When wheelspin is worse than useless

When a vehicle is being towed out of a muddy and slippery ditch, what should the stricken driver best do to help the rescuer? Many readers

First and foremost, do nothing that might cause the stuck vehicle to sink any deeper, like spinning the wheels in hopes of sharing the load. A spinning wheel won’t help; it is more likely to just...dig.

Usually, the car is in a muddy ditch because the road itself is wet and slippery. So that’s the surface the rescue vehicle is using while it tries to gain enough traction to pull both itself and the stuck vehicle. Twice the weight and half the grip? Not a good formula.

Further, the stuck vehicle is very likely tilted towards one side.  Any use of its own power will be asymmetrically delivered (because of the differential) to the wheel on the side with the least traction.

The other wheel might even still be on the tarmac, with much better grip, but receive no power at all, however hard you press the accelerator.

So, if the rescue car cannot muster quite enough power and grip to move the stuck vehicle (with its engine and handbrake off, gear in neutral), it might seem reasonable to hope that if the stuck vehicle starts its engine, engages gear, and takes a share of the work (however little) that might help solve the problem.

And it might. What drivers at both ends of the tow rope need to recognise is that power is not usually the main problem. Lack of traction is.

Revving either or both engines harder will deliver more power, but possibly too much, so instead of using what little traction there is to inch the cars forward, the drive wheel(s) simply spin, and lose traction instead of increasing it.

Wheel spinning (on a stationary vehicle) is also likely to dig the stricken vehicle deeper into the morass.

So, if some help from the towed vehicle is required, it is often more effective in second gear and at low revs, just enough to turn the wheel slowly, not enough to spin it.  

Both the towing and the towed drivers should be aware that if the wheels spin (on either or both vehicles) the grip and pulling power will be reduced, however loud the engines are getting.

If you often drive on sticky or slippery roads, one of the most effective aids (whether you are the victim or the saviour) is a piece of kit called a “tank strap”. It is a highly specialised tow rope that is both immensely strong and kinetic.   

It is so strong that if an immense force is applied, it is more likely to rip the reinforced steel mounting point off a car than to snap.  Although the belt-shaped webbing of the strap does not feel “elastic” if you try to stretch it with your hands, it does stretch if tons of force are applied. And when elastic/kinetic materials stretch under load, they develop a counterforce trying to return to their original shape.

With a conventional tow rope, it is important to ensure the rope is taut (under load) before applying more power. The towing vehicle is thus stationary, with no momentum, when the pull starts.  If a moving (running) start is attempted, the rope will probably snap.

In contrast, a robust kinetic strap won’t break, so the towing vehicle can start quite close and with a lot of slack strap and start to accelerate immediately, with only its own weight to propel. 

By the time all the strap slack has been taken up and it becomes taut (taking on the load of the stuck vehicle), the towing vehicle has a lot of momentum (its weight x speed), in addition to whatever grip and power it is still applying.

Further, the strap starts to stretch, and the strap itself exerts even more force as its material is ever more determined to recoil. While all this is happening, the stuck vehicle might not move at all. But there comes a point when all the additional forces come together – the weight of the towing car’s momentum, the speed it is going, the power of the towing engine and the grip of its tyres, and the recoil of the strap material.  The combined force is considerably more than a straight and standing-still tow can achieve.

The final result looks a bit like magic...just as the towing vehicle starts to surrender to the load, the stuck vehicle, quite gently, starts to move and then pops out of trouble like a cork from a bottle.

To give you a sense of the potential force, kinetic straps were a military invention to pull battle tanks (weighing between 10 and 80 tonnes) out of mudholes deep enough to defeat even their super-traction toothed tracks. Hence the straps’ nickname.

Kinetic straps are now readily available auto accessories. They cost quite a bit.  But they can achieve quite a lot. And they are, of course, multi-purpose for any task that requires (or simply benefits from) a strap that is so strong that few activities in a civilian context are likely to break it. 

They are as robust and sometimes stronger than steel winch cables. And their kinesis can be effective enough to enable a 4WD station wagon to unplug a 10-tonne truck loaded with building stones.

None of this disqualifies the potential value of bystanders giving the stricken car a push or, if the road is quite firm, adding weight over the drive wheels (and even bouncing up and down) to improve traction. 

And if the stuck vehicle has dug deep enough that its floor is touching the road surface, the huge level of drag that is created can of course, be reduced by digging away the road or jacking the vehicle up and putting grippy materials under its drive wheels.

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