Steering a Car and Volvo Recall

Have you heard of  Ackermann  and the geometry associated with him !!

Driving a car looks pretty easy and simple – in cinemas you would have seen that in a chase, the hero would be turning the steering wheel, his head and everything else, biting his teeth, making it appear that he is under considerable stress and strain in accelerating the vehicle faster……… 

Without going into the details of the mechanics,  the first thing that strikes the eye is the steering wheel.  The two wheeler appears to have a much simpler pattern.  It has the handle bar, which is to be held firm and movement left or right would enable the movement of the vehicle in that direction.  Steering is the similar mechanism in a car of any 4 wheeler for that matter.  It is infact a term applied to  the collection of components, linkages, etc. which will allow a vessel (ship, boat) or vehicle (car, motorcycle, bicycle) to follow the desired course. Rail transport perhaps follows a different mechanism altogether.

photo from How stuff works....

As we see, the most  conventional steering arrangement is to turn the front wheels using a hand–operated steering wheel which is positioned in front of the driver.  Again to understand how this works – you need to know something of the complex mechanism which constitutes steering column, universal joints and more which allows the vehicle to deviate from its straight path into the desired way.  To put it simply, you turn the car by rotating the steering wheel.  Unlike handle bar where the turn of the handle is directly proportionate to the angle of the wheel, things are lot different here. 

There are many types and commonest of them are the rack & pinion and recirculating ball steering.  For turning the vehicle, the wheels have to turn and for that you need to rotate vigorously the steering wheel [infact the mechanism works the other way – steering wheel – wheel – vehicle].  The first fact that would hit you hard is that when you turn your car, your front wheels are not pointing in the same direction.  Each wheel would follow a different circle.  If you are turning say left, the left hand side wheel is following a smaller radius and actually makes a tighter turn than the right wheel, which follows a relatively longer radius.  The geometry of the steering linkage makes the inside wheel turn more than the outside wheel.

For the ease of turning, EPS [electrically powered steering] or simply power steering is the contraption that uses an electric motor to drive either the power steering hydraulic pump or the steering linkage directly. The power steering function therefore does not enforce any strain on the engine, results in energy saving and is relatively easy to make a turn.   Power steering enables the driver to steer by directing  some of its power to assist in swiveling. There are two types of power steering systems; hydraulic and electric /electronic. A hydraulic power steering (HPS) uses hydraulic pressure supplied by an engine-driven pump to assist the motion of turning the steering wheel. Electric power steering (EPS) is more efficient than the hydraulic power steering, since the electric power steering motor only needs to provide assistance when the steering wheel is turned, whereas the hydraulic pump must run constantly.  A higher % of power steering systems are hydraulic, where it is powered by high pressure hydraulic fluid.

Volvo is a famous brand.  Volvo means ‘I roll’ in Latin.  The auto manufacturer was founded in 1927 and the first car rolled out of the factory in Gothenburg.   Zhejiang Geely Holding Group is a holding company and parent of Geely Automobile Holdings Ltd.  The Volvo is presently owned by  China’s Geely.

It is reported that Volvo is recalling 8,406.  S80 cars for a potential power steering fluid problem that could increase the risk of a crash.  It is reported that the recall pertains to 2007 model cars built between March 8, 2006, and May 16, 2007,  and that Volvo wishes to inspect a potential problem that could lead to a burst hose at the steering gear that would cause a sudden loss of power steering.  Volvo dealers will inspect the cars and make repairs as needed at no charge and the recall is expected to begin during August.

Ackermann steering geometry is a geometric arrangement of linkages in the steering of a car or other vehicle designed to solve the problem of wheels on the inside and outside of a turn needing to trace out circles of different radii. It was invented by the German Carriage Builder Georg Lankensperger in Munich in 1817, then patented by his agent in England, Rudolph Ackermann in 1818 for horse drawn carriages.  Modern cars do not use pure Ackermann steering, partly because it ignores important dynamic and compliant effects, but the principle is sound for low speed manoeuvres.

Regards – S. Sampathkumar.