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Friday June 23rd 2017

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Passenger Car Layouts

Normally car manufacturers tend to make their cars have an equal weight distribution for each wheel. This is because this is the overall best weight distribution. Makes the car both good at acceleration, braking and steering. However, if you want to achieve a purpose there are different distributions that would help. The different layouts play an important role in weight distribution. So let’s analyse them together:

Front engine, front wheel drive

Engine, clutch, gearbox and final drive build together to form a single integral assembly. Normally transversal engine arrangement is used. Longitudinal-mounted engine is the other possible layout. Weight distribution tends to be more than 50% in the front axle

Advantages: Having engine, clutch, gearbox and final-drive in the front makes the front heavy and this helps acceleration (If compared with a rear engine , front wheel drive. Which does not exist). More passenger room since the propeller shaft is eliminated. Tend to understeer, which makes them safer than oversteer (supposing an average driver and no traction-stability control, understeer is easier to notice and correct). It is the cheapest, lightest and more compact arrangement.

Disadvantages: Traction is penalized when hill-climbing, weight moves backwards. Not efficient braking, when braking hard is easy to block the back wheels, it is like only having two wheels to brake with, even though these have a lot of grip due to weight transfer to the front. Steering radius is reduced because drive shafts. The front wheels have to do almost all the work (accelerate, brake, steering).

What cars use this layout: The vast majority of car manufacturers, to say some examples: Ford, Audi, Seat, VW, Toyota…


Front engine, rear wheel drive

Normally longitudinal engine. Clutch and gearbox can be attached to the engine or placed in the back with the differential (the very last one improves weight distribution).

Advantages: Improves weight distribution, if it has a 50-50 weight distribution the car handles more neutrally compared with layouts that have more weight on one axel. Good traction when accelerating and braking. Safer in a front crash, less systems in the front mean less risk on intrusion. When going up a steep slope, the weight transfer to the back improves traction. Easier to design the linkages in the propulsion system. Because there is less stuff under the bonnet, bigger engines can be fitted. Maintenance is also easier since is more accessible. Steer and drive are done by different axels, so the wear is more equal between the wheels in each axel.

Disadvantages: Reduces back seats leg room space,  a tunnel is needed for the propeller shaft. Reduces boot space. Heavier and more expensive. If stuck in mud or snow it is harder to drive away than in a front wheel drive car.

What cars use this layout: BMW, Mercedes…

Front engine, all wheel drive

Similar characteristics as the front engine, rear wheel drive. Plus:

Advantages: Better handling. Better acceleration.

Disadvantages: Increases weight and production costs in comparison of other front engine layouts. Lose of some space in the back seats legs room.

What cars use this layout: Subaru Impreza, Mitsubishi Lancer,…


Rear engine, rear wheel drive

This layout consists of placing the engine, clutch and gearbox in the back. So taking the space of the boot. In these type of layout more than 50% of the weight is on the rear axle.

Advantages: Lot of weight at the back improves acceleration and braking, rear wheels and disc brakes can be designed to take a bigger amount of breaking due to the weight distribution to the rear end.

Disadvantages: Lose of the boot (the under bonnet is small due to aerodynamic design). Lose back passenger occupancy. Relatively high proportions of weight at the rear axle will make the car unstable at speed. Car tends to oversteer. Difficulties with the arrangement of the engine cooling system. Compact engine, clutch and gearbox makes servicing more difficult. The petrol tank is usually placed at the front, which is a safety hazard in a collision. If accelerating full throttle standstill in a powerful car, steering is almost lost (In a Porsche 911 the 85-95%of the weight goes to rear axle), the same happens at speed (know as a “light front”, or heavy back).

What cars use this layout: Porsche 911,…

Rear engine, all wheel drive

Similar characteristics as rear engine, rear wheel drive. Plus:

Advantages: Better weight distribution. More neutral reactions. Better traction on bends and slippery surfaces.

Disadvantages: Heavier. More expensive and difficult to design and build.

What cars use this layout: Porsche Carrera 4S,…


Mid-engine, rear wheel drive

This type of engine layout places the engine between the two axles and the rear wheels are driven. Normally 50-50 weight distribution.

Advantages: Good acceleration due to back weight transfer. Good braking. Neutral handling.

Disadvantages: When accelerating some steering is lost. Takes a lot of space. No back seats. Difficulties with the cooling system.

What cars use this layout:  Ferrari F430, Porsche Cayman

Mid-engine, all wheel drive

This type of engine layout places the engine between the two axles and all wheels are driven. Normally 50-50 weight distribution.

Advantages: The best weight distribution in all the layout types, good for sport performance. Good acceleration, good braking, neutral handling. The front of the car can be aerodynamically optimized because there is no engine there.

Disadvantages: Takes a lot of space. No back seats. Expensive to design and engineer.

What cars use this layout: Buggati Veyron, Lamborgini Murcielago.

Conclusion

This analysis makes us more aware of the differences between the various layouts. Now is easy to understand why the vast majority of the car manufacturers use front engine, front wheel drive layout. It is important to notice that other manufactures go for the front engine, rear wheel drive distribution because of its advantages. As we have seen the rear engine placement is quite used by a particular brand, however its few advantages and huge drawbacks. The mid-engine layout is only used in the highest ranges of manufacturers and sport applications.

I hope this post made you, as it made me, more aware of the huge differences between each of the layouts. Thanks for reading.

References:

Vehicle Engine and Technology. HeinzHeisler. 2nd edition, Elsevier.

-Chassis construction. TTA207 Vehicle systems and design notes. Scot Layton. Loughborough University.

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