FWD, RWD, AWD considerations

Technical discussion on converting internal combustion to electric
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FWD, RWD, AWD considerations

Post by BigMouse » Sat, 03 Nov 2012, 23:42

I am re-posting this here in its own thread as it seems like a good topic worth discussing. The original thread is here:

viewtopic.php?title=woo-hoo-first-ev-ri ... 281#p39613

The question was brought up whether FWD was "recommended" for EV conversions. My reply is below:
BigMouse wrote: I'm doing a RWD conversion for simplicity, and because I wanted the option to do direct drive (though I've since decided to retain the gearbox). The major problem with FWD (in my opinion) is the question of clearance between the motor and the "long" driveshaft. You're limited on the diameter motor you can use, and often limited on the length as well due to the position of the frame rails. This makes it very difficult to choose a donor/motor combination since it's often hard to know whether there will be clearance without taking measurements on the car itself from underneath. Even then, these measurements (centre of crankshaft to axle) are hard to take as there is an oil-pan in the way and the axle is usually at an angle. RWD cars don't have many, if any, obstructions in the engine bay, and they are usually long enough to fit any standard length motor. So if you can find a RWD candidate, chances are you won't have trouble getting whatever motor you end up choosing to fit. It's also simpler to support the motor from the original engine mount points in a RWD.

I am biased towards RWD for those reasons, but I've been tempted by a few FWD cars recently.

FWD is a bit more efficient due to having parallel shafts (RWD has to convert the motion through 90 degrees in the rear diff). As coulomb said, FWD cars are more plentiful as well, and it's easier to find a light-weight one.

Some FWD cars have longitudinally mounted engines, which have all the ease of motor mounting of a RWD car, but without the efficiency benefit of a transverse FWD.

Rear engine, RWD can have either longitudinal (Porsche, supercars) or transverse (MR2) engine mounting.

A quick (probably incomplete) list of common RWD (or FWD with longitudinal engine) cars:
Any BMW (3 series are lightest of course. I'm looking for an e36 3 series Compact at the moment)
Mazda MX-5, RX-7
Nissan 300zx, 280sx, Skyline, Silvia, etc
Toyota Supra, old Celicas and Corollas. MR2 would be treated as a FWD due to the transverse engine layout.
Mitsubishi GTO
Mercedes Benz (Most except the compact ones. 190's are surprisingly light and RWD)
Porsche (these are interesting. The front engine ones usually have a rear-mounted transaxle, which means you just couple the motor to the torque tube and retain the existing clutch/flywheel. Rear engine ones are longitudinal, which is also easy)
Audi 80, 90, A4, some VW Passat (these are either FWD or AWD, but have longitudinally mounted engines)
Subarus can be converted to RWD, but the ability to get it engineered and registered is a question. From my reading, it seems like it would have to be engineered as an ICV (individually constructed vehicle), which has cost and insurance implications.
Many vans
Any ute
Falcons and Commodores

I won't list FWD cars, there's not enough room on this thread ;-)


Thoughts?

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FWD, RWD, AWD considerations

Post by acmotor » Mon, 05 Nov 2012, 08:02

Much of the consideration of EV conversion should be focused on the battery placement as it is the dominating factor in both weight and volume. In some ways an EV conversion design should start with the batteries. FWD / RWD all work if you retain a suitable weight distribution.

I have seen many FWD conversions that end up tail heavy as they place most batteries at the rear. Be aware of the pre and post conversion weight distribution. The emotor will generally be lighter than the ICE it replaces.

I think it may be a myth about FWD vs RWD gear train efficiency.
RWD contains one 90 deg (crown and pinion) at the diff however FWD contains two parallel couplings, one to turn the drive train at end of gearbox and another to drive to the crown wheel of diff unit. Either way, the geartrain efficiency differences are down in the 1% range I'd expect.
Another consideration is that the FWD system will often force the converter to retain the gearbox. Now THAT will reduce the overall efficiency.

Purpose built EV RWD is probably the simplest mechanically. e.g. iMiEV emotor with parallel drive to crown wheel of diff with live coupling shafts to wheels. There are not the massive CVs limiting steering angle as in FWD EVs.

That RX400 hybrid's RWD emotor unit looked at elsewhere on the forums would be a nice item in a conversion.

To address the topic question though... I'd say any vehicle can be converted. There is no 'better'. It just depends on your skills and preferences.
OK, showed my RWD preference. Well if you can't have AWD then RWD is next best. Image
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Post by coulomb » Mon, 05 Nov 2012, 13:40

acmotor wrote: ... however FWD contains two parallel couplings, one to turn the drive train at end of gearbox and another to drive to the crown wheel of diff unit.

I've wondered about that.

With this image that I picked up somewhere:

Image

are you saying that there is typically another gear between the output shaft of the gearbox and the large ("crown") gear of the diff?
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Post by Nevilleh » Mon, 05 Nov 2012, 18:58

I avoided FWD in my choice of vehicle because of the inefficiency of retaining the gearbox and its pretty much impossible to do a FWD car without doing so.
I have to say that direct drive to the diff in a RWD car is pretty neat, but the torque requirements are a bit higher if you want reasonable performance. My BMW has a 4.1 diff and the acceleration was pretty good - acceptably so - when I ran a 1000a controller, less so with a 500A one.
I'd go for RWD every time and BMW 3 series cars are ideal , being fairly light for their size and able to pack away lots of batteries fairly easily. I managed to get the weight distribution somewhat better than the original by having 80-odd kg of motors replacing the original engine and gearbox, 120 kg of battery in about where the old engine was, and 80 kg of battery in the boot, while still retaining a very usable boot space.

If I were to do another, a later model than my E30 would be my choice. Direct drive, 150V dc, 1000A controller and try and find a 4.44 diff would be the go.

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Post by BigMouse » Mon, 05 Nov 2012, 21:20

Nevilleh wrote:I have to say that direct drive to the diff in a RWD car is pretty neat, but the torque requirements are a bit higher if you want reasonable performance.
Performance is the key word there for me. One of the goals of my conversion (e36 BMW coupe or hatch) is to make an electric car that not only looks good, but performs well. With a gearbox, my 0-100 times are calculate around 6 seconds in the coupe, or 5.5 in the hatch with the same gearing. With direct drive and a 4.44:1 diff ratio, I would expect about 14 seconds to 100kph. All these calculations are done based on the expected post-conversion weight (~1500kg) and a (hopefully conservative) 220Nm constant torque to 7800rpm (end of constant V/F range for my motor and battery voltages) with 0.5s gear change time for the retained gearbox scenario.

Keeping the gearbox will allow me to approach/match stock M3 acceleration performance. Direct drive would be slower and would limit my hill-climbing ability.

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Post by coulomb » Tue, 06 Nov 2012, 04:38

BigMouse wrote: Performance is the key word there for me. ... With direct drive and a 4.44:1 diff ratio, I would expect about 14 seconds to 100kph. ... Direct drive would be slower and would limit my hill-climbing ability.

True, for most situations, you can get better performance with a multi-speed transmission. But it is possible to get good performance from direct drive, e.g. the Tesla Roadster and model S. The key there is a very high maximum motor speed (circa 14,000 RPM for the Teslas), and a higher overall ratio (circa 8:1 to 10:1). Oh, and a motor with a reasonably flat power curve over most of that speed range.

Of course, one wonders what a Telsa would be like if it had a multi-speed transmission as well as the powerful motor. It would be possible to get even more torque at lower speeds, though if the higher torque exceeds the tyre traction limit, then it's just a waste of money, space, and weight. Perhaps you could get near-tyre-traction-limit torque at medium speeds, and gain an advantage there.
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Post by acmotor » Tue, 06 Nov 2012, 07:53

This is my thinking on the FWD gearbox....

Image

This is a 5 speed gearbox, note the usual straight cut gears for reverse.
The gears of course are moved to mesh as required.

I see two parallel shaft transfers of power.

There can be similar in manual gearboxes on RWD but autos are straight through in at least one ratio.
Thing is, RWD direct drive has only one 90deg transfer and that in gear terms is more efficient than two parallel transfers AFAIK.
iMiEV and some other production EVs and even some dirty EVs drive parallel direct to crown gear of diff. (while we wait for hub motors)


On the topic of performance, I agree that EVs should perform well. Image
Can I make some generalised comments though....

1) a (multi ratio) gearbox makes up for lack of power or lack of power band or lack of torque at low revs. Nothing more, and you pay in $ and weight and losses to have the gearbox.

2) since W=wT thats (2.pi.f)T we know that if the torque is constant at the breaking traction limit for the tyres (optimal design) then (1) above comes in to place such that a multi ratio gearbox can try to make up for torque shortfall particularly at low revs with an ICE. However far less that max acceleration is available as road speed increases if motor just doesn't have to torque. Back to (1).
Also that the max W for an emotor can be selected based on the traction limit. e.g. say 2000Nm at rear wheels, 1000RPM@100kmph, 4:1 back to emotor gives
W = 2 x 3.14 x 4000/60(rev/sec) x 2000/4 = 209kW   
So if 2000Nm were the traction limit then a 209kW (peak) emotor would give all the possible acceleration and no gearbox would improve on that.
(this is just a simple example to illustrate the point, don't nit pick !)

3) at some point you need to stop thinking 2WD (FWD or RWD) and think AWD if you are ever going to achieve acceleration on anything other than dry straight roads.

Don't get hung up on the exact numbers but see if you get the idea...
A ball park may be down to 10 seconds 0-100k 2WD OK on dry roads and already questionable on wet / loose surface. Fine for AWD.

Down to 5 seconds strictly dry road straignt line for 2WD.
AWD still fine on wet and loose.

Sub 5 seconds likely requires specialised rubber and excellent surface for 2WD but not really practical on the general road. Very much traction limited.

So if you want real on road ability, you need to look at AWD.

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Post by PlanB » Tue, 06 Nov 2012, 14:53

acmotor wrote: That RX400 hybrid's RWD emotor unit looked at elsewhere on the forums would be a nice item in a conversion. Image


I still have dreams about my 40kg door stop here. Two of 'em with a couple of modest 80A controllers & the fabled 720v battery pack & BMS for 1800Nm AWD. Meanwhile, in the real world, Ian's blue beast is looking impressively stealthy.


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Post by weber » Tue, 06 Nov 2012, 16:16

Thanks for a brilliant post, acmotor. And thanks for asking the right question, coulomb.

I dunno about everyone else, but I did not previously realise there was a simple calculation you could do to determine (at least approximately) when a (multi-ratio) gearbox would be pointless for performance.
coulomb wrote:The key there is a very high maximum motor speed (circa 14,000 RPM for the Teslas), and a higher overall ratio (circa 8:1 to 10:1).

I disagree. I'd say the key here is high power, and the key to getting that from a motor of reasonable mass is high electrical frequency.

A high reving 4-pole followed by a big gear-reduction is only one way to achieve that. Another way is to increase the number of poles to something like 24, in which case the same thing can be done at low rpm with no gear reduction. This is the approach taken by Ultramotive with their ironless PMAC motors, as discussed here:
viewtopic.php?title=direct-to-wheel-motors&t=1340
Oh, and a motor with a reasonably flat power curve over most of that speed range.
I don't see that as necessary at all. It is torque, not power, that is limited by traction. I presume it is much the same limiting torque at all speeds, therefore a flat torque curve seems more useful. And the curves really belong to the battery/motor/inverter combination, not just the motor.

The Tesla only has constant power between about 5000 and 7000 rpm. It has constant torque from almost zero to 5000 rpm. From tyre size and overall drive ratio I calculated that 100 km/h corresponds to approximately 7000 rpm.

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Post by BigMouse » Tue, 06 Nov 2012, 16:23

acmotor wrote:Can I make some generalised comments though....

1) a (multi ratio) gearbox makes up for lack of power or lack of power band or lack of torque at low revs. Nothing more, and you pay in $ and weight and losses to have the gearbox.


I agree with everything you've mentioned but have a couple comments on point (1).

A gearbox makes up for lack of power/torque, sure. Not arguing that. Direct drive is awesome if you can find an 8:1 final drive to play with. But for real conversions, the highest ratios found are going to be in the 4-5:1 range, maybe higher in a 4WD with a transfer case (added losses). In order to direct drive with an available ration, you need that 209kW (peak) motor you mentioned. It's not going to be spinning very quickly at the max speed with direct drive, so that power needs to come from "T" more than "w". A motor with high "T" is going to be a large frame size, and you're aware. 160 or bigger to get any real torque (your 2000Nm at the wheels with a 4.44:1 final drive would still require 450Nm from the motor). A 5-spd gearbox doesn't weigh much, maybe 30kg (one number I've found for mine). My 132 frame motor weighs around 70kg. A 160 frame is around 100kg. The highest torque 4-pole WEG 160 frame has a rated torque of about 100Nm and a Tb of 270Nm. A 180 frame can do (340Nm) and weighs 164kg. On an inverter, that might be able to achieve 450Nm, but the weight is significantly more than a 132 and a gearbox.

For comparison, my 132 motor (Tb of 150Nm) on an inverter should be able to do about 220Nm (as a guess). In 2nd gear with my current 3.45:1 final drive, that's 1900Nm at the wheels. (In 1st gear, 3200Nm, well beyond the traction limit).

I <3 AWD too. I've owned quite a few Quattro Audis and I'm very familiar with the benefits you describe. Maybe someday I'll convert a Subaru Impreza and make a true performance EV. With AWD, you'd pretty much have to keep the gearbox anyway, unless you do like that guy with the autocross EV impreza with two motors and two rear diffs.

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Post by acmotor » Tue, 06 Nov 2012, 17:43

Agreed, the gearbox and whatever other drive ratio for an EV are really there to match available torque and RPM of the emotor to the vehicle traction limits and place the emotor RPM range (say 6000RPM)= expected vehicle speed range (say 120kmph).

So keep the gearbox (if you have to) to achieve the one required ratio, maybe 7:1 overall as you say (2nd gear in your case). But don't intend changing gears, at least not if pushing for best 0-100 and particularly if clutch has been removed.   Image
As you say, 1st gear exceeds traction limit in your example so why use it unless you are a tyre waster. if 3rd gear means emotor is less than max RPM then why go there ? You're homing in on just one ratio.
Remember that emotors are not wear or noise or fuel wasters when revving higher. In fact most AC motors have their efficiency sweet spot at or above their sync RPM.

If you are chasing top performance and you can't get it with one ratio then get a bigger motor !
Image

edit: I'm considering AC motors with their wide RPM flat torque operation of course. A for acceptable vs D for dissssapointing. Image
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Post by Richo » Tue, 06 Nov 2012, 20:48

acmotor wrote: If you are chasing top performance and you can't get it with one ratio then get a bigger motor !
Image


It won't fit.
The motor I have is going to be a squeeze as is.(ie a hammer may be involved Image )

Removing the box doesn't help that much as the diff is placed next to the clutch assy.
I would have to have an additional shaft to come back part way the length of a bigger motor.

My only other option is to spin the motor I have faster...
So the short answer is NO but the long answer is YES.
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Post by jonescg » Sat, 22 Dec 2012, 05:12

I had a friend, who has done a few performance mods to ICE cars before, tell me that I would snap the gearbox if I tried to mount my Evo motor to the existing bell housing. Sure, lots of torque is on offer, but the bigger concern is actually clearing the RHS driveshaft.

He suggested I try to fit an independent rear suspension diff in the front and mount the motor behind it, directly. With a typical 4:1 ratio of offer, I'd have a top speed of about 130 km/h.

Sounds attractive, but how difficult would it be to do this? I'd save some weight by removing the gearbox, and my wife could drive it since its a perfect automatic Image.

Plausible?
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Post by KDRYAN » Sat, 22 Dec 2012, 12:01

Image
This is a electric mod I used direct drive to diff.

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Post by jonescg » Sat, 22 Dec 2012, 17:59

Hey! That looks like what I had in mind, except I was planning on having the motor behind the diff. But as this picture demonstrates, I would probably struggle to fit it there. Makes keeping the gears happy, since its still turning in it's preferred direction and it's not upside down.

I would be interested to know more about this modification!

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Post by Adverse Effects » Sat, 22 Dec 2012, 22:19

you could even do this

Electric Moke Conversion

scroll down to pics

i know they have been posted here before but couldnt find them

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Post by jonescg » Sat, 22 Dec 2012, 23:34

Given the diameter of the Evo motor, I don't think that would work Image

Placing it in front would probably be ideal since it's not going to take up as much room as a radial flux motor, and not hang over the front wheels as much either. Still not as ideal as having all of the weight between the wheelbase, but pretty good. I love the thought of a direct drive though Image
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Post by coulomb » Sun, 23 Dec 2012, 01:40

jonescg wrote: I would be interested to know more about this modification!

I think that this is it (unfortunately only one picture, and the web page doesn't work):

http://www.evalbum.com/998

Also this build page:

viewtopic.php?t=851
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Post by KDRYAN » Sun, 23 Dec 2012, 02:37

The yellow direct drive unit was constructed using a super zebo dc series motor and a 1996 Toyota RAV4 rear diff. The unit functioned well but the ratio (2.98:1) was to high and unfortunately I wasen't able to obtain a higher diff ratio for that type of housing.   3.5>4:1 would have been ideal for the power and type motor I was using. The RAV 4 diff was very suitable for this mod due to the fact it had a bolt up section on the nose of the diff housing.


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Post by coulomb » Sun, 23 Dec 2012, 04:28

KDRYAN wrote: ... but the ratio (2.98:1) was to high ... 3.5>4:1 would have been ideal...

I think you mean the gearing was too tall, like a car in overdrive (good top speed but not a lot of torque). So the ratio was numerically too low. Assuming of course that you think of the ratio as a number greater than one (2.98), rather than less than one (1/2.98 = 0.336).
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Post by jonescg » Sun, 23 Dec 2012, 04:46

Yes I was thinking that 3.8:1 would be about right in my case.

0.00172 km/rev * 300000 rev/h * (1/3.8) = 135 km/h.

That said, I will have more than enough torque to push a higher top speed. Probably safer to go with a bigger ratio than smaller.

I wonder if Honda had any IRS diffs. All I can think of is a CRV, but maybe they had others? There's a good chance the splines / shafts would match.
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Post by jonescg » Sun, 23 Dec 2012, 05:00

Just found the CRV has a ratio of 4.43:1. While it would climb hills like crazy, it would have a top speed of 116 km/h. If it's an easier fit, then I can take the lower top speed... I guess.
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Post by jonescg » Wed, 26 Dec 2012, 20:34

I might have found the ideal diff - the MX5-NB series has an IRS diff with a 3.63 or 4.10 : 1 ratio, depending on the model. Either of these would work for me, with the shorter diff probably being preferable.

One of the real benefits is the short pinion, and it comes with supports which I can somehow fix to the chassis.

Image
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Post by BigMouse » Fri, 28 Dec 2012, 23:44

BMW E36 diffs from cars with 4cyl engines and auto transmissions came in 4.44:1. They're also very compact, and don't have those large mounting arms on them.

Here's an example:
Image

That's a mid-case diff too. The 4.44:1 diffs are the small-case ones.

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Post by jonescg » Fri, 28 Dec 2012, 23:52

Thanks for the tip! I think 4.44 is a bit too low for me. The motor will be spinning pretty fast at highway speeds. 3.9 to 4.1 is ideal I think.

Oh, I figured the mounting struts could be made to fit the CRX. Without them I might struggle to find a means to support the motor-diff combination.
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