What are your opinions on hub motors in a future model s and all other electric vehicles in the future? What are the pros and cons of having hub motors and do you think they will eventually become the dominate drive method in the future? Or do you think the 4 independent motors in the middle of the axle with a simple half shaft going to the wheel similar to how rimac does it will be the best solution? I think it will be the latter as it has all the advantages of the hub motor except the extra cabin space but you still must have a suspension system so you don't gain that much room, but you don't have any of the disadvantages.
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Haxster
Member
Hub motors would add undesirable un-sprung weight to the suspension. This may be OK for utility/economy vehicles. I'm a big fan of an inboard motor-per-wheel approach for best performance and handling potential.
I don't see any good advantage to 4 motors. 2 motors for an EV has advantages over 1 as Tesla has demonstrated, but more adds complexity with little real gain and adds drawbacks. Synchronizing two motors is a simpler problem than four. Putting motors in the hubs increases the angular momentum of the wheels which in turn increases the amount of energy needed to turn the tires. You want the wheels to be as light as possible, that's one reason why the super high end wheels are carbon fiber (though the biggest reason is carbon fiber is fashionable and few people really know why it's a superior material for some uses).
There will probably be some expensive sports cars with quad motors at some point, but dual motors is the sweet spot where you get the best energy economy and performance for your money. More than two motors and the energy economy is going to start dropping, higher weight from not just the motors but the rest of the added hardware to support the extra motors and probable losses from the motors getting out of sync sometimes. Quad motors would be better in poor traction conditions like snow. The traction control system can spin different tires at different speeds depending on the quality of the traction, but that's the only real world situation where it would be an advantage. It may also help on the track in high speed cornering, but few people are going to get into those situations in the real world.
There will probably be some expensive sports cars with quad motors at some point, but dual motors is the sweet spot where you get the best energy economy and performance for your money. More than two motors and the energy economy is going to start dropping, higher weight from not just the motors but the rest of the added hardware to support the extra motors and probable losses from the motors getting out of sync sometimes. Quad motors would be better in poor traction conditions like snow. The traction control system can spin different tires at different speeds depending on the quality of the traction, but that's the only real world situation where it would be an advantage. It may also help on the track in high speed cornering, but few people are going to get into those situations in the real world.
Torque vectoring for better handling (as well as better grip in some scenarios) would be an obvious benefit of four motors - the holy grail of automotive engineering to be able to control each wheel precisely, assuming you know how to do that. 
Forgetting about any fake brake-based alternatives, several companies do do Torque vectoring (at least on one axle) using differentials in the ICE world and of course there are even per-wheel-motored limited EV productions out there.
I would be very surprised if this does not happen in the EV world. Tesla is not known for their handling prowess or history, but other companies may jump to this faster due to their heritage.
Now, whether this will manifest itself as hub motors or motors in the middle, that is a more difficult question. But I will say this: I remember seeing several hub-motor concepts at least.
Forgetting about any fake brake-based alternatives, several companies do do Torque vectoring (at least on one axle) using differentials in the ICE world and of course there are even per-wheel-motored limited EV productions out there.
I would be very surprised if this does not happen in the EV world. Tesla is not known for their handling prowess or history, but other companies may jump to this faster due to their heritage.
Now, whether this will manifest itself as hub motors or motors in the middle, that is a more difficult question. But I will say this: I remember seeing several hub-motor concepts at least.
In addition to unsprung weight, hub motors suffer higher vibrational loads and poorer cooling. It's best just to have them near the wheels and link them with a CV joint unless you specifically must have them inside the wheel for some reason or another.
Thanks for the responses so far. Anyone else have any thoughts on it? What about not just for the model s but for other electric cars and compact cars, or trucks? Personally I don't think it would be beneficial over 4 inboard motors in any application. Trucks are normally not impacted as much by unsprung weight but with trucks the weight of the motors and gearboxes goes up as well because of the required power and gearing. With compact cars normally being so light, the weight percentage that would be unsprung would be pretty high altering ride and safety even more. Same with full size sedans like the model s but maybe to a slightly less degree.
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Good point about the trucks.
One more comment on the multi-motor idea in general and the Rimac in particular: it was speculated during the Richard Hammond incident (well, by Hammond himself) that something in the multi-motor torque vectoring went wrong and contributed in him going out the road.
One more comment on the multi-motor idea in general and the Rimac in particular: it was speculated during the Richard Hammond incident (well, by Hammond himself) that something in the multi-motor torque vectoring went wrong and contributed in him going out the road.
Brando
Active Member
Why bother? Dual motor Model S isn't quick enough?? You want more complexity? You want costs to go up?
Hub motors aren't the near future, because of the extra unsprung mass and need for gearing. I
do think one motor per wheel is the not very distant future, though - torque vectoring, better grip for acceleration and regeneration, and simpler mechanicals - but they'll probably stay frame mounted and run through reduction gearing and conventional half shafts for some time to come (this is how all the electric supercars and concept supercars from the SLS and C-X75 on have been set up, including the Rimac and Nio.)
Hub motors could become the long term solution, though - if they become enough lighter and more compact that it mitigates the unsprung weight issues and they can provide ample torque while direct driven and even replace the brakes. No one does inboard brakes anymore despite the potential advantage in unsprung weight, because the benefit is small and the brakes benefit from cooling.
do think one motor per wheel is the not very distant future, though - torque vectoring, better grip for acceleration and regeneration, and simpler mechanicals - but they'll probably stay frame mounted and run through reduction gearing and conventional half shafts for some time to come (this is how all the electric supercars and concept supercars from the SLS and C-X75 on have been set up, including the Rimac and Nio.)
Hub motors could become the long term solution, though - if they become enough lighter and more compact that it mitigates the unsprung weight issues and they can provide ample torque while direct driven and even replace the brakes. No one does inboard brakes anymore despite the potential advantage in unsprung weight, because the benefit is small and the brakes benefit from cooling.
Haxster
Member
Agree! Simpler mechanicals includes the elimination of the differentials. These are almost as big as the motors and probably not much lighter. I'm sure that they're cheaper, but also much less efficient at torque vectoring.
4 motors might become a thing in expensive electric sports cars, and some heavy duty applications like trucks, but that's the only place you're going to see them. The performance increase isn't worth it for normal driving. For heavy duty applications, it can give a truck more horsepower to move big loads, but the torque and HP n a passenger car with just 2 motors is way more than sufficient.
Hub motors have another problem over axle mounted motors, you need to get the power to the motors and on something that spins, you need to have contact brushes which are another thing to wear out. The contact surface on a wheel also needs to be a sealed system or water and dirt will be getting in there on a regular basis. As the contacts wear, one of more motor is likely to cut out unexpectedly, which could lead to accidents. For safety the car would need a system to monitor the state of the contact between the motor and electrical system and shut down the car if it's compromised. Even then there could be electrical failures the system couldn't anticipate.
The idea with most EVs is to reduce the number of parts that will wear to increase reliability. Expensive sports cars tend to be a lot less reliable than passenger cars because it's assumed the buyer is willing to trade reliability for that last iota of performance. All but the most die hard gear-heads want their daily driver to be as reliable as possible.
For any number of motors, an axle mounted motor makes much better sense from a performance and reliability standpoint. Hub mounted motors are one of those things that could be done, but there are a lot of drawbacks.
Hub motors have another problem over axle mounted motors, you need to get the power to the motors and on something that spins, you need to have contact brushes which are another thing to wear out. The contact surface on a wheel also needs to be a sealed system or water and dirt will be getting in there on a regular basis. As the contacts wear, one of more motor is likely to cut out unexpectedly, which could lead to accidents. For safety the car would need a system to monitor the state of the contact between the motor and electrical system and shut down the car if it's compromised. Even then there could be electrical failures the system couldn't anticipate.
The idea with most EVs is to reduce the number of parts that will wear to increase reliability. Expensive sports cars tend to be a lot less reliable than passenger cars because it's assumed the buyer is willing to trade reliability for that last iota of performance. All but the most die hard gear-heads want their daily driver to be as reliable as possible.
For any number of motors, an axle mounted motor makes much better sense from a performance and reliability standpoint. Hub mounted motors are one of those things that could be done, but there are a lot of drawbacks.
Okay, I'm missing something here. The hub motors I've seen proposed are basically the same design as the frame mounted motors - three phase AC devices with power going to the stator. So I'm not sure where the whole discussion about sounding contacts comes from.
You would need to have a flexible connection to allow for suspension movement, either from the motor to the inverter or from the inverter to the pack depending on where the inverter is.
I was thinking the motor was in the rotating part of the wheel. The motor itself would be rotating. For that you need an electrical contact with some kind of brush that allows the wheel to turn. I looked up the concept and I had it backwards in my mind. My bad.
There still is the problem of unsprung weight and making sure the wheels are coordinated with software.
Agreed. It seems to me like the software problem is rather trivial - the drive inverters always know the exact position and speed of their motor and therefore their wheel in this case, so it's just a matter of deciding what degree of difference you want to allow and programming to match.
Unsprung weight is the killer, and the reason I said it might be a distant solution as motors get lighter and more powerful, but isn't likely in the near term.
If you can replace the brake (except for an emergency/parking unit,) the whole differential and transmission, and the wheel sensors with a motor package that weighs only a little bit more, it's an obvious thing to do. We aren't there yet, but we probably will be someday.
One problem with eliminating the brakes would be deal with regulators. That's going to take changing quite a few laws in quite a few jurisdictions. There still needs to be some kind of mechanical brake for emergencies.
If the electrical system on the car failed at speed, you need to be able to stop the car in a reasonable distance. One of the things driving the size of car wheels today is the legal requirements to be able to completely stop the car from speed to zero in x distance. The wheels have had to grow bigger to accommodate larger brake discs.
If the electrical system on the car failed at speed, you need to be able to stop the car in a reasonable distance. One of the things driving the size of car wheels today is the legal requirements to be able to completely stop the car from speed to zero in x distance. The wheels have had to grow bigger to accommodate larger brake discs.
There are multiple technical challenges to in-hub motors while the benefits are comparatively marginal...
Challenges:
- Tough operating environment (shock, vibration, weather, heat from co-located friction brakes) for durability
- Space/weight constraints - ok, you fit the motor in there, now where do the friction brakes go?
- Cooling the motor - probably need liquid cooling due to space constraints, so now you're running coolant lines
- Powering the motor - those heavy gauge wires don't like to flex a thousand times a mile as you drive down a rough road
- Regen capability - these motors need to be integrated to enable regenerative braking, adding to space/weight/heat constraints
Marginal benefits:
- Less mechanical complexity (compared to 4-motor EV with inboard motors)
- more space available in the vehicle
- Weight (since you're removing drive shafts)
Unless I'm missing something, I don't see why a company would ever pursue this kind of solution.... just my 2 cents.
Challenges:
- Tough operating environment (shock, vibration, weather, heat from co-located friction brakes) for durability
- Space/weight constraints - ok, you fit the motor in there, now where do the friction brakes go?
- Cooling the motor - probably need liquid cooling due to space constraints, so now you're running coolant lines
- Powering the motor - those heavy gauge wires don't like to flex a thousand times a mile as you drive down a rough road
- Regen capability - these motors need to be integrated to enable regenerative braking, adding to space/weight/heat constraints
Marginal benefits:
- Less mechanical complexity (compared to 4-motor EV with inboard motors)
- more space available in the vehicle
- Weight (since you're removing drive shafts)
Unless I'm missing something, I don't see why a company would ever pursue this kind of solution.... just my 2 cents.
I think the gearing is also an issue. The ideal rpm for an electric motor is much faster than the rpm of a wheel. It's certainly possible but it would be less efficient.
I'm sure performance EVs will have 4 motors, but not in the wheels.
I'm sure performance EVs will have 4 motors, but not in the wheels.
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