fwd vs rwd
TRANSCRIPT
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As we see most the cars that is produced inIndia is Front-Wheel Drive(FWD) cars,
only some cars are Rear-Wheel
Drive(RWD) like Omni,Ambassador e.t.cSo my question is, why doesn't India
produce RWD?
Karthik.
its because rear wheel drive cars needmore energy to push the car. and in a rear
wheel drive car, most of the enegry getswasted more that what we loose in a frontwheel drive, and that indirectly effects
your mileage and in india, people arealways behind mileage and stuff so we
hardly find RWD cars. this question wasactually answered this way in someautomobile show. im not exactly sure in
which channel
I would say it primarily revolves aroundhigher production cost, as RWD would callin for safety devices which would increase
the cost and not all indians are autoenthusiast and they want to just move
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from point A to B without much of anissue. So it would be wise for the
manufacturers to invest in producing a car
which could solve this purpose ratherthan thinking about a RWD which wouldincrease the production cost.
i was searching the internet after reading
a few post on here and stumbled acrossthis.....and it kinda put an end to some ofmy questions. hope it helps others
What Offers the Best Performance? FWD,RWD, or AWD?
Agent009 submitted on 7/9/2007 OfficialAutoSpies Timestamp: 3:13:39 PM
SOURCE: www.world-challenge.com
Front Wheel Drive, Rear Wheel, or AllWheel Drive, the age old question which is
the best way of quickly getting around atrack.
FWD fans tout predictability and overall
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consistency as hallmarks of the drivetrain. Many claim that properly set up
front wheel drive vehicle is just as fast
any one else.
RWD advocates on the other hand tend tofocus on intangible balance and control
factors of the vehicle, and the ability toplace the rear drive wheels exactly at the
appropriate angle to maximize power to
the wheels upon acceleration. Soundsgood in theory at least.
AWD zealots throw all arguments out ofthe window and claim that with all 4
wheels driving, maximum power andtraction go to the ground at all times.
While parasitic drive train drag is a factor,many times it is made up be deterioratingtrack conditions.
There are countless one off tests showing
the strong points of all of these formatsbut due to rules classification they rarely
meet on the track in an all out test ofprowess. Great for the theories, but shorton factual proof. However if you look a bitcloser, the melting pot of all racing the
SCCA Speed World Challenge Touring Car
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Series does just that.
With displacement limited to 2.8-liters
and power ranging from 225 to 275 bhp,the vehicles can be FWD, RWD, or AWD.But the most important part, is that all ofthe formats fall into this category under
very controlled circumstances. FWDAcuras and Mazdas, RWD BMWs and
Lexus, and even AWD Audis all tangle in a
fiercely competitive racing series.
So who comes out top in a series whereconsistency and handling are at aparamount? Oddly enough to many, the
FWD Acuras and Mazdas typicallydominate the crowd, followed very closely
by the AWD Audis and RWD BMW teamsnipping at the heels.
But surprising to me, is the drive trainsmostly associated by enthusiast
everywhere for true handling performancethe RWD and maybe even the AWD
consistently fall in the middle of the pack.Actually save one victory this season by aBMW and another by an Audi thesesupposedly superior forms of transport
are routinely regulated to being
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bridesmaids, not the bride.
The morale to the story for those of you
wanting to know, is that we can all bickerback and forth about who is the best. Butevery dog has his day, and in rare classeswhere everyone meets, the outcomes are
not always as expected.
Top Ten Drivers
1. Randy Pobst - TriPoint Motorsports
Mazda62. Jeff Altenburg - TriPoint Motorsports Mazda6
3. Pierre Kleinubing - RealTime Racing Acura TSX
4. James Sofronas - GMG/Greek Bros.Racing BMW 325i5. Peter Cunningham - RealTime Racing
Acura TSX6. Kuno Wittmer RealTime Racing Acura
RSX7. Chip Herr - STaSIS Engineering Audi
A48. Charles Espenlaub - Tindol Motorsports-Mazda69. Michael Galati - Tindol Motorsports-
Mazda6
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10. Brandon Davis - RealTime Racing Acura TSX
* 25. Andrew Wojteczko(R)- AutoAnalyzer Racing- Lexus IS (only two racesthis season)
Manufacturer Points
Mazda 47 PointsAcura 32 Points
Audi 19 PointsBMW 17 Points
*Lexus 0 Points (only two races thisseason)
Prior to the fuel crises in the 1970's, rearwheel drive was king. Just about everyvehicle, from economy to luxury, came
with rear wheel drive. The shift from rearwheel drive to front took about a decade.
Since the mid eighties, just about everyeconomy car, family sedan, minivan andeven many sport coupes came with front
wheel drive. Luxury marks such as BMWand Mercedes-Benz continued on with
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rear wheel drive but Cadillac eventuallymoved every vehicle to front wheel drive.
Once again, times have changed. Over the
last few years we've seen more and morevehicles (re) introduce rear wheel drive.Why? Well, it simple.
As cars become more powerful it isdifficult to have one set of wheels doing
the steering and the accelerating. By
having the front wheels do the steering,and the rear wheels driving the car, you
get a better-balanced vehicle. Thiseliminates torque steer and improvesacceleration. Rear wheel drive offers
better weight distribution (much closer to50/50 than fwd), which in turn offers
more predictable handling. Finally, withthe advent of traction control and stabilitymanagement systems, the front wheel
drive advantage in slippery conditions hasbeen significantly reduced. More and more
rwd vehicles have the option of AWD aswell. If nothing else, this is a great way
for automakers to hedge their bets. Still,some consumers are skeptical of rearwheel drive. Perhaps they are the victimsof clever marketing by Madison Ave. that
tried to get people to accept fwd and
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forget all about the virtues rear wheeldrive. They did a great job. Perhaps too
good.
Today cars are more powerful yet yieldbetter fuel economy. As such, we can lookat fwd and rwd more objectively. Is one
better than the other? Fwd still holds anadvantage in terms of packaging
efficiency, offering greater interior room
in a smaller package. Rear wheel driveprovides better handling and acceleration
and with the addition of traction control,virtually eliminates the fwd advantage inthe snow.
In the end, it depends on what you want
from your car. If it's performance, you'relooking at rwd. If you're indifferent,perhaps looking for a small car with
greater interior volume, it's front wheeldrive for you. Over the last 20 years,
technology has improved both layouts,reducing the advantages of fwd to a point
where rwd is a viable option for mostpeople. Ultimately, you've got morechoice, and when more choice is offeredwe all win.
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Front or Rear Wheel Drive?
One of the many issues to consider when
designing a recumbent, is whether tomake it front or rear wheel drive (FWD orRWD). The choice is not clear cut. Eachapproach has its strengths and
weeknesses, and like all good design, youhave to find the best solution fit for your
specific requirements.
FWDMoving BB
FWDTwist
Chain
RWD
Limitationssteepgrades
steepgrades
-
PSI manageable
minimal -
Chainline simple complex complex
Adjustability
easier harder harder
SteeringBehavior
oversteer oversteer
understeer
The above table attempts to compare the
relative advantages and disadvantages of
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each approach, however these aregeneralizations, and the devil, as always,
is in the details.
LimitationsFirst and foremost the significant
limitation of FWD designs is theirpotential to loose traction on steep grades(hills).
Minimum 60:40 weight distribution
On level ground, the FWD recumbentshould have more weight distributed tothe front wheel than the rear. Ratios of
60:40 or higher are reccomended.
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Loss of traction of steep grades
The problem is that as the grade becomessteeper, the weight distribution changesto favor the rear wheel. The illustration
above is extreme, and most riders even on
hilly terrain don't consider FWD traction asignificant issue. If however you are
planning to ride off road, the frictioncoefficient of gravel, mud and dry grass is
much less than tarmac, so traction willbecome a limiting factor. You can reducethe impact of grade on weight distribution
by keeping the seat height low andincreasing the wheelbase.
Pedal Induced Steering
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Effect of Trail on FWD
One significant issue for FWD designs is
the effect of trail on dynamic stability andpedal induced steering (PSI). To illustrate
this, the diagram above represents theview looking down on a 20" front wheel
that is moving forward down the page. Ithas a 75 pivot angle, 20mm of fork offsetand is leaning 30 to the left. As the frontwheel leans, the contact patch moves to
the inside of the pivot axis. This isbecause the contact patch is moving
around the outside wall of the tire. This
causes the driving force (Red) to generatea turning force (Blue) around the pivot
axis, but because the application ofhuman power using pedals is not
constant, the turning force oscillates.
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These oscillations will generatesympathetic harmonics at certain cadence
frequencies due to the shifting weight of
the legs while peddling, and theirinteraction with the dynamic trackingbehavior of caster.
On FWD twist chain designs, having a
shallow pivot axis, short trail and longtiller minimizes the impact. On FWDmoving BB designs however, the turning
force will interact with the pedal force,alternating between cooperation and
opposition. At some cadence frequenciesthis actually eliminates PSI effects andworks well, at others, particularly high
cadence it causes stability problems.
Chainline
In my experience chainline management
is the single biggest issue in recumbentdesign. You can come up with a beautifulbike or trike, but if you can't transfer
power efficiently form you feet to thedrive wheel(s), then you will have to
revise your design.
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FWD moving BB chainline
The FWD moving BB design is used bythe TT, Cruzbike, Speculoos,Python
lowracer and Flevobike, as well asthe Hipperion trike. When properlydesigned, pedal induced steering can be
kept to a minimum. This is a challenging
configuration, but in the case of thePython lowracer, it can produce a verylight bike. The direct, unencumberedchainline is also the most efficient, the
chain routing used in the other designshas been reported to consume over 5
watts.
http://traylorfwd.home.mindspring.com/http://www.cruzbike.com/http://les-velos-de-laurent.skynetblogs.be/http://www.python-lowracer.de/http://www.python-lowracer.de/http://www.flevobikeusa.com/http://personal.eunet.fi/pp/davinci/fill.htmlhttp://www.cruzbike.com/http://les-velos-de-laurent.skynetblogs.be/http://www.python-lowracer.de/http://www.python-lowracer.de/http://www.flevobikeusa.com/http://personal.eunet.fi/pp/davinci/fill.htmlhttp://traylorfwd.home.mindspring.com/ -
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FWD twist chain chainline
The FWD twist chain design has becomeextremely popular of late, particularly in
the HPV racing scene. This design isfavored by fully fared streamliners used inHPV speed trials because it helps to keep
the frontal profile of the faring to a
minimum.
RWD chainline
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RWD is by far the oldest and most widelyused chainline design. It is constrained by
the seat height -- make the seat high and
you can have an unencumbered chainlinelike the Cycloratio -- make the seat lowand you have to route the chain over thefront wheel and under the seat. For a
tadpole trike this is less of an issue thanfor a bike, in that you only have to route
the chain under the seat, but the front
cross member and steering tie rods canalso get in the way.
Adjustability
FWD moving BB adjustablity
One of the challenges all recumbentdesigns face, is making the distance fromthe seat the bottom bracket (BB-BOS)adjustable. It is often undesirable to make
this adjustment by moving the seat
http://www.jetrike.com/cycloratio-brochure.htmlhttp://www.jetrike.com/ergonomics.html#BB-BOShttp://www.jetrike.com/cycloratio-brochure.htmlhttp://www.jetrike.com/ergonomics.html#BB-BOS -
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backwards and forwards, because thismay upset the ride quality and handling
characteristics. Most designs incorporate
some variant of a telescopic boom.However with most chainlines, thisrequires adding or removing chain links tomake the adjustment, on the theory that
once fitted to the rider, the BB-BOSdistance never needs to change. If
however you are planning to race in a 24
hour HPV event with a team of riders,quick changeovers are a requirement. The
FWD moving BB design is rather unique inthis regard, the bottom bracket can bedesign for adjustment without any need
to modify the chain. The alternative forrouted chain lines is to have a longer
chain and use an adjustable chaintensioner on the return run.
Steering Behavior
The angle between the direction a wheel
is pointing and the path along which itactually moves is called the slip angle.
Slip occurs under power when a trike isturning. It also occurs under breaking asthe tire approaches its traction limit.
Under power the weight distributionusually moves to the rear of a vehicle, but
human power being as limited as it is,
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acceleration forces are rarely an issue,unless you ride a unicycle. Under breaking
the weight distribution moves to the front
outside wheel.How the steering behaves under power is
dependent on the weight distribution andthe friction coefficient of the tire and road
surface. Steering behavior becomes morepronounced when the friction coefficientis low i.e. a verge with loose gravel while
cornering. Too much weight at the rearcauses the rear to spin out (oversteer).
Too much weight at the front causes thefront wheels to plough (understeer).Neutral handling is when the weight is
evenly distributed between the front andrear, but generally slight understeer is
considered safest.
RWD understeer behavior
RWD trikes have a tendency toundersteer. This is because the drive force
is pushing the trike forward in a straight
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line, and the front wheels slip forward asthey turn. It is only the friction of the
front wheels on the pavement, not the
driving force that turns the trike. Thelocation of the optimal CoG also creates aweight distribution that favorsundersteer.
Under the decelerating forces of brakingthe steering behavior may be completelydifferent. It is dependent on the dynamic
weight distribution, which is directlyinfluenced by the location of the rider CoG
in relation to the front contact patches,the seat height, the brake forcedistribution, and the amount of brake
force applied.
FWD oversteer behavior
FWD trikes have a tendency to oversteer.The drivetrain pulls the front of the trikearound the corner. It is only the friction ofthe rear wheels on the pavement that
prevents the rear of the trike from
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spinning out. However because the weightdistribution is already biased towards the
front wheel to improve traction, these
forces tend to cancel one another out. Incompetitive racing, particularly on shorttracks with lots of cornering, riders mayprefur the responsive feel of slight
oversteer, but it carries the added riskthat the rider may loose control. Too
much oversteer will make a trike unstable
and dangerous.I have confirmed this behavior with
Jetrike. When turning sharply enough ongravel so all three wheels drift, the trikeis generally well balanced, if however
traction is completely lost, oversteercombined and an abrupt loss of
momentum is the frequent result, butbeing a trike, easy to recover from.
Soloviov S-4 all wheel drive trike
http://www.trike.ru/http://www.trike.ru/ -
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The S-4 trike pictured above, features allwheel drive (3 x 3), which would actually
come in handy on icy winter roads. It all
comes down to what your requirementsare. For most trikes the critical safetyissue is how well it handles underextreme breaking and cornering
conditions, and that is all about thelocation of the rider CoG and weight
distribution.