lots of new ML owners, Let's discuss all-wheel drive, 4wheel drive and full time 4wheel drive.... - Benzworld.org

12-05-2001, 01:22 PM

I think this will be a fun discussion and with lots of new members we can all learn and compare thoughts. 
 
Let's first establish some definitions and then compare and contrast them... 
 
Here is my view of things: 
First some definitions: 
 Open differential - a differential that provides power to both sides, however the faster turning side will sap power/torque from slower spinning side 
 Locked differential - a differential that always provides power to both sides most typcially 50% to each 
 limited slip - typically a rear differential that sends torque/power to the slower turning wheel away from a spinning wheel, opposite an open differential. Slower wheel is one with traction. 
 posi traction - typically a rear differential that begins to form a locked differential when one side spins faster than the other. 
 
1. 4 wheel drive (part-time) - implies locked center differential, but open front and back differentials. Guanteed to power at least 2 wheels. Front or rear slippage will get all torque/power for that axle no-traction will be achieved. 
2. 4 wheel drive (full-time) - same as #1, but center differntial is 'open' not locked. Any 1 wheel on vehicle can sap all torque/power. 
3. all wheel drive - typcially a vehicle that controls the 3 differentials. Most often it utilizes clutches to create 3 closed differentials: center, front and rear. techniques can also vary torque to each side of the differentials. 
 
 any others other variants? 
 
To me, it appears that the ML is an all wheel drive using 3 Open differnentials (high range) and utilizes ABS to prevent the sapping of torque/power to a slipping wheel. 
 
Benefits of AWD is a more natural handling. 
Benefits of 3 locked differntials (locked 4wheel drive)is full power to all wheels with no delay. 
 
I believe in Low Range the ML only locks the center differential. 
 
I look forward to others thoughts and information - especially if I have made mistakes. 
 
Personally, For snow driving I prefer Locked differentials as it provides even power and balance since 1 or more wheels will always be slipping due to locked differentials and varios traction/turning radius conditions. 
 
I think AWD is better for all other conditions with the exception of maybe drag-strip acceloration unless differentials ensure power to all wheels when slippage occurs. 
 
Guy

12-05-2001, 02:25 PM

Open differential - a differential that provides power to both sides, however the faster turning side will sap power/torque from slower spinning side' Most differentials are assumed as open unless specifically mentioned otherwise. A differential allows the wheels to turn at different rates, such as when you are going around a corner. Power will leak out to the side with the least resistance (i.e. on a more slippery surface), not the faster spinning side. Therefore, the side which is traction (not on the slippery surface) can't help. Locked differential - a differential that always provides power to both sides most typcially 50% to each It's a differential that provides equal amounts of power to both wheels. limited slip - typically a rear differential that sends torque/power to the slower turning wheel away from a spinning wheel, opposite an open differential. Slower wheel is one with traction. It doesn't exactly 'send' the power. There are clutches inside which help to equalise the power between both wheels so that it doesn't get leaked out to the side with the least amount of resistance. However clutch-based systems usually do not handle continous or heavy duty use very well. Because it is a 'limited slip' this means that under certain conditions, such as zero resistance on one side, a LSD can still be overwhelmed and be useless. Note that the Acura MDX, for example, uses a dual electric clutch based system for the entire rear differential. posi traction - typically a rear differential that begins to form a locked differential when one side spins faster than the other. Posi-trac is just a brand name for a limited slip differential. '1. 4 wheel drive (part-time) - implies locked center differential, but open front and back differentials. Guanteed to power at least 2 wheels. Front or rear slippage will get all torque/power for that axle no-traction will be achieved. It can mean no centre differential or locked centre differential, depending on the vehicle. A vehicle with part-time 4WD can have locking or limited slip front/rear differentials. Because the front and rear axles are mechanically bound to each other with a 50/50 split, both are forced to turn at the same speed. This is why you can't use it on high traction surfaces (meaning dry or wet pavement). 'To me, it appears that the ML is an all wheel drive using 3 Open differnentials (high range) and utilizes ABS to prevent the sapping of torque/power to a slipping wheel. Yes, the ML does have three open differentials. ABS wheel speed sensors are used to detect wheel spin. If one wheel spins a pre-set speed faster than the others, the brake on that wheel is intermittently applied to slow it down so that it approximately matches the speed of the other wheels. This way, the differential is fooled into thinking that the wheel on the slippery surface has the same traction as the other wheels, and it won't leak any power to the path with the least resistance. Contrary to popular belief, the brake does not clamp down and completely stop the spinning wheel. 'I believe in Low Range the ML only locks the center differential.' No, the ML does not lock the differentials at all. This is why you can use low range on all surfaces. 'Benefits of AWD is a more natural handling.' I think you mean neutral handling. Depends on the torque split ratio. 50/50 is the most neutral and also the most boring :-). Some vehicles that are primarily FWD biased feel like FWD vehicles. Benefits of 3 locked differntials (locked 4wheel drive)is full power to all wheels with no delay Fully power is always sent to the wheels in the ML's system. The benefits of 3 locking differntials like in the G-class is that the power will not be able to leak out to any wheel at all. Problem is that you have to know when to turn on/off the differential locks. The ML's permanent 4WD system sends about equal amounts of power to all four wheels under normal resistance conditions anyway. There is no delay and no need for power shift to another axle like other SUVs. Here's how I catagorise things (following the way that Land Rover does). I blame it on the manufacturers and magazines who use the terms interchangably. There are variations on a theme, of course, but this is basically what it boils down to: 4WD = Low range and High Range gearing AWD = Only high range gearing Part-time 4WD: Generally no centre differential. Cannot be used on dry/wet, semi-slippery roads due to the lack of the centre differential. When activated, both front and rear axles are physically locked to each other and have to spin at the same rate. This becomes a problem when turning on sufficiently high friction surfaces. Examples: Suzuki SUVs, most 4WD pickup trucks, cheaper SUVs. Permanent 4WD: No two wheel drive mode. System is equipped with a centre differential, and hence is safe to use on all surfaces. All four wheels are powered all of the time (usually 50/50 front and rear axles). This is arguably the best system since the torque split ratio does not change and is the most predictable. All wheels 'help out' all of the time and this stabilises the vehicle + improves handling. With the extra two drive wheels, the vehicle has twice the amount of traction all of the time (even in no-slip conditions) vs. a 2WD vehicle. Examples: MB M-class SUV, Range/Land Rovers. Full-time 4WD: Basically permanent 4WD but with a 2WD mode. This was born out of customer demand (for a 2WD mode). Examples: Toyota Sequoia, Jeep's SelectTrac 4WD system, Mitsubishi Montero. Permanent AWD: Basically permanent 4WD but without low range gearing. Examples include the Audi Quattro AWD system, the MB's 4-matic AWD system, Subaru's manual transmission AWD system. Full-time AWD: System is active at all times, however in most cases, the one set of wheels (usually the rears) only receive 5-10% of the engine's power unless slippage occurs. At that point, power is progressively transfered to the opposite axle to help out. Some systems can transfer power to the rear upon acceleration to improve traction. However, they revert to 2WD mode when coasting. Examples: Acura MDX, Chrysler AWD minivans, Some Subaru AWD vehicles with auto transmissions.

12-05-2001, 02:36 PM

You have the ''two foot mode'' which allows simultaneus gas and brake use at crawl speeds. It also has downhill traction function which automatically assists in engine braking when descending hills between 2 and 12 mph, with minimal throttle application.If the system senses that that one or more wheels lose traction, it brakes the slipping wheel to redirect power to wheels with grip.

12-05-2001, 05:47 PM

One additional variation is the system used on the Subaru Outback VDC where there is a basic 45/55% front to rear torque splt (numbers approximate) giving a rear drive feel, through the use of a planetery (sp?) gearset. It also uses the antiskid brakes to stop torque leak. All in all, Drew's explanation is one of the best I've seen. For the Toyota systems, there is a comprehensive explanation on one of the Edmunds, Toyota boards, given by Steve Clifford (cliffy1), who is a very knowledgeable Toyota sales person.

12-05-2001, 06:00 PM

The Subaru VTD AWD system which is found in the WRX and the VDC wagon/sedan is similar in what you would find in other cars with permanent AWD systems. The VDC equipped vehicles add stability control and traction control, something that the WRX does not come with, of course. Nonetheless, all fall into the permanent AWD catagory.

12-05-2001, 06:00 PM

12-05-2001, 06:09 PM

12-05-2001, 06:15 PM

Drew: That's what it's supposed to look like here in the mtns of NC - it was almost 70 today; the ski resort operators are dying!! I think it's a Canadian plot; you are keeping all the snow for your selves.

12-05-2001, 07:14 PM

12-05-2001, 08:04 PM