7 ms·

> On the other hand, different axles don’t have to use the same gearing ratio. That’s how dual motor Teslas work - the front wheels are geared for higher speeds

by cure 6y ago

> On the other hand, different axles don’t have to use the same gearing ratio. That’s how dual motor Teslas work - the front wheels are geared for higher speeds, and more power is shifted towards those as you go faster. No gear-shifting required. Yep. Alternatively, the Porsche Taycan has an 2-speed transmission gearbox (cf. https://www.caranddriver.com/news/a28903274/porsche-taycan-transmission/ https://www.caranddriver.com/news/a28903274/porsche-taycan-t...). I don't think gearboxes are going to become mainstream in EVs, with the exception of performance vehicles like this one.

mrfusion 6y ago
Do the Tesla’s still have differentials then? It would be cool to eliminate those two but then you’d need a motor for each wheel I guess.
- Negitivefrags 6y ago
  They do for all the models currently on the road. In the future the Roadster and the Cybertruck both have 3 motors. The rear wheels both have one each, so presumably they can avoid the differential there.
- chaboud 6y ago
  Rivian has an independent 4-motor setup. It's a compelling case of extra parts allowing for simplicity. It also allows for the ever-entertaining tank turn: https://www.youtube.com/watch?v=yzwM8KE2L3I https://www.youtube.com/watch?v=yzwM8KE2L3I
- sandworm101 6y ago
  Differentials still have a place. They allow wheels to be perfectly balanced when negotiating corners. That can be done with independent electric motors, but never perfectly. Lot of little factors have to be considered in the software. For instance, as a car corners weight shifts, this causes the diameters of pneumatic tires to change slightly. A mechanical differential handles this tiny effect seamlessly. Independent motors on each wheel would have to sense, calculate and react to this via software. That is just one effect. There are also uneven road surfaces to deal with. Any slight error would result in one wheel dragging or trying to accelerate, resulting in instability or at least chewing through tire rubber. Differentials are cheap and reliable, most going for the lifetime of a car without any issue or even real servicing. They are likely the cheaper option than running multiple motors.
  - mrfusion 6y ago
    Good point. What about simply powering only the outer wheel? It could look at the steering position to decide when to do that?
    - sandworm101 6y ago
      So long as you are happy giving up 1/2 of your traction, that would only solve corners if the steering wheel is moving. Say your right wheel hits a bump. Even with zero steering input that right wheel is traveling slightly further than the left wheel. (Up and over the bump is longer than the strait line driven by the left wheel.) There also also times when the steering wheel moves but the car remains strait (crosswinds). The further one looks down this rabbit hole, the more appealing a simple mechanical differential becomes.
elihu 6y ago
I don't expect stickshift EVs to be common either, but still I think it's surprising that no one has made a non-luxury-car EV that isn't direct drive. Manual transmissions seem like a pretty mature technology at this point. For any car company that already makes a manual transmission car it would be easy to do. They might change the design a little (maybe 3 speeds instead of 5, and use sturdier gears and different ratios). Clearly you don't need a transmission in an EV, and leaving it out is simpler and less maintenance, but still... if it can be done and it makes for a (in some ways) better or more fun car, then it's a shame no one is doing it yet. I'm currently working on converting a Mazda RX-8 to electric, keeping the transmission and clutch. It'll be interesting to see what it's like to drive. IIRC, the motor (Netgain Hyper-9 144 volt dual-ended version) has a max RPM of 8000, which is close to the 9000 rpm redline of the rotary it replaces. The motor is a lot less powerful than what you'd find in a typical Tesla (which I attribute mostly to it being designed to run at a lower voltage), but having a gearshift should make up for at least some of the difference.
- jwalton 6y ago
  I have a friend who dropped a turbocharged engine out of an old 80s BMW into an RX-8. He ended up having to cut a hole in the firewall and completely redesigning the rack and pinion because the old rack would have gone through the middle of the larger BMW engine. He kept the original gearbox and was always worried the low revving high torque BMW engine was going to murder it. :P
  - elihu 6y ago
    Thankfully I don't have that problem. The Netgain motor is pretty big as motors go and it'll be close to the power steering unit, but still I think I've got enough room there. Most of the work so far has been making battery boxes. I've got one under each rear seat where the gas tank used to be, one in the front down low where the radiator used to be, a smaller one above it, and another that goes over the motor (which raises the CG more than I'd like, but the other batteries that basically as low as they can be probably offset it). Weight balance should be about the same. I'll have to do some welding on the back seats, because they're too low for the batteries to fit. Sacrificing about four inches of foam will make for an uncomfortable seat, but it should at least look normal.
audunw 6y ago
> I don't think gearboxes are going to become mainstream in EVs, with the exception of performance vehicles like this one. I don't think transmissions will be the norm in performance vehicles either, since Tesla Roadster gen 2 will not have it, and Rimac seems to have moved away from it for Concept Two as well. Teslas solution is to optimize the front and rear motors for different speeds. There is also an experimental motor that can change "gears" by changing how it drives the windings in the motors. I don't remember the name right now. Point is there are a few solutions that are simpler and lighter than adding a two-speed gearbox, with relatively little compromise.

by cure 6y ago

mrfusion 6y ago

Do the Tesla’s still have differentials then? It would be cool to eliminate those two but then you’d need a motor for each wheel I guess.

Negitivefrags 6y ago
They do for all the models currently on the road. In the future the Roadster and the Cybertruck both have 3 motors. The rear wheels both have one each, so presumably they can avoid the differential there.
chaboud 6y ago
Rivian has an independent 4-motor setup. It's a compelling case of extra parts allowing for simplicity. It also allows for the ever-entertaining tank turn: https://www.youtube.com/watch?v=yzwM8KE2L3I https://www.youtube.com/watch?v=yzwM8KE2L3I
sandworm101 6y ago
Differentials still have a place. They allow wheels to be perfectly balanced when negotiating corners. That can be done with independent electric motors, but never perfectly. Lot of little factors have to be considered in the software. For instance, as a car corners weight shifts, this causes the diameters of pneumatic tires to change slightly. A mechanical differential handles this tiny effect seamlessly. Independent motors on each wheel would have to sense, calculate and react to this via software. That is just one effect. There are also uneven road surfaces to deal with. Any slight error would result in one wheel dragging or trying to accelerate, resulting in instability or at least chewing through tire rubber. Differentials are cheap and reliable, most going for the lifetime of a car without any issue or even real servicing. They are likely the cheaper option than running multiple motors.
- mrfusion 6y ago
  Good point. What about simply powering only the outer wheel? It could look at the steering position to decide when to do that?
  - sandworm101 6y ago
    So long as you are happy giving up 1/2 of your traction, that would only solve corners if the steering wheel is moving. Say your right wheel hits a bump. Even with zero steering input that right wheel is traveling slightly further than the left wheel. (Up and over the bump is longer than the strait line driven by the left wheel.) There also also times when the steering wheel moves but the car remains strait (crosswinds). The further one looks down this rabbit hole, the more appealing a simple mechanical differential becomes.

elihu 6y ago

I don't expect stickshift EVs to be common either, but still I think it's surprising that no one has made a non-luxury-car EV that isn't direct drive. Manual transmissions seem like a pretty mature technology at this point. For any car company that already makes a manual transmission car it would be easy to do. They might change the design a little (maybe 3 speeds instead of 5, and use sturdier gears and different ratios). Clearly you don't need a transmission in an EV, and leaving it out is simpler and less maintenance, but still... if it can be done and it makes for a (in some ways) better or more fun car, then it's a shame no one is doing it yet. I'm currently working on converting a Mazda RX-8 to electric, keeping the transmission and clutch. It'll be interesting to see what it's like to drive. IIRC, the motor (Netgain Hyper-9 144 volt dual-ended version) has a max RPM of 8000, which is close to the 9000 rpm redline of the rotary it replaces. The motor is a lot less powerful than what you'd find in a typical Tesla (which I attribute mostly to it being designed to run at a lower voltage), but having a gearshift should make up for at least some of the difference.

jwalton 6y ago
I have a friend who dropped a turbocharged engine out of an old 80s BMW into an RX-8. He ended up having to cut a hole in the firewall and completely redesigning the rack and pinion because the old rack would have gone through the middle of the larger BMW engine. He kept the original gearbox and was always worried the low revving high torque BMW engine was going to murder it. :P
- elihu 6y ago
  Thankfully I don't have that problem. The Netgain motor is pretty big as motors go and it'll be close to the power steering unit, but still I think I've got enough room there. Most of the work so far has been making battery boxes. I've got one under each rear seat where the gas tank used to be, one in the front down low where the radiator used to be, a smaller one above it, and another that goes over the motor (which raises the CG more than I'd like, but the other batteries that basically as low as they can be probably offset it). Weight balance should be about the same. I'll have to do some welding on the back seats, because they're too low for the batteries to fit. Sacrificing about four inches of foam will make for an uncomfortable seat, but it should at least look normal.

audunw 6y ago

> I don't think gearboxes are going to become mainstream in EVs, with the exception of performance vehicles like this one. I don't think transmissions will be the norm in performance vehicles either, since Tesla Roadster gen 2 will not have it, and Rimac seems to have moved away from it for Concept Two as well. Teslas solution is to optimize the front and rear motors for different speeds. There is also an experimental motor that can change "gears" by changing how it drives the windings in the motors. I don't remember the name right now. Point is there are a few solutions that are simpler and lighter than adding a two-speed gearbox, with relatively little compromise.