Living In The Passed
Overtaking is not an exercise in proving your superiority as a driver, nor is being overtaken a threat to your pride. Last month we showed you what can happen with caravan sway – and it wasn’t pretty. Here’s the absolute ins and outs of a car and caravan successfully overtaking another vehicle.
By Colin Young, Caravan Council of Australia
Safety is by far the most important consideration when towing a caravan. You must have sufficient power available to accelerate the car and caravan combination at a reasonable rate, and to maintain a reasonable speed when climbing hills.
It is vital that 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 that you are able to slow-down at a reasonable rate… safely, and in a straight line. 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 that you can overtake a slower vehicle safely, in the prevailing atmospheric and road conditions; 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 thorough knowledge of the handling/stability performance of their tow vehicle and caravan combination when a lane-change manoeuvre is undertaken.
It is vital to realise that the mass of the car and caravan combination is much greater than the mass of the car alone – hence the acceleration performance will be considerably reduced. The propelling force available depends on the friction between the driven tyres and the road surface, and so 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), it is important to start accelerating close to the required overtaking speed, while still in the left lane, while behind the vehicle that you are overtaking.
However, caution is needed, just 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 above drawing depicts a car-caravan combination [V1 and V2] overtaking another vehicle [T] (in side and plan views).
The drawing is not to scale, as LA and LD are relatively much longer than LV and LT.
For the related diagram and to see the full experience, read it in our online magazine.
- V1 and V2 show the car-caravan combination, before and after the overtaking manoeuvre.
- LA is the acceleration distance.
- LD is the deceleration distance.
- L is the total distance needed for a safe overtaking manoeuvre to be made.
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 – increases at an exponential rate. The air drag increases with the square of the speed, so 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 blueprinted engine (without accessories), under perfect conditions on a dyno in a professional 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 loud pedal to the floor. Here’s a safe way to find out:
- Ensure that both the tow vehicle and the caravan are in good mechanical condition.
- Ensure that 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:
(a) all-up load; (b) axle(s) load and (c) ball-load.
- Ensure that 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 40-50km/h, 50-60km/h, 60-70km/h, 70-80km/h, 80-90km/h and 90-100km/h. Be alert if the caravan should start to sway at the higher speeds, and be prepared to take corrective action.
- Record – or calculate – the distance travelled between each 10km/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
- Headwind (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)
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 that it is towing – in order to prevent the dangerous situation of the tail wagging the dog; should the caravan start to sway or snake.
If you really must overtake another vehicle, ensure that you do it safely, and without having to exceed the speed limit, or such a high speed that the van becomes unstable. It is highly preferable that the driver of the vehicle being overtaken realises that you wish to pass them, and hopefully reduces their speed to assist you. If their vehicle – a truck or caravan – has a UHF radio call sign on its rear, and you also have a UHF 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 that there are no dips in the road ahead, and that there are no intersecting roads from where a vehicle could suddenly emerge. Check that the road is wide enough and does not have very rough edges. Check that the vehicle that you are about to overtake, is not preparing to attempt the same manoeuvre on another vehicle, or make a right turn. If that driver is not aware of your overtaking them, be cautious of their possible increase in speed. Check your mirror carefully to ensure that you are not about to be overtaken.
Use your turn signals appropriately, in plenty of time, to indicate your intention. 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 jack-knife.
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 may very well 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 you set off on a trip – to ensure that the van brakes are operating efficiently and evenly.
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 this information.