Front Wheel Drive V/S Rear Wheel Drive

COMPILED BY

VINEETH CS Source:

• http://en.wikipedia.org • http://petrolsmell.com/2010/02/04/passenger-car-

layouts • http://www.popularmechanics.com

2011

FRONT WHEEL DRIVE V/S REAR WHEEL DRIVE

Prior to the 1970’s, almost all the automobiles had an engine in the front and had a rear wheel drive. But in 1970’s with onset of the fuel crisis, automobile manufacturers sought ways to improve the fuel efficiencies of the vehicles, changing the drive from the rear axles to the front considerably increased the efficiency of the automobiles. This was due to decrease in transmission losses and the weight reduction of the vehicles due to absence of transmission shafts.

There has been a return to the rear wheel drive recently especially in sports model cars, with the engine taken to back (its more accurately called as a mid engine design). Most of the modern luxury cars have all wheel drives with active transmission control system. The all wheel drive is the best system, which incorporates the advantages of both the rear axle and front wheel drive. In this document we will focus our attention on the front and rear wheel drives and why there was the switch from the rear wheel drive to the front wheel drive and back again!

Fig: A BUGATI VEYRON-The fastest production car, has a 1000HP (Mid Engine, All wheel Drive)

FRONT WHEEL DRIVE

It is an automotive layout in which the engine drives the front axles alone. In fact the most common arrangement found in majority of the automobiles is a front engine, front wheel drive. This is preferred to due significantly high transmission efficiencies and the setup has less weight due to less components. Most car manufactures like Toyota, Tata prefer front wheel drives for their base models.

NOTABLE 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.

• Weight: Fewer components usually mean lower weight. This also means higher efficiencies.

• 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.

DISADVNATAGES:

• Torque steer: It is the tendency for some front-wheel drive cars to pull to the left or right under hard acceleration.

• 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.

• Traction can be reduced while attempting to climb a slope in slippery conditions such as snow- or ice-covered roadways

• Requires Constant Velocity Joints as steering is done on the same axle. CV joints are costlier than the Universal joints used in rear wheel drive.

• They have larger turning radius compared to rear wheel drive automobiles of the same wheel base.

• Almost all the work is done by the front axles and hence they may become prone to failure.

Fig: TATA INDICA (Front Engine Front Wheel Drive)

Fig: AUDI A5 (Front Engine Front Wheel Drive)

REAR WHEEL DRIVE

It is an automotive configuration in which the rear axles are driven by the engine. Unlike the front wheel drive, there are two different configurations of the rear wheel drive, namely The Front Engine Rear Wheel Drive and the Rear Engine Rear Wheel Drive. Both the configurations are utilized in different automobiles and have advantages as well as disadvantages. Hence to understand their working properly we shall discuss them separately.

1) FRONT ENGINE REAR WHEEL DRIVE:

The front engine rear wheel drive was the most commonly used drive system prior to 1970’s. Typically the clutch and the transmission box are located close to the engine, while the differential is located near the rear axle. This being said, to identify which kind of drive system is used by an automobile, there is only a need to check the position of the differential. All wheel drives will have two differentials, while others will have one on the corresponding drive wheel.

NOTABLE ADVANTAGES:

• Even weight distribution: This improves traction as all the four wheel are nearly loaded equally

• Weight transfer during acceleration — During heavy acceleration, weight is placed on the rear, or driving wheels, which improves traction.

• No torque steer.

• 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.

• 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.

• Universal joints can be used instead of the complicated Constant Velocity Joints.

• Safer in a front crash.

• Bigger engines can be fitted as there is more space under the bonnet.

DISADVANTAGES:

• The possibility of a slight loss in the mechanical efficiency of the longer drivetrain (approximately 17% coastdown losses between engine flywheel and road wheels compared to 15% for front wheel drive).

• 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.

• Due to the above to reasons the fuel efficiencies of front engine rear wheel drive automobiles generally lower.

• 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.

• 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.

Fig: BMW M3 (Front Engine, Rear Wheel Drive)

2) REAR ENGINE REAR WHEEL DRIVE:

This layout consists of placing the engine, the clutch box and the transmission in rear along with the differential. Since almost all the boot space is taken by the engine and so the boot is usually located in the front. In this type of layout more than 50% weight is on the rear axles.

NOTABLE ADVANTAGES:

• Lot of weight at the back improves acceleration. Hence they are suited for drag racing. • They have superior braking characteristics as heavy weight at the back means that the

rear wheels can take majority of the braking load. • Compared to the front engine rear wheel drive, they have less weight and better

transmission efficiencies.

DISADVANTAGES:

• Relatively high proportions of weight at the rear axle will make the car unstable at speed and the car tends to oversteer.

• Difficulties with the arrangement of the engine cooling system due to engine being at the back, convective cooling of the engine becomes difficult.

• The petrol tank is usually placed at the front, which is a safety hazard in a collision.

• There is decreased room for rear passengers. FIG: PORCHE 911 (Rear Engine Rear Wheel Drive).

PERFORMANCE COMPARISON:

A few performance comparisons between near identical vehicles differing only with the type of drive, were done by a team at www.popularmechanics.com

1) FORD FOCUS VS MERCEDES BENZ C-CLASS:

2) TOYOTA CAMRY Vs INFINITI G35:

3) PONTIAC BONNEVILLE VS CHRYSLER 300C:

CONCLUSION

Front wheel drive favors the small segment automobile as its cheaper and more economical and offers considerable advantages in spacing. On the other hand business class and sports class models tend to have rear wheel drive due to better traction and better engine flexibility.

Based on the tests done by popular mechanics team, they concluded that:

Neither front-wheel drive nor rear-wheel drive is really better than the other. Today's sophisticated traction and stability control systems are so good they can mask or enhance the true driving dynamics of a vehicle. That said, through most of this test we found the effectiveness of these systems had more to do with a car's performance than which wheels were actually doing the driving.

Thus for most modern cars its irrelevant as to which type of drive is adopted as they are actively controlled by computers continuously monitoring and adjusting the various parameters for optimum performance.