The modern vehicle is a miracle of evolution. From horse and cart to the modern motor vehicle, carrying people and cargo in safety and comfort.
The basic layout of 4 wheels evolved from the horse and cart. A wheel in each corner gives the maximum load carrying and stability.
This has limited stability but its load carrying ability is compromised.
The load has to be carried within the triangular area of the 3 wheels otherwise it becomes unstable. Also if one of the wheels goes in a pot hole the stability is lost. Adding a fourth wheel dramatically increases the area and hence its load carrying ability.
Is very light and manoeuvrable, but has little load carrying ability. A pony and trap gets its stability from the pony being attached to the trap. A two wheeled bike will only stay upright when in motion.
Neither would be used for loads of any size.
This is highly manoeuvrable but extremely limited to its load. A wheelbarrow may carry a small load but it will get virtually anywhere.
A vehicle’s overall manoeuvrability is decided by it’s length. The longer it is the more room it needs to turn.
It’s stability depends on how wide it is. Also to a certain extent how high it is. Remember our bicycle from above.
With the horse and cart (and a car) it makes very good sense to steer from the front with the steering done through the front wheels.
A forward position allows for a good view of the road ahead and any immediate obstructions. With both horse and cart and the early motor vehicle forward position allows for observation and manipulation of the power source. By steering through the front axle you would steer in the direction you would want the vehicle to go.
If you steered from the back axle, to go left you would have to steer right.
Also if you were up against a wall to steer away from it you would have to turn into it which would not be possible.
Swivelling the whole of the front axle requires lots of room. If one of the front wheels was stuck for any reason it would cause the vehicle to swerve off its path. They got round this problem by allowing the wheels to pivot on the ends of the axle rather than the axle to pivot on its centre.
This is called Ackerman Steering.
The power source moved from an external animal to an internal engine. When the power source was an animal it was attached to the vehicle and where the horse went the cart followed. But with a mechanical power source the problem is where to put it and how to apply it to the vehicle wheels.
In terms of placing it you have 3 choices, to the front, in the middle or at the rear.
- The Front.
In the early days this would have been unenclosed and at the front in order for the driver to observe. Eventually the noise, heat and reliability of this unit led to it being enclosed in a separate engine compartment.
By placing the unit at the front this solid lump increases protection available to the driver. This also maximises the airflow/cooling to the engine unit. By having the engine at the front this places the most weight over the wheels through which the steering is transmitted. The benefits of this are improved cornering ability.
- The Middle.
By placing an engine in the middle of the vehicle this gives a good load distribution. This improves the handling ability and grip to the drive wheels. Active safety is improved by giving more traction so the ABS and Traction control work better. Passive safety gets bigger crumple zones.
The downside of this is the engine is now occupying space in what would normally be the passenger compartment. So placing it in the middle tends to be for sports cars where seating is limited.
- The Rear.
The engine can be placed at the rear and will give a better grip under acceleration. However this will tend to give a tendency for the rear end to spin out during cornering. So when this arrangement is used it is only on the smaller vehicles like the Volkswagen Beetle.