4x4

4×4 puzzle: the solution

We posed a 4×4 problem, and here’s the solution.

UNDERSTANDING HOW YOUR 4X4 works is the key to driving it effectively. The problems are the same no matter how big or small your 4X4 is, and so we can use our TRAXXAS TRX-4 (review here) to describe a common 4X4 problem:

Read: problem description – how would you solve this?

And here’s the solution. The steps are:

Identify the problem – diagonal wheels are spinning because open differentials send drive to the wheel that’s easiest to turn (slight simplification, but it’ll do for now). That’s clear, but why? Well, both front right and rear left wheels are on ledges, which are hard to drive over, acting like wheel chocks. And it’s unfortunate that  the wheelbase of the vehicle is exactly the length that means the two wheels hit the ledge at the same time. Well, that was by design this time, but it often happens in the bush too. It is interesting to watch to different-wheelbase vehicles tackle the same obstacle.

Next, what are the potential driving solutions?  We do have cross-axle locking differentials, but for the purpose of the exercise we aren’t using them. We could take a different line, but again we’ll imagine it’s a narrow track like on the one the LC79 is on above and that’s not an option. That leaves momentum, which doesn’t work for us as there simply isn’t enough ‘runway’ to get any momentum. That means a huge bounce over the obstacle which works on an RC car, but in real life is dangerous and liable to break things. We don’t have a winch either.

Finally, we accept we need to track build with our block. But which of the four options is best?

Option D just gets the rear left wheel up higher quicker, but the car is off the block before it gets into its stuck position. Option D does allow a little bit more momentum for the rear wheel before it hits the stopping point, but as we’ve done nothing about the front right wheel there’s not enough benefit to actually help, and Option D doesn’t make the back wheel’s job any easier.

Option B is the same, but for the front wheel. The vehicle is just as cross-axled as before, and the extra bit of ‘runway’ for momentum doesn’t help.

What we really need to do is to stop the vehicle teetering on two diagonal wheels. So, Option C works because it lifts up the rear wheels so both are in contact with the ground and therefore both have traction. It also tips the vehicle towards the front left wheel so that spends less time in the air. Unfortunately, the vehicle’s front wheels disturb the block as they pass, and so it’s no longer in the right position when the rear wheels get to it.

That’s why Option A is the best solution. Like Option C, there’s now two wheels in contact with the ground and therefore pulling forwards, dragging the rear axle over the ledge. Once the front wheels are through they can do their work of dragging the rest of the car over, and there’s no problem with the front wheels disturbing the block for the rears, as once the front are passed the block isn’t needed any more. Notice how the ‘low pressure’ aggressive tyres bite into the side of the block, exactly like aired-down mud terrain tyres work in full-size vehicles.

General rules for track building

• Understand why you can’t go forwards – if you’re hung up on a rock (clearance) that’s different from a loss of traction. Don’t just start blindly building.
• Avoid if possible – use driving techniques, it’s easier and quicker
• Fix clearance issues first.  Then once only rubber is touching the ground look at traction.
• Aim to have all four wheels with weight on them, as equal as possible, and the vehicle level.
• Consider winching over.

If you have to build:

• Back up a bit – there’s no point building exactly where you got stuck. Sometimes even backing up 10 centimetres is all you need.
• Pack under the spinning wheel or one with least weight on it – often by using material from under a stationary wheel.
• Build firm – ideally use rocks not logs, especially when wet. A very nice looking build can easily be destroyed by the weight of a vehicle or wheelspin.
• Try traction ramps – these are brilliant track building devices.
• Be aware of the environment – maybe you should turn around rather than destroy a track, creating erosion problems or worse.

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