Padkos, panic and physics

Planning a road trip this holiday season? Don’t leave without reading these wise words from JIM WARD…

December is just around the corner. It’s that time of year, when South Africans begin their annual migration, travelling far across this enormous country. Many of you will be joining the great trek. The situation described below will happen to many. 

You will typically leave much later than planned, pack too much into the vehicle and may have been delayed or upset at work shortly before leaving – all factors that can affect your reaction times. Your boss will ask you to finish that big project before close of business on Friday, so you won’t have time to check tyre pressures, fill the windscreen washer or check the trailer lights. It’s all a rush.

You get home from work after fighting through a nightmare of heavy traffic, light rain, widespread loadshedding, five cows and a recycling trolley on the freeway, and several busy intersections with no robots working. You are directed precariously over junctions by untrained car park attendants from the DRC and Malawi, one of whom is wearing a faded City Power luminous vest. The taxi drivers ignore the enthusiastic car park attendants…

The four-way stop is a chaotic mess. You forget that your partner asked you to go via the shops to buy essential groceries and a special gift for someone (whom you never liked anyway). 

Eventually you arrive home hours later than planned, having promised to be home by 3pm, knowing you still must drive to Margate that afternoon, or one of your precious timeshare days you’ve paid for all year long will be wasted. 

Doing your utmost to be quick, you frantically pack, change, then remember that one trailer tyre needs changing as it’s gone flat. You inflate and fit the spare tyre, shower, change again, then knock over the second fresh cup of coffee made for you – balanced on a mudguard – breaking your partner’s favourite cup, and then change again (“Are you really going in that?”). 

Finally, as the light fades, you climb into the car at 6:40pm with two gangly teenagers who dislike your music, your partner and Aunt Mabel (from that side of the family)  who mutters sotto voce for the next seven hours about the air conditioning and what nice padkos they used to make in her day (when she’s not going on about how gifted her son Deon is, and his nice Mercedes).

Everyone in the car knows that Deon lost the C220 in the divorce and is now living in Brakpan with a stripper called Remora, using her tatty green Daewoo to get to Cash Converters each day, but no one says anything.

Auntie Mabel also mentions periodically that it would have been easier to fly, and at the Driehoek toll plaza you suddenly remember that she is not anyone’s actual aunt at all. You only took her in after Oom Bertus died, mainly because you felt sorry for the dog.

But the scene is set – your mind is unsuitably cluttered and diffused and it’s time to take a calming, deep breath and think about stopping distances… or change drivers.

Weight increases momentum, and let’s face it, Mabel’s three suitcases are all heavy, so the loaded car weighs 1,400kg. Then there is the 600kg trailer, meaning your combination with five people has an all-up weight of 2,000kg, which is only braked by the four wheels on the car. The added trailer weight means the car’s brakes must stop 43% greater mass, using the same braking system. 

Let that register for a second… A truck and trailer has brakes on every axle. Only 66% of your wheels have any brakes at all, and you could be travelling 50% faster than that truck.

Barrelling along, you discern through the greasy windscreen (that you never had time to clean) an unmarked, dark blue flatdeck trailer without taillights, sticking into the road. The chevron is mostly missing and what’s left is too dirty to reflect. Luckily, the steady rain made you slow down a little, but you are still travelling at 110km/h and your reaction time is slower than normal due to fatigue, in-car distractions and wondering if you turned the garage light off. All of this means that your car covers 50m before you react. 

The brakes are in good condition and the tyre treads are fair, but your combination has 2,000kg of mass because of the trailer.

Kinetic energy is a function of speed and mass, and the additional trailer weight means that stopping distances increase, while the wet tarmac reduces the coefficient of friction between tyres and road, turning this into an emergency braking situation, which is very scary. The car will skid if you lock up. 

You were already covering 30.56m/s at 110km/h. During the 1.5 seconds* it takes a driver to react, your car/trailer combination covers almost 46m, while an additional 170m is needed to brake a 2,000kg mass from 110km/h on wet tarmac. Adding together your reaction and braking distances means that you need at least 216m to stop. If the ABS is overwhelmed, or the car starts to aquaplane – sliding along with locked wheels on a film of water – this distance increases even further. The unbraked trailer will effectively push the car further (incidentally, underinflated tyres will aquaplane at lower speeds than harder tyres.)

A football pitch is 105m long, so your car and trailer needs more than two football pitches to come to a halt. That is, if your trailer doesn’t jackknife and try to overtake you by skidding sideways (because it’s unbraked, heavy and its skinny little tyres break adhesion easily). 

If the trailer you saw is closer to you than 216m, you might hit it. That event might even stop Auntie Mabel talking about talented Deon’s Mercedes.

On wet tar, a 2,000kg car-trailer combo braking from 110km/h could require 216 to 240m to stop, almost double the equivalent dry-road distance.

At 123km/h, on a dry road, your 2,000-kg combo still requires 125 to 130m to stop. That’s longer than a football pitch. A 56,000-kg GCM truck and trailer combination can brake from 80km/h in around 110m (a shorter distance than your car-trailer combo at 123km/h).

Please don’t let this be the last thought that goes through your mind this holiday season. Get everything ready early, don’t overload, take your time, check tyre pressures and drive carefully. Arriving alive matters more than arriving late. Have a fabulous festive season with Auntie Mabel.

*Driving reaction times vary; 1.5 seconds is a commonly used average for an older/slightly fatigued driver.