My dad gave me a quick lecture on the dangers of driving in the snow with a rear-wheel drive.
Anyways, all this had me wondering - what IS the difference between a FF or FWD (front-wheel drive), and a FR or RWD (rear-wheel drive)? Is there a BIG difference and is one BETTER than the other?
Well, I decided to research this hoping to find answers that I could use to eventually put my parents' minds at ease. In order for me to understand the differences between the two, I first needed to understand the layout of the two and how their performance was affected by it.
Front-wheel Drive (FF or FWD), Layout and Performance
Front-wheel-drive layouts are those in which the front wheels of the vehicle are driven. The most popular layout used in cars today is the front-engine, front-wheel drive, (FF/FWD or FWD), with the engine in front of the front axle, driving the front wheels. This layout is typically chosen for its compact packaging; since the engine and driven wheels are on the same side of the vehicle, there is no need for a central tunnel through the passenger compartment to accommodate a prop-shaft between the engine and the driven wheels.
As the steered wheels are also the driven wheels, FWD cars are generally considered superior to FR (front-engine, rear-wheel drive layout or RWD) cars in conditions such as snow, mud or wet tarmac. However, powerful cars rarely use the FF/FWD layout because weight transference under acceleration reduces the weight on the front wheels and reduces their traction, putting a limit on the amount of torque which can be utilized. Electronic traction control can avoid wheelspin but largely negates the benefit of extra torque/power.
A transverse engine (also known as "east-west") is commonly used in FF/FWD designs, in contrast to FR/RWD which uses a longitudinal engine. The FF/FWD layout also restricts the size of the engine that can be placed in modern engine compartments. This is another reason luxury/sports cars almost never use the FF/FWD layout.
- Interior space: Since the powertrain is a single unit contained in the engine compartment of the vehicle, there is no need to devote interior space for a driveshaft tunnel or rear differential, increasing the volume available for passengers and cargo. Instead, the tunnel may be used to route the exhaust system pipes.
- Weight: Fewer components usually means lower weight.
- Improved fuel efficiency due to less weight.
- Cost: Fewer material components and less installation complexity overall. However, the considerable MSRP differential between a FF/FWD and FR/RWD car cannot be attributed to layout alone. The difference is more probably explained by production volumes as most rear-wheel cars are usually in the sports/performance/luxury categories (which tend to be more upscale and/or have more powerful engines), while the FF/FWD configuration is typically in mass-produced mainstream cars. Few modern "family" cars have rear-wheel drive as of 2009, so a direct cost comparison is not necessarily possible. A contrast could be somewhat drawn between the FF/FWD Audi A4 and the FR/RWD BMW 3-Series, both which are in the compact executive car classification.
- Improved drivetrain efficiency: the direct connection between engine and transaxle reduce the mass and mechanical inertia of the drivetrain compared to a rear-wheel drive vehicle with a similar engine and transmission, allowing greater fuel economy.
- Assembly efficiency: the powertrain can often be assembled and installed as a unit, which allows more efficient production.
- Placing the mass of the drivetrain over the driven wheels moves the centre of gravity farther forward than a comparable rear-wheel drive layout, improving traction and directional stability on wet, snowy, or icy surfaces.
- Predictable handling characteristics: front-wheel drive cars, with a front weight bias, tend to understeer at the limit, which (according to e.g. SAAB engineer Gunnar Larsson) is easier since it makes instinct correct in avoiding terminal oversteer, and less prone to result in fishtailing or a spin.
- A skilled driver can control the movement of the car even while skidding by steering, throttling and pulling the hand brake (given that the hand brake operates the rear wheels as in most cases, with some Citroen and Saab models being notable exceptions). A small car with the FF/FWD layout is superior for motor sport events focusing on manouvreability such as Autotesting.
- It is easier to correct trailing-throttle or trailing-brake oversteer.
- The wheelbase can be extended without building a longer driveshaft (as with rear wheel driven cars).
- Torque steer is the tendency for some front-wheel drive cars to pull to the left or right under hard acceleration. It is a result of the offset between the point about which the wheel steers (it is aligned with the points where the wheel is connected to the steering mechanisms) and the centroid of its contact patch. The tractive force acts through the centroid of the contact patch, and the offset of the steering point means that a turning moment about the axis of steering is generated. In an ideal situation, the left and right wheels would generate equal and opposite moments, canceling each other out; however, in reality, this is less likely to happen. Torque steer can be addressed by using a longitudinal layout, equal length drive shafts, half shafts, a multilink suspension or centre-point steering geometry.
- Lack of weight shifting will limit the acceleration of a front-wheel drive vehicle. In a vehicle, the weight shifts back during acceleration, giving more traction to the rear wheels. This is one of the main reasons why nearly all racing cars are rear-wheel drive. However, since front-wheel drive cars have the weight of the engine over the driving wheels, the problem only applies in extreme conditions.
- In some towing situations, front-wheel drive cars can be at a traction disadvantage since there will be less weight on the driving wheels. Because of this, the weight that the vehicle is rated to safely tow is likely to be less than that of a rear-wheel drive or four-wheel drive vehicle of the same size and power.
- Traction can be reduced while attempting to climb a slope in slippery conditions such as snow- or ice-covered roadways.
- Due to geometry and packaging constraints, the CV joints (constant-velocity joints) attached to the wheel hub have a tendency to wear out much earlier than the universal joints typically used in their rear-wheel drive counterparts (although rear-wheel drive vehicles with independent rear suspension also employ CV joints and half-shafts). The significantly shorter drive axles on a front-wheel drive car causes the joint to flex through a much wider degree of motion, compounded by additional stress and angles of steering, while the CV joints of a rear wheel drive car regularly see angles and wear of less than half that of front wheel drive vehicles.
- Turning circle — FF/FWD layouts almost always use a Transverse engine ("east-west") installation, which limits the amount by which the front wheels can turn, thus increasing the turning circle of a front-wheel drive car compared to a rear-wheel drive one with the same wheelbase. A notable example is the original Mini. It is widely misconceived that this limitation is due to a limit on the angle at which a CV joint can be operated, but this is easily disproved by considering the turning circle of car models that use a longitudinal FF/FWD or F4 layout from Audi and (prior to 1992) Saab.
- The FF/FWD transverse engine layout (also known as "east-west") restricts the size of the engine that can be placed in modern engine compartments, so it is rarely adopted by powerful luxury and sports cars. FF/FWD configurations can usually only accommodate Inline-4 and V6 engines, while longer engines such as Inline-6 and 90° big-bore V8 will rarely fit, though there are exceptions. One way around this problem is using a staggered engine.
Rear-Wheel Drive (FR or RWD), Layout and Performance
Rear-wheel drive (RWD) typically places the engine in the front of the vehicle and the driven wheels are located at the rear, a configuration known as front-engine, rear-wheel drive layout (FR/RWD layout). The front mid-engine, rear mid-engine and rear engine layouts are also used. This was the traditional automobile layout for most of the 20th century. Nearly all motorcycles and bicycles use rear-wheel drive, either by driveshaft, chain, or belt, since the front wheel is turned for steering, and it would be very difficult and cumbersome to "bend" the drive mechanism around the turn of the front wheel.
All the disadvantages of FF/FWD systems are advantages of RWD vehicles. With some of the mechanical parts removed from the front and installed at the rear, vehicle balance and handling are much improved. Using the rear tires for acceleration traction takes the load off the front, so drivers accelerating out of a corner have much more lateral grip. RWD is used on all the world’s fastest road course race cars and many performance production vehicles for this reason.
- Even weight distribution — The layout of a rear-wheel drive car is much closer to an even fore-and-aft weight distribution than a front-wheel-drive car, as more of the engine can lie between the front and rear wheels (in the case of a mid engine layout, the entire engine), and the transmission is moved much farther back.
- Weight transfer during acceleration — During heavy acceleration, weight is placed on the rear, or driving wheels, which improves traction.
- No torque steer (unless it's an all-wheel steer with an offset differential).
- Steering radius — As no complicated drive shaft joints are required at the front wheels, it is possible to turn them further than would be possible using front-wheel drive, resulting in a smaller steering radius for a given wheelbase.
- Better handling in dry conditions — the more even weight distribution and weight transfer improve the handling of the car. The front and rear tires are placed under more even loads, which allows for more grip while cornering.
- Better braking — the more even weight distribution helps prevent lockup from wheels becoming unloaded under heavy braking.
- Towing — Rear wheel drive puts the wheels which are pulling the load closer to the point where a trailer articulates, helping steering, especially for large loads.
- Serviceability — Drivetrain components on a rear-wheel drive vehicle are modular and do not involve packing as many parts into as small a space as does front wheel drive, thus requiring less disassembly or specialized tools in order to service the vehicle.
- Robustness — due to geometry and packaging constraints, the universal joints attached to the wheel hub have a tendency to wear out much later than the CV joints typically used in front-wheel drive counterparts. The significantly shorter drive axles on a front-wheel drive car causes the joint to flex through a much wider degree of motion, compounded by additional stress and angles of steering, while the CV joints of a rear wheel drive car regularly see angles and wear of less than half that of front wheel drive vehicles.
- Can accommodate more powerful engines as a result of the longitudinal orientation of the drivetrain, such as the Inline-6 and 90° big-bore V8, making the FR/RWD a common configuration for luxury and sports cars. These engines are usually too long to fit in a FF/FWD transverse engine ("east-west") layout; the FF/FWD configuration can typically accommodate at the maximum an Inline-4 or V6.
- Under heavy acceleration, oversteer and fishtailing may occur.
- On snow, ice and sand, rear-wheel drive loses its traction advantage to front- or all-wheel drive vehicles, which have greater weight on the driven wheels. Rear-wheel-drive cars with rear engine or mid engine configuration do not suffer from this, although fishtailing remains an issue.
- Some rear engine cars (e.g. Porsche 911) can suffer from reduced steering ability under heavy acceleration, because the engine is outside the wheelbase and at the opposite end of the car from the wheels doing the steering although the engine weight over the rear wheels provides outstanding traction and grip during acceleration.
- Decreased interior space — Though individual designs vary greatly, rear wheel drive vehicles may have: Less front leg room as the transmission tunnel takes up a space between the driver and front passenger, less leg room for center rear passengers (due to the tunnel needed for the drive shaft), often no seat for a center rear passenger, and sometimes less boot space (since there is also more hardware that must be placed underneath the boot). Rear engine designs (such as the Porsche 911 and Volkswagen Beetle) do not inherently take away interior space.
- Increased weight — The components of a rear wheel drive vehicle's power train are less complex, but they are larger. The driveshaft adds weight. There is extra sheet metal to form the transmission tunnel. There is a rear axle or rear half-shafts, which are typically longer than those in a front-wheel drive car. A rear wheel drive car will weigh slightly more than a comparable front wheel drive car (but less than four wheel drive).
- Improper weight distribution when loaded — A rear wheel drive car's center of gravity is shifted rearward when heavily loaded with passengers or cargo, which may cause unpredictable handling behavior.
- Higher initial purchase price — Modern rear wheel drive vehicles are typically more expensive to purchase than comparable front wheel drive vehicles. Part of this can be explained by the added cost of materials and increased complex assembly of FR/RWD layouts, as the powertrain is not one compact unit. However, the difference is more probably explained by production volumes as most rear-wheel cars are usually in the sports/performance/luxury categories (which tend to be more upscale and/or have more powerful engines), while the FF/FWD configuration is typically in mass-produced mainstream cars.
- The possibility of a slight loss in the mechanical efficiency of the drivetrain (approximately 17% coastdown losses between engine flywheel and road wheels compared to 15% for front wheel drive — however these losses are highly dependent on the individual transmission). Cars with rear engine or mid engine configuration and a transverse engine layout do not suffer from this.
- The long driveshaft (on front engine cars) adds to drivetrain elasticity. The driveshaft must also be extended for cars with a stretched wheelbase (e.g. limousines, minivans).
- When shifting down at high RPM, rear-wheel drive vehicles tend to oversteer if the clutch is not released correctly.
Ok. I'm obviously no mechanic so alot of this was a bit too complex for me to grasp, to say the least. But what I did understand was:
- Front-wheel drive vehicles transmit power through the front wheels. Rear-wheel drive vehicles transmit power through the rear wheels.
- Front-wheel drive offers an advantage in slippery conditions such as ice or snow as more weight is over the drive wheels reducing slipping during acceleration. But since most of the weight is up front, a FWD car is not as well balanced therefore it doesn't handle quite as well.
- Rear-wheel drive has advantages for traction under acceleration and with rear-wheel drive, the steering function is separated from power delivery. The front wheels steer, the rear wheels drive.
- By having the front wheels do the steering, and the rear wheels driving the car, you get a better-balanced vehicle. This eliminates torque steer and improves acceleration. Rear wheel drive offers better weight distribution (much closer to 50/50 than FWD), which in turn offers more predictable handling.
As cars become more powerful it is difficult to have one set of wheels doing the steering and the accelerating. Today's sophisticated traction and stability control systems are so good they can mask or enhance the true driving dynamics of a vehicle - the front wheel drive advantage in slippery conditions has been significantly reduced.
Ultimately, you've got more choice, and when more choice is offered we all win.
As for me...
I bought this Chrysler 300 without even knowing anything about front-wheel rear-wheel drive. Quite honestly, I bought it simply because I liked how it looked and I liked the size - not too small, not too big. The extra 'comforts' were a bonus such as the heated leather seats, dual temperature control etc. (all things I could live without, actually).
Getting in and out of a car was another important thing for me (since I'm tall), and the 300 offered a greater ease of this.
As for traction in the winter months - I have that covered by buying snow tires! It's becoming mandatory soon (if it isn't already), in Canada for all cars to have snow tires during winter anyways.
The only thing I never thought about was the fuel economy. Unlike most people, fuel consumption was never an issue for me (probably because I rarely go anywhere besides work and back). But the fuel consumption - 12.2 L/100km (23 mi./gal.) in the city and 8.1L/100km (35 mi./gal.) on the highway - is not the greatest - but it's not the worse either and compared to the 2011 Ford Fusion (the other car I was thinking of getting), with a smilar engine (3.5L/V6) that consumes 12.6L/100km in the city - I think I'll sacrifice fuel economy for comfort and style.
Resource(s): autos.ca, wikipedia, popularmechanics.com, all-about-car-selection.com