FC
01-06-2004, 10:58 AM
The board seems very slow today and I have become generally confused about these topics since I started researching SUV's.
Perhaps we could all benefit from the technical gurus that lurk here and help explain the basics of these systems as well as diffrences and advantages.
I'll start with what I know and ask about what I don't.
First differentials:
Most cars have open differentials. These allows for wheels to spin at different speeds but will render the wheel with more traction useless once the wheel with the least traction starts spinning. To remedy this mechanically, limited slip differentials are available. This is the best solution. To get past this on the cheap, manufacturers offer electronic systems that will apply the brakes on the slipping wheel to get the one with traction to move. Basically, "fooling" the open differential by making the slipping wheel seem to have more traction, and so providing power to the non-slipping wheel. It's a crappy way to pretend you have an LSD but it works for the average Joe. For performance driving, you want a real LSD.
This makes perfect sense to me.
How this applies to 4wd/AWD systems.
Most current AWD systems (MB/BMW/Audi) employ a center differential (sometimes open, but usually an LSD) to send power to the front and rear differentials. These both tend to be open differentials. Again, most advanced AWD systems use some kind of electronic control to apply the brakes and send power from side to side and front and back. These differentials allow AWD to be permanent.
By contrast, not all 4WD systems are full time. In this case power is sent to the rear as in a regular RWD car, and when necessary a transfer case is used to select 4wd. This will send power to the front and rear. On some older systems -trucks- (and maybe even new ones) one can manually lock the hubs and force the wheels to spin at the same speed. Of course this is terrible for regular driving.
No SUV's have manually locking hubs. Some allow to manually lock the fornt/center/rear differentials independently, which in effect accomplishes the same thing. It locks all wheels and sends 25% of power to each wheel all the time no matter what.
But most SUV's with 4wd don't offer this feature. So one of my questions is, for example in the case of a jeep GC (which has solid front and rear axles) what happens when you put in 4low? The center differential locks, I know that. But are the front and rear diff's open? If so, you really only have at most two wheels with traction.
Jeep's most expensive model offers Vari-Lok progressive locking front and rear diffs. Sending up to 100% of power to one wheel mechanically. But what happens if you don't have these? Are these fancy differentials just for the full-time 4wd system?
Let's now look at the Nissan Pathfinder and Toyota 4runner/Land Cruiser. They have solid rear axles but front independent suspensions. How would their 4wd systems work?
What is the advantage of a solid axle if an independent suspension can still offer a locking differential? Is it just that it is a more heavy duty off-road system? Is there the assumed understanding that every solid axle nowadays offers automatically locking hubs when ordered with 4wd? :?
I'm confused. Thanks!
Perhaps we could all benefit from the technical gurus that lurk here and help explain the basics of these systems as well as diffrences and advantages.
I'll start with what I know and ask about what I don't.
First differentials:
Most cars have open differentials. These allows for wheels to spin at different speeds but will render the wheel with more traction useless once the wheel with the least traction starts spinning. To remedy this mechanically, limited slip differentials are available. This is the best solution. To get past this on the cheap, manufacturers offer electronic systems that will apply the brakes on the slipping wheel to get the one with traction to move. Basically, "fooling" the open differential by making the slipping wheel seem to have more traction, and so providing power to the non-slipping wheel. It's a crappy way to pretend you have an LSD but it works for the average Joe. For performance driving, you want a real LSD.
This makes perfect sense to me.
How this applies to 4wd/AWD systems.
Most current AWD systems (MB/BMW/Audi) employ a center differential (sometimes open, but usually an LSD) to send power to the front and rear differentials. These both tend to be open differentials. Again, most advanced AWD systems use some kind of electronic control to apply the brakes and send power from side to side and front and back. These differentials allow AWD to be permanent.
By contrast, not all 4WD systems are full time. In this case power is sent to the rear as in a regular RWD car, and when necessary a transfer case is used to select 4wd. This will send power to the front and rear. On some older systems -trucks- (and maybe even new ones) one can manually lock the hubs and force the wheels to spin at the same speed. Of course this is terrible for regular driving.
No SUV's have manually locking hubs. Some allow to manually lock the fornt/center/rear differentials independently, which in effect accomplishes the same thing. It locks all wheels and sends 25% of power to each wheel all the time no matter what.
But most SUV's with 4wd don't offer this feature. So one of my questions is, for example in the case of a jeep GC (which has solid front and rear axles) what happens when you put in 4low? The center differential locks, I know that. But are the front and rear diff's open? If so, you really only have at most two wheels with traction.
Jeep's most expensive model offers Vari-Lok progressive locking front and rear diffs. Sending up to 100% of power to one wheel mechanically. But what happens if you don't have these? Are these fancy differentials just for the full-time 4wd system?
Let's now look at the Nissan Pathfinder and Toyota 4runner/Land Cruiser. They have solid rear axles but front independent suspensions. How would their 4wd systems work?
What is the advantage of a solid axle if an independent suspension can still offer a locking differential? Is it just that it is a more heavy duty off-road system? Is there the assumed understanding that every solid axle nowadays offers automatically locking hubs when ordered with 4wd? :?
I'm confused. Thanks!