4 x inwheel (permanent magnetic) drives?

4 x inwheel (permanent magnetic) drives?

why has the model S no all wheel inwheel drives? It would be a really great combination for this kind of car!

Timo | 20 février 2011

Unsprung weight weakens handling, and such hub-motors tend to be rather ineffective so your range would suffer.

Brian H | 20 février 2011

Also, 4X the places to go wrong. Why give Murphy a helping hand?

EVBob | 21 février 2011

you are right but this is not a solution, because this answer is against progress.
e.g. its better to use one cylinder instead of 4 cylinder.
sorry this isn't a answer!!

Volker.Berlin | 21 février 2011

@EVBob, I beg to differ, and agree with Brian H. Given that we are talking about the first vehicle of its kind by many aspects, we do not need to add another "first". It is already revolutionary as designed by now.

Now, the priority should be to make it real, and make it a robust, practical solution that has as much value as possible in most car users' daily life, with as little worries as possible. In other words, time to delivery and low maintenance/high reliability/high customer satisfaction are huge values and by far outweigh the advantages of an exotic solution at this stage of the development.

Once the Model S is on the road and real-life data has been collected, based on that experience another model may try a more exotic motor placement.

And btw, what do you want to gain from putting the motors into the wheels? I do not see any advantage, but I do see disadvantages: Unsprung weight and very tight constraints to the construction of the motors due to unnecessarily tight size, weight and form limits. The idea looks "cool" at first sight, because it is an unexpected way of placing the motor(s), but at closer inspection it does not look so good any more.

Ramon123 | 21 février 2011

PML Flightlink of England seems to be the leader, and the claim that it would increase unsprung weight is disputed by those who have studied their inwheels - in fact, their inwheels, which includes braking and regen, are claimed to be just as light as an ordinary disk/caliper assembly - as I recall around 35 pounds,
which means that they would not only not increase unsprung weight , but decrease overall weight. Exactly why Tesla designers choose not to use them will require the question to be posed to them. Everything here is sheer speculation and guessing. I'm certain that the design team is well aware of inwheel motors. Apparently the Fisker Karma is using two of them and although their capacity is high, the juice from their small battery pack (about the size of the Volt) isn't enough to move their vehicle into the sub 8 second to 60 sprints. That car turns on its gas engine when the driver hits the performance mode and uses both sources of juice - even then the acceleration is slower than the Model S.
As for the idea that more than one motor would increase the chances
of failure, It should be obvious that actually having more than one motor vastly decreases the odds that a malfunctioning motor will leave you stranded, as any outboard motorboat owner can tell you. Four wheels so equipped would make traction control and
four wheel drive a snap and also allow for better acive suspension
Why not ask the design team why they chose not to use them? It might have been something as obvious as cost.

dsm363 | 21 février 2011

It could be as simple as they need to get the car out the door and they're familiar with the current setup. Maybe after production starts in a few years, they could revisit the in-wheel option for version 2 but I doubt it.

Volker.Berlin | 22 février 2011

"It should be obvious that actually having more than one motor vastly decreases the odds that a malfunctioning motor will leave you stranded..."

True. But cost/maintenance is increased by the same factor. Assuming that one motor averages 20 years before breaking down for some reason, then that's an acceptable life span. Having four motors of the same kind brings down the average time between maintenance to one fourth, i.e., five years. So, chances are you never get stranded due to motor failure (although probability is still not zero), but initial cost as well as maintenance frequency and cost will be four-fold. Everybody has to decide for him/herself if that is worth the added reliability of the system. The reasoning is very similar to having a RAID (Redundant Array of Independent Disks) vs. a single hard drive in your computer...

Tesla has to try and choose the option that fits best most of its potential customers, and I think it is plausible to choose one motor for the initial version of a car like the Model S.

Timo | 22 février 2011

@Ramon123, it is 31kg / motor + disk brakes. In other words around twice as much as usual unsprung weight. Also 4*31 is 124kg which is quite a bit more than twice the weight of the Model S motor + transmission + PEM.

So, you lose in weight, efficiency, power and handling with hub-motors. You gain a bit better /wheel traction control, torque at low speeds and better regen.

I mention low speeds because based on the chart your max speed would be around 150km/h:

1300RPM cutoff, normal wheels in 17inch rims (based on specs the motor diameter is 420mm or 16.8 inch):

wheel circumference around 1.9m

1.9m * 1300RPM = 2470 meters / minute = 148.2 km/h.

Not very fast.

Volker.Berlin | 21 février 2011

"You gain a bit better /wheel traction control, torque at low speeds and better regen."

And still, this is not particularly an argument for hub motors. You could achieve the same by installing a second "regular" motor for the second axle. Before this thread takes off into a discussion of 4 wheel drive vs. rear wheel drive, that discussion already takes place over here:

VolkerP | 22 février 2011

great research & calculation, Timo!

For a really fast hub motor driven EV go google for Eliica. This experimental car uses eight wheels for a total power of 480kW and a top speed of 370km/h (~230 mph). They still are in search of a corporate sponsor to produce a small series of 200 units.

TESLA is not about building prototypes, it is a game changer for the global car industry. Technology setup for Model S is complete. Now they focus on starting production in large volumes.

Timo | 22 février 2011

Started to think about that torque. That Protean Electric hub-motor generates about peak 900Nm of torque (around 400Nm continuous), sounds much but that is direct drive torque.

Roadster engine generates around 400Nm of torque, but at 150km/h it does...(14000/200 = x/150 : x=70*150 : x= 10500RPM)

That means there is a reduction gear of ratio 10500/1300 =~ 8:1. This means Roadster torque when measured at the drive axle after transmission is actually 8*400 = 3200Nm. Even with four of those 900Nm peak torque motors you get just 3600Nm so in order to beat Roadster engine you *need* all four hub-motors. If you use just two you actually lose in torque, and if you use two traditional motors you can't match torque those two produce with four hub motors, not even close. In top of that AFAIK Model S engine will have a bit more oomph than Roadster engine.

Vawlkus | 22 février 2011

There's also the extra complications if something goes wrong.

What happens if one of the 4 mototrs fails for some reason? If you travel along on three, your car won't be going in a straight line anymore, the one motor on it's own will push the car hard in the opposite direction, since it's counterpart isn't offsetting it's thrust.

There's also the issue of speed controlers, which I believe do not exist, that can handle the potential difference between the front and rear wheels regularly. If, for example, the front motors are a little slower than the rear ones is your car going to fold up in the middle? Probably not, but you are going to stress it in a way that it's not intended to handle.

This has already been thrashed to death in other threads. Tesla will be looking into AWD, but NOT until after the Model S is on the road.

Douglas3 | 22 février 2011

"Having four motors of the same kind brings down the average time between maintenance to one fourth, i.e., five years."

That is an overly-simplistic and excessively conservative calculation. In reality the reliability of hardware degrades over time, with dramatic reductions in reliability near end-of-life. So while four motors may indeed be 4X more likely to go wrong, the expected lifetime of the set will be closer to that of one motor.

msiano17 | 22 février 2011

There is still to many objects to be debated regarding in-wheel motors. Nothing of which sounds extremely beneficial where the advantages significantly outweigh the disadvantages.

For example; there are several pros and cons to purchasing an EV over ICE. But the absolutely outstanding advantage is that there is no more dependency on fossil fuels (there are several advantages this seems most prevalent for most buyer).
Ergo, the advantage of buying an EV, even the first edition which may have a few glitches, far outweigh the disadvantages.

Point being, I do not see any advantage that significantly stands out for in-wheel motors.

Timo | 22 février 2011

Hub-motors do not have much benefits in passenger cars, but when you move to bigger vehicles, especially military ones with eight or more wheels and requirement for very rough terrain they start to make a lot of sense. AWD with very good control of wheels, there would be no drive axle to restrain wheel movement, with hydraulics you could adjust ground clearance when needed, even lift unused punctured broken wheel off the ground and continue with the rest etc.

World largest trucks used in mines already have hub-motors. They are serial hybrids. I'm a bit surprised this tech has not already been adapted by military for eight-wheel armored personnel carriers.

Or some forest multi-purpose saw-mill/lumber transport vehicles (whatever you call those do-it-all vehicles).

EVBob | 26 février 2011

always the argument of the Unsprung weight problem i cant hear it anymore.
a rim of wheel has around 10kg and a hub motor of this weight would have 100kW it would be to much for just one wheel so a hub motor with 50kW(5kg) would be enough!
and why does telsa use a induction motor instead of a permanent magnetic motor it cant be the price.
tesla could do it better!!!!

Timo | 26 février 2011

You have a wrong belief of how light those hub-motors are. 50kW motor would weight over 30kg and takes up entire 17" rim. Low-speed, low torque engines do not raise much desire, especially when they hurt the performance of the vehicle.

Induction because it gives better overall efficiency and better operating range than permanent magnet motors (and are also much cheaper and easier to manufacture).

Oliver in Seattle | 26 février 2011

Also, US DOT regulations require mechanical braking on all four wheels. In hub motors would require additional mechanical brakes. This would defeat the purpose of putting the motors in the wheels. Volvo has a concept C30 with in hub motors but the system would not meet safety regulations. Until such time as the regulations change, the concept is DOA.