18 replies to this topic

[mariner]

This is an extract from the Autocar test of the revised Polestar 2 which has switched from  FWD to RWD

 

 But on our 70mph motorway touring efficiency test, the improvement was 26%. That’s partly because, as EV makers have learned, a car is always more efficient at a steady cruise when being driven on from the rear axle (where the majority of its mass is carried, with more of the drive torque being translated into forward motion) than it is if pulled from the front.

 

I am not saying it's not true but I wonder why as CV joints are still needed.

 

BTW I can see the re -run of the " fuel economy lies debate " coming soon.

 

Claimed fuel economy

3.9-4.2mpkWh

 

Test fuel economy - average

3.2mpkWh

 

[desmo]

I'd like to see the thermodynamic sums explaining that.

[Greg Locock]

Absolute tosh.

[Magoo]

I was curious enough about the statement to look up the original article to see if I could get a glimmer of what the author might be trtying to say. No help there. 

 

Rear-drive packaging can be more efficient with an EV, which the writer may have picked up in a media presentation and then misconstrued. 

[Magoo]

3.2 miles per kWh is not very remarkable.

[ARTGP]

When cruising at 70mph, you are using so little power/torque (one only needs to overcome drag, rolling resistance, and drivetrain friction) that the driven wheel is irrelevant. 

 

 RWD could be more efficient in the edge case scenario (lead foot syndrome). In that use case, the rear weight transfer means the rear axle is better at putting power to the ground than the front axle. This is a roundabout measure of efficiency, although it's not very economical to drive this way. 

Edited by ARTGP, 12 September 2023 - 03:40.

[kikiturbo2]

I always thought that for an EV FWD would be preferable as it would enable higher recuperation under (heavy) braking

[Ross Stonefeld]

Can't you just harvest the front and propel the rear?

 

Are most EVs FWD or RWD or AWD or... ?

[Bloggsworth]

In the BTCC the rear wheel drive cars have to run a punatively high 1st gear or they would run off into the boondocks at the start of every race, so clearly when rearward weight transfer is involved, RWD is advantageuos. I suppose that one could posit a similar case when the car is going up hills as there will be an element of rearward weight transfer there; but whether overall there is some thermal efficiency benefit over a long journey I don't know. I always thought that the almost universal change to FWD was related to ease and cost of manufacture, being able to shove the whole engine/drivetrain up from underneath in one go etc.; personally, though I drive an FWD car, I'm still uncomfortable with one pair of wheels doing nearly 70 to 80% of the work...

[Wuzak]

In the BTCC the rear wheel drive cars have to run a punatively high 1st gear or they would run off into the boondocks at the start of every race, so clearly when rearward weight transfer is involved, RWD is advantageuos. I suppose that one could posit a similar case when the car is going up hills as there will be an element of rearward weight transfer there; but whether overall there is some thermal efficiency benefit over a long journey I don't know. I always thought that the almost universal change to FWD was related to ease and cost of manufacture, being able to shove the whole engine/drivetrain up from underneath in one go etc.; personally, though I drive an FWD car, I'm still uncomfortable with one pair of wheels doing nearly 70 to 80% of the work...

 

The reason front wheel drives with transverse engine and gearbox are more efficient than front engine/rear wheel drive cars is that they don't have to change the drive through 90 degrees.

 

This doesn't apply to rear wheel drive EVs, so there should be no difference in efficiency between front and rear wheel drive.

[desmo]

Yeah, if dynamic f/r weight shifts are significantly affecting your traction enough to impact mileage, you are a very, very sporty driver!

[Bloggsworth]

Yeah, if dynamic f/r weight shifts are significantly affecting your traction enough to impact mileage, you are a very, very sporty driver!

Did I say significantly? No, I don't think I did...

[Magoo]

Someone's going to figure out how to run on three wheels like a Pinewood Derby car. 

[Greg Locock]

First order approximation is that rolling resistance is proportional to load, so for a given vehicle weight the distribution and load transfer  and number of identical tires don't have any effect.

[Magoo]

That was strictly tongue-in-cheek, but it works like the blazes on Pinewood Derby cars. 

[Greg Locock]

Sure, aero. Also it stops the tires fighting each other on irregular surfaces as a 3 wheeler is statically determinate. 

Edited by Greg Locock, 15 September 2023 - 21:24.

[Magoo]

Can't you just harvest the front and propel the rear?

 

Are most EVs FWD or RWD or AWD or... ?

 

Put the short tires on the front and the tall tires on the rear and let gravity do the work. Downhill all the way. 

 

Front-drive EVs tend to be adaptations of existing ICE platforms. Clean-sheet EVs with 2WD tend to be  RWD as the motor is roughly the size of a differential, a boon to packaging. From there it's easy to create an AWD version by adding a second motor at  the front. Since it's all electronics, traction control and torque vectoring are simple. 

 

Hub motors once held some promise, mainly as a means to quick 'n easy adaptations of ICE pickup platforms to EV. Lordstown Motors, RIP.  

[Magoo]

Okay, here are two guys who do know what they're talking about.

 

Auto industry past, present, and future, spilling the tea on Jose Lopez, Roger Smith, et al. Autonomy, Tesla, and the great EV flip. Rambles toward the end, as we seniors tend to do. 

 

These are two of the people who, several years ago, got me interested in Tesla. Blame them. 

 

 

 

Edited by Magoo, 16 September 2023 - 12:43.

[Lee Nicolle]

 

This is an extract from the Autocar test of the revised Polestar 2 which has switched from  FWD to RWD

 

 But on our 70mph motorway touring efficiency test, the improvement was 26%. That’s partly because, as EV makers have learned, a car is always more efficient at a steady cruise when being driven on from the rear axle (where the majority of its mass is carried, with more of the drive torque being translated into forward motion) than it is if pulled from the front.

 

I am not saying it's not true but I wonder why as CV joints are still needed.

 

BTW I can see the re -run of the " fuel economy lies debate " coming soon.

 

Claimed fuel economy

3.9-4.2mpkWh

 

Test fuel economy - average

3.2mpkWh

 

Independent suspension is the very obvious answer.

An electric Ovlov is an Ovlov, irrelevant to motoring