I have a friend that thinks that rwd is sooooo great. I'd like to get a good explantion of how the physics work from an expert. What are the pros and cons to each setup???

 

this is great! keep it coming, more technical aspects please...

 

yeah! serious! good topic! keep the debate heated!

 

In regard to the sporty AWD vehicles we're talking about (front-engined), the singular problem that puts a monkey wrench in its handling dynamics relative to a sporty front-engined RWD vehicle is its fore/aft mass balance.

Typical front-engined sporty AWD cars are balanced at ~60/40. Typical front-engined sport RWD cars are balanced at ~53/47.

But this is where the entire issue begins.

I'm sure many people know how to draw wheel tracking diagrams to show the necessity of differentials (to show different wheel tracking lengths), and some people know how to draw wheel tracking diagrams to show the necessity of Ackerman (to show different front wheel turning angles), but few people seem to know how to draw wheel tracking diagrams to show a vehicle's 'attitude' in a steady-state corner.

In a conventional front wheel-steered vehicle, the 'pivot' of the turn lies on a point that extends along the rear axle. Lines perpendicular to the front wheels extend and intersect with this rear axle line, giving the pivot point. As you can see now, all these lines we've drawn are radii of this center. (It's much easier to see if you've already seen one on paper, which you probably already have since its a fairly common illustration.)

What people generally fail to appreciate from this diagram is the fact that the radii between the front and rear ends are different. This is to say that in any given corner, the front end tracks a wider arc than the rear end. This effect is more pronounced at tighter arcs and less-so at large arcs. (I'll come back to this later.)

This is, in a nutshell, the same effect that plagues extremely long vehicles, such as buses. Doubtlessly you've seen buses negotiate a tight turn by swinging the front end very wide in order to keep the rear end from running over the curb (which is to say, coming in very tight). While this effect manifests this way for buses, what it means for a sporty vehicle of any kind is that 'centrifugal force' (yes, I'm quite familiar with the strictly-speaking physics quibbles; the term is still employed because 1) people understand it, and 2) the effects are accurate from a behavioral point of view, even though it isn't from a causative perspective) is higher on the front end of vehicle than the back end. Why? Same angular velocity, different radii. Given the same mass on both ends, the front end is already subject to higher 'centrifugal force'. Given more mass on the front end, there is much more 'centrifugal force' for the front tires to try to work against. The point that I'm making is: the front contact patches are always relatively overworked in a front-steered car.

Being that both sporty AWD and sporty RWD cars are both still at least a bit front-heavy and so both still suffer from this effect, why did I call the difference in mass distribution "where the entire issue begins," instead of being the entire issue?

Because we haven't considered what drive torque adds to this problem.

A typical sporty AWD car splits torque (not RPM, for those of you who have yet to grasp what a differential does) equally between the two axles. Contact patches on both ends of the car are forced to share what grip they have between lateral acceleration (cornering) and keeping the car moving forward. The effect is to decrease the available grip for lateral acceleration on both ends. In contrast, a sporty RWD car puts down all of its drive torque through the rear axle. Only the rear contact patches are forced to share their grip between lateral acceleration and keeping the car moving forward. Only the rear contact patches have their capacity for lateral grip decreased. The front contact patches have all their grip available for lateral acceleration.

So let's tally things up.

Front-engined typical AWD:

The front contact patches are always relatively overworked in a front-steered car + the relatively heavier front end + both ends have their capacity for lateral acceleration equally reduced = A highly overworked front end = Predominant understeer, all else being equal*

Front-engined typical RWD:

The front contact patches are always relatively overworked in a front-steered car + not all that heavy of a front end + only the rear end has its capacity for lateral acceleration reduced = A not nearly as overworked front end = More neutrality, all else being equal

* - There are things that can be done to AWD cars to reduce the relative grip of the rear end to produce neutrality. But the key here is "reduce". It produces a more neutral-handling car, but the front-end lateral acceleration limits remain the same, that is, lower than that of a RWD car.

(continued in next post, due to post size restrictions)

 

(continued from previous post, due to post size restrictions)

Some of you might be tired of reading, but the story does not end here. There exist 'perfect' AWD configurations. I'll go on to say that these 'perfect' AWD configurations are theoretically unbeatable with RWD, all else being equal.

Going back to the central problem, which is that of a severely overworked front end, a front-engined AWD car can deal with this in three ways:

1) Raise the amount of available grip in the front end by resorting to wider front wheels/tires, or simply stickier tires. A fellow autocrosser employed this solution in his GC8 2.5RS by using imported (non-DOT, read: illegal) super-sticky Yokohamas in combination with STX-legal Azenis' in the rear. While this solution might be workable for autocross/track, super-soft tires are generally not workable for the streets. The more lasting solution is wider front wheels/tires, but the constraints of the wheelwells as imposed by the engine bays of most cars makes it difficult to put in truly wide front wheels/tires.

2) Reduce the amount of torque that gets directed through the front end with a torque-biasing center differential. Which basically makes the car more RWD-like. This is the solution employed by the Skyline GT-R, and now the WRX STi with DCCD, the latter of which is the reason I am on this board today.

3) Force the rear wheels to play a role in turning the car, which is the genius behind Mitsubishi's AYC, which is basically (conceptually) Honda's ATTS employed on the rear axle. Too bad those of us in the States won't get it.

Looking at the problem in another way, a rear-engined AWD car does not overwork the front tires as-is. This is why AWD was such a convenient leap to elevate the Porsche 911 to supercar levels with the 959 (and now the 911 Turbo). This is why the Countach went AWD with the transformation to the Diablo, and continuing with the Murcielago and Gallardo. The EB110 and Veyron, being automotive grand complications, were born blessed with this configuration from the start. It has been rumored that the future NSX will go this route but with a bit of a twist: hybrid AWD.

Technical enough?

In response to Jballs:

Everything else being equal (comparable power-to-mass, comparable mass-to-tiring, and comparable sporting intent), front-engined RWD generally gives higher limits and is more entertaining to drive in general sporting conditions (dry). But to extract this performance potential, it requires a driver who is somewhat practiced at true performance driving. OTOH, RWD feels more agile and balanced at all driving levels, which is the reason why most premium marques such as MB, BMW, Lexus, Infiniti, etc. all went with RWD even in decidedly non-sporting cars. Audi is the lone deviant, which is because all their previous production technology has been based on FWD or AWD, and being the lone AWD entrant differentiates themselves from everyone else.

But when conditions turn extremely slick and there remains the need to travel quicker than the next guy, AWD is absolutely the way to go. Trust me, I tried to race on tarmac that had turned a bit muddy, and it was absolutely no fun (alternatively, you might call it a 'fun overdose') in my torquey RWD car w/race tires. 'Launch' was a non-word, and through the cones I would alternate between locked-up understeer plows (braking) and trying to keep from my spinning my rear tires excessively and putting myself into doughnuts. At least in AWD I would have had the option of spinning all four tires equally and producing more forward movement than otherwise. And AWD did indeed rule that day.

AWD also rules under compromised driving abilities, such as inexperience, or coping with severe driving stress. I'm sure as you've all heard, the WRX is the choice of getaway vehicle by punks and holdup artists in Aussieland. Punks are not the sort that you would expect to be Mario Andretti's, and the situation of a getaway is decidedly extremely stressed. In a movieland parallel, the choice of a S8 for 'Ronin' was an inspired selection that added to the authenticity of the film (not that it was all that authentic in the first place, but the detail helped as opposed to taking away from) In an imperfect parallel from the olden days of automobiledom, the FWD (very oddball in those days) Citroen Traction Avants of the Great Depression era were well-known French gangster cars renouned for their aplomb under stress.

This is not to associate AWD cars with unsavory characters, mind you. For the very same reasons, police cars should ideally be AWD, if we weren't so insistent on buying cheap full-sizers from domestic makes.

 

rentankonko, that is a naive view of trends in racing regulations.

Quattro was banned in IMSA only because the regulatory mechanisms for ensuring close racing (weight penalities) did not work the way it was meant to (slowing the Audi's), at least in the timeframe that would have mattered. The Audi's would simply drop the weight in the rear and help their mass balance. Was there a point that it would have began slowing them down instead of helping? Sure. But nobody was willing to wait until the rules were completely retweaked to deal with one team, when it was easier just to single out a particular technology that they had based their entire effort on. In short, the Audi dominance was assasinated, but that is not to say that AWD alone was the reason why they dominated. AWD is only the technology that IMSA chose to assasinate them with.

In something that is closer to what any of us will be concerned with, go check out SCCA Solo and Solo 2, which is amateur competion with showroom and production-based cars. A good chart that I'm way too liberal at busting out is the classification of stock vehicles. These classifications are not at all arbitrary, rather, they are based on cumulative raw results of over a thousand events held across the country per year, over many years. Here is a page somebody threw up with these classifications:

http://www.moutons.org/sccasolo/Lists/2003/stockc.html

What you should notice is not where the highest AWD car is classed, but what power-to-mass and mass-to-tiring it possesses relative to its performance competitors. (In stock class the tiring is limited to whatever DOT-legal tries that can be shoehorned onto stock-dimensioned rims). You should notice that most AWD cars only perform as well in competition with RWD cars that are heavier relative to their power and heavier relative to their tiring.

This will probably change as the STi (and eventually the US GTR) is released, but this is what holds true for AWD as it relates to anything any of us will ever have.

 

That's a strange misinterpretation.

http://www.scca.org/amateur/solo2/na...esults/ds.html

That's the results for the 2002 (latest) National Championships for D-stock. The results for the 2001 NC goes along the same lines.

Notice that 10 out of 39 entrants were AWD. Then notice that 12 out of 39 entrants were RWD. It can hardly be said that RWD is olny doing well against AWD because it is overwhelming the number of competitors in its class.

If you go out to each regional site and go through all of their results (way too many), you will see that there are significant numbers of AWD entrants in in both D-stock and G-stock. They do alright in D-stock, but they are often outclassed in G-stock, hence few make it to the National Championships.

That example was used because this is not the first time I have dealt with this issue. The Audi/IMSA story is invariably the one that always gets cited and misinterpreted by AWD-is-superior proponents. If you've got an example where Subaru AWD (or any other AWD) was regulated out of competition, by all means, share with us.

If you can't come up with any, then I would think that your claim that "AWD has been LEGISLATED out of those competitions" is a bit hollow.

In an aside that adds to the point of my previous postings about technology being regulated out of competition, consider the Tyrell P34 (6-wheeler) and the 1978 Brabham 'fan car'/Elford-Chapparal 2J (mechanical air extraction). All of technologies were regulated out of competition not because these technologies were superior in themselves (they had as many drawbacks as they did advantages), but rather that they provided dominance by exploiting loopholes/uncovered areas in a set of regulations that were originally conceived to control speeds and development costs under the conventional paradigm.

(They're always trying to do this, see current F1 crisis. For an example in the failure to do that, see FISA's ill-fated Group B era)

In other words, if the sanctioning organizations weren't imposing rules specifically designed to curb speeds and costs of developing 'conventional' cars, there would be no advantage to these 'unconventional' technologies.

You realize, of course, that this invites the question, "what's wrong with hunting ducks with Stinger missles if it works and gives you tasty roast duck at the same time?"