All Wheel Drive. It’s All You Need!

by Zach Billings on June 24, 2011

Subaru STiLets start with Front Wheel Drive (FWD), the poster child for uselessness. In terms of everyday practicality and useability in adverse weather conditions, it could be worse, but it’s not even close to the best.

Sure, the engine is at the front, pushing weight down on the drive wheels. This is good for snow and rain traction, but how useful can two drive wheels really be in a blizzard? Don’t have blizzards where you come from? That’s unfortunate but lets think rain. The road is wet and you go to accelerate into traffic on a busy street while taking a right turn. What happens? The passenger side wheel starts to spin. Most of the fools on the road that we call drivers don’t know why that happens. It’s because with only two drive wheels and an open differential – which nearly all FWD cars have – the inside wheel has less weight over it and will therefore spin if you have little traction, too much throttle, or both. This issue rears its ugly head even worse when trying to drive a FWD car quickly. Even with a limited slip differential (LSD) it’s not likely to catch the wheel spin before you’ve finished that quick turn in your idiotic Civic Si. You’re still spinning off some of your power on one wheel and if you throttle too hard you might spin both of them mid-corner, causing you to also run wide into that tree you thought you were too l33t to hit.

The other thing to consider with FWD is trying to accelerate from a dig (a stop). When you accelerate hard in any car, a percentage of the weight shifts to the rear wheels. That’s funny… I thought the front wheels were the ones that needed grip… Wheel spin. At least Rear Wheel Drive (RWD cars) don’t have that problem. They are better off with hard acceleration because you’re putting more weight over the drive wheels, yielding traction. Ever watched a powerful muscle car launch at the drag strip? They do a wheelie, putting 100% of the car’s weight over the drive wheels. Advantageous, if you have enormous drag slicks on the rear wheels, but if you aren’t using a dedicated drag car, chances are you don’t. Most RWD cars tend to have more power than FWD cars because they can tame it. Issue is, you still have too much power for two wheels, and if they don’t spin in a straight line they definitely will if you go near the throttle mid-corner. What’s that? You don’t plan to drive your car so hard that it would matter? Fine. Tell me if you never get wheel spin on a raining day when you have to pull out quickly. Furthermore, try driving to work in the snow. Fail.

Enter All Wheel Drive (AWD). The drivetrain for real people. I’ll begin with the cons, for those who would try to claim me biased. AWD cars are generally a little less fuel efficient then their 2WD counterparts because you do lose a little bit of energy through the extra mechanics in the driveline. AWD cars do, on average, tend to have a few hundred to couple thousand more in lifetime maintenance costs. There are more mechanical parts so that makes sense. It’s also not a myth that if you blow a tire in an AWD car you have to replace all four (at least if there are more than a couple thousand miles on the tires). Having one tire (the new one) that’s slightly larger in circumference can in fact damage the center differential over time and cause mechanical fatigue to the whole drivetrain.

Reading the above, you will notice that these are really all about money. You end up spending a few grand more over the life of the car than you would on a 2WD car. Funny that people find that so offensive, when they will readily spring for a few grand extra when initially purchasing a car. You’re paying for the performance, driveability, and practicality of AWD!

Practical. Where are you going to go that your AWD car can’t. Snow? No problem. Hell, if you have an above average AWD system you can still do 5-10 over in a blizzard with no concern. Going camping where there are dirt roads you’ll have to drive on? A FWD car will likely manage, but not if you run into any mud. RWD? Forget it. AWD will go through large amounts of mud, dirt, snow, and gravel without issue. Four drive wheels means four places to be searching for grip. If stuck in the mud, you’re going to need a given threshold of power to get out. If you have to apply that power through two wheels, you may exceed the threshold of grip before reaching the threshold for motion. Distribute the power to four wheels and each one now needs half the grip that two did to do the same job. What a novel idea.

When it comes to every day driveablity, AWD is not likely to let you down. If you accelerate into traffic while turning, you now have two outside wheels with good traction to do that with. AWD cars also tend to have a LSD in at least the rear, if not front and rear. This means you’re also maximizing power to the inside wheels, in the unlikely event that you throttled so hard in regular traffic that the two outer wheels aren’t enough. If you’re a competent driver, AWD may save your ass in an emergency avoidance maneuver – of course if your natural tendency is to mash the brake pedal like most idiots on the road, it’s not going to matter. Proper emergency avoidance often involves turning and using the gas simultaneously. Do this in a FWD car and you’re definitely spinning off that throttle input on the inside wheel. Do it in a RWD car, and you will likely spin out, making the crash worse. RWD cars do not handle turning and throttling together unless you’re trying to drift, but that never goes well if you’re not on a track. AWD cars can often handle significant throttle input during hard cornering for avoidance without loss of control. Non performance-oriented cars may have a lower gross grip threshold per tire, but at least everything is distributed four ways. Furthermore, AWD cars generally have somewhat sophisticated center differentials, which can change where the power goes, front to back, maximizing control when it’s needed.

Take a performance AWD car into consideration, and all of the above factors are just amplified. Grip is usually high at each tire, leading to an enormous gross grip threshold. This means you can be pretty liberal with your throttle provided you’re not at the threshold of lateral grip. My 2005 Subaru STi has over 350lb-ft of torque and even the most aggressive acceleration will not cause a bit of wheel spin. Even in the wet. With an advanced, performance-oriented differential system, the Subaru STi will constantly and drastically change which wheels my power is going to in order to maximize grip by giving the power to the wheels that have it.

AWD is truly the king of the drivelines, with only very specific circumstances causing exception. Don’t believe me? Go try an AWD car for a while then talk to me.

 

{ 2 comments… read them below or add one }

Kollin February 7, 2012 at 12:50 pm

So AWD is superior if you’re offroading or worried about traffic accidents. Gotcha.

Zach Billings February 14, 2012 at 12:18 pm

Or if you’re racing. Or it’s snowy. Or icy. Or salty.

If the idea of a car is to grip the road so you can turn, go and stop, it stands to reason that, at any time you’re using throttle, AWD is superior because it distributes your power four ways rather than two.

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