Front-Wheel Drive vs. Rear Drive

[Jonathan Sek]

I can't believe it's been since April '08 that I joined this forum. Just want to say, thanks for all the info from everybody.

I've searched all over this forum, and have not found answers as to what the great benefits were to make the switch from simple, motorcycle style rear-wheel belt drive to FWD. And seeing the difficulty the driver had in controlling the current 1800lb. version at the XPrize lane changing demo, along with Wilbur's comment to "reduce weight", got me thinking.

When I 1st layed down my deposit over two years ago, I remember my major comment to people about the Aptera was to the effect that "this EV is really innovative from an aero standpoint and I like the fact that it is as a 3-wheel design with a simple belt drive, like a Harley." I have owned several motorcycles and my hobby is bicycle racing (in which we are weight freaks), so the original design really appealed to me. I remember being chagrined when finding out about the shift to FWD (my BMW is RWD).

I wonder if the XPrize driver could have handled the vehicle better if it had been the RWD, lighter and wider front track pre-production version?

One other thing. What would be wrong with front disc wheels and a fender for rain instead of fairings? It would allow a wider track w/o increasing the overall vehicle width. My racing time trial bicycle uses disc wheels made of carbon fiber (CF is not practical for mass market wheels, I know).

[NeilBlanchard]

Hi Jonathan,

There are several reasons that front wheel drive are better than driving the single rear wheel:

* The center of the road is greasy, and especially in the rain it will be slippery, this would be a bad thing.

* Front wheel drive allows a lot more regenerative braking to happen, since the weight always shifts the front of any vehicle, you can get most of the braking from the front.

* On a three wheeled vehicle, the center of gravity needs to as close to the pair of wheels and away from the single wheel, so putting the motor up front and driving the wheels with the most traction is good.

* In snow and ice and other low(er) traction situations, front wheel drive is just better; partly for the other reasons above.

I don't think that the handling problems can be attributed to the front wheel drive. There have been several front wheel drive prototypes that have had no such handling problems.

I think/hope/pray that they have solved the handling problems, and that they can manage excellent efficiency; or at least enough (for now) to not get eliminated from the X-Prize. I think there is too much weight, and some of the excess is too far back in the vehicle? I think that most of the issues they had can be attributed to soft front suspension spring and dampening rates, and/or the lack of an (adequate?) anti-sway bar?

I'll point to the performance of the FVT eVaro through the accident avoidance test -- it is a front wheel drive, three wheeled vehicle, and while may be heavier than the 2e (it does have a smaller battery pack: 14kWh vs 20kWh), it does have a 4-cylinder (1100cc) driven 20kW genset, as well. I think it went through the test in the 48-49mph range? And no drama.

[KarenRei]

The rear belt drive was also noisy, too. It was efficient, mind you.

[PatQ562]

I had poor results with an attempt, many years ago, to replace an objectionably noisy roller chain drive with a toothed belt on a primitive EV. It was quieter, but tended to jump teeth at full power, and when tensioned to prevent this, seemed to place a high load on bearings, causing drag. Coasting was impaired and current draw seemed higher, so I have since held a poor opinion of belt drives.

However, I looked it up, and one source at least, supplies the following stats:
Roller Chain: 95-98% efficiency
Spur & Bevel gear: 93-96%
V-belts: 85-97%
Toothed belts 95-97.5%.

So noisy chain is still the best by a small margin, followed closely by toothed belt, whereas gears can be a couple of points less. However, toothed belt efficiency drops off with smaller pulleys (as required for substantial reduction) and the 2-stage scheme shown on early Apteras would have doubled the losses. Furthermore, a belt's efficiency drops a lot at light loading, where an EV spends a lot of time. The belt must be rated for peak power, which will mean spending a lot of time at 10% of this load.

The other big objection to belts is their perishable nature. Perhaps motorcyclists with drive-belt experience can report on how long they last. My impression is that several years, and several tens of thousand miles is considered good service.

For all these reasons, I for one welcomed the shift to enclosed, oil-bath, shaft-driven FWD. Neil has recapped the RWD handling issues, which only increase as we provide higher power and torque.

Pat Q

[AR-51]

This is a good web-site on three wheel drive. Basically you get better acceleration with rear wheel drive and front wheel drive is best for control.

http://www.clevislauzon.qc.ca/Professeurs/Mecanique/ethierp/3-wheels/stabil1.htm

[Jonathan Sek]

The rear belt drive was also noisy, too. It was efficient, mind you.
Since many seem to be in doubt about Aptera's future, perhaps someone could expound (or ref. a web site) about the trade offs of hub mounted electric motors so I can evaluate other potential future designs. A reference by Karen said that they were expensive. I've noticed other EV protos mentioning them.

[lapwing]

I have given this drive subject a lot of thought, and the X-prize testing, has confirmed some of my deep reservations about S01.

I really liked the rear wheel drive version of the earlier Aptera, for all the right reasons. Weight distribution, stability, weight distribution.

The single biggest fixed load in an ultra-light electric vehicle is the battery pack. In an Aptera configuration it should be between the two front wheels.

Putting the battery mostly behind the seats is a very big oops as S01's marginal handling in the X-prise demonstrated.

Design wise I would love to see 3 wheel motors (of which there are several companies with good prototypes) The two front ones should be in the 14-18KW range (continuous) with a 10-15KW rear. (actually even 3 x 15Kw would do fine) A battery pack in the 15-25KW.hr range between the wheels - perhaps allowing a large range of 300#-700# battery options.

Charger, DC/DC converter, Aux battery, and all the other "stuff" under the seats, including aircon compressor, vehicles "computer" and entertainment.
I think that the 3 phase motor inverters should be liquid cooled, and mounted near the respective motors.

There is nothing inherently wrong with 3 wheels, provided the designers realize that they are designing something more akin to a Lancair (http://www.lancair.com/Main/) than a Prius.

Dynamics & weight distribution is what the Commuter Car (http://www.commutercars.com/) guy got right, because he had to, and it shows. By comparison the current S01 Aprera is poorly conceived (albeit well executed)

edit:

http://www.worldcarfans.com/10607246585/pml-builds-640hp-electric-mini

http://www.proteanelectric.com/live/

http://www.iskra-ae.com/projects_files/In_wheel_electric_drives.pdf

http://www.gizmag.com/go/4666/

http://www.hybridcars.com/components/michelins-reinvents-wheel-with-motors-25308.html

[evmavin]

I have given this drive subject a lot of thought, and the X-prize testing, has confirmed some of my deep reservations about S01.

I really liked the rear wheel drive version of the earlier Aptera, for all the right reasons. Weight distribution, stability, weight distribution.

The single biggest fixed load in an ultra-light electric vehicle is the battery pack. In an Aptera configuration it should be between the two front wheels.

Putting the battery mostly behind the seats is a very big oops as S01's marginal handling in the X-prise demonstrated.

Design wise I would love to see 3 wheel motors (of which there are several companies with good prototypes) The two front ones should be in the 14-18KW range (continuous) with a 10-15KW rear. (actually even 3 x 15Kw would do fine) A battery pack in the 15-25KW.hr range between the wheels - perhaps allowing a large range of 300#-700# battery options.

Charger, DC/DC converter, Aux battery, and all the other "stuff" under the seats, including aircon compressor, vehicles "computer" and entertainment.
I think that the 3 phase motor inverters should be liquid cooled, and mounted near the respective motors.

There is nothing inherently wrong with 3 wheels, provided the designers realize that they are designing something more akin to a Lancair (http://www.lancair.com/Main/) than a Prius.

Dynamics & weight distribution is what the Commuter Car (http://www.commutercars.com/) guy got right, because he had to, and it shows. By comparison the current S01 Aprera is poorly conceived (albeit well executed)


What you said makes no sense, you propose putting the pack forward in the car for weight distribution but you say rwd is better for weight distribution, well moving the motor to the rear puts more weight to the rear which is the opposite of what you suggest. FWD is not the issue it is far better, the issue is the Aptera design changes and what seems to be lack of experience and gross oversight.

[Jonathan Sek]

I'm not an expert on trike weight distribution, but I know from bicycle racing, & auto racing on the track, that you don't want too much weight up front. When I am braking hard on the bicycle, I shift my body weight way back behind the saddle in order to not under weight the rear tire. Even though most of the stopping power is up front, I can't use most of it, as steering is affected.

Auto racing on the track, rear wheel drive is great as you can power through turns and the front wheels only have to steer. RWD allows use of throttle response to control fore/aft weight distribution during acceleration & deceleration. Once, I was coming over a hill in my BMW (which has near perfect 50/50 weight distribution), I was carrying too much speed and there was a 90 degree sharp right hand turn at the hills downside. So, I stepped on the brakes. This caused the rear end to get light, I started fish-tailing, over corrected and did a 360 degree spin into the dirt. I got the black flag of shame and had to go into the pits for a lecture.

During the Aptera's XPrize emergency handling exercise, it appears the driver is braking, the rear end would be getting lighter and it starts to fish tail. I wasn't behind the wheel though, but that's what it appears on the video.

[lapwing]

What you said makes no sense, you propose putting the pack forward in the car for weight distribution but you say rwd is better for weight distribution, well moving the motor to the rear puts more weight to the rear which is the opposite of what you suggest. FWD is not the issue it is far better, the issue is the Aptera design changes and what seems to be lack of experience and gross oversight.

I do not prefer rear wheel drive, but it is workable.

I do prefer, all wheel drive, if using wheel-motors.

Front wheel drive would be fine if using wheel-motors & if the pack was moved to the front, between the front wheels.

In the earlier rear wheel drive version, pack weight was between the front wheels which I prefer. This meant the weight of the inverter and motor, the whole vehicle body and interior and two people etc, were holding down the back wheel. That is quite a bit of weight. However, the pack up forward low is key to good three wheel vehicle dynamics.

I want the battery pack up forward between the front wheels, because this is the lowest part of the vehicle. There are two tires to carry the load, and roll under hard cornering can be managed by a simple torsion bar, between two independently suspended wheels.

The CG of the battery pack, should be as low as possible. Pack CG would be behind the axle some, dependent on other weights

[wpatters]

Front drive is for regenerative braking.

Adding cv joints, drive shafts and all the other add ons for front drive is a disaster.

A toothed drive belt is much lighter and more efficient that the crap they have now.

[KarenRei]

Indeed, it is. It was also noisy, unfortunately.

[wpatters]

IM not so HO.

Good design adds lightness and simplicity.

[KarenRei]

IM not so HO.

{{clarify}}

[SEGsby]

I think he meant "In My 'not so' Humble Opinion"... *shrugs*

[PatQ562]

I too would prefer to see the pack weight underneath the seats and floor (?) and up front between the axles, AND front wheel drive, but this would require downsizing of these components to fit the limited space, which appears to have been sacrificed in favor of more crush space and general weight creep. It is a problem however to have batteries in the very front where accidents could create shorts or a shock hazard. There's always tradeoffs.

If you put batteries under the floor you raise the passengers which is not ideal.

Pat Q

[evmavin]

Front drive is for regenerative braking.

Adding cv joints, drive shafts and all the other add ons for front drive is a disaster.

A toothed drive belt is much lighter and more efficient that the crap they have now.


RWD has terrible regen, the belts have maintenance issues and snap often, I have replaced many on EVs. FWD on an EV is very easy to do and not complex, it is reliable and offers excellent performance and regen, it is also low weight. I have had two FWD EVs with regen and one with RWD belt and I would never buy a belt drive vehicle again! You are over simplifying what is needed in the rear belt drive for an AC drive VS DC. The problem is not FWD, it is the talent and design mistakes at Aptera.

[lapwing]

.................................It is a problem however to have batteries in the very front where accidents could create shorts or a shock hazard. .................
Pat Q

This is simply not true. Shock hazard is a function of pack construction and fusing.

There are many products that allow automatic "fusing" of the battery pack when needed as for example in a crash. These can be an electronically switched breaker/solenoid type device, perhaps coupled to a signal from airbag deployment. Or these can be of the inertia type, where any sudden impact severs the connection. You will already find these employed in current hybrids, often both types. In a 320V battery pack about 10 or 20 of these would make the voltages no more dangerous than ordinary car batteries. anything below about 30V can be considered non lethal even to bare hands. Couple that to the fact that the chassis is composite and ....... well I just don't see an issue

Fire is always a potential issue (as it is with gasoline) and again design and choice of materials makes this way safer than you would expect.

Look for testing of LiFePo4 type batteries on U-tube etc. Thundersky, A123, or Life-batt types.

Choose the right chemistry and this is a non issue. Couple this to a composite chassis and there is even less likelihood of shorting and fire in the event of an accident.

[NmGfan]

I had poor results with an attempt, many years ago, to replace an objectionably noisy roller chain drive with a toothed belt on a primitive EV. It was quieter, but tended to jump teeth at full power, and when tensioned to prevent this, seemed to place a high load on bearings, causing drag. Coasting was impaired and current draw seemed higher, so I have since held a poor opinion of belt drives.

However, I looked it up, and one source at least, supplies the following stats:
Roller Chain: 95-98% efficiency
Spur & Bevel gear: 93-96%
V-belts: 85-97%
Toothed belts 95-97.5%.

So noisy chain is still the best by a small margin, followed closely by toothed belt, whereas gears can be a couple of points less. However, toothed belt efficiency drops off with smaller pulleys (as required for substantial reduction) and the 2-stage scheme shown on early Apteras would have doubled the losses. Furthermore, a belt's efficiency drops a lot at light loading, where an EV spends a lot of time. The belt must be rated for peak power, which will mean spending a lot of time at 10% of this load.

The other big objection to belts is their perishable nature. Perhaps motorcyclists with drive-belt experience can report on how long they last. My impression is that several years, and several tens of thousand miles is considered good service.

For all these reasons, I for one welcomed the shift to enclosed, oil-bath, shaft-driven FWD. Neil has recapped the RWD handling issues, which only increase as we provide higher power and torque.

Pat Q

I find the toothed belt on my NmG to be very quiet. The only noise I hear inside and outside the EV is the motor whine. I have replaced one, at 8,500 miles, due to interference cause by a poorly trimmed belt guard (now fixed after belt replacement). That task only took 45 minutes.
The profile match between the belt teeth and pulley cog must be very precise. For example, Gates recommends use of their pulleys with their Polychain Carbon GT belts, or an approved qualified aftermarket supplier. They will not honor their warranty if an unapproved pulley set is used. Most noise and tensioning problems are related to not following the belt manufacturers recommendations for pulley matching and over-tensioning the belt. For most motorcycle-like toothed belt applications, of less than 100lbft torque, use NO tension but rather a very precise close slip fit. As torque loads head up over 100lbft towards 350lbft, toothed belts need tensioning to reduce "play" to as little as 1/4" deflection.

:happy0025:

[JustWilliam]

On the subject of "in wheel" or "hub" motors-

An enemy of ride and handling remains excessive unsprung weight. This is why high end and performance cars use very light, very strong wheels.

Such motors have obvious appeal, and could be fantastic depending on the vehicle and it's mission. But they need further development to be ready for "prime time".

[SEGsby]

I don't know of anyone who's actually using them in a commercial vehicle that's available yet? *shrugs*

[lapwing]

You are correct in that there are no in wheel production vehicles at present, but Then that applies to most all electric vehicles other than NEV's, and they don't count.

I have seen extensive "prototyping" of in wheel motors and weight/handling is less and less of an issue as the motors evolve. Companies like BMW/Mini/PML and Miev/Misubishi have good prototype examples of what can be achieved.



http://www.treehugger.com/files/2006/08/the_hybrid_mini.php

http://www.youtube.com/watch?v=rnJz0RtdTiE

http://en.wikipedia.org/wiki/Eliica

EDIT: It seems highly likely that a number of well developed active suspension prototypes make the added weight a non issue. Anyway to give you some idea I have in my hands an LEMCO 15KW PM electric motor with 30KW max and it weighs 11kg. The frame is cast iron. While not an in wheel motor, it demonstrates these motors can be light compact and powerful.

Also I have seen a number of motorbike projects with in wheel rear wheel drive. Ride dynamics and handling are just fine.
http://endless-sphere.com/forums/viewtopic.php?f=10&t=7718

http://www.enertrac.net/index.php

While I don't think three of these will do the job on the Aptera, 4 of them would probably work great on a model "T".

The key advantage, is that the hub motor frees up space that allows improved battery placement and capacity
:)



http://www.treehugger.com/files/2005/12/miev_mitsubishi.php

http://cache-02.gawkerassets.com/assets/images/gallery/12/2008/12/medium_3151785710_ee3ddf3ebd_o.jpg

[lapwing]

I did have one other point, about in wheel motor dynamics.

The Chinese are way ahead of everyone on this one.

In China wheel motors form the vast majority of electric two wheel bicycles and scooters, and they have made a lot of them. Not always the best quality, but highly efficient and effective. The in wheel electric scooters are available here in the US. http://www.vectrix.com/index.php

http://www.enertrac.net/index.php

Bottom line is that we all have pre-conceived notions of what does and does not work in cars. Chances are much of what we know, does not need to apply to a safe urban commuter like the Aptera.

EDIT: PS : How much does a solid front axle on the average F150 weigh? There are sure a lot of them on the road and they handle just fine.

Dealing with in-wheel motor weight, may just be a matter of donning higher profile tires.

What do you think?

[wilsodo]

Just William is quite right in the importance of unsprung weight. What is as important in a lightweight vehicle, is the ratio of unsprung to sprung weight. As I recall, optimally, it should be something like 13 percent.
Also, proper three wheel design requires two thirds of the vehicle weight over the end of the car with two wheels. Aside from poor suspension design, this is a major reason the Aptera doesn't handle well.
Fwd lends itself nicely to a reverse trike if you put the proper weight over the wheels.
My ECO-Holland design does have the batteries up front and down the tunnel. I moved the passenger compartment back to accommodate and increase crash safety. What do you think?

[lapwing]

Just William is quite right in the importance of unsprung weight. What is as important in a lightweight vehicle, is the ratio of unsprung to sprung weight. As I recall, optimally, it should be something like 13 percent.
Also, proper three wheel design requires two thirds of the vehicle weight over the end of the car with two wheels. Aside from poor suspension design, this is a major reason the Aptera doesn't handle well.
Fwd lends itself nicely to a reverse trike if you put the proper weight over the wheels.
My ECO-Holland design does have the batteries up front and down the tunnel. I moved the passenger compartment back to accommodate and increase crash safety. What do you think?

I don't agree that the Aptera suspension is a bad design. It's about the fact that they have battery weight to high and way back in the vehicle.

I agree with your 13% un-sprung weight, but I'll bet very few drivers could tell the difference between 20% unsprung and 13% un-sprung weight handling wise. Tire selection would have way more noticeable effects, in particular 50 profile vs say 65. Either would be perfectly acceptable in a commuter like Aptera.

So lets say Aptera is in the 1500# range (it's currently over that).

That gives unsprung weight of 13% at 195# or 20% 300#.

I know for a fact that 100# of unsprung weight per wheel, is easily achievable in a 20kw nominal in wheel motor (about 55#-60# for the motor).

Good discussion by the way. Thanks!

[wilsodo]

You may be right about Aptera's suspension design. I just have a different understanding of how forces are transferred to the pavement. Having the battery CG aft would lead to softer springs in the front which would contribute to body roll. They probably stiffened them up to pass the lane change tests which would lead to a harsher ride.
I may be obsessing over minimizing the wheel weight but I do so in the interest of maximizing handling, ride comfort and safety.
Which hub motor do you prefer? Are they AC or DC? Regen?

[JustWilliam]

You are correct in that there are no in wheel production vehicles at present, but Then that applies to most all electric vehicles other than NEV's, and they don't count.

I have seen extensive "prototyping" of in wheel motors and weight/handling is less and less of an issue as the motors evolve. Companies like BMW/Mini/PML and Miev/Misubishi have good prototype examples of what can be achieved.



http://www.treehugger.com/files/2006/08/the_hybrid_mini.php

http://www.youtube.com/watch?v=rnJz0RtdTiE

http://en.wikipedia.org/wiki/Eliica

EDIT: It seems highly likely that a number of well developed active suspension prototypes make the added weight a non issue. Anyway to give you some idea I have in my hands an LEMCO 15KW PM electric motor with 30KW max and it weighs 11kg. The frame is cast iron. While not an in wheel motor, it demonstrates these motors can be light compact and powerful.

Also I have seen a number of motorbike projects with in wheel rear wheel drive. Ride dynamics and handling are just fine.
http://endless-sphere.com/forums/viewtopic.php?f=10&t=7718

http://www.enertrac.net/index.php

While I don't think three of these will do the job on the Aptera, 4 of them would probably work great on a model "T".

The key advantage, is that the hub motor frees up space that allows improved battery placement and capacity
:)



http://www.treehugger.com/files/2005/12/miev_mitsubishi.php

http://cache-02.gawkerassets.com/assets/images/gallery/12/2008/12/medium_3151785710_ee3ddf3ebd_o.jpg
" It seems highly likely that a number of well developed active suspension prototypes make the added weight a non issue. "

Really? Active suspensions not only add weight and complexity, they draw power as well. In my book, that is very MUCH an issue.

[SEGsby]

@lapwing

I like the idea of adding hub motors to an antique, like a Model T. :)

[lapwing]

" It seems highly likely that a number of well developed active suspension prototypes make the added weight a non issue. "

Really? Active suspensions not only add weight and complexity, they draw power as well. In my book, that is very MUCH an issue.

I agree. active suspension is not needed in an Aptera.

However, in general active suspension, can be very effective improving handling and ride quality, according to Michelin. I have no specific knowledge of these systems though.

[lapwing]

@lapwing

I like the idea of adding hub motors to an antique, like a Model T. :)

Me too. And I think it would handle pack weight quite well also.

[AndyH]

I don't know of anyone who's actually using them in a commercial vehicle that's available yet? *shrugs*

Does this count?

Video - Part 1
http://engineeringtv.com/video/Raytheon-BluWv-Hy-DRA-Part-1

Video - Part 2
http://engineeringtv.com/video/Raytheon-BluWv-Hy-DRA-Part-2

http://communities.canada.com/windsorstar/blogs/vanderblogger/archive/2008/04/21/on-spring-and-raytheon-s-bluwav-lithium-ion-powered-military-suv.aspx

Interior PM/BLDC motors, Chevy hub, completely submersible hub motors and battery packs...

There's enough power in each of the four hub motors to allow 45-50mph travel speed with three motors destroyed...

[SEGsby]

It's very awesome technology, and nice to see the military taking advantage of the benefits of electric motors for silent warfare...

But is this something I'll be able to buy and drive across country on the highways? *shrugs*

The iMiEV Sport was supposed to have hub motors at one point. I put in in the vaporware category with so many other empty EV Promises... :(

[AndyH]

Nope - not likely to be in a dealership 'near you' anytime soon. ;)

There are plenty of Chinese scooters running hub motors. The 5KW unit on mine will get too hot if I let it. I'd expect that to be a main weakness of a sealed motor.

Oh well - good hunting!

Andy