Drivetrain pros/cons and future trends?
Discussion
The compactness and controllability of electric motors has led to a greater diversity of drivetrain layouts; ordinary family hatches come with front, rear and twin-motor 4wd propulsion, while three and four motor drivetrains are on sale, enabling tricks like torque vectoring and the videogenic 'tank turn'.
My e-C4 is FWD, and to be honest it does suffer a little from torque steer and traction control intervention. I'm always tempted to use the immediate torque, and then the front tyres often can't quite handle it.
Compared to FF ICE cars, an EV has closer to 50/50 weight balance as the battery is under the passenger compartment, so has less weight over the driven front wheels and worse natural traction (although the more neutral balance should benefit pure lateral behaviour).
Given the sophistication of modern stability control and the controllability of electric motors, and a 50/50 weight balance, I can't really see any benefits to front-drive over rear. Most FWD EVs seem to be shared platforms with ICE models, apart from perhaps the Zoe.
I had a day in a Taycan 4S, whose combination of perfect traction, perfect precision, immense acceleration and strong and easily measured out braking made it an absolute point-blank overtaking weapon, creating passing spots on the shortest straight.
However most reviews suggest that for EVs that come in both RWD and 4WD versions, the 2WD version is preferable - good enough traction, less over the top performance, and some potential adjustability.
In principle, a four motor layout seems ideal, as you can then do whatever you want with the drive in software, so total flexibility - and the potential to use torque vectoring to overcome the larger moment of inertia of battery EVs.
So I guess two questions, what experiences do people have of different layouts, and any predictions for whether EVs will end up standardising on one or two layouts in future?
My e-C4 is FWD, and to be honest it does suffer a little from torque steer and traction control intervention. I'm always tempted to use the immediate torque, and then the front tyres often can't quite handle it.
Compared to FF ICE cars, an EV has closer to 50/50 weight balance as the battery is under the passenger compartment, so has less weight over the driven front wheels and worse natural traction (although the more neutral balance should benefit pure lateral behaviour).
Given the sophistication of modern stability control and the controllability of electric motors, and a 50/50 weight balance, I can't really see any benefits to front-drive over rear. Most FWD EVs seem to be shared platforms with ICE models, apart from perhaps the Zoe.
I had a day in a Taycan 4S, whose combination of perfect traction, perfect precision, immense acceleration and strong and easily measured out braking made it an absolute point-blank overtaking weapon, creating passing spots on the shortest straight.
However most reviews suggest that for EVs that come in both RWD and 4WD versions, the 2WD version is preferable - good enough traction, less over the top performance, and some potential adjustability.
In principle, a four motor layout seems ideal, as you can then do whatever you want with the drive in software, so total flexibility - and the potential to use torque vectoring to overcome the larger moment of inertia of battery EVs.
So I guess two questions, what experiences do people have of different layouts, and any predictions for whether EVs will end up standardising on one or two layouts in future?
740EVTORQUES said:
Four wheel drive means you can have active brake regenerative on both axles, although only more powerful cars will have big enough front inverters to have the desirable front biased brake balance on regenerative alone.
Motors that are rotating but not in use still generate losses, including hysteresis losses in the iron of the stator. I've mentioned before that BMW have opted for a wound-field rotor arrangement on some cars that uses two carbon brushes running on a pair of continuous rings on the rotor to energise the rotating field.The benefit of this is that when the front (or rear) motor is not in use, the hysteresis losses can be eliminated by turning the rotating field off completely. Obviously a brushed rotor sounds somewhat archaic, the use of a pair of continuous rings means there are no discontinuities that would tend to wear the brush down.
The front motor can the be instantly energised at the application of regen, and operate on the front wheels to achieve the bias you mentioned.
Direct control of the rotating field also allows for precise field-weakening at high rpms, a feature of most EV motors that puts the motor drive system into a constant power regime, with torque decaying as the rpms increase i.e. for high speed cruising without the need for extreme acceleration. Hysteresis losses increase with speed, so there is also the benefit of marginally improving efficiency at high speed cruise by weakening the field.
Edited by GT9 on Sunday 28th April 07:47
samoht said:
...
Given the sophistication of modern stability control and the controllability of electric motors, and a 50/50 weight balance, I can't really see any benefits to front-drive over rear. Most FWD EVs seem to be shared platforms with ICE models, apart from perhaps the Zoe.
Zoe is a Clio with a slightly different body, they were built on the same production line.Given the sophistication of modern stability control and the controllability of electric motors, and a 50/50 weight balance, I can't really see any benefits to front-drive over rear. Most FWD EVs seem to be shared platforms with ICE models, apart from perhaps the Zoe.
nufcfan said:
Good question. I’m currently toying with a new EV purchase and debating AWD vs oneof the 2 wheel drive models. I do live in an area that gets quite icey in the winter so always kept clear or RWD but I’m not sure it’s as big an issue with EVs.
EVs tend to have better traction control than ICE, and more weight over the rear axle, so that would help in a small way, but the greater torque of an electric motor would offset that. You would have to be gentle with the throttle, *and* with lift-off due to regenerative braking only operating on two wheels.I'd go for AWD with Michelin Cross-climates.
Edited by gmaz on Sunday 28th April 09:47
nufcfan said:
Good question. I’m currently toying with a new EV purchase and debating AWD vs oneof the 2 wheel drive models. I do live in an area that gets quite icey in the winter so always kept clear or RWD but I’m not sure it’s as big an issue with EVs.
If the weather gets bad enough for long enough to justify worrying about it then winter tyres are the answer. AWD will help with traction but at the end of the day it's the tyre that makes all the difference when turning and stopping.What cars are you considering out of interest?
nufcfan said:
Thanks. Bit of a BMW fan so possibility the iX3 or maybe AWD ix1. IX is probably a bit big for needs but the ix3 is RWD only.
Alternatives looking at Q4 in either rwd or AWD.
Should be able to pick up a set of good quailty Vredestein winters for < £700 for the iX3. The placement of the battery pack and motor does also mean that unlike your usual RWD BMW plenty of weight is towards the back of the car (60% rear) so traction is less of a problem. Alternatives looking at Q4 in either rwd or AWD.
Seams to me 4wd system was pushed by Audi on ICE for performance, basically to solve the traction loss when accelerating 0-60. The car weight shifting to the rear (loosing front wheel grip ) combined with the engines north -south mount. 4WD had other advantages which lead others manufacturers to follow. The down side being weight and drive chain windup.
Fast forward to EV which have no such issues as motor and traction control systems are far better. The advantages verses cost/weight if not really there, a nice to have for sure
Fast forward to EV which have no such issues as motor and traction control systems are far better. The advantages verses cost/weight if not really there, a nice to have for sure
ashenfie said:
Seams to me 4wd system was pushed by Audi on ICE for performance, basically to solve the traction loss when accelerating 0-60.
Quite.As with many things, a combination of increased mass, resulting in a desire for more power, resulting in a desire for higher efficiency for lower fuel consumption/CO2.
Resulting in the need to distribute propulsion to all four wheels, adding complexity and mass.
The pathway that we've already trodden with ICE powertrains has shifted wholesale from what I would call a pure ICE to a highly complex, turbo-charged, overly-torquey, overly-geared, not very characterful conundrum.
By overly torquey I mean that it is unavoidable that too much wheel torque appears, often when you don't want it, in the pursuit of trying to have enough wheel torque at other times.
Requiring complex electronic aids that can only do so much to contain the wheel torque spikes that result from repeatedly building up and releasing inertia in the powertrain while delivering a saw tooth power profile that creates all sort of traction and loss of stability problems when conditions are less than ideal.
It's basically trying extremely hard to mimic a single speed electric powertrain but failing, so why the f
k not just fit a single speed electric drivetrain and remove all of the failings of the mimickery, including the noxious byproducts and poor energy utilisation.I've often pondered why aliens have often been reported flying over our planet but quickly buggering off.
I think they've looked at what we are doing, basically pumping colossal amounts of waste heat and other undesirable stuff into the air that is crucial for our existence in order to push metal boxes at high speed through that same air, so we can enhance our existence, but often worsen our existence, or those around us, by doing so.
They've basically seen the madness of it all from a great height and said WTF.
I've driven a whole bunch of EVs, and I've found some of the smaller single motor ones powering the front wheels to be challenging when you want to make a quick getaway, especially in the wet. The smaller cars with single motors but rear wheel drive like bog standard Q4 e-tron felt better to drive in all conditions, keeping the front wheels for steering only.
Personally, I've ended up choosing a dual motor car, as the all wheel drive feels reassuring when pushing on, and when cruising along, it will disconnect (or switch off?) the front motor, so you don't lose out on efficiency at speed.
I would like to try the quad motor EVs when they come out here.
Personally, I've ended up choosing a dual motor car, as the all wheel drive feels reassuring when pushing on, and when cruising along, it will disconnect (or switch off?) the front motor, so you don't lose out on efficiency at speed.
I would like to try the quad motor EVs when they come out here.
great thread. proper school day for me GT9!
I've long questioned the need for 4wd for most people. I spend a lot of time in the Alps with a 4wd Rav4, it's on good winters and I don't think it ever engages the rear tyres. certainly no dash lights pop up as with the application of brakes by ESC. I also have an old Jimny for the local farm tracks, which are muddy and rutted. fitted with AT tyres I've never needed to select 4wd.
Neither of those cars are driven with vigour. But if they were, then 4wd would assist traction but not cornering or braking (cue pedants citing engine braking over 4 wheels). Anyway, interesting how most cars I see in ditches in the Alps are 4wd! on summers.
Vast majority of cars will never go above 90mph, so single motor one gear is surely best for 99% of UK cars? rwd too.
I've long questioned the need for 4wd for most people. I spend a lot of time in the Alps with a 4wd Rav4, it's on good winters and I don't think it ever engages the rear tyres. certainly no dash lights pop up as with the application of brakes by ESC. I also have an old Jimny for the local farm tracks, which are muddy and rutted. fitted with AT tyres I've never needed to select 4wd.
Neither of those cars are driven with vigour. But if they were, then 4wd would assist traction but not cornering or braking (cue pedants citing engine braking over 4 wheels). Anyway, interesting how most cars I see in ditches in the Alps are 4wd! on summers.
Vast majority of cars will never go above 90mph, so single motor one gear is surely best for 99% of UK cars? rwd too.
CABC said:
great thread. proper school day for me GT9!
I've long questioned the need for 4wd for most people. I spend a lot of time in the Alps with a 4wd Rav4, it's on good winters and I don't think it ever engages the rear tyres. certainly no dash lights pop up as with the application of brakes by ESC. I also have an old Jimny for the local farm tracks, which are muddy and rutted. fitted with AT tyres I've never needed to select 4wd.
Neither of those cars are driven with vigour. But if they were, then 4wd would assist traction but not cornering or braking (cue pedants citing engine braking over 4 wheels). Anyway, interesting how most cars I see in ditches in the Alps are 4wd! on summers.
Vast majority of cars will never go above 90mph, so single motor one gear is surely best for 99% of UK cars? rwd too.
What if you want a high performance EV in the UK? Can single motor EVs provide the rapid acceleration that buyers of performance EVs want?I've long questioned the need for 4wd for most people. I spend a lot of time in the Alps with a 4wd Rav4, it's on good winters and I don't think it ever engages the rear tyres. certainly no dash lights pop up as with the application of brakes by ESC. I also have an old Jimny for the local farm tracks, which are muddy and rutted. fitted with AT tyres I've never needed to select 4wd.
Neither of those cars are driven with vigour. But if they were, then 4wd would assist traction but not cornering or braking (cue pedants citing engine braking over 4 wheels). Anyway, interesting how most cars I see in ditches in the Alps are 4wd! on summers.
Vast majority of cars will never go above 90mph, so single motor one gear is surely best for 99% of UK cars? rwd too.
raspy said:
What if you want a high performance EV in the UK? Can single motor EVs provide the rapid acceleration that buyers of performance EVs want?
Depends on whether it's a strict 2-seater or something a bit bigger.Right now, or at least within the next couple of years, I'd say yes it's possible, as long as you choose a car that doesn't follow the mainstream design of large skateboard battery propelling a full-size 4/5 seater metal body.
Once you get into the 2 ton mass bracket, I'd say there just isn't enough grip available at the rear axle for the required power to weight to speed ratio.
The obvious blueprint we've already seen is the i3s, although not really what you'd call high performance.
Increasing the budget, I'm thinking the likes of Caterham's Project V look like they have the right ingredients.
Personally, I've driven several test/rental fwd EV's and one red. I've owned two AWD and much prefer them. EV torque, especially if rolling 10-20mph is significant and you need AWD to put the power down. Obviously that's only relevant if you have a powerful EV, but this is PH so I'll assume that's what everyone has or at least wants for the purposes of this thread 
For the torque vectoring performance and off road benefits, I do hope continually reducing motor prices and tighter package leads to motor per wheel becoming standard for the more powerful offerings. Moving the motor very close to each wheel also frees up the space traditionally required for duff and axles, which could be very useful for land rover type vehicles looking to gain rude height and/or more useful interior space. Something that rivian has already done an excellent job of proving in terms of performance and ability.
For the torque vectoring performance and off road benefits, I do hope continually reducing motor prices and tighter package leads to motor per wheel becoming standard for the more powerful offerings. Moving the motor very close to each wheel also frees up the space traditionally required for duff and axles, which could be very useful for land rover type vehicles looking to gain rude height and/or more useful interior space. Something that rivian has already done an excellent job of proving in terms of performance and ability.
GT9 said:
Depends on whether it's a strict 2-seater or something a bit bigger.
Right now, or at least within the next couple of years, I'd say yes it's possible, as long as you choose a car that doesn't follow the mainstream design of large skateboard battery propelling a full-size 4/5 seater metal body.
Once you get into the 2 ton mass bracket, I'd say there just isn't enough grip available at the rear axle for the required power to weight to speed ratio.
The obvious blueprint we've already seen is the i3s, although not really what you'd call high performance.
Increasing the budget, I'm thinking the likes of Caterham's Project V look like they have the right ingredients.
Right, so if I want to drive a 5 metre long EV with space for 5 people and their luggage and can do 0-60 in under 4 seconds, I'm going to still need a dual motor EV in years to come. Right now, or at least within the next couple of years, I'd say yes it's possible, as long as you choose a car that doesn't follow the mainstream design of large skateboard battery propelling a full-size 4/5 seater metal body.
Once you get into the 2 ton mass bracket, I'd say there just isn't enough grip available at the rear axle for the required power to weight to speed ratio.
The obvious blueprint we've already seen is the i3s, although not really what you'd call high performance.
Increasing the budget, I'm thinking the likes of Caterham's Project V look like they have the right ingredients.
raspy said:
Right, so if I want to drive a 5 metre long EV with space for 5 people and their luggage and can do 0-60 in under 4 seconds, I'm going to still need a dual motor EV in years to come.
5 people, sure, but that's not much different to the choice of modern ICEs (i.e. 4WD).As an exercise we could all try to nominate a sub 4 second 2WD car that can carry 5 people with luggage.
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