Steering system

Steering and suspension Report

The steering system is one of the major parts of any automotive machine as without this it would only be able to travel in straight lines instead of being able to negotiate turns and manoeuvres. Since its invention steering systems have advanced at an incredible rate with many technological advances such as power assisted steering and different methods and arrangements of steering systems.

A steering system works by transforming rotational motion to linear motion. This is carried out via the driver turning the steering wheel in a rotational motion which turns a steering column that travels to the engine bay to in between the wheels. At the end of the steering column is a gearing mechanism.

There are a few different types of steering mechanism but the most common steering gear set, is a rack and pinion arrangement. The pinion gear is turned by the steering column which sits on top of a toothed flat gear which is moved side to side in a linear motion with the rotational motion of the steering column, this causes the steering rack to move which when attached to the steering pivots via tie rods onto the wheel hub assembly causes both of the wheels to turn. In a rack and pinion system the gear ratios play a big part in how easy a car is to steer. If the ratio is high the steering wheel will be easier to turn but it will have to be rotated more to get the wheels to move, where as if the ratio were too low the steering wheel would not have to be moved much but it would be very hard to turn, which would make driving manoeuvres really hard.

In 1810 a German inventor called Rudolph Ackermann found that is the steering pivots were brought into line with the centre of the wheel axle the car would turn better as both wheels would be turning at different angles. This works better than both wheels turning the same angle because there is alot more tyre scrub on the wheels as one is travelling a further distance than the outer wheel. The Ackermann set up is very useful for cars cornering at high speeds as it helps to keep tyre temperatures low, but its disadvantage is that it slightly restricts performance in low speed manoeuvres (image and some info from http://en.wikipedia.org/wiki/Ackermann_steering_geometry) Another steering mechanism is a recirculating ball mechanism that uses the motion of the steering to lift a block up and down rather like a bolt turning and pulling a nut up and down. The block has teeth cut into it that engage with a gear attached to the pitman arm. So when the block is lifted the gear turns the pitman arm, which when

Attached to the tie rods causes a linear movement that is transferred to the wheels. The function of the recirculating balls makes this whole process smoother. The wheel being turned pushes balls through the worm gear that drives the steering rack up and down this eliminates the backlash/play that would occur if a normal gear were used as the balls are always in contact while the gears would have a moment where the teeth are not in contact as the motion is reversed. The balls also reduce friction in the system and therefore also reduce wear. (Image and some info from http://auto.howstuffworks.com/steering3.htm) One of the main advancements in steering technology is the invention of power assisted steering which greatly helps the driver as it makes the steering a lot lighter than a conventional steering system. This is done by adding a hydraulic system and slightly altering the existing designs of the original steering system.

A powered rack and pinion system looks like this:

When the steering wheel is turned not only the pinion shaft is rotated, but the spool/rotary valve is rotated too which directs high pressure hydraulic fluid from the power steering pump through the spool valve and into the relevant power cylinder, this aids the pinion gear in moving the rack with less force required by the driver. When the wheel is turned the spool valve releases the pressure from the opposite side and runs the high pressure fluid to the relevant side of the power cylinder steering the car in the opposite direction. A design constraint of a power steering system is that it must only act on a cylinder when the driver is moving the steering wheel. This is controlled by the spool/rotary valve, inside the rotary valve there is a torsion bar which holds the valve closed until the driver adds torque to the steering wheel, in which case it twists and releases fluid into the correct power cylinder, the more torque the driver adds the more the valve opens. There are a number of disadvantages to power assisted steering such as the high repair costs if it breaks and also the fact that it is run via a drive belt connected to the engine therefore slightly reducing the cars overall performance and fuel economy. (http://auto.howstuffworks.com/steering4.htm)

There is a new method of steering being developed in the industry right now called drive by wire. This method could replace all of the mechanical parts, from the steering wheel and the steering gears to even the accelerator brake and clutch. It works by replacing the shafts and mechanical systems with electrical systems that take the movements the driver makes at the steering wheel and sends them to a motor that will turn the wheels, this saves room inside the engine bay and means the car will be adaptable to every person the drives it. As of now the drive by wire system is not road legal because if the electrics fail the driver will have no control over anything the car is doing. (http://en.wikipedia.org/wiki/Steer-by-wire)

In most modern cars there are a number of safety features worked into the steering mechanisms in a car to ensure the driver remains safe during a collision. Starting from the steering wheel there is the airbag that inflates within milliseconds to stop the driver hitting the steering wheel. The steering column also has some safety mechanisms fitted into it such as it may have a collapsible section that will crumble rather than the steering column heading towards the driver, the steering column may also be designed so that the universal joints allow the steering column to fold up into a reverse ‘Z’ shape upon impact (see picture of steering column from a Hyundai accent (http://hyundai.co.in/accentwebsite/images/safety/sf_008.gif)) . In older vehicles that did not have any of these features if the impact was sufficient enough to the front end the driver would be impaled on the steering column.

The function of the suspension in a car is to improve the handling and passenger and driver comfort whilst the vehicle is in motion. The are many different types of suspension some are still in use today and others have become obsolete mostly due to new research and improved designs.