Hub motors revisited

[dieqast]

Hey everybody,

My question is if Tesla ever played with the MIEV-idea, the use of 2 or 4 small electrical engines in the wheels instead of one big engine + connectionshafts? My opinion is this is a more compacter(thuss lighter), practical and efficient solution as you'll need less parts. Or has Tesla done some homework to prove that the 1 engine-solution is better? Also with the lower point of gravity and more space for batteries it looks like something ideal for this great innovating company to work with in the future.

Thanks in advance.

[TEG]

This has been discussed at length before. Certainly Tesla knows all about that approach and chose not to use it. Some reasons mentioned were unsprung weight, cost, complexity, safety and regulatory.

Some old forum discussions can be found in these topics:

Of hub motors and dual motors - Tesla Motors Club Forum
In-wheel vs drive train in the Whitestar - Tesla Motors Club Forum
Radical tire & wheel technology - Tesla Motors Club Forum

[dieqast]

This has been discussed at length before. Certainly Tesla knows all about that approach and chose not to use it. Some reasons mentioned were unsprung weight, cost, complexity, safety and regulatory.

Some old forum discussions can be found in these topics:

Of hub motors and dual motors - Tesla Motors Club Forum
In-wheel vs drive train in the Whitestar - Tesla Motors Club Forum
Radical tire & wheel technology - Tesla Motors Club Forum

Thanks for the links, I don't know the figures, but my hypothesis is that the efficiency of 4 seperate motors is better than mechanical efficiency of the drivetrain. My guess is that mechanical efficiency gets up to 80%-85% and the loss in efficiency between 1 and 4 seperate elektric motors is say low 90%'s. This is just guessing though, I haven't done any research on this. And my opinion is that the unsprung weight won't effect handling proportionately. Other than that, Tesla should stick to their current design for future models.

[vfx]

Actually unsprung weight was the big reason not to use wheel motors in a sports car. Fine for a Milk Float / delivery van but crappy for a car that's supposed to have handling a priority.

Check out Martin's comments under "Other Ideas" at the bottom of this page:
Tesla Motors - think

[Brent]

I know of two vaporware cars that intend to use hub motors. (This idea comes up so frequently that there might be many more.) Perhaps someday someone will produce something for the masses that settles the unsprung weight issue:

Welcome to Lightning Car Company - The UK's Premier Electric Sports Car

Electric Mini: 0-60 in 4 Seconds: It Has Motors In Its Wheels : TreeHugger

[TEG]

Don't forget Zap-X:

ZAP-X Crossover SUV
Yahoo! Message Boards - ZAP - Wheel Hub Motors - Need an Engineer

[Hunter]

Thanks for the links, I don't know the figures, but my hypothesis is that the efficiency of 4 seperate motors is better than mechanical efficiency of the drivetrain. My guess is that mechanical efficiency gets up to 80%-85% and the loss in efficiency between 1 and 4 seperate elektric motors is say low 90%'s. This is just guessing though, I haven't done any research on this. And my opinion is that the unsprung weight won't effect handling proportionately. Other than that, Tesla should stick to their current design for future models.

Before I start, it makes some difference here which Tesla drivetrain we're talking about. For simplicity I'm going to stick with 1.5, which just has a single reduction gear.

First note that these hub motors are not likely to run at wheel speed either, so they also need reduction gears. So it's not right to act like you skip all drivetrain losses by using hub motors. It seems to me that the only part you are getting around is the differential. These vary to a certain degree, but I would expect this part to be at least 95% efficient. And I would anticipate the four little motors would be less than 95% as efficient as the one big motor, but I can't find much in the way of specs around. In any case, whichever way you go it's going to be very close...you won't save more than a percent or so.

Of course, the whole question (while interesting) is basically moot...efficiency wasn't even the big reason Tesla cited for going with the one motor. You offer no explanation why you don't think unsprung weight matters. It does. In addition to the links TEG gave, this Tesla blog on driving dynamics explains it well (and is good read for lots of other reasons). That plus the cost, weight, and complexity make great reasons to go with the single motor, and even if the hub motors were a couple percent more efficient it wouldn't be enough to override these concerns.

[W8MM]

There's also a reliability argument involved.

MTBF (mean time before failure) responds inversely to the square of the number of parts in an assembly.

If a single hub motor were to achieve the same MTBF as a big motor, one could expect the failure of one of the group of four hub motors compared to the failure of the big motor to occur 16 times sooner, on average.

It seems to me that trying to make a hub motor that is pounding up and down over pot holes 16 times more reliable than a big motor isolated in the central chassis is a pretty tall order.

[doug]

Personally, I don't think hub motors are necessarily better.
However, as a scientist and engineer, I'd really like to see someone try to make a performance vehicle with them.
They pose an interesting control problem and I'd like to see that experiment in action.

[doug]

From SEMA 2008:
Highlights from SEMA 2008 - The New York Times > Automobiles > Slide Show > Slide 5 of 13

The Hi-Pa Drive F-150 uses regular wheels and tires, but each wheel hub has been replaced with an electric motor, good for about 150 horsepower each, for a total of 600 HP.
Photo: Jerry Garrett for The New York Times

Electric Ford F-150

Updated: Hi-Pa Drive Ford F-150 Is All Electric - PickupTrucks.com News



2009 Ford F-150 by Hi-Pa Drive - 2008 SEMA Show Photos & Coverage - Truck Trend

It's too bad it looks like PML is going out of business.
Hi-Pa Drive from PML flightlink
" Hi-Pa Drive is a PML Flightlink product. "