5 ms·

Why bother replacing the gearbox?

Nasrudith 7y ago
Because there is no need to transmit power mechanically when you can stick wires to electric motors and drive directly to reduce complexity, improve efficency, and save weight.
- deleted 7y ago
  [deleted]
dsfyu404ed 7y ago
Because an automatic transmission is just a waste with the power "curve" of an electric motor. An automatic transmission basically replaces the function of a clutch by letting you put power to the wheels at super low rpm while keeping the engine spinning a decent rpm. An electric motor does not need this, it is just wasted energy in that application. Some sort of direct gearbox with multiple speeds would probably be useful in a work vehicle application. However, one from an automotive application would be totally overkill because an electric motor's power band is too wide to need more than two or maybe three widely spaced gears. A purpose built high/low gearbox (like on a drill) would be better. Sure, you can always spec a bigger motor but that has trade-offs. Most early EV conversions had fairly low horsepower motors and retained the manual transmission and used it to avoid overheating the motor when leaving a stop wide open (which you did most of the time when you only had ~50hp). Obviously that's a non-issue with the higher power motors that are commonly used today.
- shoes_for_thee 7y ago
  Wouldn't the advantage of being able to cruise at low RPMs massively increase range with the same size battery? And wouldn't not replacing the transmission result in lower costs for the retrofit? If the upfront cost of a retrofit is the major hurdle preventing adoption, then reducing complexity of the retrofit would seem more important than reducing overall complexity of the drivetrain... particularly since you'll still be better-than-parity complexity following an electric retrofit. I understand the non-trivial power-loss of connecting a motor to a transmission, but to me this seems like an acceptable trade... particularly as someone who enjoys rowing through my gears.
  - dsfyu404ed 7y ago
    >Wouldn't the advantage of being able to cruise at low RPMs massively increase range with the same size battery? Not really. An electric motor does not lose much efficiency with increased speed. >And wouldn't not replacing the transmission result in lower costs for the retrofit? In a longitudinal application it would be cost neutral to retain a manual transmission. In any application it would be dumb to keep an automatic transmission. In a longitudinal application replacing any transmission is as easy as a couple carrier bearings and a different drive shaft. If I was really trying to cut cost but perform a large volume of conversions on FWD cars with transverse mounted engines (i.e. basically all FWD cars) and automatic transmissions I'd keep the transmission housings but replace all the clutches, planetaries, etc with a single solid shaft. While machined parts are expensive they're still cheaper than coming up with another solution for the differential.
rebuilder 7y ago
I think the point was that if you're looking at an expensive replacement anyway, it might be worth doing a somewhat more expensive replacement to convert to an electric drive train.
mikeash 7y ago
In addition to the other reasons given, it’s possible that the gearbox wouldn’t hold up with the new motor. Tesla originally built the Roadster with a two-speed transmission but the high torque of the electric motor kept shredding them, and they finally shipped the car with a single fixed gear ratio. I think there are a couple of super high end EV models with transmissions, but almost all EVs just have a fixed ratio.
djaychela 7y ago
It's that model's Achilles heel - they go wrong for a pastime, a combination of weak design, poor manufacturing tolerances and no way to measure bearing preload.

Because there is no need to transmit power mechanically when you can stick wires to electric motors and drive directly to reduce complexity, improve efficency, and save weight.

deleted 7y ago
[deleted]

dsfyu404ed 7y ago

Because an automatic transmission is just a waste with the power "curve" of an electric motor. An automatic transmission basically replaces the function of a clutch by letting you put power to the wheels at super low rpm while keeping the engine spinning a decent rpm. An electric motor does not need this, it is just wasted energy in that application. Some sort of direct gearbox with multiple speeds would probably be useful in a work vehicle application. However, one from an automotive application would be totally overkill because an electric motor's power band is too wide to need more than two or maybe three widely spaced gears. A purpose built high/low gearbox (like on a drill) would be better. Sure, you can always spec a bigger motor but that has trade-offs. Most early EV conversions had fairly low horsepower motors and retained the manual transmission and used it to avoid overheating the motor when leaving a stop wide open (which you did most of the time when you only had ~50hp). Obviously that's a non-issue with the higher power motors that are commonly used today.

shoes_for_thee 7y ago
Wouldn't the advantage of being able to cruise at low RPMs massively increase range with the same size battery? And wouldn't not replacing the transmission result in lower costs for the retrofit? If the upfront cost of a retrofit is the major hurdle preventing adoption, then reducing complexity of the retrofit would seem more important than reducing overall complexity of the drivetrain... particularly since you'll still be better-than-parity complexity following an electric retrofit. I understand the non-trivial power-loss of connecting a motor to a transmission, but to me this seems like an acceptable trade... particularly as someone who enjoys rowing through my gears.
- dsfyu404ed 7y ago
  >Wouldn't the advantage of being able to cruise at low RPMs massively increase range with the same size battery? Not really. An electric motor does not lose much efficiency with increased speed. >And wouldn't not replacing the transmission result in lower costs for the retrofit? In a longitudinal application it would be cost neutral to retain a manual transmission. In any application it would be dumb to keep an automatic transmission. In a longitudinal application replacing any transmission is as easy as a couple carrier bearings and a different drive shaft. If I was really trying to cut cost but perform a large volume of conversions on FWD cars with transverse mounted engines (i.e. basically all FWD cars) and automatic transmissions I'd keep the transmission housings but replace all the clutches, planetaries, etc with a single solid shaft. While machined parts are expensive they're still cheaper than coming up with another solution for the differential.

rebuilder 7y ago

I think the point was that if you're looking at an expensive replacement anyway, it might be worth doing a somewhat more expensive replacement to convert to an electric drive train.

mikeash 7y ago

In addition to the other reasons given, it’s possible that the gearbox wouldn’t hold up with the new motor. Tesla originally built the Roadster with a two-speed transmission but the high torque of the electric motor kept shredding them, and they finally shipped the car with a single fixed gear ratio. I think there are a couple of super high end EV models with transmissions, but almost all EVs just have a fixed ratio.

djaychela 7y ago

It's that model's Achilles heel - they go wrong for a pastime, a combination of weak design, poor manufacturing tolerances and no way to measure bearing preload.