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> Tesla has an ordinary open differential but will apply the brake on an overspeeding wheel. Is that different than the traction control used by other automak

> Tesla has an ordinary open differential but will apply the brake on an overspeeding wheel. Is that different than the traction control used by other automakers? > Tesla's all wheel drive system has a separate motor for the front and rear wheels. That is very cool. And it sounds much more complex - I wonder if the complexity is worth the benefit.

CarVac 10y ago
Separate motors for front and rear wheels is far less complex than adding more moving parts. For AWD, you need a driveshaft and a differential. The differential has a lot of parts, there are a lot of bearings that sap power, and it adds a lot of weight. Using separate electric motors for front and rear instead increases the available power, enables unlimited torque ratios from all-forward to all-rear, and has many fewer moving parts.
- hackuser 10y ago
  Thanks. But isn't coordinating the motors complex? If the power delivered to the wheels is out of sync for a moment, I would imagine it could be catastrophic, and the system has to deal with variables of turns, traction, acceleration/braking, etc.
  - CarVac 10y ago
    Why would it be? It's not like trying to coordinate rowers in crew; you have full, absolute, instantaneously-responding control over the motors, as well as information on exactly how fast they're going, how much force they're applying, how fast each individual wheel is turning, what the steering angle is, etc. And then what's controlling it is a computer, not a human. Stability control, which already exists in every car, deals with all of those variables and is able to correct for driving with judicious braking (or release of the brake) on individual wheels, even when some or all may be slipping or locked up, the car going sideways, on varied surfaces. Adding more things for a computer to control makes it easier, if anything.
  - tapeman 10y ago
    Keeping the front and rear motors in sync is a cake walk compared to controlling the brakes for traction control. "If the power delivered to the wheels is out of sync for a moment, I would imagine it could be catastrophic" The control software is probably updating every 3-15 milliseconds, so even if the software had a massive hiccup, it would have to last for multiple frames for it to actually get translated to the pavement. The drivetrains of cars are actually pretty mushy. Everything's mounted on bushings, there's backlash in the gears, and tires are rubber. All of this adds together to buffer out any kind of spikes. Much like a capacitor and resistor can be used to buffer electricity. So if, say, the front wheel was 5% underpowered for a frame, then all that would happen is the gears in the front differential would go slack for a few milliseconds. If on the next frame, the power was corrected, the differential would go back to being taut. If it was just 5% underpowered, you likely wouldn't even feel this. If the front motor totally locked up for one frame, you'd still probably only feel a little jitter.

CarVac 10y ago

Separate motors for front and rear wheels is far less complex than adding more moving parts. For AWD, you need a driveshaft and a differential. The differential has a lot of parts, there are a lot of bearings that sap power, and it adds a lot of weight. Using separate electric motors for front and rear instead increases the available power, enables unlimited torque ratios from all-forward to all-rear, and has many fewer moving parts.

hackuser 10y ago
Thanks. But isn't coordinating the motors complex? If the power delivered to the wheels is out of sync for a moment, I would imagine it could be catastrophic, and the system has to deal with variables of turns, traction, acceleration/braking, etc.
- CarVac 10y ago
  Why would it be? It's not like trying to coordinate rowers in crew; you have full, absolute, instantaneously-responding control over the motors, as well as information on exactly how fast they're going, how much force they're applying, how fast each individual wheel is turning, what the steering angle is, etc. And then what's controlling it is a computer, not a human. Stability control, which already exists in every car, deals with all of those variables and is able to correct for driving with judicious braking (or release of the brake) on individual wheels, even when some or all may be slipping or locked up, the car going sideways, on varied surfaces. Adding more things for a computer to control makes it easier, if anything.
- tapeman 10y ago
  Keeping the front and rear motors in sync is a cake walk compared to controlling the brakes for traction control. "If the power delivered to the wheels is out of sync for a moment, I would imagine it could be catastrophic" The control software is probably updating every 3-15 milliseconds, so even if the software had a massive hiccup, it would have to last for multiple frames for it to actually get translated to the pavement. The drivetrains of cars are actually pretty mushy. Everything's mounted on bushings, there's backlash in the gears, and tires are rubber. All of this adds together to buffer out any kind of spikes. Much like a capacitor and resistor can be used to buffer electricity. So if, say, the front wheel was 5% underpowered for a frame, then all that would happen is the gears in the front differential would go slack for a few milliseconds. If on the next frame, the power was corrected, the differential would go back to being taut. If it was just 5% underpowered, you likely wouldn't even feel this. If the front motor totally locked up for one frame, you'd still probably only feel a little jitter.