4x4 and lockers How it works on GW463? - Mercedes-Benz Forum
 
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post #1 of 6 (permalink) Old 02-02-2008, 10:08 AM Thread Starter
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4x4 and lockers How it works on GW463?

Hi,
Essentially I have these questions:
1. In 4x4 high, how is the power distributed to the wheels?
2. In 4x4 low............................................... .............?
3. What Happens when the central lock in engaged as far as distribution of power to the wheels is concerned?
4. And last when the rear then front lockers are engaged?

Thank you
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post #2 of 6 (permalink) Old 02-02-2008, 04:02 PM
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it's full time 4wd so in either high or low you have 3 differentials... one in the middle to the front and rear drive shafts, and one at each axle left and right... so all are getting some power. Lift any one tire off the ground or on super slippery stuff and that tire will just spin... car goes nowhere.

If you lock the center differential it's now like the 460 in 4wd. Front and rear drive shaft must turn... but in cross axle situations (or slippery under 2 tires, or slipping from too much throttle) and you could spin one front and 1 rear and you don't go anyway.

If you lock the rear then both tires must spin at the same speed, same for the front. which means they will try to push you straight ahead. Steering is more awkward and affected as such.

Not wise on dry pavement to lock any of them if you aren't careful and know what you're doing... though it can be done. Not really wise to lock any of them in rain either... only really intended to be locked in dirt and snow.

Low and high on the t-case are basically like the front gears on a mountain bike... normal driving, high range... steep driving, or technical driving, or really slow driving, then low range is nice (granny gear if you will).
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post #3 of 6 (permalink) Old 02-03-2008, 12:35 PM
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Quote:
Originally Posted by rey445588 View Post
Hi,
Essentially I have these questions:
1. In 4x4 high, how is the power distributed to the wheels?
2. In 4x4 low............................................... .............?
3. What Happens when the central lock in engaged as far as distribution of power to the wheels is concerned?
4. And last when the rear then front lockers are engaged?

Thank you
1. each wheel gets 25% of the torque as long as the traction on each wheels is equal
2. each wheel gets 25% of the (increased by factor 2.16) torque as long as the traction on each wheels is equal
3. each wheel gets 25% of the torque as long as the traction on each wheels is equal
4.each wheel gets 25% of the torque as long as the traction on each wheels is equal

As Eric posted already, the 463 has 3 differentials - front center rear. The axle differentials allow the wheels to rotate at individual speeds. The center differential allows the to drive shafts to rotate at individual speeds.
Differentials react to resistance (traction) - if one output side of the differential has less resistance, the diff allows that side to speed up. The amount of torque generated at that slipping wheel is determined by the amount of resistance. Power and torque are not lost at the slipping wheel - its just less, never zero. The non slipping always gets the same amount of torque as the slipping wheel.

The ability of differentials to allow for individual speeds can be detrimental when traction becomes unequal. Slippery ground or uneven terrain can lead to slipping wheel which then will speed up - and the differential(s) will allow the speeding up of the slipping wheel (after ll that's what they were designed for). http://www.4x4abc.com/4WD101/one-tire-spinning.html
Since one or more slipping wheels may not allow enough torque to be created, a vehicle can seize movement.

By disabling one or more of the differentials (locking) wheels can not rotate at individual speed any longer. That way the amount of torque generated by all 4 wheels is usually high enough to keep the vehicle moving. With a disabled (locked) differential the amount of torque generated at both wheels is no longer equal. The wheel with high traction (resistance) creates a high amount of torque and the wheel with less traction creates a low amount of torque. Both together are usually sufficient to move the car.
Drawback is, as already mentioned, that the vehicle will be very hard to steer.
http://www.4x4abc.com/jeep101/engage-diff-lock.html

Examples:
all diffs unlocked (open)
One slipping wheel: the axle diff allows the wheel to speed up (low torque on both wheels)
the center differential senses the low resistance of the axle with the slipping wheel and allows the drive shaft leading to the axle to speed up as well (low torque on both drive shafts). Result can be no more forward movement.

Two slipping wheels (one on each axle - usually diagonally opposed): the axle diffs allow the wheels to speed up (low torque on all 4 wheels)
the center differential distributes low torque to both axles. Result will be no more forward movement.
center diff locked
One slipping wheel: the axle diff allows the wheel to speed up (low torque on both wheels)
the center differential can no longer react to the low resistance of the axle with the slipping wheel (low torque on the drive shaft leading to the axle with slipping wheel - high torque at the drive shaft leading to the axle with non slipping wheels). Car will most likely still move (if the combined torque is sufficient)

Tow slipping wheels (one on each axle - usually diagonally opposed): the axle diffs allow the wheels to speed up (low torque on all 4 wheels)
the center differential distributes low torque to both axles. Result will be no more forward movement.

center diff locked plus rear axle diff locked:
One wheel with low traction at the locked axle: wheel is prevented from slipping, the axle diff generates high torque at the wheel with high traction, low torque at the wheel with low traction = high torque at the drive shaft leading to that axle.
The center differential distributes torque front/rear depending on resistance (might be higher to the rear [more than 50%] than with open center diff). Car will move.

One wheel with low traction at the open axle (front) and one wheel with low traction at rear: the front axle diff allows the wheel to speed up (low torque on both wheels). The wheel with low traction at the locked rear axle: wheel is prevented from slipping, the axle diff generates high torque at the wheel with high traction, low torque at the wheel with low traction = high torque at the drive shaft leading to that axle. The center differential distributes torque front/rear depending on resistance (very little to front - a good amount to the rear since the front has little resistance. Car will probably move.

center diff locked plus front and rear axle diff locked: One wheel front and rear with low traction. The wheel with low traction at the locked rear axle: wheel is prevented from slipping, the axle diff generates high torque at the wheel with high traction, low torque at the wheel with low traction = high torque at the drive shaft leading to that axle.
The wheel with low traction at the locked front axle: wheel is prevented from slipping, the axle diff generates high torque at the wheel with high traction, low torque at the wheel with low traction = high torque at the drive shaft leading to that axle.
The center differential distributes torque front/rear depending on resistance. In this case a good amount to front and rear. Car is very likely to move.

center diff locked plus front and rear axle diff locked: One wheel front and two wheels rear with low traction.
The wheel with low traction at the locked front axle: wheel is prevented from slipping, the axle diff generates high torque at the wheel with high traction, low torque at the wheel with low traction = high torque at the drive shaft leading to that axle.
Both wheels with low traction at the locked rear axle: wheels are prevented from slipping, the axle diff generates very little torque at both wheels = low torque at the drive shaft leading to the rear axle.
The center differential distributes torque front/rear depending on resistance. In this case a good amount to front and very low to rear. Car could possibly move.

With all differentials open the torque per wheel is never more than 25%. Never more than 50% per drive shaft. When wheels slip torque is even lower per component = very low stress.
With all differentials locked all the torque could be generated at one wheel = very high stress at that axle shaft and components involved. If the parts are not designed for the torque lockers generate, they could fail.
On the G (both 460 and 463) all components are able to handle this high stress. Meaning heavier parts that are much more expensive than on most 4x4.
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post #4 of 6 (permalink) Old 02-03-2008, 10:46 PM Thread Starter
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You're great guys. Thank you for taking the time to reply and thank you for the info.
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post #5 of 6 (permalink) Old 02-04-2008, 09:29 AM
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Would there ever be a need for locking any combination of the 3 possibilities and not all. Example; center only, or center rear, or front only etc. etc.

Also, when off road or in snow, with all or some locked, is high ranget ok and higher speeds (within reason)?

Thanks

Last edited by surt; 02-04-2008 at 09:30 AM. Reason: term
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post #6 of 6 (permalink) Old 02-04-2008, 11:10 AM
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Quote:
Originally Posted by surt View Post
Would there ever be a need for locking any combination of the 3 possibilities and not all. Example; center only, or center rear, or front only etc. etc.

Also, when off road or in snow, with all or some locked, is high ranget ok and higher speeds (within reason)?

Thanks
At higher speeds I would leave traction control (more drive stability) to the computer system alone (ETS). At slower speeds (below 30 mph) on fresh snow activating the center will create substantially more stability (possibly the rear in addition). Depends. But always keep in mind that as soon as any of the lockers are activated, ABS, ETS, BAS and ESP are no longer functional - and that might be a serious drawback. Might. Depends.

Off-road there are several scenarios where rear only (without the center) and front only (without center or rear) are needed. Plus many combinations of activating and deactivating those 3 lockers lockers during the (usually very technical) drive are benefical. The three activation switch unit can be modified to accommodate that (the old 460 had independent activation levers for that reason).
Those examples are very specific and for the serious user only. Requires in depth knowledge of the 4x4 system and some good hands on training.
http://4x4abc.com/4x4training/expert-training.html
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