4x4ForLife wrote:Ah ha. Now I get it. I am really technically/mechanically challenged and have never managed to get my head around the whole differential thing. Now does anyone know how a Limited Slip Diff works, since that is what my Tata Xenon has?
This may help explain:
4WD - four-wheel drive
Also known as part-time all-wheel drive, this system has an open differential on the front and rear axle and a transfer case on the output from the gearbox. Typically 4WD is normally driving the rear axle with the front axle only coming into play in 4WD mode. The transfer case is the device that splits the torque between the front and rear axles. It typically has some sort of selectable internal differential or viscous coupling to allow the front and rear drives to turn at different speeds if need be. Some trucks and SUVs have a selector with 2H, 4H and 4L on it - it looks like a second gear shift. This is actually controlling how the front and rear outputs of the transfer case get locked together. In 2H mode (2-wheel drive, high), it essentially disconnects the front output completely and only drives the rear axle. In 4H mode (4-wheel drive, high), it engages the front output via the viscous coupling so that the axles can turn at different speeds, and now sends torque to both open differentials. In 4L mode (4-wheel drive, low) it engages a second set of reduction gears and locks the front and rear axles together so they must spin at the same speed. This would be bad for on-road driving because it does not allow any difference in speed between the front and rear wheels, so you'd often get dragging and slipping which would make the car essentially unsafe to drive. However, locking everything together like this and reducing the gear ratio makes perfect sense for off-roading, which is why it's an option. However, with open differentials, it's still entirely possible to get stuck with a 4WD vehicle. If you're off-roading and the front-left and rear-right wheels both leave the ground together (for example), then the torque will all be sent to those wheels and they'll spin helplessly in the air. Locking, limited-slip or Torsen differentials solve this but add weight, complexity and cost to the system.
Locking hubs On older 4WD systems, the front wheels could only be engaged to the transfer case by locking hubs. Essentially the transfer case was always sending torque to the front driveshaft and had no viscous coupling. To get into 4WD mode, the driver had to stop and get out, and lock the front wheels to the axles so they could be driven. In newer 4WD systems, the lockable hubs are still present on some models, but are designed more for mechanical sympathy and fuel economy than anything else. With the hubs onlocked, the whole front part of the drive system isn't being dragged along for the ride, which causes mechanical wear and a drop in fuel economy.
AWD - all-wheel drive type 1
Finally, all-wheel drive or full-time 4WD. Found mostly on sportier cars, but also on some SUVs, there are two types of AWD, both designed to try to overcome the problems with 4WD. The simplest form has two open differentials - one on each axle - and a viscous coupling between. The engine drives the gearbox which drives two output shafts. One goes to the front open differential and the other goes to the viscous coupling, the output of which is connected to the rear open differential. Under normal conditions, this type of AWD system functions exactly like a 2WD car, driving only the front axle (unlike a 4WD which normally drives the rear axle). Lower end Subarus and some of the Honda trucks use this system. The front wheels turn at a certain rate, and the rear wheels are dragged along for the ride. Both halves of the viscous coupling are spinning at the same speed so no torque is sent to the rear axle. If the front wheels begin to slip and spin, the input to the front of the viscous coupling begins to spin faster than the rear and because of its torque-converter-like design, this causes the rear output to want to speed up. At this point, the drivetrain is now transferring torque to the rear axle and the car starts to function in AWD mode. Actally, AWD is a bit of a misnomer at this point, because unless the car has limited-slip differentials front and rear, it's still only really driving two wheels in this mode - the one on the front and the one on the rear axles that have the most traction. That leads us nicely on to.....
AWD - all-wheel drive type 2
This is the other type of AWD found on higher-end Subarus, rally cars, expensive sports sedans and such. Very similar to the type 1 AWD, it replaces the viscous coupling with a Torsen differential, and replaces the open differentials front and rear with either Torsen or limited-slip differentials. This is the only true all-wheel-drive system because it will always drive all four wheels. It's also bloody expensive and it saps gas-mileage because of all the extra drag induced in the driveline. But then if you're into performance off-roading, gas-mileage really isn't your primary concern.