Automobile layout

In automotive design, the automobile layout describes where on the vehicle the engine and drive wheels are found. Many different combinations of engine location and driven wheels are found in practice, and the location of each is dependent on the application for which the vehicle will be used. Factors influencing the design choice include cost, complexity, reliability, packaging (location and size of the passenger compartment and boot), weight distribution, and the vehicle's intended handling characteristics.

Layouts can roughly be divided into two categories: front- or rear-wheel drive. Four-wheel-drive vehicles may take on the characteristics of either, depending on how power is distributed to the wheels.

Front-wheel-drive layouts

Main article: front-wheel drive
FF layout

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, 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, FF (front-engine, front-wheel-drive layout) cars are generally considered superior to FR (front-engine, rear-wheel-drive layout) cars in conditions such as snow, mud, or wet tarmac. The weight of the engine over the driven wheels also improves grip in such conditions. However, powerful cars rarely use the FF layout because weight transference under acceleration reduces the weight on the front wheels and reduces their traction, limiting the 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 designs, in contrast to FR which uses a longitudinal engine. The FF layout also restricts the size of the engine that can be placed in modern engine compartments, as FF configurations usually have Inline-4 and V6 engines, while longer engines such as Inline-6 and 90° V8 will rarely fit. This is another reason luxury/sports cars avoid the FF layout. Exceptions do exist, such as the Volvo S80 (FWD/4WD) which uses transversely mounted inline 6 and V8 engines, and the Ford Taurus SHO, available with a 60° V8 and front-wheel drive.

Characteristics

Front-wheel drive gives more interior space since the powertrain is a single unit contained in the engine compartment of the vehicle and there is no need to devote interior space for a driveshaft tunnel or rear differential, increasing the volume available for passengers and cargo. [1] There are some exceptions to this as rear engine designs do not take away interior space (see Porsche 911, and Volkswagen Beetle). It also has fewer components overall and thus lower weight. [1] The direct connection between engine and transaxle reduces 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. [1] In front-wheel-drive cars the mass of the drivetrain is placed over the driven wheels and thus 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. [1] [2] [3] Front-wheel-drive cars, with a front weight bias, tend to understeer at the limit which, according to 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. [3] [4]

According to a sales brochure for the 1989 Lotus Elan, the ride and handling engineers at Lotus found that "for a given vehicle weight, power and tyre size, a front-wheel-drive car was always faster over a given section of road." [5] However, this may only apply for cars with moderate power-to-weight ratio. [2] [6] [7] According to road test with two Dodge Daytonas, one FWD and one RWD, the road layout is also important for what configuration is the fastest. [3]

Weight shifting limits the acceleration of a front-wheel-drive vehicle. During heavy acceleration, weight is shifted to the back, improving traction at the rear wheels at the expense of the front driving wheels; consequently, most racing cars are rear-wheel drive for acceleration. However, since front-wheel-drive cars have the weight of the engine over the driving wheels, the problem only applies in extreme conditions in which case the car understeers. On snow, ice, and sand, rear-wheel drive loses its traction advantage to front or all-wheel-drive vehicles which have greater weight over the driven wheels. Rear-wheel-drive cars with rear engine or mid engine configuration retain traction over the driven wheels, although fishtailing remains an issue on hard acceleration while in a turn. Some rear engine cars (e.g., Porsche 911) can suffer from reduced steering ability under heavy acceleration, since the engine is outside the wheelbase and at the opposite end of the car from the wheels doing the steering. A rear-wheel-drive car's centre of gravity is shifted rearward when heavily loaded with passengers or cargo, which may cause unpredictable handling behavior. [4]

On front-wheel-drive cars, the short driveshaft may reduce drivetrain elasticity, improving responsiveness. [4]

Advantages

  • 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. [1]
    • 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. [8]
  • Cost: Fewer material components and less installation complexity overall. However, the considerable MSRP differential between a FF and FR 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 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 Audi A4 FrontTrak (which has an FF layout and front-wheel drive) and a rear-wheel-drive BMW 3-Series (which is FR), both which are in the compact executive car classification and use longitudinally mounted engines. [1]
  • 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. [1]
  • Assembly efficiency: the powertrain can often be assembled and installed as a unit, which allows more efficient production.[ citation needed]
  • 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. [1] [2] [3]
  • Predictable handling characteristics: front-wheel-drive cars, with a front weight bias, tend to understeer at the limit, which (according to 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. [3] [4]
  • 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).
  • It is easier to correct trailing-throttle or trailing-brake oversteer. [3]
  • The wheelbase can be extended without building a longer driveshaft (as with rear-wheel-driven cars).

Disadvantages

  • Front-engine front-wheel-drive layouts are "nose heavy" with more weight distribution forward, which makes them prone to understeer, especially in high horsepower applications.
    • If a front-engine front-wheel-drive layout is fitted with a four-wheel-drive, plus enthusiast driver aids, such as active front differential, active steering, and ultra-quick electrically adjustable shocks, this somewhat negate the understeer problem and allow the car to perform as well as a front-engine rear-wheel-drive car. These trick differentials, which are found on the Acura TL SH-AWD and Audi S4 3.0 TFSI quattro, and Audi RS5 4.2 FSI quattro, are heavy, complex, and expensive. [9] [10] While these aids do tame front end plow, cars fitted with these systems are still at a disadvantage when track tested against rear-wheel drive vehicles (including those with added four-wheel drive [11]). [12]
  • 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. [13] [14] [15] [16] [17] [18] [19]
  • In a vehicle, the weight shifts back during acceleration, giving more traction to the rear wheels. This is one of the main reasons 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 such as attempting to accelerate up a wet hill or attempting to beat another RWD car off the line.
  • 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.
  • 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 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 or F4 layout from Audi and (prior to 1992) Saab
  • The FF 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 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.
  • It makes heavier use of the front tyres (i.e., accelerating, braking, and turning), causing more wear in the front than in a rear-wheel-drive layout.
  • Under extreme braking (like for instance in a panic stop), the already front heavy layout further reduces traction to the rear wheels. This results in disproportionate gripping forces focused at the front while the rear does not have enough weight to effectively use its brakes. Because the rear tyres' capabilities in braking are not very high, a significant number of cheaper front drive vehicles use drum brakes in the rear even today.
  • The steering 'feel' is more numbed than a RWD car. This is due to the extra weight of drive shafts and CV joint components that increase unsprung weight.