Gearing down on a budget

97guardmetallic said:
What is the difference between the Detroit truetrac and the yukon grizzly locker. pros and cons of both?
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The first thing to know is that a Detroit Truetrac is a limited slip differential. It is NOT a locker and there are a lot of differences between a locker and a limited slip differential.

For flat roads and/or trails that are slick from snow or ice LSDs like the Truetrac are superb and the way to go. If there's ice on an uneven off-camber or steep trail, lockers that can't be turned off can be a nightmare. They earned the 'low-side' finder nickname in those conditions since they'll cause you to slide toward the low-side of the trail if you have a locker that can't be disable.

But if you're doing trails that are difficult/uneven enough so tires are lifted partially or completely up off the trail, LSDs are pretty well useless which is where lockers are the way to go. That's why I replaced both of my Detroit Truetrac LSDs with lockers. For the uneven trails I prefer, lockers are the only thing that help, LSDs are useless.

Here are two lengthy articles I wrote 15-20 years ago I am copying and pasting below. Sorry they're a bit wordy but most who have read them say they give a good explanation of lockers and LSDs.

@Chris should we make the two below articles into stickies somewhere?

Why do I need a locker? I thought I had 4X4!

Q: Why do we need lockers, I thought I had four wheel drive!!??

A: We need lockers for tough terrain because the differentials installed into our front and rear axles have a characteristic that can turn our 4x4 Jeeps into 4x2 Jeeps in low traction situations.

A factory differential (sometimes called an "open differential") has a characteristic in that when one wheel loses traction, that wheel will start spinning ineffectively. You've seen that happen before, I'm sure. The bigger problem is that one spinning wheel on an axle causes both wheels to lose power to the point you can't move your Jeep... so even though the wheel on the other non-spinning side may have excellent traction, that non-spinning wheel won't receive enough power to keep you moving. Why? Because the differential screws up and only "sends power" to the wheel that is spinning. That is not technically totally accurate but it illustrates the problem very nicely, with a more technically accurate explanation as follows.

So the engine, via the drive shaft and differential, is seeing very little resistance from the axle with the spinning tire. So what? If the engine sees little resistance, it develops little torque. Low resistance to the engine, low developed torque. Lots of resistance to the engine causes it to develop lots of torque. Connect a dynamometer to an engine without a load on it and it'll show very little developed torque. Now put a brake of some kind against the engine output shaft (to add resistance) and the amount of torque developed by the engine will suddenly and dramatically rise. No resistance, very little developed torque... high resistance, a high amount of developed torque.

When a wheel starts spinning, the reduced resistance the engine sees from the axle causes the engine to dramatically reduce the torque the engine develops and sends to the axle. Here's what else is going on that is a key to understanding this whole thing... a stock factory differential ALWAYS ALWAYS ALWAYS splits the torque it receives from the engine 50:50 to each wheel. Exactly 50-50, every time. So when one wheel is spinning and the engine torque decreases dramatically because of that, the OTHER wheel that still has good traction is seeing 50% of the greatly reduced torque. In fact, the torque sent to the axle is reduced so greatly that when the differential divides it 50-50, there is insufficient torque for the wheel with good traction to keep you moving. This means you're stuck!

So when you're driving on a dry paved road, both tires are receiving equal amounts of power and the high traction they're seeing from the dry road helps the engine to develop maximum torque. Both tires are pushing equally with lots of available torque. Now jack up a wheel (or lift it with a rock on a trail you just drove over) and you're not going to move even though the other tire is still on the ground. The tire in the air is spinning like crazy, causing the torque that axle receives from the engine to go right into the toilet.

So, when one tire on the axle is spinning, you don't have enough power for the other tire to keep you moving. For obvious reasons, all this is a huge problem for a 2wd vehicle (just one axle to drive you). It works exactly the same for a 4x4 but you have one more axle to assist in keeping you moving. But if one tire per axle has poor traction, you are stuck... since one spinning tire per axle is enough to reduce all developed torque from the engine down below the point the Jeep needs to move forward.

OK, we know what the problem is now, what's the fix? One, you could STEP ON THE BRAKES a little... which would stop the tire(s) from spinning so more torque would be developed, which should be enough for the OTHER tire that still has good traction to get you moving again. Stepping on the brakes forces the engine to work harder so it develops more torque which is sent to the wheels... so that gets more torque sent to the wheel (both actually) that still has traction so you may be able to get unstuck. Yes, stepping on the brakes (to a point best learned by practice) works rather well in these situations. Just about all drivers used to know that technique when few roads were paved... but it's just about a lost art now.

So what does a locker do? It mechanically LOCKS the left and right wheel together to overcome the above problems. It won't allow one wheel to start spinning while the other sits doing nothing. The left and right wheels are mechanically locked together.

Automatic lockers keep the left and right sides locked together except when you turn left or right, where it will automatically unlock the outside wheel during the turn until after the turn is completed at which time it locks up again. When the locker unlocks for a turn, the outside wheel is allowed to rotate faster than the inside wheel so it doesn't hop and skip during the turn. The inside wheel is driving during a turn with an automatic locker-equipped vehicle. The locker automatically locks again once both wheels are turning at the same RPMs again.

The problem with an automatic locker is that most are not very street friendly when installed in the rear axle. Because they keep the left and right wheels locked together except when forced to unlock for a turn, they can cause unusual handling characteristics like rear-end waggle, tire chirping, disconcerting loud bangs and snaps from the locker, and even sideways sliding down slippery off-camber slopes where they earned the nickname "low-side finder"... which can sometimes produce a high 'pucker-factor' at times. For offroading however, locker's negatives are far outweighed by their benefits in challenging conditions.

But in many Jeeps like the TJ, an automatic locker is fine when installed in the front axle. Since the front axle doesn't receive torque in 2wd, a front automatic locker unlocks easily enough for turns that you may not even notice its presence. Only in 4x4 is the front axle receiving torque which makes it harder for the locker to unlock for turns. About the only conditions where an automatic front locker would not be good in the type of 4x4 system a Wrangler TJ has would be on icy or snow-covered roads where you need 4x4. In 4x4, a front automatic locker would cause understeer (make the Jeep want to drive straight in a turn) which would not be good if the road was slick from snow or ice.

A manual locker is "open" (unlocked) until you actuate it. The ARB Air Locker and the cable-actuated Ox-Locker are examples of manual lockers. These are good because they remain unlocked until you choose to lock them. This eliminates the handling problems automatic lockers have on the streets.

By the way, a locker is installed inside the differential and it replaces the "spider" gears that make a differential work they way it does.

So some Jeepers add lockers in the rear, others add them to the front. I happen to think locking the rear axle first does the most good, but I have installed automatic lockers into both axles which works pretty darned well. But if your rear axle is the notoriously weak Dana 35c that comes stock on all Wranglers except the Rubicon and Unlimited, avoid installing a locker into it and install the locker into the front axle instead. Since the front axle rarely receives more than 50% of the torque that the rear axle does, it can usually handle a locker without problem with reasonably sized tires. But if your rear axle is the optional and far stronger Dana 44, by all means install a locker into it if your trails are tough enough to make a locker desirable.

So what's a limited slip differential? First, it is not the same as a locker. It is more like an automatic brake for the spinning tire... it performs like when you use the step-on-the-brakes technique so the spinning side gets coupled to the non-spinning side for more resistance so more engine torque is generated by the engine so the non-spinning tire receives more torque to help get you moving again. It operates as a brake somewhat by coupling the added resistance of the side with more traction/resistance to the side that has less traction/resistance. A LSD depends on some tire spin to get it working so it's not as efficient for challenging terrain as a locker is. But then generally speaking, a LSD is far more "driver friendly" on the streets, which is why we all don't just have lockers in our Jeeps.

4x4 & & getting torque to all 4 wheels answers

All four tires are driven equally by the 4x4 system and all four pull equally if they all have equal traction. If all four wheels have equally good traction, then all four will pull equally well. So even if your Jeep does not have a locker or limited slip differential, each of the four wheels will receive approximately 25% each of the torque from the engine, when traction is equal under each of the four tires.

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 an equal amount which usually means insufficient power to keep you moving.

The engine can develop no more torque than the tire with the least amount of traction can accept before spinning.

So the short answer is 'yes', all four tires 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 always (!) splits whatever torque it receives 50:50 to both sides. Why does the amount of torque get reduced at all? Because the engine only develops torque when it is working into resistance. Run an engine with a torque meter connected and watch what happens to the amount of torque produced 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 force to the engine and the amount of torque will increase in direct proportion to the amount of resistance the engine is working into. The more braking force applied to the engine's output shaft, the more torque the engine produces.

So because the engine only produces torque when working into resistance, a spinning tire reduces the amount of power (torque) the engine produces because the engine is working into less resistance caused 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 insufficient 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 up a couple clicks can often help by acting as a poor-man's limited slip differential. Pulling the parking brake up a few clicks when you already have a limited slip differential will help improve its operation. A key drawback to a LSD whether it be clutch or gear based is that when one tire is spinning up in the air, it doesn't help create enough torque for the other tire still on the ground to keep you moving. Without you helping it by stepping on the brakes or pulling the parking brake handle up if it's a rear tire that is spinning, a LSD doesn't do much for you. On flat terrain they help a lot, but on uneven terrain when both tires can't always be on the ground, a LSD is not very helpful. That's a situation where a locker reigns supreme over a LSD.

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 is in all Wranglers together with low-end Cherokees and Liberties. A part-time 4x4 system locks the front and rear driveshafts together inside the transfer case so they drive the front and rear axles together in lock step. 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".

In reality however, the front and rear axles really don't even turn exactly the same RPMs when you're in 4wd so you still get "wind-up" if you drove in 4wd on the street even if you drove in a perfectly straight line. Why? Because 1) you can't drive in a perfectly straight line and 2) the front and rear axle ratios are usually .01 different from each other. Like a 3.73 and 3.74, 4.10/4.11, etc.. Why the .01 ratio difference between the front and rear axles? Because the front and rear axles usually have different ring gear diameters which makes it nearly impossible for the gear manufacturers to economically make the front and rear axle ratios exactly the same. And no, they are not made .01 different on purpose to make the front or rear pull more when in 4wd, that is an old wive's tale.

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 an "open" 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.

Hope this helps!
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