Air Lockers vs. No Air Lockers

[PeasBugs]

The Ford limited slip vs. the air lockers that Sportsmobile has as an option. What is the difference? What are the pros and cons? Do you really gain that much performance?

[jage]

Mmmm... 4 wheel drive... where to begin...

OK, on any standard drive axle, the power only goes to one wheel. This means that in a 2wd you are only getting power to one wheel. This is most observable on ice or in snow.

4wd gives you another drive axle. But with the new axle you also only get power to one wheel at a time.

Turns out this is the wheel with the least resistance, on that axle.

So, four wheel drive is ACTUALLY two wheel drive.

This becomes important in a situation such as this:


Now there is not much weight on the front driver-side tire, nor the rear passenger side tire. In some situations they will actually be hanging in the air. Since the power goes to only the wheel with the least resistance, the two hanging in the air get the power and you go nowhere.

A similar situation will happen if you pull over to the side of the road in winter, say putting both passenger tires on solid ice, with both drive tires on the pavement. When you try to drive away, the tires on the ice have the least resistance (once they start to spin) and you'll get nowhere fast, despite having 4x4 and two tires on good pavement.

With me so far? :shock:

Now these are extreme situations but they illustrate the point. So what a limited slip application does is give partial power to the other wheel. In other words on the ice or hanging in the air, the other wheel on the axle with the limited slip would get a little of the power. You wouldn't be at a dead standstill, but you can't always move out of an extreme situation like the illustrations.

However in situations like mud and loose dirt, this can provide more power at more places and keep you from getting stuck, or help you get unstuck.

Limited slip doesn't really engage or disengage, but there's a technique using your brakes to make it work better (I've actually gotten out of a twisted no traction situation with my Jeep Wrangler using only limited slip and the brake to prove it would work).

So... that brings us to a locker. A locker basically locks both wheels together so you get FULL power to both wheels. In the tire in the air situation you can drive forward almost normally (which can be bad as you can put the tires in the air even further away from the ground without noticing... it becomes quite like a tetter totter.)

The SMB locker is an ARB which is a top locker that is turned on and off by air pressure. In other words when off you have the equivalent of a limited slip and when on the wheels are locked together. In the ice on the side of the road example you wouldn't even notice there was ice with even one locker engaged.

For loose material, non-extreme situations, like deep mud, getting 100% power to both axle wheels is great.

Unless you're going to do extreme wheeling lockers front and rear are overkill, in my opinion. Getting a locker in back gives you some piece of mind... if you're starting to get stuck or going through something you know will be rough, you turn it on and save yourself worry and trouble.

I think you would be more than adequate skipping the locker. Get stuck six or so times and you can always opt to put one in.

[PeasBugs]

jage: Thanks Do the lockers use the Extreme Air onboard air system or do they have their own system?

[jage]

They would use the onboard system... it's probaby required, check the SMB site.

[EMrider]

Very helpful overview.

Just curious, is the locker option only applicable to the rear axle if you have the SMB 4x4 package? Is the front axle limited slip or a locker with SMB 4x4?
Thanks
R

[jage]

The sheet says locker $1500 each axle if I recall correctly.

[PeasBugs]

I was just looking at a 4x4 sheet from Sportsmobile and it also mentions "Dynatrac full floating rear axle" $3700. Anyone know what that is or what a "full floating" rear axle is?

[geoffff]

The power of the engine is transmitted from the driveshaft to two wheels that must be able to turn at different speeds through a differential. A differential is a torque splitter -- that is, half the torque always goes to each wheel. So if one wheel is in the air, it takes practically zero torque to spin it, and therefore the wheel still on the ground also gets practically zero torque. If one wheel is spinning on ice and one is on pavement, both wheels get the ice-slipping amount of torque, not the pavement-slipping amount of torque (because the ice wheel can't take anymore torque). This is why a standard car with an open differential is like "1 wheel drive", and a standard 4x4 with two open differentials (and a locking transfer case) is like a "2 wheel drive".

Torque is kind of like water pressure. You know what happens in some old houses when you flush the toilet when someone's taking a shower. The shower then gets less pressure. If someone then went and disconnected the toilet completely from the wall so water was freely spurting out of the fill pipe, the shower would then get zero pressure.

A locker gives you the option of connecting the two wheels so that they always turn at the same speed rather than absorbing the same torque. Now with a locker it doesn't matter that one wheel is on ice or in the air. All engine torque can go to the wheel on good ground.

A limited slip is a brake inside the differential that somewhat connects the two wheels, and acts somewhat but not quite like a locker.

Our van has the Ford limited slip on the rear and an ARB air locker on the front, as was recommended to us by Sportsmobile (if I wanted a locker). A locker really isn't necessary unless you enjoy getting yourself into tricky situations.

When we first got our Sportsmobile, I climbed pretty far on a 4x4 trail with my hubs accidentally unlocked. So without knowing it, I was only using 2wd and the limited slip! The van still did a pretty impressive job of giving enough torque to the good wheel.

Being a brake, limited slip differentials do wear out -- limiting the slip less and less over time.

The ARB air locker uses its own compressor, not the onboard ExtremeAire compressor. So, I have two air compressors on my van.

-- Geoff

[geoffff]

With a standard semi-floating axle, the axle supports the weight of the van.

With a full-floating axle, the wheels support the weight of the van, and the axle just turns the wheels (while bearing no weight).

Full-floating is stronger.

You can identify a van with the full-floating axle, as the axle noticeably goes completely through the wheel, sticking out somewhat.

[PeasBugs]

Thanks for all of the valuable information. I am starting to get a better picture of what I would truely use and what might be overkill for me. THanks