I just stumbled across a thread - now closed, and for good reason - in which the relative merits of FWD vs AWD were "discussed".

Normally I'd just let you kiddies fight amongst yourselves, but my name was taken in vain a couple of times, so I intend on setting the record straight.

But we'll use science instead of bull****, and who knows? Maybe one of you will listen and actually learn something.

Let's begin:

The single most important fact in vehicle dynamics - that's the science of what makes a car handle is this:

"The harder you push a tire into the ground, the more it sticks, but the ratio of stick vs push falls off the harder you push on the tire"

Almost everything related to handling revolves around this fact.

What it means is this - placing more load (and in a non-aero-downforce world, that means "weight") on a tire makes it grip more. But the amount of grip you get diminishes as you add load (weight) to it.

So let's say you have a tire. Add 100 lbs of weight to it, and you get 100 lbs of grip. Add another 100lbs of weight (so 200lbs now) and you get 80 lbs of grip (180 total) Another 100 lbs, you get 60 lbs of grip (300lbs weight -> 240lbs grip) and so on and so forth.

Adding weight always adds more grip, but the curve tails off. Different tires have different curves, but they all round off this way.

OK, so we're sitting at a start line in a FWD car. The lights drop, we let out the clutch, and we hammer the gas. What happens?

Well, you get pressed back in your seat for one.

But what does that mean from a vehicle dynamics standpoint? It means that **weight is being transfered rearwards**

Y'all have seen "The Fast and Furious", right? Remember the big ol' V8 from the end of the movie? Big honkin' wheelie? That's extreme rearward weight transfer under acceleration.

THERE IS NOTHING YOU CAN DO TO STOP THIS - IT'S PHYSICS!

Because of this, when we accelerate, we take weight off the front wheels and put it on the back wheels. But this is a FWD! Those front wheels are what are driving the car!

So what happens? The harder we accelerate, the more we unload the fronts. The more the fronts are unloaded, the less grip they provide. Eventually, so much weight is transferred rearward that the amount of power we're trying to put down exceeds the amount of grip we can provide, and the wheel starts spinning.

Once that happens, the amount of grip plummets (a spinning wheel has much less grip than a non-spinning wheel) and the rate of acceleration drops. When _that_ happens, weight transfers forward, the grip level rises, and eventually the tire bites again - and acceleration increases, weight goes rearward, the tire unloads, and away we go again.

What does a FWD sound like on a hard launch? Chirp chirp chirp, as the tire unloads and loads again as the weight transfer seesaws back and forth.

Or if you have a modded FWD that is making enough power, the thing just unloads and spins, because there's enough power to keep the wheel spinning even when the car has basically slowed to a stop - there's not enough grip to re-establish the tire.

OK, how about an AWD then?

Launch the car, and the exact same thing happens, as far as weight transfer is concerned. But this time, we have a pair of drive wheels back there. As weight transfers rearward, the rear tires GAIN grip, not lose it. You can put down a lot more power from just that fact alone.

But we're not done yet...

The second thing to consider is that for a given power level, an AWD has 4 contact patches to use, not just 2. Let's say that a FWD can put down 200 HP before it starts spinning tires, and let's also assume that the FWD has an LSD so we don't have to worry about an open diff acting as a fuse. That means that each tire can put down 100 HP.

Well, all else being equal, that means an AWD can put down 400 HP **on the same tires** before you get wheelspin, because each drive tire can support 100HP, and we';ve got 4 of them (actually, a little more because the rears gain capacity with weight transfer, call it 450...)

But we're still not done yet...

If you think about the nature of the tire/load curve, you'll come to the conclusion that you get the most grip out of any set of tires when they are all equally loaded. Think about it - as you transfer load from one end of the car to another (or from one side of the car to another) the end that is gaining load is gaining grip, but at a slower rate than the tires that are being unloaded, for a net loss of grip.

But... DSMs are inherently nose-heavy. They start off something like 60/40 front/rear weight distribution. Well, if you start off 60/40, and you transfer 10% of the weight rearward, what do you get? Perfect weight distribution, and the maximum possible grip from a set of tires.

Compared to an AWD, all else being equal, a FWD can only put down between 25 to 40 percent of the power that the AWD can.

Now, this doesn't come without penalty. An AWD is by necessity heavier than a FWD of the same chassis type - it's got more parts, and most of those parts are heavy. So now we've got two variables to worry about, power capacity and overall weight.

Power capacity you worry about exiting turns. If you are down on power, you cannot accelerate as fast as the guy who has more power than you. Weight, however, bites you everywhere. A heavier car is punished in acceleration, in braking, in steady-state cornering, and in transitional cornering. More weight is *always* bad.

So if we're talking about a race where we have to turn the car, slow the car, and accelerate the car, it is entirely possible that a light, underpowered car will beat a heavy, powerful car, no matter what their respective drivelines are.

If we are talking about cars that are underpowered (from a spinning the tires perspective) then the grip advantage from having AWD doesn't buy you anything over a FWD, unless the surface gets so slick that there's suddenly "enough" power to spin tires and make you care again. If the AWD car has to give up weight to the FWD car, then the FWD car will probably be faster - it depends on how much weight, grip, and power we're talking about.

But as you start adding power, the inability of the FWD to put it down (especially on corner exit - but why that is I leave as an exercise for the students) starts becoming more and more of an issue. Eventually, as power increases, the FWD cannot make any more gain from it, and the weight penalty doesn't make up the difference any more, and the AWD will be faster.

When you're racing nearly-stock cars, and when you can reliably count on both the cars being underpowered and the AWD version being heavier than the FWD, the FWD may well be faster (it depends on how much lighter the FWD is and how underpowered the cars are)

Who cares about nearly-stock cars? The exciting cars are the modded ones. And having AWD lets you use freakishly large amounts of power - AND my AWD is 2826lbs dry - lighter than most DSM FWDS - so there's no weight penalty to make up any difference.

A FWD may well start out a little faster (excepting standing starts) but the more you mod it, the less and less true that is. Weight transfer off the drive wheels inherently limits the performance potential of a FWD car - and the harder you accelerate, the worse it gets. A FWD car is its own worst enemy from an engineering perspective.

The OEMs make FWDs for one reason and one reason only - packaging. With a FWD driveline, all the drive parts are forward of the firewall. You don't need to accomodate a transmission or driveshaft hump in the cockpit, so you can make more interior room for the same external dimentions. There is NO performance argument for using a FWD, not if you can use a RWD or AWD driveline and get down to the same (or close enough) weight

In SM, the class I run in, the FWD cars are given MASSIVE weight breaks (they can be a lot lighter) in order to keep them competitive. Without a built-in weight advantage like that, they'd never be able to keep up.

So then, the bottom line is that assuming you can make the power, and especially if you can lose the weight, AWD trumps FWD.

Any questions?

DG
http://farnorthracing.com