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Thread: RWD versus FWD

  1. #1
    BMW Power alpinum's Avatar
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    Default RWD versus FWD

    Datorita unor discutii survenite pe un alt thread deschid acest subiect pentru a lamuri pe cei interesati care este diferenta intre RWD -Rear Wheel Drive si FWD -Front Wheel Drive. Vom aborda putin filozofia BMW si de ce BMW si Mercedes au ales aceasta tractiune.
    /// M - seven gears for the sixth sense

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    BMW Power alpinum's Avatar
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    Default Acceleration

    Acceleration
    Accelerate with near full power in a front-wheel drive car and you'll quickly notice the resulting effect of rearward weight shift. The front tires will likely lose traction and spin – even on clean, dry pavement. By contrast, hard acceleration in rear-wheel drive cars increases the rear wheels' grip on good road surfaces because of the rearward weight shift.
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    BMW Power alpinum's Avatar
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    Default Braking

    Braking
    Stopping ability is enhanced by the superior weight distribution of RWD. With the rear wheels carrying a greater percentage of the car's weight load than on a front-wheel drive car, they can apply more braking force to the road and help shorten stopping distances. Since RWD contributes to even tire wear, it is more likely that tires on a RWD car will have greater tread depth. Unless tires on a FWD car are rotated religiously, the front tires may become worn and less effective in braking.
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    BMW Power alpinum's Avatar
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    Default Responsive Cornering

    Responsive Cornering
    Near equal weight distribution helps give front and rear wheels more balanced traction. This balance gives neutral handling characteristics that make cornering maneuvers easier. Rear-wheel drive's more equal weight distribution also aids handling agility through a lower moment of inertia. FWD cars usually have higher moments of inertia, contributing to understeer and sluggishness in cornering. As a result, RWD cars feel more responsive, lighter, and more nimble.
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    BMW Power alpinum's Avatar
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    Default Balanced Force Distribution

    Balanced Force Distribution
    With FWD, both steering and propulsion forces tax the front tires' slip-resistance during cornering. That's part of the reason why FWD cars tend to understeer or plow forward, changing directions less quickly than the turning angle of the front wheels. Since RWD separates the tasks of cornering (front wheels) and propulsion (rear wheels), it more equally distributes the traction-threatening forces to all four wheels.
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    BMW Power alpinum's Avatar
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    Default Torque Steering

    Torque Steering
    Torque steering is a negative side-effect of FWD caused by the delivery of power to the wheels that steer the car. During acceleration in a curve or from a standstill, the force of torque steering can pose a hazard by changing the direction of the front wheels unless the driver is alert and can exert counteractive force on the steering wheel. RWD does not exhibit torque effect because the engine is isolated from the steering gear.
    /// M - seven gears for the sixth sense

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    BMW Power alpinum's Avatar
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    Default Longer Wheelbase

    Longer Wheelbase
    RWD allows a longer wheelbase and a more forward positioning of the front wheels. The longer wheelbase provides better handling while the forward position of the wheels reduces the possibility of the front spoiler scraping on dips.
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    BMW Power alpinum's Avatar
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    Default No CV Joints

    No CV Joints
    FWD cars have four CV (constant velocity) joints connecting the engine to the front wheels. In comparison, RWD cars use universal joints which wear out much slower than CV joints.
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    Default Sumary

    Summary

    RWD pros:
    Fore-aft weight distribution more balanced. Braking performance enhanced. Tire wear more even. Cornering easier, more responsive. Lighter than AWD configuration for better acceleration and cornering performance and better fuel-efficiency. Better hard acceleration performance on good surfaces than with FWD. Better cornering ability because steering and propulsion are applied at separate axles. Greater agility because of lower resistance to changes in direction (lower moment of inertia). Longer wheelbase for smoother ride. Absence of torque steering effect common with FWD. No CV joints to replace.

    FWD pros:
    Good traction during mild acceleration on slippery surfaces. Lighter weight helps fuel-efficiency. Interior room enhanced by lack of longitudinal driveshaft. Less expensive to manufacture.

    AWD pros:
    Traction enhanced on all road surfaces under all weather conditions. Faster acceleration "off the line" due to all wheels driving. Better road grip during cornering in adverse weather or slippery road conditions.
    Last edited by alpinum; 04-01-2006 at 04:34.
    /// M - seven gears for the sixth sense

  10. #10
    BMW Power alpinum's Avatar
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    Desigur dezbaterea FWD vs. RWD este una la nivel de principiu, caracteristica finala a automobilului este data de ansamblul punte motoare, latime anvelope (fata spate) , ecartament (fata spate), cinematica, elastocinematica, arcuri, amortizoare si distributia greutatii. Fiecare din aceste puncte poate influenta pro sau contra caracteristica viratoare. Sa ne rezumam totusi la diferentele date de puntea motoare si influenta generala a acesteia asupra distributiei greutatii.

    RWD pentru un caracter sportiv
    Acceleratie: Prin transferul greutatii catre axa spate, RWD este evident cea mai buna deoarece cu cat accelerezi mai puternic cu atat creste controlul pe spate. In cazul FWD totul se petrece exact invers: cu cat accelerezi mai tare cu atat mai mult pierzi din tractiune.
    Putere si stabilitate in curbe: Daca torqul de la motor este transmis prin roti separate de la servo, atunci masina si comportamentul ei are de suferit datorita limitei de sarcina laterala. In cazul FWD roata are sarcina extrem de dificila sa fie si “tractoare” si sa si suporte sarcinile masinii plus fortele impuse de catre forta gravitationala. Sa luam FWD: la o viteza mare daca se apasa frana puternic si in acelasi timp se rasuceste volanul, rotile din fata vor incepe sa danseze rau de tot creand instabilitate.
    Control mai bun.In cazul uni sasiu perfect echilibrat (cazul BMW), rotile din fata si cele din spate derapeaza in mod egal si echilibrat cand se ia o curba. Aceasta se numeste neutral handling. Cu RWD, torqul fiind transmis prin intermediul axei spate determina ca rotile din spate sa fie impins cu putere fara a derapa ca in cazul FWD cand dupa luarea unei curbe stranse cu o viteza putin mai mare nu numai ca spatele fuge in afara curbei dar atrage dupa sine si efectul reversiv si anume acela ca fundul masinii sa se balanseze inca o data in sens opus –adica centrul curbei- urmand ca abia dupa aceea sa ai din nou un control deplin asupra masinii. In cazul FWD, cu cat vrei sa aplici mai multa putere rotilor din fata, cu atat mai mult ele vor incepe sa derapeze si sa scoata fundul masinii in afara curbei. Desigur si FWD poate fi stapanit insa asta cere anvelope foarte bune a caror durata de utilizare este mai mica, ca sa nu mai vorbim de solicitarea mult mai mare a tuturor componentelor angrenate in acest proces in comparatie cu cele din cazul RWD.
    Sa nu uitam insa ca un factor extrem de important in stabilitatea si “handlingului” masinii este distributia greutatii. Marea majoritate a masinilor dispun de o distributie a greutatii 40-45% si 60-55%.
    In cazul majoritatii autoturismelor cu tractiune RWD distribuirea greutatii se face cat mai aproape de 50/50 iar in cazul BMW PERFECT 50/50 ca de ex. noua serie 3.
    Intrucat in cazul FWD marea majoritate a componentelor cu o greutate considerabila sunt cumulate in fata este foarte greu sa obtii o distributie a greutatii in mod egal.

    In concluzie, pentru ca FWD sa treaca neutru prin viraje, trebuie ca acceleratia laterala sa fie redusa. In plus, anvelopele sunt solicitate la limita ceea ce inseamna ca nu pot transmite forta longitudinala, respectiv nu poti acelera decat foarte tarziu asa ca iti ia mai mult timp sa parcurgi virajul. Sau, daca intri mai incet si accelerezi masina va subvira deoarece se reduce capacitatea de transmitere a fortei laterale, raza virajului si implicit distanta pe acesta se va mari deci va creste timpul necesar parcurgerii acestuia.

    RWD permite accelerarea usoara, chiar daca pierde aderenta deoarece raza virajului se reduce prin supravirare.
    Last edited by alpinum; 04-01-2006 at 06:17.
    /// M - seven gears for the sixth sense

  11. #11
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    Default Why are rear-drive cars more fun?

    Why are rear-drive cars more fun? Every enthusiast may know the answer, but I didn't. So I called up a helpful GM suspension expert, Vehicle Chief Engineer Ed Zellner. There are, I learned, five basic reasons:

    1) "Balance": The car rides on four patches of rubber, each about as big as your hand. An ideal car would distribute its weight evenly, so each tire had to bear the same load, and none would give way earlier than all the others. The ideal weight distribution, then, would be split about 50/50 between front and rear (actually, 48/52 to help with forward pitch during braking). "A rear-drive car can typically approach that," says Zellner. Engineers can move the front wheels forward, so that the engine – which doesn't have to be connected to those wheels -- sits behind the front axle. Meanwhile, the driveshaft and rear differential (necessary to send power to the rear tires) add weight in the rear. Front-drive cars, which must connect the engine and transmission to the front axle, typically have their engines mounted way forward and can't do much better than a 60/40 front/rear weight distribution.

    2) Center of Gravity: This is the point the car wants to "rotate around" in a turn. On a rear-drive car, it's "about where the driver sits," says Zellner. In a turn, in other words, the car seems to be rotating around you – you're at the center. It's a natural pleasant effect, suggesting you're in control, the way you're in control when you're walking or running around a corner and your weight is centered inside you. (Analogy No. 2: It's like wearing stereo headphones and having the sound centered between your ears!) A front-drive car, in contrast, with its massive front weight bias, wants to rotate around a point in front of the driver. So in a corner, the driver isn't just rotating around his spine. He's moving sideways, as if he were a tether ball on the end of a rope, or Linus being dragged when Snoopy gets hold of his blanket. Not such a pleasant feeling, or a feeling that gives you a sense of natural control.

    3) "Torque Steer": One of the most annoying habits of many powerful front-drive cars is that they don't go straight when you step on the accelerator! Instead, they pull to one side, requiring you to steer in the other direction to compensate, like on a damn boat. This "torque steer" usually happens because the drive shafts that connect the engine to the front wheels aren't the same length. Under power, the shafts wind up like springs. The longer shaft -- typically on the right -- winds up a bit more, while the shorter left shaft winds up less and transmits its power to the ground more quickly, which has the effect of pulling the car to the left. (This winding-up phenomenon occurs the moment you step on the pedal. After that, the wind-up relaxes, but "torque steer" can still be produced by the angles of the joints in the drive axles as the whole drivetrain twists on its rubber mounts.) Engineers try various strategies to control this veering tendency, but even designing shafts of equal length (as in all Cadillacs) doesn't completely solve the problem because the engine still twists a bit in its mounts and alters the angles of the drive shafts. True, some manufacturers -- Audi, for example -- are said to do a particularly good job of repressing torque steer . But even a top-rank company such as Nissan has problems -- its otherwise appealing new front-drive Maxima is said to be plagued by big-time, uninhibited torque steer. Rear-drive cars, meanwhile, don't really have a torque-steer problem that needs repressing. Their power goes to the rear through one driveshaft to a center differential that can a) have equal-length shafts coming out from it and b) be more firmly mounted.

    4) Weight Shift: Suppose you just want to go in a straight line. What's the best way to get traction? Answer: Have as much weight over the driving wheels as possible. Front-drive cars start with an advantage -- but when any car accelerates, the front end tips up, and the rear end squats down. This transfers weight to the rear wheels -- away from the driving wheels in a FWD car but toward the driving wheels in a rear-drive car, where it adds to available traction. In effect, the laws of physics conspire to give RWD cars a bit more grip where they need it when they need it. (This salutary effect is more than canceled out in slippery, wet conditions, where you aren't going to stomp on the accelerator. Then, FWD cars have the edge, in part, because they start out with so much more of their weight over both the driving and the turning wheels. Also, it's simply more stable to pull a heavy wheeled object than to push it -- as any hotel bellhop steering a loaded luggage cart knows. In snow, FWD cars have a third advantage in that they pull the car through the path the front tires create, instead of turning the front tires into mini-snowplows.)

    5) "Oversteer" and the Semi-Orgasmic Lock-In Effect: In a rear-drive car, there's a division of labor -- the front tires basically steer the car, and the rear tires push the car down the road. In a FWD car, the front tires do all the work – both steering and applying the power to the road – while the rears are largely along for the ride. That, it turns out, is asking a lot of the front tires. Since the driving wheels tend to lose traction first, the front tires of front-drive cars invariably start slipping in a corner before the lightly loaded rear tires do -- a phenomenon known as "understeer." If you go too fast into a curve -- I mean really too fast -- the car will plow off the road front end first. In rear-drive cars, the rear wheels tend to lose traction first, and the rear of the car threatens to swing around and pass the front end -- "oversteer." If you go too fast into a corner in an oversteering car, the car will tend to spin and fly off the road rear end first.
    What's the best way to fly off the road? Safety types prefer frontwards -- understeer. Why? To control an oversteering skid, where the rear wheels are heading for the weeds, you have to both slow down and counterintuitively turn the wheel in the opposite of the direction you're turning. In a front-drive car, with the front wheels slipping, you slow down and keep turning the way you'd been turning to get around the corner in the first place -- a more natural maneuver, since you're pointing the car in the direction you want to go. This is why, for safety reasons, even rear-drive cars sold to average consumers tend to have their springs and other suspension bits set up to make them understeer -- to make the front tires slip first, despite the car's innate oversteering tendency. Only by applying lots of power in a corner can you actually break the rear end of a bread-and-butter rear-drive car like the Mustang loose -- a maneuver favored by sports car freaks, but one you try at your own peril.
    Big American manufacturers (all heavily invested in front drive) like to say that for 99 percent of drivers, driving at normal speeds, FWD's inherent understeer and better traction in the wet makes it preferable -- both safer and easier to drive quickly. It's only the 1 percent of speed freaks who enjoy breaking the rear end loose and then catching it with a bit of "reverse lock." Here's where I emphatically dissent.
    It's pretty clear to me, after driving hundreds of different vehicles over several decades, that rear drive offers a big aesthetic advantage to ordinary drivers at ordinary speeds in ordinary conditions. Why? The lock-in effect I mentioned earlier. Suppose you go into a corner in a rear-drive car at a reasonable, safe, legal speed. Nothing's about to skid. But you can still feel the front end starting to plow wide a bit. What to do? Step on the gas! Don't stomp on it -- but add a bit of power, and a miraculous thing happens. The front end swings back in, the car tightens its line. Cornering traction seems to increase. And the car feels locked into a groove, balanced between the motive power from the rear and the turning power in the front.
    You don't have to be a race driver to feel this. You can be a defensive driver and feel it. You can be driving a 1973 Ford Maverick with leaking shocks and you'll feel it. Accountants feel it on the way to the office and housewives feel it on the way to the Safeway. Even Ralph Nader probably feels it. It's a good part of what makes driving a car a sensual act. (What's happening, technically? None of the tires is at its limit of adhesion. But the added speed is making the front tires --which [since they are undriven] have plenty of surplus traction -- apply more force to the road surface to change direction. Meanwhile, the rear of the car is shifting outward, ever so slightly -- not a Bullitt-style power slide, but a subtle attitude adjustment that cancels the plowing effect. The power "helps you through the corner," as Zellner puts it.)
    This doesn't happen in a front-drive car. The best an ordinary driver can hope for in a FWD car is that it "corners as if on rails" -- no slippage at all. No plowing -- but also no semi-orgasmic "lock in." More typically, if you hit the accelerator in a fast corner, things get mushy up front (as they did that evening near Jayne Mansfield's house). The lesson the FWD car seems to be teaching is: Try to go faster, and you're punished. Front-drive cars are Puritans! In a rear-drive car, you hit the accelerator and things get better! Rear-drive cars are hedonists. (This is assuming you don't hit the accelerator too hard.)
    /// M - seven gears for the sixth sense

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    BRAVO !

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    mrrrrr! mikilol's Avatar
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    ...un topic foarte util ....banuiesc ca nu sunt singura care apreciez efortul tau de a posta aceste explicatii FOARTE utile mai ales pentru cei care inca mai au idei preconcepute vis-a-vis de RWD...

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    Junior Member r2d2's Avatar
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    Nice...
    Cea mai scurta distanta dintre doua puncte este in constructie.

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    O masina RWD e mai usor si mai precis de condus pe carosabil uscat sau in derapaje controlate/provocate.. nu neaga nimeni asta, doar de-aia ne-am luat marea majoritate BMW..

    Singura problema la RWD e ca daca ti-a fugit fundul intr-un mod brusc si neasteptat/necontrolat, e mult mai greu/imposibil de redresat..

    Nu ma refer la piloti, ci la 90-95% dintre soferii patriei..
    Iar aici ma includ si pe mine, desi am reflexe, am talent la condus, am 10 ani experienta etc

    Si daca tot vorbesc strict de experienta/practica - nu de teorie (ca aia e frumoasa :D) - pot sa fac o comparatie intre Nova si E30.. Nova era incomparabil mai "stabila" iarna, decat ursuletul
    Cu Nova am intrat in zapada de 30cm, cu damburi de 50cm (in parcarea dela servici :D), am urcat pe drumuri forestiere unde nu as mai fi avut curaj sa urc cu E30, poate nici macar cu E36 (am ASC)

    Cu ursuletul 318i am facut o prostie cat mine de mare: la 3 noaptea - drum cu 4 benzi absolut gol - am retrogradat din a 4-a in a 3-a la ~50km/h, accelerand usor - vrand sa ii arat pasagerului din dreapta cum patineaza rotile la viteza aia, in linie dreapta!
    Masina a avut o reactie absolut violenta si neasteptata, spatele fugind instantaneu; am compensat din volan (nu am pus frana, doar am luat piciorul depe acceleratie), iar masina a revenit la traiectorie dar mai violent decat derapase; am compensat din nou si m-am trezit cu ursu' fix la 180 de grade , mergand/franand deja cu spatele la 40 la ora..

    Cu Nova nu am patit niciodata asa ceva!

    Acum insa, cu ASC, DSC etc lucrurile s-au mai skimbat.. din pacate insa nu toata lumea isi permite aceste sisteme de control a tractiunii..
    Viata e dura, dar aspra..

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