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I prefer to have an optional locking differential on the drive wheels for snow. In my experience, the most common problem with snow is getting enough traction

I prefer to have an optional locking differential on the drive wheels for snow. In my experience, the most common problem with snow is getting enough traction to move out of parking spot or up a steep incline. Like most AWD vehicles, the AWD Tesla has an open differential which does not help much in these scenarios. When traction gets dicey it behaves like a 2WD with torque spread between one front and one rear wheel.

imeron 8y ago
It has a diff-like behavior by actively braking the wheels from software. See Youtube video https://youtu.be/xbaNQQaFrnE https://youtu.be/xbaNQQaFrnE
- fingerlocks 8y ago
  Thanks! My side hobby is restoring & retrofitting 80s vehicles, so I had not considered new tech like this. I just assumed it worked like every other open diff.
  - pedrocr 8y ago
    That's how every other open-diff car already works. Everyone does that. EVs just have the extra advantage that they're much more precise at applying power so the traction control can be much better.
    - fingerlocks 8y ago
      I was referring to the brake based traction control system, not the differential
      - pedrocr 8y ago
        That's also what I mean of course. All modern AWD systems and even just FWD/RWD cars use the ABS to do traction control with open-diffs. The base AWD system in a Subaru is three open-diffs and then ABS braking is used on any wheel that's slipping to make sure the torque still reaches the other wheels.
        fingerlocks 8y ago
        Right, as I mentioned, I'm an 80's VW van enthusiast. I've transplanted subaru engines into VW vans, but I always clip the unused sensors (like traction control) from the wiring harness. I haven't touched anything newer than 2005 yet. So, despite "everyone doing it" by your words, I was just unaware of how it worked because it's not relevant to me. My VW syncro van has both forward and rear locking differentials anyway.
- londons_explore 8y ago
  I'd like to see high speed video of this working. Friction is non-linear so you're going to need very fast feedback loops on the brakes to simulate a locked differential. One can't just apply a fixed amount of braking to every wheel and have stuff magically work out.
  - sliken 8y ago
    Yes. On normal cars this is achieves by sensors and brakes that I believe work at 100 times a second and up. Tesla have the advantage that they have similar control over acceleration. This technology has been common for years, and early on it had lots of downsides, but it's most recent versions is quite good. It's becoming more and more popular, even on enthusiast cars. The main issue is while limited slip differentials do work, they are expensive, add weight, and they add friction. So the software solution gets you better MPG, better HP, and more control. There's many different names for it, traction control, torque vectoring, traction assist, etc. Even Subaru who bet the heaviest on AWD (selling only AWD until they sold the BRZ), and had one of the best respected AWD systems is moving to open differential with the brake assist, on at least some of their cars. Not sure about the STI and WRX though.
    - bluedino 8y ago
      It also works just fine for Toyota's full size pickup, the Tundra. No 'real' locking differential but it can climb trhough hills/rocks just fine
- justtopost 8y ago
  This is great unless you drive you car hard. Then the system overheats your brakes before you even try to slow down. No thanks.
sliken 8y ago
Indeed, most cars are single wheel drive (FWD or RWD with an open differential). Some more expensive sporty cars have limited slip differentials, mostly to avoid inside drive wheels spinning in sharp corners. It helps in the snow as well of course. My Subaru WRX had a limited split in the center and rear, so it had three wheel drive when slippery. The higher end STI had an option for a front limited slip differential. Apparently when rallying on tight courses they would pick three wheel drive because it allows rotating around the inner front wheel for very sharp turns. It's great fun, plant a wheel (literally motionless), the car spins around it, until you let go of the wheel and the car launches in that direction. On more open courses the rally drivers would select the four wheel drive to get the most of the traction when going mostly straight. The Tesla AWD is actually 4wd, but under software control. I wouldn't say as nice as a locking differential, but pretty close and able to get some power to all four wheels.
- fingerlocks 8y ago
  The Subaru differential system is really a beautiful thing all on it’s own. It’s funny, just after posting my comment I thought “should have mentioned the limited slip for the subie fans”. And to my chagrin, I’m deep in a worm hole learning a whole lot about the Tesla AWD system tonight.
- lagadu 8y ago
  LSDs are no longer the domain of expensive sports cars though, nowadays they made it all the way to the cheaper cars, like the hyundai i30n and the mk8 fiesta st. It's pretty cool.
  - gambiting 8y ago
    But those are still the "sport" versions of pedestrian cars. No one is fitting a limited slip diff to a base model of the i30 or the fiesta. The i30n will be 1% of all i30 sales if even that.

imeron 8y ago

It has a diff-like behavior by actively braking the wheels from software. See Youtube video https://youtu.be/xbaNQQaFrnE https://youtu.be/xbaNQQaFrnE

fingerlocks 8y ago
Thanks! My side hobby is restoring & retrofitting 80s vehicles, so I had not considered new tech like this. I just assumed it worked like every other open diff.
- pedrocr 8y ago
  That's how every other open-diff car already works. Everyone does that. EVs just have the extra advantage that they're much more precise at applying power so the traction control can be much better.
  - fingerlocks 8y ago
    I was referring to the brake based traction control system, not the differential
    - pedrocr 8y ago
      That's also what I mean of course. All modern AWD systems and even just FWD/RWD cars use the ABS to do traction control with open-diffs. The base AWD system in a Subaru is three open-diffs and then ABS braking is used on any wheel that's slipping to make sure the torque still reaches the other wheels.
      - fingerlocks 8y ago
        Right, as I mentioned, I'm an 80's VW van enthusiast. I've transplanted subaru engines into VW vans, but I always clip the unused sensors (like traction control) from the wiring harness. I haven't touched anything newer than 2005 yet. So, despite "everyone doing it" by your words, I was just unaware of how it worked because it's not relevant to me. My VW syncro van has both forward and rear locking differentials anyway.
londons_explore 8y ago
I'd like to see high speed video of this working. Friction is non-linear so you're going to need very fast feedback loops on the brakes to simulate a locked differential. One can't just apply a fixed amount of braking to every wheel and have stuff magically work out.
- sliken 8y ago
  Yes. On normal cars this is achieves by sensors and brakes that I believe work at 100 times a second and up. Tesla have the advantage that they have similar control over acceleration. This technology has been common for years, and early on it had lots of downsides, but it's most recent versions is quite good. It's becoming more and more popular, even on enthusiast cars. The main issue is while limited slip differentials do work, they are expensive, add weight, and they add friction. So the software solution gets you better MPG, better HP, and more control. There's many different names for it, traction control, torque vectoring, traction assist, etc. Even Subaru who bet the heaviest on AWD (selling only AWD until they sold the BRZ), and had one of the best respected AWD systems is moving to open differential with the brake assist, on at least some of their cars. Not sure about the STI and WRX though.
  - bluedino 8y ago
    It also works just fine for Toyota's full size pickup, the Tundra. No 'real' locking differential but it can climb trhough hills/rocks just fine
justtopost 8y ago
This is great unless you drive you car hard. Then the system overheats your brakes before you even try to slow down. No thanks.

sliken 8y ago

Indeed, most cars are single wheel drive (FWD or RWD with an open differential). Some more expensive sporty cars have limited slip differentials, mostly to avoid inside drive wheels spinning in sharp corners. It helps in the snow as well of course. My Subaru WRX had a limited split in the center and rear, so it had three wheel drive when slippery. The higher end STI had an option for a front limited slip differential. Apparently when rallying on tight courses they would pick three wheel drive because it allows rotating around the inner front wheel for very sharp turns. It's great fun, plant a wheel (literally motionless), the car spins around it, until you let go of the wheel and the car launches in that direction. On more open courses the rally drivers would select the four wheel drive to get the most of the traction when going mostly straight. The Tesla AWD is actually 4wd, but under software control. I wouldn't say as nice as a locking differential, but pretty close and able to get some power to all four wheels.

fingerlocks 8y ago
The Subaru differential system is really a beautiful thing all on it’s own. It’s funny, just after posting my comment I thought “should have mentioned the limited slip for the subie fans”. And to my chagrin, I’m deep in a worm hole learning a whole lot about the Tesla AWD system tonight.
lagadu 8y ago
LSDs are no longer the domain of expensive sports cars though, nowadays they made it all the way to the cheaper cars, like the hyundai i30n and the mk8 fiesta st. It's pretty cool.
- gambiting 8y ago
  But those are still the "sport" versions of pedestrian cars. No one is fitting a limited slip diff to a base model of the i30 or the fiesta. The i30n will be 1% of all i30 sales if even that.