If you're a compu-whiz and can get a NY Times article for free
02-06-2002, 10:01 AM
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If you're a compu-whiz and can get a NY Times article for free
In the 1/27 Sunday Times, there's an excellent chart called "An All-Wheel Drive Overview" that lists the many AWD variations currently existing for all makes and models. I cut it out but can't seem to squeeze it into my A: drive to get it onto the forum!
FYI, for quattro the normal front/rear split is given as 50/50, the extreme potential split is 100/0 or 0/100. One wheel can drive the car.
FYI, for quattro the normal front/rear split is given as 50/50, the extreme potential split is 100/0 or 0/100. One wheel can drive the car.
02-06-2002, 10:28 AM
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cough
All-Wheel Drive in Many Flavors
By BOB KNOLL (NYT) 681 words
BY now, most drivers know the crucial distinctions between rear drive and front drive. But even experts' heads may spin when they try to account for the many variations of all-wheel-drive (and four-wheel-drive) systems.
For consumers, the key questions are these: How many wheels are driving the car, and for how much of the time?
In a conventional car or truck, with either front or rear drive, when one wheel slips the other loses traction, too. If one wheel is stuck in snow, its counterpart also spins. This problem is solved by limited-slip differentials -- which have been optional on many cars for decades -- that link both driving wheels and split the available torque between them. In the same snowy situation, the unstuck wheel may gain enough traction to pull the car free.
With conventional ''part time'' four-wheel drive -- the type that has long been optional on trucks -- a transfer case locks the front and rear axles together. This allows greater traction since the power is sent to both sets of wheels. But unless there are limited-slip differentials in both axles, traction is really available at only two wheels, one in the front and one in the rear. Thus, if the vehicle's right wheels are on ice, it won't be able to move. (Part-time systems also have a low gear range that multiplies the torque for challenging low-speed conditions.)
Unless the transfer case between the front and rear axles also has some form of limited-slip device, the vehicle cannot be driven on dry pavement without the wheels binding unpleasantly on corners. Thus, ''full time'' four-wheel-drive systems must permit some slippage, so the front and rear wheels can rotate at different speeds when cornering. Manufacturers accomplish this in different ways -- some mechanical, some hydraulic, some electronic. But unless the differentials between the driving wheels also have a limited-slip design, all the driving wheels will not be able to provide traction in challenging conditions.
The most sophisticated and expensive four-wheel-drive systems are often called permanent. When sensors detect that any wheel is slipping, torque is directed to the other wheels. No action is required by the driver (except to engage the low range for arduous off-roading).
All-wheel drive works much the same way, except it lacks the low gear range. And while some trucks and S.U.V.'s let the driver switch systems -- to select full-time or part-time settings, or regular two-wheel drive -- an all-wheel-drive car does not offer any choice.
These vehicles do not have a transfer case, but they have a center differential that can allow the front and rear axles to operate independently, or some sort of clutch or hydraulic system that distributes the torque as needed to provide extra traction.
The design and function of this center differential are the main differences among the all-wheel-drive systems available today. Audi's quattro system, for instance, drives both the front and rear axles all the time, and in normal driving the torque is split equally between front and rear. BMW, which wants all of its cars to have its familiar rear-drive feel, splits the torque in its all-wheel-drive models with a bias toward the back wheels. Others, like the Honda CR-V, power the front wheels all the time unless they start to slip, and then torque is sent to the rear wheels.
In any good design, the power is shifted transparently, so the driver is never aware that the vehicle is compensating for a variation in traction.
More manufacturers are taking advantage of the near-universality of antilock brakes, opting to forgo the rather complicated variable center differential, or the differentials on front or rear axles. Instead, they rely on the brakes and traction control to slow the slipping wheel and transfer power to wheels with traction. Mercedes's 4Matic system, and BMW's all-wheel drive, work this way.
Those companies' electronic controls also permit, in theory, a single wheel to propel the car. Thus, if three wheels were stuck but one had traction, all would not be hopeless.
By BOB KNOLL (NYT) 681 words
BY now, most drivers know the crucial distinctions between rear drive and front drive. But even experts' heads may spin when they try to account for the many variations of all-wheel-drive (and four-wheel-drive) systems.
For consumers, the key questions are these: How many wheels are driving the car, and for how much of the time?
In a conventional car or truck, with either front or rear drive, when one wheel slips the other loses traction, too. If one wheel is stuck in snow, its counterpart also spins. This problem is solved by limited-slip differentials -- which have been optional on many cars for decades -- that link both driving wheels and split the available torque between them. In the same snowy situation, the unstuck wheel may gain enough traction to pull the car free.
With conventional ''part time'' four-wheel drive -- the type that has long been optional on trucks -- a transfer case locks the front and rear axles together. This allows greater traction since the power is sent to both sets of wheels. But unless there are limited-slip differentials in both axles, traction is really available at only two wheels, one in the front and one in the rear. Thus, if the vehicle's right wheels are on ice, it won't be able to move. (Part-time systems also have a low gear range that multiplies the torque for challenging low-speed conditions.)
Unless the transfer case between the front and rear axles also has some form of limited-slip device, the vehicle cannot be driven on dry pavement without the wheels binding unpleasantly on corners. Thus, ''full time'' four-wheel-drive systems must permit some slippage, so the front and rear wheels can rotate at different speeds when cornering. Manufacturers accomplish this in different ways -- some mechanical, some hydraulic, some electronic. But unless the differentials between the driving wheels also have a limited-slip design, all the driving wheels will not be able to provide traction in challenging conditions.
The most sophisticated and expensive four-wheel-drive systems are often called permanent. When sensors detect that any wheel is slipping, torque is directed to the other wheels. No action is required by the driver (except to engage the low range for arduous off-roading).
All-wheel drive works much the same way, except it lacks the low gear range. And while some trucks and S.U.V.'s let the driver switch systems -- to select full-time or part-time settings, or regular two-wheel drive -- an all-wheel-drive car does not offer any choice.
These vehicles do not have a transfer case, but they have a center differential that can allow the front and rear axles to operate independently, or some sort of clutch or hydraulic system that distributes the torque as needed to provide extra traction.
The design and function of this center differential are the main differences among the all-wheel-drive systems available today. Audi's quattro system, for instance, drives both the front and rear axles all the time, and in normal driving the torque is split equally between front and rear. BMW, which wants all of its cars to have its familiar rear-drive feel, splits the torque in its all-wheel-drive models with a bias toward the back wheels. Others, like the Honda CR-V, power the front wheels all the time unless they start to slip, and then torque is sent to the rear wheels.
In any good design, the power is shifted transparently, so the driver is never aware that the vehicle is compensating for a variation in traction.
More manufacturers are taking advantage of the near-universality of antilock brakes, opting to forgo the rather complicated variable center differential, or the differentials on front or rear axles. Instead, they rely on the brakes and traction control to slow the slipping wheel and transfer power to wheels with traction. Mercedes's 4Matic system, and BMW's all-wheel drive, work this way.
Those companies' electronic controls also permit, in theory, a single wheel to propel the car. Thus, if three wheels were stuck but one had traction, all would not be hopeless.
02-06-2002, 11:51 AM
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Max front/rear allocation
I've always been under the impression that the Audi center differential sends up to a max of 2/3 power to the front or back wheels, not 100%. Trying to remember where I read this, though it seemed authoritative at the time.
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02-06-2002, 12:38 PM
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The quattro torque split is incorrect...
The Torsen in current quattros will do something like a 35/65/35 split. Older models were closer to 25/75/25. The old V8 would do 0/100/0. Also, I'm not sure if all new quattros have 4 wheel EDL. The allroad does, but my '99.5 A4 doesn't.