Diff lock vs inter axle lock

@quatto i quoted the piece mr @Lepton1 already wrote explaining what the switches do, I would hope the explanation of the "what" would lead to the extrapolation of "why"... However:

If you have a vehicle with 10 wheels, on 6 axle ends; 2 wheels on 1 axle end are the only ones being powered: you don't have to break traction (loose driving surface, spotty slick driving surface) much before you lose momentum. If 4 wheels on 2 axle ends have power, you'll have that much more likelihood of retaining momentum and control. If 8 wheels on 4 axle ends; that much more likely not to break all traction and lose momentum.
Howsoever: when the rear 8 (assuming tandem axle full lockers) wheels are all turning as one, those front 2 skinny tires will have a lot more resistance when they attempt to change the directional course of the vehicle; as the rear 8 are all driving straight ahead.


Some people learn by book learning, some by doing.[/QUOTE]

And some need to be yelled at profanely and clocked upside the head.... some.

 

I'm sure most are familiar with 4x4 pickups? Try and turn on dry pavement when you have it locked in 4x4. Similar principal to that. Hard to turn and lots of binding in the axles and driveshafts. Except in a tractor trailer it is much worse because you have 2 or more diffs pushing you straight.

 

Cornbread language...

Youre making a left turn. In a left turn, the right side wheels will be the outside whesls since they are on the outside of the turn. The left wheels will be the inside wheels.

In a turn, the outside wheels have to travel a longer distance. Thus, they have to spin faster than the inside wheels. This difference in speed with a locker will cause the drive axles to bind up. So instead of turning smoothly, the outside wheels will bounce (bouncing is never good), or the drives will slide. This is why cars and trucks are made with "open" diffs, or positive traction/ limited slip. A locker LOCKS the driveline completely.

 

Could you recap what you told him here so that we could all benefit from your experience?[/QUOTE]
In the scenario where the young man blew out the drive shaft he was coming down a steep grade on a narrow two lanes paved road on a right curve with limited visibility, then had to turn hard left up a 20% grade on dirt with a cattle guard up about 100' from the road. Cattle guards on dirt roads are notorious for getting deep trenches in front and back, huge potholes, and that was the case here.

In conversation with him I found out that his plan of attack was to carry as much speed as possible off the paved road to get up that hundred yards steep dirt grade. He started in 6th on a 10 speed, but quickly lost power and downshifted to 3rd and then floored it to get over the cattle guard. The slick metal of the cattle guard is where it blew up.

I explained my plan in such a situation:

1. Engage the power divider as soon as you get to the dirt.

2. Downshift to low range at the high limit of your rpm's. Pick a gear you can start the climb with the engine revving up high. With my truck that's about 2000 rpm.

3. Never floor it. Watch the drives in your rear view. If they start any kind of spin or you feel loss of traction EASE OFF the throttle a millimeter at a time.

4. The goal is to top out in the same gear you started, light on the throttle, and don't be afraid to top out at 900 rpm's. Even though you are at that point well below the torque or horsepower curve, you won't damage the engine. Just keep momentum and stay light on the throttle to build momentum on the flat or less steep part. Think of it the same as when you first start out at idle when you let out the clutch, then ease into the throttle.

If you do have to downshift on a steep grade like this, do it early and skip at least two gears to catch it high in the rpm's.

 

Full throttle isn't going to hurt the driveline EXCEPT in cases like this. Most likely the wheels slipped as he crossed the cattle guard, and grabbed again once he hit the dirt. A slip/grab at full torque is what did in the driveline. It wouldn't have mattered if the approach had been pavement, he'd have likely still spit out the shaft.

The driveline is completely capable of handling the full torque of the engine, even in lower gears, provided it's a constant load. If there is even the slightest chance of a wheel slip, you need to be in a lower gear at higher RPM, and there are two reason for doing so. The first is that if you do have a wheel start to spin, you're operating in the part of the power curve where torque is rapidly falling off. If the spin happens in the meat of the curve, there's a lot more torque to accelerate that spinning wheel. Secondly, at higher rpm, not only is there less torque and a falling curve, you are also close to the governor, so the wheel can only accelerate slightly before the engine starts to reduce power. The end result is a bit of slippage at the tires without blowing them away into a full out spin, as the rapidly falling power allows the tire to regain traction much sooner.

 

Can we get an "Amen"!!???

The last thing I want to do is get anywhere close to maximum horsepower in slick ice, thick mud, or pulling a steep grade. If you have a turbo boost gauge in your truck try to stay at or below 50%, which translates to having my foot into the throttle about 1/3 the way to the floor.

The last thing you want is to overpower your traction, especially up a hill. Ease into or off the throttle. In thick mud or snow if you loose traction and spin the drives you end up digging deeper and building a wall of snow or mud in front of your tires.

There's been more than a few times I lost traction on a slick climb. The best option is to back all the way to the bottom and try again. This is something I have done quite a few times and it gets tricky at night on a dirt hill in the middle of nowhere, having to get out and place lights behind to guide me on the backup.

 

That's why CAT motors were so good off road.

 

@Beau If you have both interaxle and diff locks and you only use diff locks you are not getting power to the rear axle. Yes the differential is locked but the driveline between the two axles is not. That is what the interaxle is for.

 

Yup if you have both Interaxle first then diff lock.