AN ACCIDENT WAITING TO HAPPEN

SOME 45 YEARS AGO, one of the fuel companies ran deliveries from its depot in Cape Town Harbour. Then, wide new roads and big traffic circles were constructed on the reclaimed harbourfront. Soon afterwards, the fuel company experienced a series of puzzling tanker turnovers. Were the new roads to blame? Or was it something else? Investigators struggled to solve the mystery.

At that time, I was the young design engineer for Consani Engineering in the Westen Cape, which made most of the country,s tankers. The company was called in to assist. Some digging revealed that all the semi-trailer tankers involved: ➜ Had done the job for a long time; ➜ Were operated by experienced drivers; ➜ Overturned while exiting a traffic circle at normal speed; ➜ Had previously negotiated the same traffic circle without incident, either fully laden or at higher than normal speed. What had changed? I will get to that. But first let’s backtrack to talk about semi-trailer stability. Essentially, it depends on three factors: ➜ Positioning of the load. ➜ Suspension. ➜ The current international convention of the semi-oscillating fifth wheel.

Firstly, on the subject of loading, the road ordinance lays down strict rules of maximum sizes of vehicles and maximum weights that a combination of axles can carry. Maximum allowed weights include the weight of the trailer, so manufacturers make trailers as light as possible.

Now, for a maximum load on a horse/ trailer combination, the position of the centre of gravity is fixed. It cannot be moved forward or backwards without overloading one or more of the axles. Part loads, however, can be located anywhere along the semi-trailer and still be legal. Here is where the problem starts.

Some drivers are not aware that locating the load upfront, instead of over the trailer’s rear wheels or bogie, renders the semi much more unstable.

The fifth wheel, which attaches the semi-trailer to the mechanical horse, is usually a semi-oscillating fifth wheel, allowing the rear of the trailer to move up

“Rollover without the Risk” (How your world works, February 2016) highlighted how simple training helps truck-trailer drivers avoid expensive and often deadly rollovers caused by incorrect driving methods. But, says a reader, we’re missing something important: the role of the fifth-wheel towing mechanism patented a century ago this year. Inventor Herman G Farr got it back to front, says Cape town engineer R J Consani Pr Eng (Int).

and down with ease, while stopping the trailer from rocking sideways. If the load is placed over the fifth wheel, in straightline travel, sideways roll is curtailed and transmitted to the horse. The driver can “feel” the trailer roll.

But when the horse/trailer takes a sharp corner (often at nearly right angles to the trailer), all that changes. The front-rear movement restriction of the horse-mounted fifth wheel now effectively becomes a sideways roll in the trailer. There’s no restriction or feeling transmitted to the horse, up to about 30 degrees. If the load is perched over the fifth wheel, when accelerating out of a sharp turn the likelihood of tipping is highly increased.

Which brings me back to the mystery of the overturning fuel tankers. Further investigation showed that every vehicle that overturned was returning to the depot with a part load. The manager of the petrol company’s transport division had instructed all drivers to empty the compartmentalised semi-trailer starting from the rear and working towards the front. This meant that the semi-trailer could end up with a load only in the front compartment, directly over the fifth wheel, hence the instability.

Every trailer that overturned was in this condition.

We immediately advised the petrol company to change their unloading sequence and the problem went away.

The decision of the transport manager was understandable, in the light of a pending change in the road ordinance to enforce a minimum of 20 per cent of the gross vehicle weight on the driving axle(s). By leaving fuel in the front of the semi, this would ensure more weight on the driving wheels of the horse – but it caused major instability. Yet even with a part load on the rear of the trailer the driving axles were over the 20 per cent of total vehicle weight.

Now to the second risk factor, suspension. In general, the further apart the springs, the more stable the suspension against rollover. Also, the higher the centre of gravity of the load plus trailer over the plane on which the springs act, the more overturning moment will be applied to the springs in cornering and hence the more the trailer will tilt, causing more instability.

Finally, on the subject of international convention of the fifth wheel, I may be shot down in flames, but what I am going to say is the truth. The international convention of placing the semi-oscillating fifth wheel on the horse and the rubbing plate on the trailer, from a rollover point of view, is wrong. The semi-oscillating fifth wheel should be on the trailer and the rubbing plate on the horse. When the horse and trailer travel in a straight line the effect would be no different from what it is now. However, in a tight corner, if the fifth wheel were on the trailer it would stop the trailer from “rolling”, as the oscillation direction would still be front to back relative to the trailer. The fifth wheel would allow the horse to “roll”, but this would not cause instability. The same problem exists when a horse/trailer jackknifes – the horse is just over 90˚ relative to the trailer and the fifth wheel does not stop the trailer from tilting over. In fact, it is biased to tilt the trailer to 30˚.

As I see it, the only disadvantage of placing the semi-oscillating fifth wheel attached to the trailer would be a more restricted angle of entry to the coupling.

The current fifth wheel system is so entrenched into the transport system that I doubt it will ever change, but if it did, it would need to do so in the countries we follow, namely the USA and Europe.