KTNP
Corrugations or Washboards (RSA, Aus or USA terminology)
This thread on roads is interesting and it appears there are many angles to this discussion. I went off in search of some research on the matter and came across Dr Karl an Australian Road Engineer and Off road specialist.
In short he talks about “HOW” the corrugations form – surely this is the element to attack – prevention is better than cure! His answer is simply keep the road surface smooth, which I would suggest is through regular maintenance and not by managing the effect of a rough road such as reducing tyre pressure to levels below 1.4.
Also, an experiment conducted in Australia showed that a 2WD causes corrugations more than a 4WD!
Lets hope that the Minister of Tourism does not approve a tar road for the TR and MM route, as that is not desirable as is just as likely to corrugate if not maintained properly.
If you want to see the full text visit his site at Extract from
http://www.abc.net.au/science/k2/trek/default.htm
He notes the following quoted from the site with my comments within the [] brackets:
One thing I have discovered in trying to understand natural phenomena is that there are usually a few explanations - not just one single explanation. I strongly suspect that we (ie, road engineers) don't fully understand corrugations!
Now the first thing to realise is that Outback dirt roads are not the only surfaces that get corrugations.
You can see corrugations on bitumen and even concrete roads, you can also see corrugations on steel railroad lines, there are corrugations on the overhead metal rails that feed power to electric trains and trams, skiers in the snow country often find rough corrugated washboard patterns on a well-travelled ski trail.
[Well this does show it is not just a
Kgalagadi road that develops this pattern, railway lines, snow slopes – skiers don’t even have wheels!].
A major factor in making corrugations was the road speed of the vehicle.
So here is Mather's theory of corrugations.
It's based on the fact that you can never make a road perfectly smooth. There will always be tiny little bumps. Once his wheel got up to about 6-7 kph, it would bounce up when it hit a tiny bump. As the wheel came down and hit the sand, it would spray sand both forwards and sideways off the track, leaving behind a little crater. This crater would then be the valley of a corrugation. As the wheel came up out of the valley, it would jump into the air again, and so the pattern of valley-and-mountain would repeat itself.
[So maybe dropping the speed limit in the park to 5km/h is the answer, but could make for a rather challenging visit. The 120kms between TR and MM would take 24 hours of driving at 5km/h! Clearly this won’t work]
Mather saw that the first few corrugations to appear on the "smooth" road were quite shallow, and very close to each other. But as the corrugations got deeper, they gradually moved away from each other, until their height and their distance apart had settled into a stable pattern. Once this stable pattern of corrugations was set up, then the entire pattern of corrugation would migrate down the road in the direction of travel of the wheel. In the Australian Outback, engineers have seen corrugations heading in opposite directions on each side of the road from (say) a cattle grid, with each set heading in the direction of travel of the cars.
A bump on the road that makes one car's wheels bounce, will also make any other cars' wheels bounce. These bouncing wheels will all tend to land at the same point. And that's how the corrugations form.
[Maybe naïve but surely if this is the “how” the answer is simply to ensure a smooth non bumpy road with and through regular maintenance such as regular grading or dragging,(as we cant make a smooth road once off.]
The faster the road traffic, the further apart are the corrugations. French engineers working in the fast and flat desert roads of North Africa found corrugations that were about one metre apart.
[KTNP corrugations appear to be some 8cm to 12cm apart. Also worth considering is perhaps requiring vehicles to travel at varied speeds instead of getting everyone to drive the same 50km/h, as this would mean each car is only making it identically worse regardless of the weight or tyre pressure due to all bouncing being more or less exactly the same].
The other explanations for corrugations involve engine resonance, wind (either natural or created by the moving vehicle), wheel hop, braking, acceleration, the road itself shrinking as it dries out, the wave of dirt pushed in front of the tyre, and the influence of shock absorbers.
[Of note here is the suggestion that a poorly constructed road base could be a further reason for corrugations. Where the road base is flexible and movements occurs will see rapid corrugation formation.]
But I was especially impressed by the Turnbull Family, who did experiments on their 3-kilometre airstrip with a 4WD, a 2WD and a semi-trailer (nope, they didn't test a bulldozer).
They ran each vehicle 10 times along their airstrip. They put each vehicle through acceleration, cruise, and slow breaking. Why did they do this? Well, they said that " ... number one son Cory chose a science project to do on holidays (he boards at Toowoomba Grammar School)..."
Cory made some good scientific observations:
The 2WD had more corrugations
[Ban 2WD from the KTNP? Not a wise tourism decision, so can't do!];
Most corrugation occurred at acceleration (all vehicles);
2nd most corrugations occurred at braking
[Not much one can do about this in the park]
Corrugation increased with traffic;
Size of corrugations was in proportion to tyre diameter (semi, 4WD).
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