In-wheel vs drive train in the Whitestar

[asdar]

I understand why they went with the single centrally located motor on the roadster. I wonder if they'll go the same way on the Sedan.

On the roadster performance was key. The in-wheel motors added un-sprung weight. Unsprung weight being any weight on the car that doesn't go through suspension to get to the road.

I don't know that unsprung weight would be as big a deal in a family sedan with a probably top end around 100 mph, and acceleration in the 0-60 in 6-8 second range.

I'm not sure what the differences statistically between in-wheel vs transaxle. What I've been able to dig up points to about 10% more efficient for in wheel and quite a bit better braking reclamation for extended range.

 

[tonybelding]

Mitsubishi are going with hub motors, and the PML Mini QED concept car used hub motors.

GM started using hub motors in their Hy-Wire concept car, but later backed off that. The Sequel has hub motors in the rear, but only a single, centrally located motor to drive the front wheels.

I will be very surprised if Tesla use hub motors in the White Star. I think their philosophy will be that a single motor and more-or-less conventional drive train worked well in the Roadster, and it'll work well in the White Star.

 

[Michael]

Wouldn't having wheel motors also allow better battery placement, better weight and more efficient interior space management?

Although Martin stated that using wheel motors adds unsprung weight, I've also heard that the wheel motors used on the PML Mini were only 30kg. I'm wondering whether this weight is really increasing the unsprung weight when we factor out the components that are actually replaced by using the in wheel motors, and if it's only a slight increase, why wouldn't this be considered acceptable on a family sedan that doesn't need to have the same focus as a sport vehicle.

 

[WarpedOne]

I suspect that wheel motors have its own pile of unsolved engineering problems. It my all sound and look nice and beautiful until you go down to designing it.

1. Designing it to last for at least 100 000 miles in summer, winter, rain, frost, dirt, sand, etc.

2. It may not overheat even at slowly going uphill when the motors are giving 20kW each and only turn very slowly having very little air movement. Watercolling?

3. Insulating a 200V and 100A (20kW) electric line so it resists all vibratons, moisture and watter, heat and cold for some years.

4. Regen braking only works while the wheel is still rotating fast enough. When it slows down there is no meaningful braking force left but the wheel stil turns and the car is still moving. To completly stop, some reverse power has to be applied. Whick drains the battery. Empty battery, no brakes? Bad bad bad. So some form of frictional brakes is mandatory and it will take available space from hub motor and add some more unsprung weight.

Though electric vehicles are not mainstream, there were some before and none of them yet had wheelhub motors. I figure wheelhub motors and required onboard support systems would end up being heavier and took more space than Tesla's current drivetrain that fits completely between rear wheels.

Also having a simple transmission is not a bad thing. I guess a 185kW at 13500rpm requires lower current than 45kW hubmotor rotating at 40rpm and going 200km/h). Lower current usually means lower losses and better efficieces.

 

[tonybelding]

And now Mitisubishi are backing away from hub motors too. Check it:

http://www.evworld.com/view.cfm?section=communique&newsid=13353

 

[asdar]

I understand a bit better now, but this could be all wrong.

Part of the problem is that the hub motors aren't the variable voltage AC motors of the type the single center mounted motor is. That's a new engineering challenge to give the same type of freedom to the wheel mounted motors that the center have.

That was pretty good timing on the Tesla blog they went over motors just a couple days after I started this.

I've just seen a few reports of the development of motors like the one Tesla uses.

 

[Future]

I understand a bit better now, but this could be all wrong.

Part of the problem is that the hub motors aren't the variable voltage AC motors of the type the single center mounted motor is. That's a new engineering challenge to give the same type of freedom to the wheel mounted motors that the center have.

That was pretty good timing on the Tesla blog they went over motors just a couple days after I started this.

I've just seen a few reports of the development of motors like the one Tesla uses.

Furthermore, One big electric motor is a lot more efficent than four compact ones. This loss in efficency may compromise for the larger battery pack that can be present in a car using hub motors, resulting in a heavier, mediocre handling car that has no real advantages over one with one single efficent motor. From what I can gather from Tesla, The new WhiteStar sport sedan will sport (lol pun intended) a placment of the motor that is designed from the ground up for electric cars. Most of the EVs today use chassies (?) built for ICE vehicles. With the small size and impressive torque output of an electric motor, Tesla engineers will be able to optimise a high performace, efficent design that would be otherwise impossible in any other vehicle.

Is anyone else pumped for WhiteStar to come out? Its going to be a blast to drive!!!  ;D

 

[TEG]

http://www.worldchanging.com/archives/005118.html
(When Tesla was) "asked whether they thought about using in-wheel motors, since putting a small motor in every wheel instead of having one big motor with a drivetrain connecting it to the four wheels can greatly reduce mechanical complexity and weight, as well as improving reliability. (This is one thing EV's make possible which simply can't be done feasibly with combustion engines.) Interestingly, they did consider it, but JB said it would have made safety certification extremely difficult. It's perfectly safe, but the certification regulations are written assuming you have one motor and a drivetrain, so there are some certifications (such as the one for Anti-Lock Braking) you can't pass in a car with no drivetrain. These rules would need to be re-written to allow vehicles with in-wheel motors to be certified, which is obviously not going to happen without significant money and time spent lobbying--not a fight a small startup company should take on if it can avoid it."

 

[Kardax]

Could someone explain to me the advantage of 4 hub motors versus 4 inboard motors?

Seems to me 4 inboard motors would give minimal unsprung weight and more reliability as the motor isn't exposed to all the rain, snow, mud, rocks and debris that cars face on the road.

-Ryan / Kardax

 

[TEG]

The only real benefit I can see to doing them in the wheels are for packaging reasons.
Without the need for disc brakes, you have that empty space in the wheel where you could stuff a hub motor.
If you do it inboard you have cut into your cabin space as you need some sort of motor/battery tunnel down the center of the vehicle.
Also with it inboard, you have to worry about ground clearance. You don't want too much hanging down in the center of the car that could catch on things going under the car (this can be a problem with differentials).

I guess another benefit is that you don't need CV joints with a hub motor. If you do them inboard then you need a little extra weight of half shafts and CV joints. Also the CV joints can wear out.

Now recall there are many downsides to doing it in-hub.
Among them:
Unspung weight.
High voltage, high current wires going to each wheel. These wires need to bounce, vibrate, turn, and otherwise suffer abuse that could wear them out. Also having high current & voltage at each wheel could be a safety concern in a crash or even driving through the rain or a car wash if water gets on the hub motors.

 

[Kardax]

You're right about that--putting the motors in the wheels will definitely save space in the chassis. On the other hand, electric motors aren't that big to begin with...

The need to send high-voltage wires to the wheels seems to negate the advantage of not needing a CV joint. There are quite a few cars on the road today with CV joints not having a problem, so they seem reliable enough to me...

-Ryan / Kardax

 

[TEG]

CV joints & boots are maintenance items, particularly on front wheel drive cars. My Wife's old Subarus and Taurus / Sable models always needed CV joint replacement eventually (although they are probably good for 60K miles on average)