Automobile layout

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In automotive design layout is the place where both the engine and driven wheels are.

Contents

[edit] Types

Front-engine, front-wheel drive, layout places both the engine and driven wheels at the front of the vehicle. This layout is typically chosen for its compact packaging - that is, it takes up very little space, allowing the rest of the vehicle to be designed more flexibly. In contrast with the FR layout, the FF layout eliminates the need for a central tunnel or a higher chassis clearance to accommodate a driveshaft providing power to the rear wheels. Like the RR and RMR layouts, it places the heavy engine over the drive wheels which aids traction. As the steered wheels are also the driven wheels, FF cars are generally considered superior to FR cars in conditions such as snow, mud or wet tarmac. However, powerful cars rarely use the FF layout because weight transference under acceleration unloads the front wheels and sharply reduces their grip, effectively putting a cap on the amount of horsepower which could realistically be utilized. Electronic traction control can avoid wheelspin but largely negates the benefit of extra mileage.

[edit] Characteristics

Front wheel drive gives more 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] 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 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] 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 for instance 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 tire 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]

In a front wheel drive car it is easier to correct trailing-throttle or trailing-brake oversteer.[3]

Some high power front wheel drive cars may exhibit torque steer.Torque steer can be addressed by using a longitudal layout, equal length drive shafts, half shafts, a multilink suspension or centre-point steering geometry.[8][9][10][11][12][13][14][15]

Lack of weight shifting will limit the acceleration of a front-wheel drive vehicle. During heavy acceleration, weight is placed on the rear, or driving wheels, which improves traction. This is the main reason 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. The weight shifting and weight distribution of rear wheel drive cars cause oversteer and the related problem of fishtailing. 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 this configuration provides outstanding grip and traction as the engine weight is over the drive wheels. 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.[4]

On FR cars the long driveshaft adds to drivetrain elasticity.[4]

[edit] Advantages of front-wheel drive

  • 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]
    • However, the exhaust pipes can now be routed through the unused space once occupied by the drive train.
  • Cost: Fewer components overall.[1]
  • Weight: Fewer components usually means lower weight.
  • Improved fuel efficiency due to less weight.
  • 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 for instance 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]
  • The 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 early Saabs being an exception).[16][dubious – discuss]
  • 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 tire size, a front wheel drive car was always faster over a given section of road."[17] However, this may only apply for cars with moderate power-to-weight ratio.[2][18][7][dubious – discuss] 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]
  • 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).

[edit] Disadvantages of front-wheel drive

  • Torque steer is the tendency for some high power 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 (which falls at a point which is aligned with the points at which 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, cancelling 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.[19][20][21][22][23][24][25][26]
  • 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 all- or 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.
  • The driveshafts may limit the amount by which the front wheels can turn, thus it may increase the turning circle of a front-wheel drive car compared to a rear-wheel drive one with the same wheelbase.

[edit] References

  1. ^ a b c d e f g h Inside Line: What Wheel Drive?
  2. ^ a b c d William, Milliken (1995). "Merits of Front-, Rear-, and Four-Wheel Drive", Race Car Vehicle Dynamics. SAE International, 730. ISBN 1560915269. “Front-wheel drive has been most successful in the lower power/weight range and in sutuations in which superior derectional stability on low coefficients is important. There has never been a successful front-drive Grand Prix car nor a competitive Indianapolis car of more than 300 hp.” 
  3. ^ a b c d e f g h "What's It Like To Drive", describes a test between two Dodge Daytonas, one FWD and one RWD
  4. ^ a b c d The Hidden Virtues of Front Wheel Drive
  5. ^ Lotus Elan M100 Sales Manual
  6. ^ Frere, Paul (1992). "From Slipping to Sliding", Sports Car and Competition Driving. entleyPublishers, 67pp. ISBN 0836702025. “Front-wheel drive which, due to the reduced front wheel grip under acceleration, is practical only for cars of moderate power-to-weight ratio” 
  7. ^ a b Prost, Alain (1990). "Controlling a car at the limit", Competition Driving. Hazelton Publishing, 50pp. ISBN 0905138805. “Front-wheel drive. In this instance, both power and steering are directed through the front wheels, the rears remaining free. Following the principle of weight transfer once more, the lightening of the front wheels under acceleration considerably reduces their effectiveness and thus limits the usable power. Consequentally, this type of transmission is generally less effective on racing circuits, a few rare exceptions notwithstanding, but has its advantages in road events where maximum power is not called into play so often” 
  8. ^ Jens Dornhege. Torque Steer Influences on McPherson Front Axles.
  9. ^ What is Torque Steer?. MPH Magazine.
  10. ^ Handling. AutoZine Technical School.
  11. ^ Technobabble: Multilink and the Beam. Sport Compact Car - November '98.
  12. ^ Suspension Geometry. AutoZine Technical School.
  13. ^ Why use Quaife?.
  14. ^ Storm Transmission Modifications.
  15. ^ Paul Yih. Vehicle State Estimation Using Steering Torque. Stanford University.
  16. ^ Modern Racer: Front-Wheel-Drive Oversteer
  17. ^ Lotus Elan M100 Sales Manual
  18. ^ Frere, Paul (1992). "From Slipping to Sliding", Sports Car and Competition Driving. entleyPublishers, 67pp. ISBN 0836702025. “Front-wheel drive which, due to the reduced front wheel grip under acceleration, is practical only for cars of moderate power-to-weight ratio” 
  19. ^ Jens Dornhege. Torque Steer Influences on McPherson Front Axles.
  20. ^ What is Torque Steer?. MPH Magazine.
  21. ^ Handling. AutoZine Technical School.
  22. ^ Technobabble: Multilink and the Beam. Sport Compact Car - November '98.
  23. ^ Suspension Geometry. AutoZine Technical School.
  24. ^ Why use Quaife?.
  25. ^ Storm Transmission Modifications.
  26. ^ Paul Yih. Vehicle State Estimation Using Steering Torque. Stanford University.
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Automobile layouts
Layouts
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Engine position
Drive wheels
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