All-Wheel Drive.........

[imp]

is a trend nowadays. I know of several schemes that have already been employed in production, not experimental or prototypes, to allow for speed variation between front and rear wheels.

 

The AWD mini-vans of early 1990's, Ford and Chevrolet, offered as optional, used differing approaches. Similarly, SUV's are being sold with AWD. The earliest I recall here in the U.S. were called "full-time" 4WD. They were, typically, the Ramcharger, Jeep, and Blazer. These used a transfer case containing a third differential, and were driven using a chain, which seemed to be the "weak link". Full-time died out, but AWD resurrected it, basically.

 

GM used a fluid clutch in the transfer case to allow "slippage". Ford opted for an electro-mechanical clutch which applied automatically as a difference in wheel speed between front and rear was sensed.

 

Which system has proven to be the best? I personally lean toward the fluid clutch myself, since after all, if they are suitable for the best washing machines.............! imp

[Rocket Man]

i don't think you can call one type of transfer better than another.

a design is a design but the fabrication is the important part.

certain types of clutches inherently last longer than others but some are necessary for sheer power transfer.

a fluid clutch is limited by the coolant where a mechanical clutch can handle higher temperatures.

 

an older toyota landcruiser has barely any electronics and it has a mechanical clutch. for relibility, it's proven itself several times over.

some 4wds have a torque converter, it's essesntially a self regulating, fluid based gear set with a variable slip. if you drop the clutch under high revs in a high gear, the torque converter can drop into a 10:1 ratio and vary according to the output shaft speed untill the internal pressures dictate a 1:1 ratio.

 

for actually taking 4wd into the scrub where it's meant to be, you want something sturdy.

for a 4wd being wasted in the city that might see the scrub once a year in the hands of an inexperienced driver, the raw power transfer is all important.

some cars simply have 4wd as a saftey feature boosting both efficiency and traction so basically anything goes if it's built right.

 

there's all sorts of technology going into cars these days, if your car has as much power as a mashing machine, you might just want to go with the research those engineers put in and fit it with a fluid clutch.

[imp]

i don't think you can call one type of transfer better than another.

a design is a design but the fabrication is the important part.

certain types of clutches inherently last longer than others but some are necessary for sheer power transfer.

a fluid clutch is limited by the coolant where a mechanical clutch can handle higher temperatures.

 

an older toyota landcruiser has barely any electronics and it has a mechanical clutch. for relibility, it's proven itself several times over.

some 4wds have a torque converter, it's essesntially a self regulating, fluid based gear set with a variable slip. if you drop the clutch under high revs in a high gear, the torque converter can drop into a 10:1 ratio and vary according to the output shaft speed untill the internal pressures dictate a 1:1 ratio.

 

for actually taking 4wd into the scrub where it's meant to be, you want something sturdy.

for a 4wd being wasted in the city that might see the scrub once a year in the hands of an inexperienced driver, the raw power transfer is all important.

some cars simply have 4wd as a saftey feature boosting both efficiency and traction so basically anything goes if it's built right.

 

there's all sorts of technology going into cars these days, if your car has as much power as a mashing machine, you might just want to go with the research those engineers put in and fit it with a fluid clutch.

 

Thank you for the information, Rocket Man. My concern for use of electronic sensing of speed difference between front and rear wheels, and then applying and releasing an electromechanical clutch, is how fast (repetitively) do you want the system to respond? If applied and released rapidly, the effect might be like our "ABS" systems which supposedly stop a vehicle more quickly.

 

Incidentally, a new mandate here requires ALL new passenger vehicles sold in U.S. in 2012 and beyond be equipped with electronic stability control! imp

[Rocket Man]

the speed variation between front and back is necessary. the front wheels will always travel further in a turn.

the most efficient way to acheive this is with triple differentials. however, if one wheel leaves the ground, it spins under power and the others can coast in any direction.

the clutches on the transfer are just there to stop this happenning. if you go into a slide, you cant apply power without it.

using it independantly on the front and back gives you a more controllable car. the furthest you can take this is independant power control on each wheel through the standard tripple diff. that would ential an acive clutch of some sort inside every diff.

 

then you need to consider what the car needs to do.

if the car needs a diff lock, you can't use a fluid clutch inside it.

if it's for the local hoon, you need something that can handle more torque.

if it's a volvo, a fluid clutch is more efficient and has a more linear torque taper.

 

abs can't stop on gravel as a result of near sighted programming and slow response. the faster it can respond, the safer it is.

abs relies on torque sensing. if the torque drops off sharply, it opens the calipers then eases them back on to the last "safe" level. abs is there to put the highest force on the wheels which is just ahead of the skid point. abs holds your limit just back from there. the jittery brakes are a sign of a crappy system.