CAN YOU TOW AND DODGE?

Go! Drive and Camp Camp Guide - - On The Open Road -

What’s to be done if that kudu re­mains de­fi­ant, re­gard­less of how hard you are brak­ing and hoot­ing? Ei­ther you are go­ing to hit it or you will have to swerve.

Surely all of us know that the slower you drive, the more con­trol you have over your tow­ing com­bi­na­tion... but Fran­cois and Marèt demon­strates em­pir­i­cally with a sim­ple cal­cu­la­tion how a slower tow­ing speed can make an ob­serv­able dif­fer­ence when a driver sud­denly has to re­duce the speed of his tow­ing com­bi­na­tion in or­der to swerve.

For the pur­pose of this ex­er­cise, let’s as­sume that you’re driv­ing a dou­ble-cab bakkie with a tare weight of 1 900 kg, lug­gage that weighs 300 kg, two oc­cu­pants weigh­ing 100 kg each, and a half­filled fuel tank of 40 kg (this comes to a to­tal of 2 340 kg): In the first test, we hitch a trailer with­out run-in brakes. The trailer has a tare of 360 kg and it’s car­ry­ing a load of 390 kg (giv­ing a to­tal of 750 kg).

If this tow­ing com­bi­na­tion is driv­ing along a level road at 120 km/h and it has to brake sud­denly, it will take 5,3 se­conds (in­clud­ing your re­ac­tion time) to come to a com­plete stop over a dis­tance of 101 m. If towed at 90 km/h, it would’ve taken 4,2 se­conds to come to a stop over a con­sid­er­ably shorter dis­tance.

Now let’s sup­pose we hitch a car­a­van to the same dou­ble-cab bakkie with a tare of 1 120 kg and 100 kg of lug­gage in­side (so a to­tal of 1 220 kg). In this case, you do need over­run brakes for the car­a­van.

Even though this com­bi­na­tion is con­sid­er­ably heav­ier, the over­run brakes should do their job. If towed at 120 km/h, the bakkie and the car­a­van can be brought to a halt within 4,2 se­conds over 83 m. At 90 km/h, the com­bi­na­tion can be brought to a halt af­ter 3,3 se­conds and 52 m.

A QUICK DUMMY?

You may be­lieve that you’ll be able to brake in an emer­gency. But what hap­pens if you also have to swerve to avoid hit­ting some­thing in the road ahead of you? How easy is it for your tow­ing com­bi­na­tion to start skid­ding over the road sur­face – or even worse – to start top­pling over?

“The sta­bil­ity of a ve­hi­cle de­pends on a num­ber of fac­tors. The most im­por­tant of th­ese is the ra­tio be­tween the wheel align­ment of the tow­ing ve­hi­cle and its cen­tre of grav­ity above the road sur­face,” says Fran­cois. “The cen­tre of grav­ity is that point where you can as­sume all of the weight of the ve­hi­cle to be cen­tred. The po­si­tion of this point of course de­pends on the de­sign of the ve­hi­cle and the way in which you load the lug­gage into it. For ex­am­ple; the cen­tre of grav­ity of a car with a roof car­rier loaded with heavy lug­gage moves up­wards.

“The Static Sta­bil­ity Fac­tor (SSF) of the ve­hi­cle is an in­di­ca­tion of its re­sis­tance to roll. The static sta­bil­ity is cal­cu­lated by di­vid­ing the width of the wheel align­ment by dou­ble the height of the cen­tre of grav­ity. The big­ger this value, the more sta­ble the ve­hi­cle will be while mov­ing.

“A typ­i­cal sedan has a SSF of about 1,4, while that of an SUV is about 1,17 and >

that of a bakkie is 1,18,” ex­plains Fran­cois.

“The SSF serves as an in­di­ca­tion of the ten­dency of a ve­hi­cle to fall over when in­volved in an ac­ci­dent. A ve­hi­cle with an SSF of 1,03 or less stands a 40% chance of fall­ing over, while the chance of crash­ing with an SSF with 1,45 or more is only 10%. The SSF of most tow­ing ve­hi­cles ranges from 1,13 to 1,24, so they con­se­quently stand a 20-29% chance of rolling over.”

THIS IS HOW THINGS PLAY OUT

Fran­cois made a few more cal­cu­la­tions to il­lus­trate how tow­ing speed, in com­bi­na­tion with the sta­bil­ity of your tow­ing ve­hi­cle and car­a­van, af­fect your abil­ity to avoid a col­li­sion.

Sup­pose the head­lights of a tow­ing com­bi­na­tion with a high av­er­age SSF of 1,25 sud­denly re­veal a sta­tion­ary truck 45 m ahead in the road. Will there be enough time for the driver tow­ing at 120 km/h to switch lanes suc­cess­fully (that is, to swerve within 3 m), pro­vided he is able to sud­denly turn at a sharp enough an­gle to al­low the tow­ing com­bi­na­tion to only just avoid skid­ding or rolling over? But at 90 km/h, things look bet­ter (see illustration).

As you con­tinue to load lug­gage and other items higher and higher, both in your bakkie and the car­a­van, the height of the cen­tre of grav­ity in­creases and the SSF value de­creases. The mo­ment that the SSF value has dropped to be­low 0,6, it will be im­pos­si­ble to avoid hit­ting any­thing that you no­tice ahead of you in the road – not even at a speed of 90 km/h.

When a tow­ing com­bi­na­tion is mov­ing ahead at 120 km/h, the com­bi­na­tion will al­ready have moved ahead 26,7 m within the driver’s 0,8 sec­ond re­ac­tion time. This means you are al­ready 18,3 m from the truck, while you need 25,9 m to swerve out in time. So it takes you 52,6 m in to­tal to re­act.

Sup­pose the tow­ing com­bi­na­tion is mov­ing ahead at 90 km/h, then it will have moved 20 m in the re­ac­tion time of 8 se­conds and the driver will be able to per­form the ma­neu­vre in 19,3 m – with a lee­way of 5,7 m be­fore it will hit the sta­tion­ary truck.

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