Why Front-Wheel-Drive Cars are More Efficient

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Why Front-Wheel-Drive Cars are More Efficient
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Front-wheel drive dominates today’s automotive landscape, powering everything from subcompact hatchbacks to full-size SUVs. However, it wasn’t always this way. Back in the day rear-wheel drive was king, but over the last 30-odd years carmakers have made a dramatic shift from back to front. Why the flip-flop? No, they weren’t impersonating John Kerry; in reality, they didn’t have a choice.

FWD-RWD-04.jpgAsk any automotive enthusiast and they’ll tell you rear-wheel drive is the only way to go. It offers numerous benefits over its front-powered competition, including better handling and grip in certain driving situations, more balanced weight distribution and easier servicing. The ability to do doughnuts in snowy parking lots is another plus.

Of course this layout has its disadvantages, too. And probably the biggest one is efficiency, though not necessarily what you’re thinking. Yes, improved fuel economy was arguably the No. 1 reason for the transition to front-wheel drive over the last three decades, but that’s not the only reason automakers switched things around.

“The fuel efficiency [benefit] is ethereal” said Jim Hall, Managing Director of 2953 Analytics because “It’s primarily about packaging.” He said front-wheel drive allows a company to make cars smaller and lighter. This is why they’re more efficient than rear-wheel drive ones, not necessarily because rear-wheel drive is hugely more wasteful, though it’s generally a bit heavier.

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“The incremental weight tends to be the incremental weight of the prop shaft” said Hall, noting that it also takes extra power to turn a long, heavy driveshaft that routs torque to the back-end.

Reiterating his earlier point, Hall said “Front-wheel drive is all about packaging.” He explained that in a typical front-engine, rear-drive vehicle the transmission tends to eat up a lot of interior volume and that becomes a big issue on small cars that have limited cabin space. This is why there are almost never any compact cars with rear-wheel drive. Also, rear-axle assemblies can gobble up precious trunk space, another downside in a small car.

FWD-RWD-14.jpg“You have to take a significant amount of mass out of a vehicle to appreciably affect fuel economy” said Mark Barrett, Group Vice President of Engineering and Procurement at AAM, a large Detroit-based component supplier. He also noted that significant weight reduction is difficult because “safety, overall NVH [noise, vibration and harshness] and road feel” are all conflicting interests. Still, Barrett said the switch to front-wheel drive coincided with the move away from body-on-frame to lighter unibody construction, a change that dramatically cut the mass of cars. Today’s pickup trucks are still built on heavy-duty full frames, but car’s don’t necessarily need that extra beef.

“The big focus is improving engine technology, improving transmission technology and driveline technology” Barrett said. “Cylinder deactivation, direct injection, variable valve timing, the whole variety of engine technologies” are being used by automakers to improve fuel economy.

SEE ALSO: What’s the Best All-Wheel-Drive System for You?

Barrett said the least efficient part of a vehicle is the engine but “Transmission technology then becomes the second area of focus.” Automakers are aiming for 10 to 15 percent improvements in powerplant efficiency. Barrett said “We’re squeezing 2 or 3 percent gains out of our products” and that’s pretty impressive because with axles and differentials there’s not a whole lot to work with. Engines have many systems that can be optimized and improved but the driveline is pretty much just gears and bearings.

BFWD-RWD-06.jpgack to front-wheel drive versus rear-wheel drive. All things being equal, can the latter ever be as efficient as the former? “From a pure technical standpoint, yes” Barrett said, but from an economics point of view “probably not.” When it comes to front-wheel drive “You really can’t beat it from a cost and efficiency standpoint” he said.

As touched upon earlier, rear drive is usually heavier. “Mass efficiency on front-wheel drive cars tends to be better” Hall said. The driveshaft, rear axle and associated structure all add mass and additional cost. Not surprisingly that extra expense makes rear-wheel drive more economically viable at the premium end of the market, which is part of the reason why many luxury vehicles use that drivetrain layout, plus it’s better from a handling standpoint.

But efficiency still matters, even with premium vehicles. Automakers and suppliers are working tirelessly to stretch every drop of fuel as far as possible, and AAM is doing some interesting things in this area.

“We supply the independent rear-drive axle for the Cadillac ATS” Barrett said. To make it as efficient as possible “We attacked it from every aspect.”

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“Sounds simple at the high level” he said, but the devil really is in the details. For instance, AAM used a mix of bearings – both ball and high-efficiency tapered units – to cut friction. They laser welded the ring gear to the case inside the differential, which eliminated the need for bolts and saved more weight. This was particularly challenging because parts can distort from the heat of welding. Engineers also tinkered with the oil seals to reduce drag; they even examined the lubricant used inside the axle assembly. Barrett said “What are the additive packages in the lube to absolutely minimize the friction in the axle?” No stone went unturned; no part was ignored.

SEE ALSO: What is Motor Oil?

The end result of all this hard work is pretty impressive. “It’s the highest efficiency IRDA (independent rear-drive axle) on the market” Barrett said, meaning it tops competing systems used by Lexus and Mercedes-Benz. All told their efforts resulted in a 2 to 3 percent improvement in fuel economy. That works out to about .2 miles per gallon, which “at first glance doesn’t seem like a lot, but it’s HUGE” Barrett said.

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Aside from the Cadillac ATS, AAM also supplies GM with rear-drive assemblies for the Chevrolet Camaro, both V6- and V8-powered models as well as the insanely quick ZL1.

With government regulators demanding ever-better fuel economy rear-wheel drive is under siege. You could argue it’s at risk of going extinct in mass-market vehicles, but if companies like AAM can continue refining the layout it will remain a viable option in the future.

 GALLERY: Front-Wheel Drive vs. Rear-Wheel Drive

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GALLERY: 2013 Cadillac ATS

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  • http://www.facebook.com/profile.php?id=778555533 Mirko Reinhardt

    What about rear-engined RWD cars? No long driveshaft there.
    And could Mr. Cole please elaborate a bit about where rear-engined FWD cars fit in the picture? (Could I consider a Dymaxion?)

  • huwtm

    You need to change the title of the story. It’s misleading.

  • George Bailey

    I hate FWD in all but the econoboxes. Larger cars look ungainly with their long front overhang and their front wheels pressed up against the front doors. The profile view of the red Ford Taurus shows exactly what I’m writing about. Its short wheelbase stance and long snout makes the car look as if it is a constipated dog. Plus the dynamics of hanging the engine out in front of the front axle affects handling, tire wear and turning radius of FWD cars. Ugly is as ugly does.

  • huwtm

    That’s not an issue of the engineering, that’s US design. There are lots of European and Japanese FWD That don’t suffer the massive over hang front and back to try and emulate old fashioned cars. A properly designed FWD body has a short hood very little over hang front and rear. But in the infinite LACK of wisdom by US designers they think people like you won’t buy a car unless the hood is a mile long and the boot arrives the day after tomorrow.

  • Ahead of the curve

    The BRZ and Miata are both lighter than most FWD cars.  Your average FWD car is about 3100 lbs now.  The article is full of bs when it says that FWD are more efficient because politics is trying to force the regular people into driving FWD econoboxes while the elitists get to drive $100.000 cars.  

  • Bob Beck

    “This is why there are almost never any compact cars with rear-wheel drive”. Really? Get your head out of your A$$. They were ALL Rear wheel drive except the Mini from the beginning of car production until the 70’s. Datsun 411,510,610, B210,1600, 2000, BMW 1600, 2002, 320i, Toyota Corolla, Celica, Supra. Triumph 10, MG Magnette, Vega, Pinto, Capri, Mustang II and the list goes on. These all got 30 MPG or better hwy. before fuel injection. Your theory is BS. FWD is only Cost effective to the OEM. It costs the consumer more to maintain and repair and the OEM’s save on production costs.

    I’m sick and tired of torque steer and the high cost of repairs of FWD. I’m going back to RWD as soon as possible.

  • DoomsdayJesus

    My BRZ weighs under 2800 lbs. That is LIGHT for a car that’s about the size of a 370Z, which is about 600lbs more (engine, wheels frame are all considerably heavier). But it still gets better than 30MPG on the freeway at 70 MPH, which is pretty damn impressive.

    And from what I hear, they are pretty damn good in the snow. I have a manual, but the automatics have a snow traction mode which supposedly works terrific.

  • DoomsdayJesus

    This article makes some pretty ridiculous assumptions. It also ignores tradeoffs in safety that you get from better handling, less torque steer, or the obvious benefits of AWD. And Subaru and Audi can get some pretty damn good mileage on their AWD models.

    I have a Subaru Outback that gets 30MPG on the freeway, which is unparallelled in an AWD car that size, weight, and with that cabin interior. You can thank the CVT and tall manual 6th gear for that one. And my Subaru BRZ gets over 30MPG at 70 MPG with pretty decent cabin and trunk space for a 2+2 RWD sports coupe. Even 370Zs have pretty impressive trunk space.

    Not to mention, it wouldn’t surprise me if carbon fiber drive shafts came down in price and seriously cut weight penalties. There are already a number of aftermarket options.

  • Emperor_of_Night

    I was going to write precisely this. But I know the problem with RR layout is that it leaves very little room for cargo space. Of course the ideal would be a car with a rear mounted flat four or flat six engine that can fit under the rear trunk, which would leave enough space even for a smaller front trunk that can house the spare tire, like the Volkswagen Type 3 of the 70s did. I wonder why the hell a car like that doesn’t exist right now.

  • Emperor_of_Night

    RWD vs FWD is a never-ending discussion. I’ve driven both RWD and FWD cars and can tell you that I find FWD cars to be easier to handle and safer to push to the limits. What I DON’T like about FWD is the weight unbalance and how the front tires get quickly wasted. Until they find out how to minimize understeer and make the front tires last longer, I think I prefer the FR layout and its better weight distribution.

  • Joe

    What About Cars Like The Mr2? Its Basically Fwd Layout With The Engine And DrivE Wheels In The Back. Trade Off Would Be Space But My 88 Supercharged Mr2 Still Gets 30Mpg Combined, Better Then Alot Of Modern Ff Layouts.

  • huwtm

    Its got nothing to do with MPG. It’s pure space efficiency and handling that separates them on the road. (especially mid engines) As for building, it can be cheaper and lighter, to have front over rear.

  • PacerRacer

    I think the rear engine layout makes it harder to pass safety tests…less of a front crumple zone- check out the crash test of an old Beetle or Vanagon on YouTube and you will see what I mean. Not for sure but that’s what they said on a documentary about the Volkswagen Bus called “The Bus”