Technical RWD vs FWD

[gounc14]

Ok so maybe I'm wrong, but I've always sort of assumed that RWD is quicker off the line than FWD, but a good friend of mine is completely convinced that an FF car, physics speaking, should be quicker off the line than an FR car, and I don't know all my physics well enough to dispute him. I was wondering if someone can post a detailed description of the physics differences between FF, FR, AWD, and Mid-Rear. We both agreed that the Mid-Rear should be the best off the line, but after that we disagreed on everything, and as he is an engineering student, he generally is exposed to a great deal more of the technical engineering/physics information than I am. Please help enlighten us guys =)

[innesheavyindustries]

Correct me if I'm wrong, but...

My assumption was that AWD was the quickest off the line, considering that the power of the motor is transmitted to four wheels instead of two. Two wheels can only transmit so much power before exceeding their grip factor, resulting in a burnout. Theoretically, an AWD system (dividing the power equally) should be able to launch faster because of the combined grip of the four tires allows for more power to be exerted.

An obvious refute for that question is "why aren't the SCCA drag race cars AWD?"I don't really have a good answer for that, except I assume that their MR configurations are due to the fact that AWD trannies are notoriously weak.

MR, FF, and FR, from what I assume, would vary very slightly, with MR being the fastest of those three.

[PhaneSoul]

but... excluding awd, wouldnt you think fwd would be faster considering the weight factors? the more weight put on tires, the better the grip, but that probably could cut down the power put out from all extra weight, i guess you would have to run a bunch of tests on all the possibilities to find out which is better

[Adikt]

well when you accelerate all the weight is transfered to the back tho, i think it goes

from best to worstAWDMRFRFF

[PhaneSoul]

now i kno what awd, fwd, and rwd is, but whats mr, ff, and fr

[Adikt]

MR = Mid engine, Rear wheel driveFF = Front engine, Front wheel drive FR = Front engine, Rear wheel drive

[PhaneSoul]

thank you

[S14Life]

Now I thought an FF car would be faster off the line because less horsepower/torque is lost through the drive train..

The power would get to the wheels in an FF car before they did in an FR car, right?

This is assuming of course the 2 cars are exactly identicle (other than the drivetrain layout) and are performing in identicle conditions.

[easymmkay240sx]

The FF car will lose a lot of it's traction from the weight being tossed back (releiving pressure from the front wheels which = less contact surface area which usually = wheel spin) an FR car will have all that weight thrown back onto the rear tires which will help produce better traction.

[gounc14]

Can someone explain HOW the weight is transferred to the back tires under acceleration? I showed the guy a bunch of articles about it, and he said, justly so, that none of it was backed up with data.

[easymmkay240sx]

....

And this guy is majoring in engineering? Wow.

I'd make a bigger post about this but my side hurts and I need sleep....maybe tomorrow.

[entity]

Gounc14 said "physics speaking". I'm assuming this excludes the extra hp lost in rwds, so they would both be putting down the same whp.

For dry conditions, RWD would launch faster because the weight would shift to the rear during launch. This would press down on your rear tires and cause them to have far more traction than your front tires, which would actually be lifting(more or less) off the ground when when you launch. That's why all the cars with alot of power are RWD. Notice how much more difficult it is to chirp your tires shifting in RWD than FWD?

Anyone can burn out with FWD, I want to see those Civic drivers who think they're badasses do it with only 110lbs/ft of torque & rwd:D.

That aside, I think it's a toss up between AWD and MR. I don't know if AWD is worth the extra weight & hp loss compared to the good amounts of traction MR layouts provide.

[entity]

Can someone explain HOW the weight is transferred to the back tires under acceleration? I showed the guy a bunch of articles about it, and he said, justly so, that none of it was backed up with data.

You're sitting in a car. You press the accelerator. The wheel spins, and the front of the car tilts upward. This sifts the weight to the rear.

If you brake, your car tilts foward and sifts the weight to the front. That's why the front brakes are always bigger than the rear brakes.

If he needs any more proof tell him to fill a glass of water up all the way, put it in his car, and accelerate. What happens?

[gounc14]

Eh don't bash him, he just had a really long conversation with a supra driver who decided that because supras lack low end power, fwd is better from the start.

[gounc14]

And his theory is sound, he just doesn't take into account the front end rising under acceleration, because in theory the weight of the engine on top of the front wheels would create more traction to the front tires. But with the weight shifting to the rear end, the extra weight on the rear tires and the lack of weight to the front tires causes the rwd car to gain traction faster, right?

[entity]

That would be it, and the entire time that the rear wheels are gaining traction, the front is losing. Fwd's do have the advantage in wet conditions though because they start out with more traction.

Yay I hit Intake/exhaust.

[Rockenreno]

And his theory is sound, he just doesn't take into account the front end rising under acceleration,

Well, if he didn't take that into account, obviously his theory is not sound.Quote » because in theory the weight of the engine on top of the front wheels would create more traction to the front tires. But with the weight shifting to the rear end, the extra weight on the rear tires and the lack of weight to the front tires causes the rwd car to gain traction faster, right? [/quote] The weight transfers to the rear during acceleration, just press on the gas in any car and you will feel this. It's basic physics... how else would cars do wheelies? You've seen drag races I assume? Watch the cars all lift off in the front and push down in the rear. The answer is very clear cut, there should be no confusion. This guy must be failing physics...

So yes, RWD is better off the line that FWD as far as weight/physics go.

[PhaneSoul]

yes i totally forgot that the front lifts up under acceleration, SHOOT ME PLZ, hehe, i dropped out of physics, whent to applied bio n chem

[cstang68]

Anyone can burn out with FWD, I want to see those Civic drivers who think they're badasses do it with only 110lbs/ft of torque & rwd:D.

I drove around for 2 years in a 1980 Rx7 that had about 80 ft*lbs at the wheels. I could get scratch on the 2-3 shift. Being able to spin the tires has more to do with gearing (which acts like a torque multiplier) and tire diameter (if you have a smaller tire it will make a greater force on the tire with same amount of torque...Torque=Force*radius)

[irax]

all this is great and all, but i think everyone forgot the most basic thing that i think they overlooked.

almost all cars shift there weight to the back at launch this is caused by the amount of tq just being twisted thrugh the drive shaft and to the differential.

but from there on out every one has it right, because the weight tranfers to the drive wheels you get better ecceleration

and everyone also forgot

RR carsthe only one i can think of is the beatle

the reason it was such a fast car with little whores power is because the motor is on the rear axle and its rwd so all the weight is not only on the drive tires, but on launch the rest of the weight transfers there...

but then again, the more the weight transfers back, the less forward its going, and thats why they have wheelie bars on dragcars that have just disgusting amount of tq...

id like to see a RF car...

[C-Kwik]

I drove around for 2 years in a 1980 Rx7 that had about 80 ft*lbs at the wheels. I could get scratch on the 2-3 shift. Being able to spin the tires has more to do with gearing (which acts like a torque multiplier) and tire diameter (if you have a smaller tire it will make a greater force on the tire with same amount of torque...Torque=Force*radius)

Actually, I would say it has more to do with the amount of torque available, along with the weight of the flywheel, difference in flywheel vs input shaft speed and overall traction. Not to mention a clutch that can handle the kind of immediate and abrupt loading associated with a hard shift.

[SilviaS14KA24DE]

does your friend own a car? does he look at cars accelerating off the line?

[extheflow]

if this guy was FWD he would still be at the line.

[RalphCare]

if this guy was FWD he would still be at the line.

couldnt have said it better myself.

i used to work at Striclty European Motors (SEM) its a shop that repairs volkswagens and whatnot... we had the fastest watercooled FWD volkswagen in the world (that wasnt considered a "frankenstein" car)

it runs 9's now with 776HP, and im pretty sure that even with gobs of torque, and whatever suspension you throw on there.. that looking at the videos i have, you can tell that any RWD car with half that horspower is faster off the line.

http://www.strictlyeuropeanmotors.com

look at the videos and judge by yourself.

[Shift_Oversteer]

it's true, a buddy of mine has a ff stealth r/t (222hp stock) and even when I had my sohc ka if i launched it right id have him through 1st gear simply because of traction issues.

[Ligouri Rd]

Technobable follows, read at own risk. This may not be work safe, unless your boss doesn't mind you catching up on sleep.

All moving objects must be analyzed by their centre of mass. Force is transmited at the bottom of the tyre. The centre of mass will always be above the bottom of the tyre and the vertical distance from the centre of mass to the bottom of the tyre is the moment arm. When force is applied at the bottom of the tyre, the centre of mass reacts with the same force wanting to stay still. This reaction force coupled with the moment arm creates a torque which wants to rotate the car towards the rear.

Statically (not moving) most cars have around a 60/40(fwd) to ideally a 50/50 (rwd) weight distribution. Even RR Porshe's and MR NSX's try to achiceve a 50/50 distribution, of course those are roadcourse cars not drag cars.

Friction force is directly proportional to the force with which an object is pushed down (normal force) by the coeficient of friction and the frictional area. More force down and more area to put it down = more traction.

When a car accelerates all the weight of the car the torqing back at the centre of mass adds or subtracts from the static weight distribution. At the rear there is more force pushing down, which flexes the rear tyres and increases the contact patch and increases the amount of force that you can apply at the wheels before over coming the friction limits of the tyres.

With the same power, Fwd vs Rwd vs Awd and given enough traction Rwd will always be faster. The power is transferred exactly were the most traction available is. If you flip a top fuel rail end for end no matter what you do rwd will win. Its a circle: the more traction you have, the more power you can put down which means more acceleration which means more weight transfer which creates more traction.

In the real world you can never have enough traction. You will always break the friction limits before you reach the power limits. Unless you lift the front tyres there will be some traction at the front wheels available to put more power down. If depends on setup. On a rail the front wheels are only along for the ride. On a skyline there is not enough traction to transfer all of the weight to the rear so you drive the front to compensate.

Dan HerodesMechanical EngineerOxford Instruments Measurement Systems

[420_240sx]

Alls I know about it is that I was embarrassed burning out off a light in a FWD Escort GT lol....its so annoying, the beginning "hop" of the line....i know you guys know what I am talking about

[karay240]

Btw, the reason that non-street drag cars don't use AWD is not because the transmission tends to be the weak link. it's b/c if you got slicks on the car, it grips good ehough that no one can justfy the extra weight of the AWD system.

Having said that, the drivetrain that's quickest off the line is AWD for a street car with DOT tires, then MR, FR, FF, respectively.For a purpose built drag car, then MR, FR, AWD, FF, respectively. . . That's just a rule of thumb, and is nowhere near the absolute. There are waaaaay too many variables to think about.

The reason I put FR being faster than AWD on slicks comes from the logic that many skyine owners convert to RWD for 0-400m drag cars. This is not to say that there are no professional AWD skylines, however, this seems to be the norm.

[Onizuka]

Actually there is an assload of AWD sub 10 second GTR's (I dare to say the gross majority of them). Why in the hell would they convert to RWD? The car IS RWD until it senses its loosing traction (it has a computer controled electro magnetic center dif). When you have 1000-1400 hp in a street car, 4 wheels putting it down is better than 2.

[Ceptos]

i didnt read all the way through your post ligouri so i could be repeating some things.

the weight transfer has nothing to do with tourqe of the engine or the dive and squat of the car. it is simply caused by acceleration. in physics there is no such thing as de-acceleration, you are accelerating backwards, so the weight is transferred to the front tires. acceleration forwards means weight on the rear tires. acceleratoin vectors work in only one direction, so when you change directions you are actually accelerating, this causes the weight shift to the outside tires around a corner. and all that weight in the front of a FF car means more weight transfer at the front wheels...but thats a whole other thread =)

something that might be noteworthy...you lose traction faster than you gain it, so your overall traction will always decrease when weight is transfered.

yes FWD cars lose less hp to the wheels because there is less rotational mass. awd adds a lot of this as well as unsprung weight and weight in general, so it might not be effecient for all racing applications. my friend has a gsx (awd eclipse) with all kinds of work done to it, i think he said something like ~600hp at the crank and ~450 at the wheels. i dont know if that was right or not, i think his g/f is the one that told me that.

fwd was not created for preformance, it was made to be less expensive to make, burn less gas and i guess be more predictable in weather. getting rid of understeer and creating balance is achieved somewhat differently than it is in a rwd car. but like karay said, there are too many variables to think about to set anything in stone.

for anyone still in highschool, take honors physics! its a fun class, college physics...ehh