are circumstances where you could lose traction on one axle but not another and have more than what I've listed powered - its a "worst case" scenario.

 

[73m819]

With the HET, it works like this -

Transfer high, switch off = high range, rear axles only powered, interaxle diffs unlocked
Transfer high, switch on = high range, all axles powered, interaxle diffs unlocked
Transfer low, switch off = low range, all axles powered, interaxle diffs unlocked
Transfer low, switch on = low range, all axles powered, interaxle diffs locked

View attachment 632343

In the 1st diagram above, the 6 rear tires are powered if there is equal traction. If there is loss of traction, only one of the rear six tires is powered, on any axle. In the 2nd diagram, all 8 tires are powered if there is equal traction. If there is a loss of traction, any one of the rear tires and one front axle tire are powered. In the 3rd diagram shows 8 tires powered if there is equal traction. When traction is lost, one tire on each axle is powered. There are circumstances where you could lose traction on one axle but not another and have more than what I've listed powered - its a "worst case" scenario.

Your chart and drive layout is wrong, I have watched number 4 axle spin (1 tire and not going anywhere), flipped the traction control to engaged, this engaged the number 1 axle and only number 1 plus number 4, with this combo the het pulled right out of the hole (very wet grass pulling up to a road embankment), when I moved three hets to storage, the terrain was not the best for air bag suspension, just locking in number 1 axle would not do the trick, dropped the transfer into low, still ONLY number 1 and 4 spun (only ONE tire per axle), I flipped the traction control lever to engage, now I had all 4 axle powered, at times I still had 1 or 2 axles spin, still ONLY one tire spun per axle.

First, I was surprised that the hets do not have locking rears, a 8x8 really a 8x4
Second, I was surprised that the hets did so bad dealing with the terrain.
Third, I was surprised how fast the hets can loose traction do to bad terrain.
Forth, I found that it was real easy for a axle (one or both sides) to loose ground contract due to the straight frame and air bag suspension.
I came to the conclusion that the HEMTT is a far better truck for traction then the HET due to the articulated axle sets which keep ALL the axles in contract with the ground in very rough terrain.
Two of the hets were pulling a non running 931a2, the third pulled my pu, after the traction issues with the first two, we just shut the operation before someone got hurt or something broke, delt with things later.

 

[Castle Bravo]

Your chart and drive layout is wrong, I have watched number 4 axle spin (1 tire and not going anywhere), flipped the traction control to engaged, this engaged the number 1 axle and only number 1 plus number 4, with this combo the het pulled right out of the hole (very wet grass pulling up to a road embankment), when I moved three hets to storage, the terrain was not the best for air bag suspension, just locking in number 1 axle would not do the trick, dropped the transfer into low, still ONLY number 1 and 4 spun (only ONE tire per axle), I flipped the traction control lever to engage, now I had all 4 axle powered, at times I still had 1 or 2 axles spin, still ONLY one tire spun per axle.

Its not wrong - that is how it works and it is what you're describing as well. When the HET is in high range, the switch engages the front axle but leaves the interaxle differentials unlocked. This provides power to all 8 tires, but because the interaxle differentials are unlocked and the axle differentials are open, if there is unequal traction, the tire with the least traction will be powered. What the switch does is dependent on what range the transfer case is in.

First, I was surprised that the hets do not have locking rears, a 8x8 really a 8x4

4 powered axles of 4 total is a 8x8, an 8x4 would be 2 powered axles of 4 total in that "#x#" parlance.