Why is 4WD less efficient than 2WD?
Discussion
A comment on another thread about Audi's got me thinking earlier - someone (paraphrasing) said that they would avoid the 4WD version of a car because the fuel economy was crap compared with the 2WD version. Same car, same engine, same gearbox, just a different drivetrain. Thinking logically, the force required to move the car is the same, and while there will be a small amount of extra transmission losses from the transfer box and differential, why is there such a difference in economy between 2 otherwise identical cars? Feels like there should be a really simple explanation, but it's passed me by. Officially (under the old test) there is 5MPG difference between an A4 Quattro and standard 2WD A4 (190hp diesel, 2015 reg), although I believe in real world driving the difference is much greater.
TGCOTF-dewey said:
It's not a small amount of transmission loss.
+ extra mass to carry.
How much is lost in the transmission? Especially in cars with Quattro, BMW XDrive etc as they only activate the 4WD when the 2 driven wheels start slipping as far as I understand. Extra weight is about 100kg or so, or 1 extra passenger. + extra mass to carry.
I've owned 3 4WD saloon cars, the rest (many) have been either RWD or FWD.
Now don't get me wrong, I have an appreciation for a good AWD system, but the truth is I've never really needed it. My 2WD cars have never left me stranded due to lack of grip and I've found a good pair of winter tyres on a FWD car will perform as good in snow as a 4WD car with summer tyres.
With the extra weight and drive train loss meaning higher fuel consumption and reduced power to the road I see no real benefits for me. I am of course just talking about normal cars here, not (proper) off road vehicles which are used accordingly.
Now don't get me wrong, I have an appreciation for a good AWD system, but the truth is I've never really needed it. My 2WD cars have never left me stranded due to lack of grip and I've found a good pair of winter tyres on a FWD car will perform as good in snow as a 4WD car with summer tyres.
With the extra weight and drive train loss meaning higher fuel consumption and reduced power to the road I see no real benefits for me. I am of course just talking about normal cars here, not (proper) off road vehicles which are used accordingly.
Condi said:
TGCOTF-dewey said:
It's not a small amount of transmission loss.
+ extra mass to carry.
How much is lost in the transmission? Especially in cars with Quattro, BMW XDrive etc as they only activate the 4WD when the 2 driven wheels start slipping as far as I understand. Extra weight is about 100kg or so, or 1 extra passenger. + extra mass to carry.
Condi said:
How much is lost in the transmission? Especially in cars with Quattro, BMW XDrive etc as they only activate the 4WD when the 2 driven wheels start slipping as far as I understand. Extra weight is about 100kg or so, or 1 extra passenger.
It’s not just when they slip, x-drive are full time 4x4 that can move power away from the front wheels if it’s not required. The smaller Audis use the system that only supplies 4x4 when the front wheels slip but the bigger ones are on all the time, again moving power forwards when rear drive isn’t required.
Essentially you’re dragging all the 4x4 gubbins about all the time plus frictional losses.
I had a bmw 420d x-drive that got about 42mpg in my use, other people doing similar journeys seemed to get 50+ without really trying in non x-drive cars.
If you want to appreciate the drag in the transmission, just jack a wheel up and feel how much resistance there is when you try to turn it. It isn't a huge amount, but every inch of movement has to overcome that drag. 4wd has far more bearings and gears moving so there is far more transmission drag.
Condi said:
TGCOTF-dewey said:
It's not a small amount of transmission loss.
+ extra mass to carry.
How much is lost in the transmission? Especially in cars with Quattro, BMW XDrive etc as they only activate the 4WD when the 2 driven wheels start slipping as far as I understand. Extra weight is about 100kg or so, or 1 extra passenger. + extra mass to carry.
The line gets blurred with newer and older version of various AWD systems. Some manufacturers have given their 4WD/AWD system more rear bias to mimic a RWD car (but with 4WD/AWD).
Condi said:
A comment on another thread about Audi's got me thinking earlier - someone (paraphrasing) said that they would avoid the 4WD version of a car because the fuel economy was crap compared with the 2WD version. Same car, same engine, same gearbox, just a different drivetrain. Thinking logically, the force required to move the car is the same, and while there will be a small amount of extra transmission losses from the transfer box and differential, why is there such a difference in economy between 2 otherwise identical cars? Feels like there should be a really simple explanation, but it's passed me by. Officially (under the old test) there is 5MPG difference between an A4 Quattro and standard 2WD A4 (190hp diesel, 2015 reg), although I believe in real world driving the difference is much greater.
Firstly, most manufacturer mpg results are lies. And in reality on the road, it makes very little difference to economy.Gassing Station | Engines & Drivetrain | Top of Page | What's New | My Stuff