Command-trac 4Hi usage questions

I would keep searching for a Select-Trac.

You NEVER want to be in 4wd with Command Trac on ANY hard surface where the tires are not free to slip/slide unless you want to replace tires and drive components a premature death.

While you are searching also look for one with the vari-lock axles.

Are you able to order a new Jeep and still get the "family" price or is it just limited to dealer stock?

The dealer can type what you are looking for into the computer locater and they can find what you need. They can search for used jeeps also..

No... it's always there. Wet pavement too. I won't even use 4wd on hard dirt... some 4wd have a slight difference between the front and rear diff ratios (like 4.10 and 4.11) which will actually cause it to bind up unless there is some slip or give. Slight tire diameter differences from front to rear contribute as well.

Yes, see above.

Like I said, I normally don't. If you are intent on going fast, then you'll put up with faster tire and drivetrain wear and less efficiency. It's not as bad as dry blacktop. Just be aware that in a turn, the

When I say hard dirt, I mean *shiny*... like emulsion-coated, no-dust-blowing roads.

Not in the slightest... like driving on ball bearings. Have fun. __ Steve .

You can shift in and out of 4HI as required at speeds up to 55 MPH. You can safely run 4HI on short, straight stretches of high traction surfaces (even asphalt), especially when there is loose stuff ahead to allow unwinding.

Don't worry too much about using 4HI on anything but asphalt or concrete. Even very hard packed dirt will allow enough wheel slip.

Steve

Sandst> I'm thinking about buying a Jeep Liberty. I know Selec-Trac may be a

I purchased a Cherokee with command trac and had the Dealer change over to selectrac at around 30,000 mile and like the selectrac much better. So you don't have to worry about road surface. The change over cost was around $1,900, so you should get the selectrac option when you purchase the car new and save yourself some money in the long run. You are not likely to regret it even if it may mean a few more $ per month on payments.

Yes, there is a problem. Not an immediate problem to be sure, but a long term problem.

The trouble is, the SelecTrac is a Full Time system, meaning it has a differential of sorts built in. The differentials in the axles accomodate speed differences from left to right, the differential in the tcase will accomodate speed differences front to rear. There are inherent speed differences because the front and rear are not geared precisely the same, they are off by a few hundredths, but over time and distance something has to give. When driving on dirt, or snow, the give comes from the tires slipping on the ground, but when driving on pavement there is no slip to speak of, and the drive chain inside the tcase stretches. This stretching is a bad thing. A very bad thing.

No, the problem doesn't occur only when turning. It occurs worse when turning.

A liberty has a 4wd option?

ROTFLMAO.

Now if they would just accessorize with this...

Well, except the IFS would give it away - but then a liberty buyer probably couldn't spell IFS anyway.

L.W.(ßill) Hughes III wrote:

IMO, get SelecTrac if you want AWD-type performance, i.e., shift it & leave it. However, keep in mind that SelecTrac does not release the front hubs - everything spins all the time, even in 2HI. The only thing 4HI does is engage the front/rear differential and sends some power to the front wheels. When everything spins, gas mileage generally takes it on the chin, especially in cold weather-short distance driving. On the other hand, if the new CommandTracs still work like the older ones, the front hubs and axles release & stop spinning when in 2HI, giving slightly better gas mileage. Others may disagree, but I was apalled at the crummy mileage from my XJ with SelecTrac - ~10-14 mpg - rivaling my old V8 Grand Wagoneer with CommandTrac which got 10-12 mpg. I also think that in 2HI, The additional resistance of the spinning front components with SelecTrac has the disadvantage of making the rear end a little looser in slippery situations than in CommandTrac 2HI. Maybe this is why my XJ Limited came with a limited slip rear end. [Do all SelecTracs come with a L-S rear axle?] With a shorter wheelbase vehicle, this can become an issue rather quickly. So if conditions even look slippery, I throw it into 4HI since it doesn't really use any more fuel than when in 2HI.

Thanks to all for a perspective on this issue. Although everyone doesn't agree I can see the issues better now. Previously, I didn't pay much attention to the shift-on-the-fly into/out of 4Hi capability - thanks for drawing attrention to this. Assuming that feature works flawlessly, some of my misgivings about Command-Trac don't bother me as much. If I were to order from the factory, I'd probably request Selec-Trac but right now I'm tempted by the end-of-the-model-year employee discounts and factory rebates as dealers try to clear their 2005 inventory - and their inventory, in my area, is nothing but Command-Trac.

Sandstone

My parents just bought a 2005 Liberty Limited. It was advertised that it came with Command-Trac. It actually has Selec-Trac. (2WD-4PARTTIME-4FULLTIME-N-4LO) Or did Jeep set up Selec-Trac differently with the Libbys? So check it out to see what it really has! Maybe you will also get lucky! ;-)

If you got selectrac with paying extra for it, then you really got a deal. It has been my experience that most salespeople selling jeeps don't know the difference themselves and will tell you that it is ok to use command trac fulltime wet or dry. For myself after having selectrac I would never go back to command trac.

Larry

My 02 WJ with Select-Trac does not have posi-traction.

What is with the delay in the front from kicking in?

That is the perfect example of the limitations of the 'Selectrac' full time, it's one wheel drive only. With Commandtrac, you at least can get one front and one rear wheel going due to the center lockup and with some practice, you can get all 4 spinning or at least both rears and one front.

You hit the brake pedal to load up the spinning tire and the friction will equal the torque needed to break the other side free so both wheels on the same axle will spin. The owners manual mentions using the E-brake to do this, but I find a fast hit with the brake pedal works pretty well.

This is an old trick with open diffs and goes way back.

Mike

Billy Ray wrote:

The shift on the fly feature works really well in both of our Jeeps. I live in Canada and just treat 4 hi like any other shift. When I see snow on the road I shift in, when it clears, I shift out. I also find 2 better at snow or slush covered intersection corners. I shift back to

Running in 2 high is usually best for mileage.

Mike

Sandst>

When your Selectrac is in the 4Hi or 4Lo position, that is a "part-time"

"All four wheels are pulling if all four wheels have equal traction. If all four tires have equally good traction, then all four will pull equally well.

A standard open differential always (always!) splits the torque 50:50 between both sides. The problem is that when one tire starts spinning due to poor traction, that reduces the amount of torque seen by either side by an exactly equal amount. So if one side starts slipping, the other side's power (torque) is reduced by a corresponding amount which usually means insufficient power to keep you moving. The axle can deliver no more torque than the side with the least amount of traction can take before slipping.

So the short answer is 'yes', all four pull in 4wd. But the moment one side starts slipping, neither side gets enough power to keep you moving even if the non-slipping side still has traction.

Why does the power (torque) get reduced to both sides and not just the side that is slipping? Again, because the differential will always (!) split whatever torque it sees 50:50 to both sides (few people know that). Why does the amount of torque get reduced at all? Because the engine only develops torque when it is working into a resistance. Run an engine with a torque meter connected to it and watch what happens to the amount of produced torque as resistance to the engine is varied. When the engine is working into zero resistance and just running free, it produces nearly zero torque. Apply a braking action to the engine and the amount of torque will increase in direct proportion to the amount of resistance it is seeing. The more braking force applied to the output shaft, the more torque the engine produces.

So because the engine only produces torque when seeing resistance, a spinning tire reduces the amount of power (torque) the engine produces because the engine is only seeing the nearly zero amount of resistance produced by the spinning tire. This works the same way with 2wd and 4wd, it's just that with 4wd, you have more of a chance that at least one of the two axles will have enough traction to allow the engine to produce enough torque to keep you moving. Which is why you get stuck in the first place... when one side is spinning, there is insufficent power being delivered to the other side to get or keep you moving.

And all of this is why a limited slip differential (LSD) can help since it helps to "couple" (via a clutch or a gear-based device) the resistance the side with good traction is seeing to the other side with poor traction, increasing the amount of resistance seen by the engine... thereby increasing the amount of torque delivered to both sides. Which is why the simple technique of stepping on the brakes a little when a tire is spinning can often get you unstuck... because it too increases the resistance the engine is working into which increases the amount of torque that is delivered to the wheels. Or if a rear tire is spinning constantly, pulling the parking brake on a couple clicks can often help by acting as a poor-man's limited slip differential.

What does a locker do? It mechanically locks the left and right wheels together so when one turns, they must both turn at the same speed. This arrangement prevents one side from spinning uselessly while the other side does nothing. Automatic lockers are always locked but they unlock automatically to allow the outside wheel in a turn to 'ratchet' faster as the outside tire must do through the turn. Once the turn is completed, the locker re-locks both sides together. A manual locker is one that doesn't lock the left and right sides together until it is actuated either via a push-button or lever. An ARB Air Locker is air-pressure actuated, others like the Detroit Electrac are electrically actuated. The Ox Locker is actuated via a lever and cable. Manual lockers have an advantage for on-road driving in that when unlocked, they act like an "open" axle... i.e. one without a locker or limited slip differential... which means it drives like an unlocked vehicle until the locker is activated via the push-button or lever.

Part-Time and Full-Time 4x4 systems...

A part-time 4x4 system called Commandtrac which is in all Wranglers together with low-end Cherokees and Libertys locks the front and rear axles together inside the transfer case. Because they are locked together, the front and rear tires must rotate at the exact same rpms. However, the front tires must rotate faster than the rear tires during any turn so a part-time system fights that... which makes a part-time system inappropriate on a paved road because the high level of traction on a paved road prevents the tires from slipping which would otherwise allow the front and rear tires to grudgingly rotate at different rpms. Offroad this is not a problem since the poor traction of an offroad trail allows the tires to slip as needed. But when they try to slip/rotate at different rpms on a high-traction surface, the entire drivetrain is stressed which is bad for it. This problem is called "wind-up".

Finally, a full-time 4x4 system like Selectrac is available on Grand Cherokees, Cherokees and Libertys couples the front and rear axles together, but they are not mechanically locked together like they are with a part-time 4wd system. The front-to-rear axle coupling can be done via either a differential like the Selectrac system uses (just just like what is in the center of the axle) or a fluid (viscous) coupler. The benefit to a full-time 4wd system is that because the front and rear axles are not mechanically locked together, the front and rear tires/axles can rotate at different rpms from each other. This allows a vehicle with a full-time 4wd system to drive in 4wd "full time" on a paved road without problem since there is no 'wind-up' problem to harm the drivetrain. You cannot get a full-time 4x4 system in a Wrangler from the factory."

H> What is with the delay in the front from kicking in?