HORRENDOUS PRICE OF OVERTAKING BADLY

THE dangers facing caravanners overtaking other vehicles have been highlighted by the Caravan Council of Australia. General manager Colin Young said risking lives to save a few seconds can come with enormous risks. "Are you prepared to pay this horrendous price?" he asked. Mr Young has drawn up a blueprint to help caravanners overtake safely.

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SAFETY is by far the most important consideration when towing a caravan. You must have sufficient power available to accelerate the car-caravan combination at a reasonable rate and to maintain a reasonable speed when climbing hills.

It is vital you are able to accelerate quickly to get out of trouble, and especially to avoid getting into trouble, for example when overtaking another vehicle.

It is also vital you are able to stop at a reasonable rate safely, and in a straight line.

With so many significant variables involved, it is not possible to provide a definitive procedure to determine the overall distance required for overtaking, for every tow-vehicle-caravan situation.

Risking your life ‒ and that of your passengers and other road-users ‒ to save a few seconds can come with enormous risks. Are you prepared to pay this horrendous price?

The most important aspect is that the driver must have good eyesight, especially for the perception of distance on the road.

The number one rule is that if you are not 100 percent confident you can overtake a slower vehicle safely in the prevailing atmospheric and road conditions then do not attempt to overtake.

The second most important aspect is that the driver must have a good appreciation of the acceleration performance of their tow-vehicle when towing a loaded caravan.

The third most important aspect is that the driver must have a good appreciation of the handling/stability performance of their tow-vehicle and caravan combination when a lane-change manoeuvre is undertaken.

A while back, a gentleman named Mr Newton derived a formula, stating that: Force = Mass multiplied by Acceleration [F = m x a].

In reverse, Acceleration = Force divided by Mass [a = F / m].

"Acceleration"is what we need to increase speed, in order to overtake a slower vehicle.

"Mass" is the total mass of the car-caravan combination.

"Force" is the forward thrust - "tractive effort" - supplied by the car tyres’ traction on the road surface.

It is vital to realise that the "Mass" of the car-caravan combination is much greater than the "Mass" of the car alone, hence the "Acceleration" performance will be considerably reduced.

The "Force" is the horizontal force available, hence it will be considerably reduced on an incline.

The "Force" available depends on the friction between the driven tyres and the road surface, hence it will be considerably reduced on wet roads and dirt roads.

The critical factor in overtaking is to spend as little time as possible in the wrong lane (on two-lane roads), hence it is prudent to start accelerating close to the required overtaking speed while still in the left lane and behind the vehicle you are overtaking.

However, caution is needed in case the vehicle ahead of you suddenly slows down, or some other situation requires you to stop the planned manoeuvre and you need to brake heavily.

The drawing depicts a car-caravan combination [V1 & V2] overtaking another vehicle [T] (in side and plan views). It is not to scale, as LA and LD are relatively much longer than LV and L.

• V1 and V2 show the car-caravan combination, before and after the overtaking manoeuvre.
• T shows the vehicle being overtaken.
• LV is the length of the car-caravan combination.
• LT is the length of the vehicle being overtaken.
• LA is the acceleration distance.
• LD is the deceleration distance.
• L is the total distance needed for a safe overtaking manoeuvre.

The most influential factor in determining "L" is the speed of "T", hence the required increased acceleration and speed of "V"  in order to overtake safely and move back into the left lane.

The relative speed between "V" and "T" is also instrumental in determining the overtaking distance required, along with the time spent in the wrong lane.

Of course, the "LA" and "LD" distances and times are much longer than these "just to clear" figures, and the two (before and after overtaking) "Safety Factors" must be added.

As the speed of "T" increases, the manoeuvre ‒ the required "L" ‒ and the danger increase at an exponential rate.

The air-drag increases with the square of the speed, hence the effective tractive effort is reduced. The "inertia" (momentum) of the combination also increases with the square of the speed.

The engine power required increases with the cube of the speed.

Total Resistance = Rolling Resistance (which does not vary with speed) + Air-Drag Resistance
Power Required = Total Resistance x Relative Air Speed

Determining Actual Acceleration Performance of the Combination:

Advertised powers and torques may be quite different to those actually achieved in the real world.  while they may be honest figures, they may have been measured at the flywheel on a blue-printed engine (without accessories), under perfect conditions on a dyno in a professionally controlled atmosphere laboratory.

These figures will inevitably reduce as the engine becomes worn, as the distance travelled increases.

Hopefully you will never be caught in a tight spot (read: dangerous situation) when overtaking.

However, it is best to be well-prepared and know what the ol' girl will do when you push the pedal to the floor.

Safety First:

• Ensure both tow-vehicle and caravan are in good mechanical condition.
• Ensure all tyres are correctly inflated to the prescribed pressures.
• Ensure the caravan brakes are operating efficiently and evenly.
• Load the tow-vehicle as you normally would for a trip.
• Just for this test, load the caravan to its legal limit in a reasonable and typical manner.
• Use a certified weighbridge to measure the all-up load, axle(s) load and ball load.
• Ensure none of the vehicles’ ratings are exceeded.
• With a passenger to record the readings, locate a suitable safe flat road ... ideally the middle of
a level, deserted, three-lane (in each direction) freeway.
• Check the accuracy of the speedometer using a good GPS unit.
• With the aid of a stopwatch, carefully and safely conduct a series of wide-open-throttle
acceleration runs, recording the times taken to go from 40 - 50 km/h, 50 - 60 km/h, 60 - 70
km/h, 70 - 80 km/h, 80 - 90 km/h and 90 - 100 km/h. Be alert if the caravan starts to
sway at the higher speeds.
• Record ‒ or calculate ‒ the distance travelled between each 10 km/h segment. This will enable you to appreciate how long  (time and distance) it will take to overtake another vehicle, when you wish, or need, to.

The available acceleration rate is reduced as the:

• Mass of the 'van increases
• Mass of the tow-vehicle increases
• Effective frontal area (actual area and the aerodynamic co-efficient) of the combination increases
• Head-wind (if present) increases,  i.e. as the relative speed increases
• Tyre (especially under-inflated), brake and wheel-bearing (not adjusted) drag increases
• Available engine power decreases (with wear, and poor tuning)

www.caravancouncil.com.au has, under Technical Articles on the home-page, an independent doc on selecting a suitable tow-vehicle.

It high-lights the important issues to be considered (without mentioning any vehicle makes and models).

While a heavier tow-vehicle will have a lower acceleration ability, compared to a lighter tow-vehicle with the same engine power, it is essential to have a tow-vehicle with sufficient mass  compared to
the caravan it is towing  to prevent the dangerous situation of the tail-wagging the dog should the caravan start to sway or snake

Cautions:

If you must overtake another vehicle, ensure you do it safely and without having to exceed the speed limit or such a high speed that the caravan becomes dangerously unstable.

It is highly preferable that the driver of the vehicle being overtaken realises you wish to overtake them, and hopefully reduces their speed to assist you.

If their vehicle ‒ truck or caravan ‒ has a CB radio call-sign on its rear,and you also have a CB radio, it is advisable to alert them of your intention.

Be extremely careful and double-check the road ahead to ensure you have sufficient distance of clear road to safely overtake another vehicle.

Check there are no dips in the road ahead and there are no intersecting roads from where a vehicle could suddenly emerge. Check the road is wide enough and does not have very rough edges.

Check the vehicle you are about to overtake is not preparing to overtake another vehicle or make a right turn. If that driver is not aware of you overtaking them, be cautious of their possible increase in speed.

Check your mirror carefully to ensure you are not about to be overtaken.

Use your turn-signals appropriately and in plenty of time to indicate your intention to overtake. However,
do not rely on the turn-signals of the vehicle being overtaken, as a safe-to-overtake signal.

Do not swerve sharply into the right lane, or sharply back into the left lane, as this may well induce a hazardous sway ... possibly ending in a dangerous jackknife.

On the other side of overtaking:

When being overtaken, you must consider the other party and realise that speeding up in such a situation will not only risk lives but lead the other party to extreme frustration and an even more-dangerous effort to pass you.

As a courtesy, you should gently slow down to make the overtaking safer.

Of course, if the overtaking driver has made a serious error of judgment and is in a hazardous situation, it is imperative to brake heavily to better-allow the driver to quickly cut back in front of you, so as to avoid a lethal head-on collision.

It is important  before setting off on a trip  to ensure the caravan brakes are operating efficiently and evenly.

Disclaimer: This information is provided in good faith, in an effort to improve road safety. It is believed to be correct, but no liability whatsoever is accepted for any issues arising from using it.

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