Last Thursday, I found myself behind the wheel of a slightly aging Toyota Corona 1.8.
Now, there is nothing as unnerving as finding oneself at the receiving end of one’s own nuggets of wisdom (and advice), but what happened is what happened.
So, what exactly happened? That particular car had lived a rather hard life, driving over bad roads and the first few moments of driving it proved that fact.
The engine was fine, very OK. In fact, you could drive it endlessly in fifth and there would not be so much as a shiver from it.
But the suspension and steering were something else altogether. The shocks were beyond kaput (although the springs were fine, almost), causing the vehicle to sag at an ungainly angle, and that was not even the real problem.
No, the issue was that driving the vehicle at any speed above 50 km/h called for generous endowment in the trouser department.
The steering had an alarming amount of play off-centre. It had also become disturbingly non-linear and the turning circle had degenerated into truck-like dimensions; sometimes I would execute a three-point turn where other cars would simply perform a single U-turn.
There was a terrible shake, especially from the front sub-frame, every time I declutched and came on the power, the sort of shake you get when trying to take second gear conditions in fourth gear, as though the car wants to stall.
And again that was not the biggest problem. Driving in a straight line called for constant sawing at the helm.
Adjustments in road camber or imperfections on the tarmac surface would cause the car to wander aimlessly, darting left and (especially) right like a nervous cockroach.
It also had a tendency to pull suddenly to the right without warning, particularly dangerous given that in Kenya we keep left, so pulling to the right means going right into the face of oncoming traffic.
This could be controlled by incessant adjustments that were necessary to maintain a straight line, but things came to a head when, at one point, I was forced to brake hard.
The car did not just pull to the right, it actually started turning right. Frightened out of my wits, I knew something had to be done. That something was to check the wheel alignment.
The mechanic who looked at it, after seeing the diagnosis from the aligning kit (which looks like a mechanical device used by a dentist from hell), shot me a glance that was a cross between deep sympathy and disbelief that I had not killed myself yet, and he pronounced the vehicle a death-trap.
The boys who were aligning the wheels were trying unsuccessfully not to laugh. However, they did their job.
And a good job they did too, or so I thought. The play disappeared, the steering became linear (almost, but better than before), and the worrisome trembling was minimised to a point where you could barely notice it (but it was still there), so I assumed things were now OK.
The mechanic, in bidding me goodbye, told me to keep my speed at 100 km/h or less, as he was still not very confident about my ride for the day.
Off I went. Somewhere ahead, along unfamiliar roads and in very heavy rain (the sort of Noah’s-flood monsoon-like squall that reduces visibility to less than 15 per cent and renders people homeless), I drew up behind a slow moving truck.
I performed the ritual necessary for a wrong-side pass (also called overtaking).
Downshift into fourth, check mirrors (couldn’t see diddly-squat), confirm no one is coming the other way, downshift again into third, change lane ,and floor it.
That is when I saw the huge speed bump. Apparently, trucks don’t slow down for nothing and I had to conform to the prevailing road conditions (the speed bump), which meant that I had to stomp on the brake pedal as hard as my delicate ankles would allow and without aquaplaning.
Things happened very fast from that point on: the car speared off to the right, so I yanked the tiller hard to the left to counter the bizarrely erratic behaviour of the fore end, but this only led to some sort of understeer as the tyres scraped noisily over the wet tarmac (it was that bad) and the car started to leave the road on the wrong side, refusing to obey and maintaining its unintended north-easterly bearing.
The only way out was to act counterintuitively and release the brakes, so I did. This corrected the course but I still hopped over the bump at about 70 km/h, throwing the aft end of the car alarmingly into the air.
All this took the best part of four seconds (I told you things happened really fast). My passenger gasped “Sorry about that, dude, but did you not see that bump?”
I did not answer because it is at that moment that I realised that the car had even worse problems than I had imagined, and the basis of this week’s article (and the next) started forming in my mind.
The steering system
The steering system of a car is one of the most elaborate pieces of mechanical hardware, more so if it is power assisted (hydraulic or electric) and the vehicle is front-wheel drive.
To best understand it, in today’s classroom we will trace a path from the driver’s palms gripping the steering wheel rim to the rubber tyres gripping the road, and everything in between, but only everything common to all types of automotive steering systems.
This is because in subsequent lectures we will discuss the various steering types and try to troubleshoot the reason I almost ended up in a maize field during a heavy downpour.
The steering wheel rim is connected to the steering column, the “tree trunk” that disappears into the vehicle’s dashboard.
This steering column is nowadays collapsible by law. In the olden days, it was just a solid steel rod that would impale you, penetrating your chest and coming out your back in case of an accident.
The collapsing characteristic is either telescopic (like a TV/radio antenna) or by a universal joint along its length that folds under duress.
The Steering Box
The steering column terminates in a steering box, where the steering ratio is determined (low ratios for smaller cars, higher ratios for bigger and heavier vehicles), and the circular motion of the steering column is differentiated or transformed into the linear motion of the steering linkages.
The vehicle may or may not have power assistance (discussed hereafter), so from the box, the steering effort travels through the steering arm to the connecting rod, tie-rods, knuckles, and finally the king pin axis, to which the front wheels (or back, depending) are attached.
The reason behind the gear reduction within the steering box is to minimise the torque necessary to turn the wheels. Motor cars are generally heavy. We all know that, so turning them must require a lot of effort.
Through gear reduction, the torque applied by your arms at the wheel is multiplied within the steering box before being applied at the wheels, so that, even without power assistance in the steering system (not all cars have the P/S or PAS option checked when at the dealer forecourt), driving a car should not be left to body builders and farm hands only.
Nobody ever mentions this, but I think another reason for the gear reduction is to give a huge margin for error in the case of the inept and the wilfully wayward.
To get my meaning, have a go at the wheel of a go-kart and you will understand why keenness, accuracy, and skill behind the wheel are not only advisable, but a must.
We will look at the various types of steering boxes (rack and pinion vs. recirculating ball) next week, but for now, let us discuss a man called Ackermann.
Ackermann and His Principle
You may or may not have noticed this, but when a car is turning, the inside wheel turns at a sharper angle compared to the outer wheel.
This is because it inscribes a circle of smaller radius compared to that of the outer wheel. This whole setup is what we call Ackermann’s Principle.
If the two tyres turned at the same angle, the inner wheel would push across its width rather than rotate along its circumference.
When a car turns in a full circle, the tyre tracks are supposed to show two concentric circles. But with the wheels at a common angle, the circles (of similar radii) would overlap, and the car would turn in an ellipse rather than a circle.
This is not even possible at constant steering wheel lock. Ackermann’s Principle is made possible by the geometry of the steering linkage.
The geometry, in simple terms, is thus: each (front) wheel is attached to the steering system at two points.
The two points on each wheel, connected to each other and to the corresponding point on the opposite wheel, form a symmetrical trapezium, with the length towards the front of the car being slightly longer than the one towards the centre of the car.
To get the picture, the line joining the centres of the two wheels passes between these two lengths. Each corner of this trapezium is a swivelling joint.
So when turning, the trapezium distorts, losing its symmetry and the angles on one side become sharper (acute) while those on the opposite end widen, or tend towards the obtuse (high school geometry: the sum of all angles within a quadrangle must add up to 360).
The loss of symmetry explains the lack of similarity between the two tyre angles while turning.
Self-centreing of the steering wheel
Now this you must have noticed: whenever you apply lock (i.e. rotating effort) to the steering wheel, it tends to bring itself back to the centre point, that is, it straightens itself out when you let go.
There is no magic behind this, nor is there a pump or a device that does this for you; it is pure mechanics (or physics, or both).
The caster angle, along with Ackermann’s Principle and gear reduction, is one other fundamental concept of the steering system.
The pivot point steering each wheel is positioned slightly ahead of the wheel itself, and this causes a tendency for the wheel in question to align itself with the direction of travel, hence the self-centreing. It is a virtual force that does this, just like centripetal force.
Power Assistance in the Steering System (PAS)
There are two major types: hydraulic and electric/electronic, although, with the advancement of technology, a hybrid system of the two can now be realised.
Hydraulic systems are run by a pump that is driven by engine power through the fan belt (which also turns the alternator and water pump) and a pulley on the pump.
It is hard to explain how exactly the pump works without diagrams and excessive jargon, but in a nutshell, it works a bit similarly to a centrifugal supercharger, only instead of blowing air, it pumps hydraulic fluid.
The vanes within the pump draw in fluid at low pressure (from low torque driver input — we are not that strong) and dispense it at higher pressure, with the help of the engine.
The engine speed determines the amount of flow within the pump, so the general pump design allows for sufficient flow at idle, and the use of relief valves controls the pressure ceiling at high engine speeds when the pressure gets too high.
There are problems with this kind of setup. First, it is hard on the fuel and wastes engine power, seeing that the pump runs full-time, even when going straight or when the car is not in motion.
Then, you do not want your car engine to stall if you are using this kind of jury-rig. The system will stop working (the pump is powered by the engine, remember) and the effort at the steering wheel will be twice as heavy as you not only have to work the steering system manually, you also have to work the entire power-assistance mechanism too.
Disposal of used or dirty hydraulic fluid is also becoming an issue in this heavily policed, environmentally conscious 21st century, and this builds the case for an electric/electronic system.
There is no fluid to top up (or dispose of, or leak), it is infinitely tunable to the vehicle type, road condition, and to a driver’s liking and the system only need work when the wheel is turned.
Most power-assisted steering systems come with a fail-safe mechanical linkage; it is actually compulsory in motor vehicle application.
But where did these systems even come from? The advent of front-wheel drive, and the increase in tyre width (bigger contact patch, bigger effective area for frictional forces to apply) and size made most steering systems quite an effort to work manually without increasing the ratios in the steering box to ridiculous and unworkable numbers.
So power was needed as a palliative to the unplanned workout at the helm. There is plenty more to discuss about steering systems, so future lectures are not to be missed.
Speedsensitive power assistance, variable ratio steering (also called active steering), four-wheel steer, steerby- wire technology — all these will be looked at and explained, but as a parting shot, please take note of this: the Corona that gave me a big scare was not mine.
