RWD VS. FWD & AWD

LXSC

Registered User
I've always assumed that RWD is the prefered drivetrain in regards to speed and performance. I'm not talking about handling or control but just good 'ole go power (acceleration and speed). Is this true? What are the positives it has over FWD and vice versa? I thought I'd mention AWD as well but I don't really think it's a big player in terms of limits of performance upgrading you can do to an AWD system. After driving a 2003 Audi A4 (1.8 T with quattro (AWD)), I realize that you need more power to get all four wheels going as opposed to just two.
 
benifits of front wheel drive are only increased traction during slow speed in bad weather, and smaller packaging. It has no other benifits. Handling is worse because the front wheels are being called on to handle, power inputs, steering inputs, and braking inputs all at the same time. Having 60% of the weight over the front wheels is not good for breaking either. When braking hard up to 80% of the braking with a FWD car is done with the front wheels.

AWD can be very good, although it does add more weight and rotational mass to a car. AWD handling can be great if the bias is set up properly.

RWD allows for some of the best driver control and near 50-50 weight balance. Unlike FWD, during acceleration all the weight is transfered to the drive wheels helping traction. Oversteer or understeer are available without the use of an emergancy brake.
 
FWD Vs RWD is a pretty complex issue. One of my coworkers is a FWD fanatic, no not one of THOSE fanatic's either. He is a semi-pro rally racer, who usually finishes in the top of his class, driving a FWD VW Golf. So he has enlightened me on some of the aspects of FWD I hadn't realized before.

First there are the weight distribution aspects, that 1badSC mentioned, then there are also the friction distribution (I am sure there is another name for this) aspects. Each tire can only provide so much force, RWD cars use the rear wheels to push the car forward, and the front tires do the steering. In a FWD car the front tires do everything, the rears are just along for the ride. So, generally, if you push right to the edge of traction, the RWD car can do better since it distributes steering and acceleration forces amongst 4 tires Vs only two. See the editorial by Csaba Csere In July 2004's car and Driver.

In acceleration obviously RWD has an advantage since weight transfers to the rear in any car, loading the drive tires in RWD, and unloading them in FWD.

Which brings me to a very important point. There are basically two things that determine how much traction a tire has, its construction (compound, tread etc) and the force pushing it down on the road (weight.) Assuming you can put the same tires on FWD as RWD the tire itsself is a non issue. What is different is the weight on the tires. You take off and the front end lifts there is less weight on the front tires, they have less traction. you hit the brakes hard and the front end dives, it has much more weight than usual on it. This is why front brakes are usually bigger, and wear much faster, they do most of the work.

When you hit the brakes hard going around a corner, the front tires are loaded, and have much more grip. They generally keep going thdirection they are pointed, but the rears are unloaded, and if they lose traction to the point of slippage will tend to travel straight rather than continue around the curve. This is where your rear end slides out - oversteer. The opposite experince would be trying to turn too sharp without hitting the brakes, the front tires do not have enough traction to pull the car, so they slide straight - understeer. Cars can be tuned to respond more or less to these phenomenon, but that is another issue.

The point here is to realize that in a dynamic situation, when a car is moving around, acceleration, slowing down, the weight distribution changes, even going around a corner the outside of the vehicle gets "heavier." This changing weight distribution changes the amount of traction each tires has, and in a FWD car each tire has a different job than in a RWD.

Another difference is that RWD cars are "pushed" around and FWD cars are "pulled" around. Try to slide a pen around on a desk by pushing the end of it, like a RWD car, if you try to make it go in a certain direction it can be tricky, remember you can't push side to side since most rear tires do not steer, you can only push forward. Now if you PULL the pen from the front end, and you CAN control direction (since front tires DO turn) you can pull the front end of the car in which ever direction you want.

So in the same situation FWD and RWD behave differently, you have to drive them differently to get the most out of them. As stated above RWD has the advantage in straight line acceleration. FWD CAN have an advantage in cornering. I never would have thought this to be the case, but I was told (repeatedly) that it can happen.

I will never forget the first time I really saw it. I was watching some sort of road course racing on Speedvision, I forget the actual series, but what was important were the cars, there were essentially two models running, BMW M3's and Acura Integra's. Obviously the cars were modified to have similar power to weight ratio's, tuned suspensions, etc etc. It was a very close race, all the cars seemed to be moving around the course in a cluster, neither having a distinct advantage, until there was an accident. One of the cars in the lead somehow came to rest in the middle of a curve (clipped a wall or something.) What happened next suprised me. ALL of the BMW's spun out trying to make it around the stalled car, ALL of the Acura's made it around without a hitch. The BMW's hit the brakes and the rears slide out. The Acura's also had to hit the brakes, but continued around the course.

In racing situations, a lot of times you can go just beyond the limits of traction of the tires and slide just a bit. What I am going to try to explain here is sort of a difficult concept, I didn't really get it until I was given a demonstration by my rally driving friend, I don't suggest anyone try it in your Moms Taurus.

The demonstration was essentially this; we were travelling in a slow arc and he was explaning how in a FWD you accelerate with the front, and steer with the rear brakes. When you are racing you want to keep your speed up, so you rarely take your foot out of the gas. So this is how you corner in a rally car, you brake for the curve, keeping your foot in the gas to keep the speed up, you turn in the direction you want to go. If the car is understeering, you want to bring the rear around more, so you push the brake more this makes the rear end slide out like you want it. If the rear end is coming around too much, you let up on the brake and the rear gains traction and falls in behind you. The whole time this is going on, the front tires are pulling you in the direction you need to go.

This is how the Acura's made it wound the wreck, but the BMW's couldn't. The BMW's locked up the rear and lost control completely. The Acura's locked the rear and slide it out far enough to go around while pulling with the front tires around the wreck.

I know it is difficult without seeing this in person, but I hope that helps. If you get a chance to see some REAL racing (road course, rally, or autocross, not the drag stuff) with FWD cars involved watch closely.

As far as good old speed and power, FWD layouts tend to weigh less overall, and have less loss through the powertrain. They also are usually weaker. So they can do more with the power they have, but they can't really make as much power as a RWD can without breaking. I know you can break anything if you try hard, but in general that is true.
 
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I would expect 80% of the braking to be done by the front wheels regardless of whether the vehicle is front wheel drive or rear wheel drive... same as with a motorcycle where the front wheel does 80% (ish) of the braking.
 
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