Awd, What's It Doing?

[tealboy]

I guess i don't understand how AWD works. My vehicle dipped into an unexpected hole and the rear hitch got hung up to the point I couldn't get traction at the rear tires. I thought the front wheels would have kicked in to pull me out, but they didn't seem to do that. The rears would spin fast, but the fronts were not spinning, even when i stomped the gas pedal. However, once i got out of the hole and tried to spin the tires on the soft surface/gravel, the fronts were grabbing and maintaining traction.

i guess i don't really understand how awd works, b/c the ditch puzzled me.

[Canadian]

Well, in part it depends on what kind of vehicle you were stuck with. Different makes/models use different systems and approaches to how AWD works.

[lexdog]

AWD System one (true all wheel drive - or full time symmetric AWD) has a conventional differential inside the transfer case - each of the wheels gets about 25% of the torque as long as traction is equal. However, the center diff cannot be mechanically locked.

To prevent a complete loss of traction when one wheel or one axle would spin, a viscous coupling or a similar device like a Haldex coupling will try to "glue" both driveshafts together to keep enough torque flowing to the axle with traction. Works kinda OK on slippery pavement when the vehicle has already sufficient momentum and the connecting device has to kick in very infrequently. Off-road or in other situations with slow speed and high demand for torque the glue box (viscous coupling or Haldex etc.) is overstressed and fails to deliver the needed torque. High torque transfers and continous use make especially viscous couplings fail. Haldex units are much more reliable but cannot satisfy the constant high demand for torque at all wheels either.

[tealboy]

that was interesting feedback, lexdog, thank you. the vehicle i was driving and referring to is the cadillac escalade. however, i own a rx300 awd that could have equally been in this same situation. do you know if there is any difference between these two systems? since mine says vsc on the side, i know which system my RX has. I got the awd for the occassional offroad use, but that's really for dirt roads that have a thin layer of slippery mud, not for holes and drop offs like today.

[Rx330driver]

THey probably do use the same concept, of how it kicks in when theres slipping. Your back tires were technically not slipping, because they werent touching the ground, thereby not activating the front tires.

[lexdog]

Im not real familar with GM's AWD system but I do know that most AWD system detect slippage when 1 wheel his a higher rpm then others. Once this this happens when the computer tries to transfer power to the wheels with lower rpms.