Preamble

A car is like any other ‘ system’ comprising some basic ‘elements’. The first two being, say, the power train and suspension and the last one being the ‘Steering/Wheels’.

It’s well said that even if the first two are flawless, the end performance of the ‘System’ as a whole can be ruined by poor quality Steering/Wheels.

In order to get the full hang of it, it’d be desirable to first have a look at some basics. So in this Article, we’ll explore a car’s ‘steering system’ coz that’s where most troubles can arise…..

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1) General

The purpose of a Steering system is obvious. Smooth and efficient driving depends not only on the basic design of the steering gear, but also how the 'nut behind the wheel' handles it. This is an art as well as a science but can be accomplished with some practice and understanding.

Steering Systems are basically of two types –

- Rack and Pinion and
- Worm gear/re-circulating ball types.

Both have their advantages and disadvantages. A Rack and Pinion System by its very principle is a superior system as it gives a better road feel and positive response to the driver inputs. However, without any power assistance aka Power Steering, they tend to be a little stiffer than its counterparts. The latter are lighter in the effort required, but are not very positive as the Rack and Pinion types. They also need more ‘ball-joints’, making it vulnerable to malfunction due to the inevitable greater wear and tear.

The ideal Steering System of a Car was conceived by a German guy called 'Ackermann' almost a 100-yrs ago. Most present day 'Rack and Pinion' Systems nearly conform to it.

The funda behind this 'principle' is that when a Car is on a curve/turning, the 'axes' of the two front wheels when 'imaginarily extended' should 'meet' at the same point on the similarly extended axes of the fixed/parallel rear wheels as illustrated in the sketch along side.

In practice, it results in the inner wheel turning-in by a greater 'degree' compared to the outer one while cornering. If this can be ensured over the 'lock to lock' sweep of the 2
front wheels, then it's assured that they will be purely 'rolling' during turns and not 'dragging' sideways at the same time - which leads to all too familiar a Tyre 'squeal' at high speed turns. The latter can still happen, in spite of a perfect Ackermann Steering, due to the Car being forced to 'slide' outwards on a turn due to 'centrifugal forces', for which precise reason the roads are 'cambered/super-elevated' at turns to 'contain’ such CF forces. Since this is not possible on flat race tracks, Motorcyclists have to lean their machines inwards into the curve.

But this is just a prelude to a Lesson in the basics of Steering Geometry.....

2) Basics of Steering Geometry

i) King-pin Inclination/Offset.

The 'King-Pin' or the 'Mc-Pherson Strut' of the present day Cars is inclined 'outwards', as seen in the sketch along side. This is done such that the 'imaginary extension' of its axis towards the road lies close to the 'inner edge’ of the tyre ‘foot-print’. Thus the distance between point ‘A’ where it meets ground and centre of the foot-print/point ‘B’ is known as the ‘Offset’. The latter plays a crucial role in deciding the stability of the car either in straight-line cruise or cornering mode.

ii) Castor/Angle.

Besides inclining the KP as above, its 'lower-end' is also slightly offset 'forwards', by 2-3*. When done so, it results in the Tyre's point of contact 'trailing' behind the imaginary point of contact of the King Pin with the Road - exactly like the funda of Castor Wheels which are only too common a sight in every day life - from office chairs to hospital trolleys and what not. Such as 'Castor' lends the 'self-centering' quality to a Car's Steering System, besides enabling it to cruise straight w/o any 'distractions' from road surface undulations.

iii) Camber/Angle.

Given the inevitable 'finite-radius' nature of the front Suspension 'lower/upper' Arms, the Wheel being 'nearly' perpendicular to its hub, it will NOT move up and down on bumps etc in a 100% 'vertical plane' - something akin to a 'horizontal'

Pendulum. If let to be so, the tyres will wear out unevenly from their 'outer edges' over a small period of time. To overcome this limitation, the 'Stock Cars' Front Wheels are given a 'positive' Camber angle of 0.5 to 1* - which results in their top edges 'sticking out' marginally compared to their bottom ones while stationary.

Having so 'cambered' them, when the front suspension is fully 'extended' like soon after going over a speed breaker, the Tyres' outer edges will be more in contact with the road. Like wise the inner edge when fully driven home such as when first hitting a speed breaker. This way, based on law of averages, it results in nearly equal wear on both the outer/inner edges of a Steering Wheel.

iv) Toe-in/out.

The ideal condition towards ensuring minimum uneven wear on the front tyres of a car will be to ensure that they run parallel to the 'line of motion' of the Car all the time - some thing which comes automatically to the rigid axle type rear wheels.

However, given the inevitable wear and tear on the ball-joints of their 'track arms' and wheel bearings, this is hard to ensure in practice, as they tend to 'flare-out' when in motion due to such 'slacks' in their linkages/bearings. Therefore, to ensure they remain parallel as far as possible while in motion, they are given a calculated 'toe-in', of the order of a few mm while stationary. Such a 'toe-in' is defined as the difference between the leading edges' distance across the front wheel rims, compared to their trailing edges. Alternatively, it can also be specified in terms of a fraction of an angular 'degree' 'per wheel' - such as in 'minutes'.