TEG

Many have asked why Tesla didn't use hub motors in each wheel.
One argument against has been "unsprung weight" which can hurt handling.

Others have asked why not have dual motors to each rear wheel so you don't need a differential.

I found an interesting answer to this here:
http://www.worldchanging.com/archives/005118.html
"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."

We shall see if Zap runs into that issue with their Zap-X car...

iisjsmith

I wonder what impact it would have made on their decision if they threw out the safety certification regulation issues.

I know they have said time-and-time-again that in-hub motors reduced handling too much to be a viable option, but the idea of dual rear motors is interesting. I completely understand their decision to go with a conventional design to fit into existing legislation, but what if they didn't have to worry about that? Is that the only thing that held them back from dual motors...or were there other variables that made it less impressive than the single motor design?

Does anyone have any links to sites that discuss the technical aspects of dual-motor drives?

WarpedOne

Why wouldn't dual inboard motors fit the legislation?

One thing I can think of now is that with two rear motors you do not need differential. Every motor is linked to its own wheel and with sudden motor failure the wheel could block sending you off the road or into the incoming traffic. This was suggested reason why GM abandon dual motors with Impact. I for one find it hard to accept it as the sole reason.

Tesla Motors gave yet another explanation. One bigger electric motor is more efficient than two smaller ones and it takes less overall space, weight and lower part count meaning less complexity meaning higher reliability.

__________________
VoltHead

iisjsmith

> Why wouldn't dual inboard motors fit the legislation?

Well it says in the first post that:

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.

As for weight I can see that two smaller motors will be heavier and bulkier than a single...but that increased weight and bulk can't be much more than what you will save from dropping the differential...not to mention the savings in complexity. Does anyone have any estimates on what a typical car differential weighs...I tried doing some google searches, but I didn't come up with anything.

I also don't buy the "sudden motor failure" reason. My guess is that they would easily have gone with a dual-motor config if they weren't tying to match existing vehicle designs as much as possible, to fit into existing regulatory pathways.

TEG

The Zytek eletric Elise used dual motors:

Michael

Regarding the statement:

'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.'

Toyota seems to have gotten around this on their AWD Highlander and Lexus Hybrid models that do not have any linkage between the front wheels and the rear wheels.

iisjsmith

> Toyota seems to have gotten around this on their AWD Highlander and Lexus Hybrid models that do not have any linkage between the front wheels and the rear wheels.

We are talking more here about the link between the two rear wheels. Tesla has said that the reason they decided to use a single motor with a differential between the two rear wheels is because such a design matches existing vehicle design, and therefore it was easier to get the Roadster certified on US roads. If they did something radical they would have to get the existing legislation changed, and that is a nightmare.

TEG

Someone's diagram of an "inboard hub motor":


(You could move the disc brake inboard as well if you really wanted to minimize unsprung weight)