Re: FWD vs. AWD vs. RWD - long

[VW155mph@aol.com]

I agree with many of the points you make, but I do have some comments.  

 
> FWD - merits:  
> 
> These cars offer a significant advantage in the area of cargo and 
> passengar space over all RWD and AWD vehicles.  Without the "hump" 
> in the middle of the car, passengers have more leg room and  without a 
> rear axle, trunks are bigger.  This is undeniable, and is the primary 
> reason FWD was pioneered.  A softer advantage over RWD is that FWD 
> equipped cars (not all, but most) have more weight over the front 
> wheels, and therefore benefit in lower traction settings where weight 
> over the drive wheels is a benefit.
> 

The weight over the drive wheels in a FWD car is only a traction benefit in low power situations.  If you try to launch a FWD hard off the line, the weight shifts back, thus causing the FWD to have wheel hop. 

> RWD - weaknesses:
> 
> As suspected, the opposite of the FWD:  Less cargo capacity and typically
> less traction.

Once again, on low speed launching, RWD will have less traction than a FWD, but on a hard launch, the weight will shift actually focusing the weight towards the back, therefore improving traction, with little, or no wheel hop.  This is the reason you still see most drag car setups as RWD.

 
> AWD - weaknesses:
> 
> The biggest weakness is the loss of effective power caused by the added
> rotating mass.  The motor must now spin 2 more wheels, a transfer case,
> two extra axle shafts, and an extra differential.  All that rotating mass
> has to be overcome by the motor and it isn't free.  On dry straight
> pavement, the RWD and FWD vehicle will be faster (even if the trunk is
> loaded with the weight equivalent to that of the extra AWD components).
> Many have noted how a lightened flywheel increased throttle response.  
> Well imagine bolting on a heavier flywheel...now you've got AWD.  This is
> the primary reason AWD vehicles are not used in CART, INDY, F1, NASCAR,
> and most other racing venues (including oddly enough SCORE desert off-road
> racing, where Ivan Stewart races a RWD truck in the sandy desert).  
> Maximum power and torque are needed on the pavement, and none of it can
> be sacrificed for the benefit of traction.  In these settings, traction is
> not the limiter, but rather either hp for top speed or torque for rapid
> acceleration, or both (F1).

I do agree with the added rotating mass, but another reason an AWD has acceleration problems is the added weight of the system.  An AWD system can add more than 300lbs to the weight of a car (except for Porsche).  The effects of the added weight can be seen in high speed situations.  I.E. Stock Eclipse GSX vs. Stock Eclipse GST.  On the track the GSX will launch and leave the FWD GST because of the AWD traction off the line, which in turn means the car can launch at a much higher rpm.  But, at the end of the 1320 the GST, if driven properly, will be right there with the GSX.  If the race were to continue, the GST would walk away from the GSX on the top end.  

Now, lets say that both of these cars are pushing out 400hp.  The GSX now has the advantage for a longer period of time.  The GSX will have traction throughout the race while the GST will be struggling to keep its tires from spinning.

> I want to point out one last mis-information that was posted.  Unless the
> vehicle is equipped with a limited slipping or locking differential (I
> don't know if the quattro does or not, but I suspect not), an AWD vehicle 
> must have either both front or both rear wheels applying positive power to 
> the ground to move.  

The Audi/VW Quattro/4Motion uses a Viscous coupling system that can divert power to whichever wheel has the most traction, whether it be in the front, or the back.

And like you said, it is all dependent on driving/racing style.

Marc A. Brown
Speed Etc. Motorsports
Club GTI International (Norteast Region)
1995 Jetta GLX (well modded)
1998 New Beetle