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Old 05-24-07, 03:25 PM   #1 (permalink)
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FWD v. RWD...and some AWD stuff too

Alight, another FYI for the Masses. Several years of this argument has finally worn me down to having to put a FAQ style dissertation up about what is the difference in Front Wheel Drive (FWD), Rear Wheel Drive (RWD), and All Wheel Drive (AWD).

To start lets go over the basics:

In order for a car to have forward propulsion, power needs to be delivered to the wheels in metered amounts of torque. This torque is the penultimate consideration of acceleration. What ever comes out of the transmission/differential is going to be used by the wheels in lower gears it’s many times the engines original horsepower. Something like 6-8 times the amount of force the engine makes hits the wheels in first gear. (Gear multiplication is another subject). Once that torque hits the wheel, that’s what gets you moving.

Due to inertia, that twisting force (along with gravitational intervention) causes weight shift. As a rule of thumb, objects rotating in one direction that are connected to a mass will cause that Mass to rotate the OPPOSITE direction. For instance, a dirt bike rider, in order square up his landing he’ll play with the rear brake and throttle to adjust the “trim” of the bike in mid air. By gunning the throttle in mid air the inertia of the tire in conjunction with the power of the engine rotating will cause the nose of the bike to lift.

The same principle pretty much works for land vehicles. Force finds the easiest way out. If that way is rotating the mass versus spinning something, that’s where it’ll go.

For Driveline comparisons:

In a straight line this inertial influence is a big part of “weight shifting” characteristics of vehicles.


Straight line performance

In a straight line the weight shift characteristic is to remove weight off the front wheels. This is true. When power is applied to the wheels rotating forward, the opposite directional force is applied to the rear of the car, along with inertia, this “squat” effect is exacerbated. That’s why it’s generally thought for ALL OUT DRAG RACING, front wheel drive is not the best choice as wheel spin is easily seen. This however isn’t the end; due to Suspension geometry one can actually put the “Easiest flow pattern” where ever they want. Saying you can cause a reverse in that natural inertial rotation by causing the linkages to actually push the mass in the same direction as the rotating assembly. This is was they call “Anti-Squat” geometry

That said however, it is one of the most stable configurations available.

There’s a theory running around that “pushing is better than pulling” this is all together wrong. In fact, it’s wholly wrong. Natural Laws dictate that a force will flow in the direction with the least resistance. Saying which ever direction gives the least amount of hurdles, energy will flow that way. What this, in effect, is saying that if you push something you’re not putting the force in the easiest flow pattern.


Examples:
I’ve used this example several times. The Log Up a Hill example. Imagine you are pushing a log up a hill nothing too hard. Pushing that log too fast will cause what to happen? The front of the log will swing back around to follow the path with the least resistance. The same is true for the force. The force is trying to move forward but this log is holding it up. Where’s it going to go? It’s going to either side of the log to bypass the log’s mass.

Same experiment with pulling that log. What’s it going to do? Nothing…it can’t DO anything. It has to follow where it’s being pulled. Saying, the travel is much more controllable and much more stable and a far better use of energy.

Putting it into perspective,

For a Rear Wheel Drive car, when you put enough power to the rear wheels the rotating force of the engine lifting the front off the ground, and then the pushing force trying to rotate the front end around is not the best use of acceleration, as you have to control it. That’s not always as easy as it seems.

For a Front wheel Drive car, the power put to the ground is much better utilized. The inertia acted on the car is in an opposite direction the wheels are turning. This means the inertia is also pushing the body backwards. This keeps the rear end planted. But not only that, when the force is applied, it’s causing everything that’s attached to it to line up in a single file and follow it. Thereby, everything goes in ONE direction

RWD trying to fight physics.


FWD Working with Physics.


Examples of Full Throttle turns

RWD Trying to fight Physics


FWD Working with Physics


Cornering and Handling:

There’s big debate as to whether FWD is better than RWD in handling. Most people believe FWD is subpar to RWD when it comes to cornering. This is ALWAYS argued upon when it comes up against high powered cars. It is time for a real explanation.

When it comes to cornering, there’s NOTHING more IMPORTANT, than GRIP. The only thing making stuff happen is grip. SECOND MOST IMPORTANT IS SUSPENSION!

What this means is it doesn’t matter how little or how much amount of grip you have, if you don’t use that grip to its full potential, you’re NOT using your platform right. In the end, the suspension is probably tied with grip as the most important factors of handling…

NOT THE DRIVE WHEELS!!!

When it does come down to handling in regards to the drive wheels, there’s HUGE advantages and disadvantages to both.

First up, with RWD having far substandard stability, it is important to set the car up right. In the dry, it’s pretty clear that with the right suspension and tuning a RWD car will be VERY VERY slightly at an advantage due to the instable nature of the drive platform. When physics act on the mass as it hurdles through a corner, it’s trying to go straight. If enough power is applied, the force will make the inside of the corner seem like the EASIER path to follow, versus off into the trees being the easiest path.

That’s going plainly by nothing else but kinematics, when you start adding rotational forces and inertia into the picture, that advantage dwindles away to nothing.

What I mean is when you add the fact that adding power ALSO raises the front of the vehicle loosens the grip and can easily make understeer WORSE, and thereby the trees are a sure thing, that’s what adding other factors means.
That’s all in the dry mind you. If you throw even a little bit of sand into the equation RWD is an abysmal disadvantage. The instability of the driveline is then highlighted as you go spinning about uncontrollably.

Also, in the dry, RWD is at a disadvantage in fast transitions. Saying, fast transitions from left to right. Something like a slalom course or a chicane, for instance. Trying to power your way through a slalom course in a RWD car will prove quite frustrating when the back end decides it’s going to go one way while the front is trying to go the other. The potential and kinetic energy starts changing rates faster and more abruptly to the point the car is whipping around. If the power wheels are at the back, it’ll just LOVE the chance to break free of all that burdensome weight it has to push.

There was a story on this forum not too long ago about a guy getting his but handed to him by a civic in a slalom course. The Guy was driving a Mazda MX5, not exactly a slouch when it comes to handling. But the civic had more stability and was able to fire through the course where the MX5 was trying to spin out.

Then there’s the evidence that RWD is not exactly superior when it comes to handling.


The S2000 lost mostly based on the instability of the car at a few of those corners.

If the chassis is set up correctly, a car will oversteer or Understeer depending on your SUSPENSION SETUP at 8/10ths Car ability.

Suspension Setups are DIFFERENT FOR EACH CHASSIS!

For a RWD chassis to get the best performance you want an understeer setup to FIGHT physics. You’re basically telling the car to understeer and nose plow because when it’s all said and done, with enough power you can cause the rear end to become the front end.

It’s the opposite for FWD, you want a slight over steer setup. Much like the Legend and a lot of other Hondas are designed to do. The Acura Legend’s Spring rates and suspension geometry are a testament to what exactly you need to do to make a car handle well. The Legend was not a bad handling car, though people who don’t know how to drive typically think it was.

SPRING RATES


1991 Legend
Front: 118Ib./in.(2.10kg/mm)
Rear: 120Ib./in.(2.15kg/mm)

As you can see by those spring rates the Legend was spring and valved to be a handling chassis. It was not a luxury boat as some youngins who drive it think it is. When modifying the suspension it’s good to remember WHERE you were from the beginning and try to keep within close range of those numbers with relationship to each other.

Another thing, it’s rare to plow your Legend over something, or to go sliding head-long into something, there’s better chance of you spinning out in the legend than anything else, simply because the chassis is designed to oversteer.

As for AWD?

All wheel drive theoretically was supposed to combine the best of both. In practice however that’s not the case. Active electronics have to mimic these characteristics and remove them when needed. However, with the right settings, an AWD vehicle will work better than both drivetrains hands down. Regardless of what TopGear said or tested, there’s a reason Audi’s QUATTRO was banned from Touring Car racing. When set correctly with the right amount of power distributed and the differentials operating as the should you’ll have more gripping force, more stability and more maneuverability than ANY other drivetrain option.

Last edited by B+O; 05-24-07 at 04:25 PM.
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Old 05-24-07, 03:40 PM   #2 (permalink)
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great write up Ivan!
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Old 05-24-07, 03:47 PM   #3 (permalink)
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Cracking up at the dude in the Mustang, you can go thinking RWD is top choice if you want, just make sure you don't have some bald tires on some rain slicked rodes, and you'll find out real fast. Give me AWD for $200 Alex!!
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Old 05-24-07, 04:27 PM   #4 (permalink)
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Quote:
Originally Posted by B+O View Post
This torque is the penultimate of acceleration.
I know what all those words mean, but they don't quite make sense to me in that arrangement...
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Old 05-24-07, 04:38 PM   #5 (permalink)
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Quote:
Originally Posted by LegendFan91 View Post
I know what all those words mean, but they don't quite make sense to me in that arrangement...
typo fixed a few minutes ago. Second to weight, Torque is the second most important/last thing you want to look at when considering acceleration.
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Old 05-24-07, 05:05 PM   #6 (permalink)
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great write up. now i understand everything. And the videos are just amazing.
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Old 05-24-07, 05:13 PM   #7 (permalink)
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Nice write up, makes me happy to own a Legend. These cars are certainly meant to handle well. I pushed my G1 to its limits and I was surprised at how well it handled.
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Old 05-24-07, 05:15 PM   #8 (permalink)
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waiting for rebuttles...
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Old 05-24-07, 05:19 PM   #9 (permalink)
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Very nice write up, I love FWD for road cource, Gimme RWD and an Off-Road cource anytime.
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Old 05-24-07, 05:21 PM   #10 (permalink)
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waiting for rebuttles...
well since Kofi Annan(aka anarch) is banned currently we prolly wont see any
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Old 05-24-07, 05:53 PM   #11 (permalink)
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video 4 is russian!
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Old 05-24-07, 05:56 PM   #12 (permalink)
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Very thorough. It seems that what you have said lines up with what Acura is doing.

Hey Russian are you doing a swap?
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Old 05-24-07, 10:00 PM   #13 (permalink)
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Very thorough. It seems that what you have said lines up with what Acura is doing.

Hey Russian are you doing a swap?
yes, I will probably be done with it and will drive the car tomorrow.
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Old 05-25-07, 04:16 AM   #14 (permalink)
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B+O, on point as usual...


Quote:
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well since Kofi Annan(aka anarch) is banned currently we prolly wont see any
he get banned for his comments to ya boy Ted about his sig?

can't disagree with that decision, but I still laughed for about 15 minutes when I read his response.
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Old 05-25-07, 04:22 AM   #15 (permalink)
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^Noice!
I just realized that since it's obvious the brakes do the deceleration, they show which drivetrain would be the best. The front brakes do most of the work while the rear brakes keep it from spinning out. Since deceleration is the opposite of acceleration the rear wheels should do most of the work while the front wheels keep it from spinning out. This would be a rear wheel biased AWD system. FWD is like have rear brakes only, you won't spin out but it would take longer to slow down. RWD is like front brakes only, you might spin out easily but it would slow down quicker.
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