A substantial amount of Australia is connected by gravel roads. If you've done much gravel driving you would have heard of corrugations. They can be absolutely terrible both from a comfort and potential damage perspective.
What are corrugations?
Corrugations refer to the lumps that develop on tracks over time. They are called corrugations because they form a similar shape to corrugated iron, like what you'd see on tin roofs. They can vary from just a few cm tall to over 100mm and the distance between varies too. Basically, they shake the life out of anything going over them!
Where are corrugations found?
Just about every gravel track will have corrugations on it within Australia. When they are graded they are smooth, but it doesn't take long to develop corrugations. You can also find corrugations on sandy tracks and along the beach. Most often, corrugations are worst exiting a corner, or going up a slight incline. People have a habit of accelerating too quickly out of both situations, and the wheel spin and incorrect tyre pressures contribute to making nasty corrugations.
What can you do about them?
Corrugations are a part of the Australian 4WD network, and there are a number of things you can do to make the ride more comfortable, reduce the stress on your vehicle and limit the damage done to the roads. In essence though, its all about balancing a number of factors together. If you don't take the time to do this, you'll have a terrible ride and something is likely to break on your vehicle.
The most important factor on corrugations is Tyre Pressures. Your suspension works extremely hard on corrugations, with shock absorbers easily hitting 100 degrees celcius from the continual dampening on bad roads. In order to reduce the stress put on your suspension (and the rest of the vehicle), you should be deflating air out of your 4WD's tyres when you hit corrugations.
By letting air out, you allow the tyres to mould better to the shape of the corrugations, and they absorb some of the shock. However, be aware that on corrugated roads, the continual flexing of your tyres side wall creates a considerable amount of heat, and your tyre pressures will rise because of this.
On the Gibb River Road, we saw our 80 Series Land Cruiser tyre pressures on the rear easily go up 6 PSI from cold to hot.
How much air you take out depends on how bad the corrugations are, what speed you are travelling and how much your vehicle weighs. The general rule is about 30% of air from cold pressures driving on road.
Corrugations and speed is a bit of an interesting one to balance. The faster you go, the smoother the road feels as you just skip the tops of the corrugations. However, this also you contact the humps less and less, and your ability to safely turn and brake is dramatically reduced. Balancing your speed and tyre pressure is imperative; you want to be going quick enough to avoid the harsh bouncing, but not so fast that you aren't driving safely for the conditions.
There's no set rule either, as corrugations vary so much. Sometimes you can get away with tyres at around 24 PSI cold, and sitting at about 70km/h in relatively bad corrugations, but it vastly depends on what is safe in any given situation. Don't put your lives at risk; take it easy and arrive alive.
On the other hand, crawling over corrugations is also insanely painstaking and not great for your 4WD either.
If you are driving on corrugated roads, you should have 4WD engaged. If your vehicle is all wheel drive anyway then that's a great option. You are better to engage the centre diff lock if possible, and drive with all 4 wheels. This gives you much better grip for cornering.
Stay out of the dust
If you are following another vehicle, you should be back far enough that the dust isn't covering your vehicle. This ensures you can see clearly in front, and can react should conditions change. It also ensures your engine doesn't suck a gut full of dust in!
Damage corrugations can do
When you think of 4WDing, perhaps gravel roads are not where you would expect a lot of carnage to happen. The truth is, corrugated gravel roads put a heap of pressure on everything on and in your vehicle, and do a lot of damage relative to your average 4WD days. The endless shaking and bouncing around stresses everything from shock absorbers and bolts to tyres, wheel bearings and anything that can fatigue over time.
It's great practice to have a look in your engine bay and under your vehicle after a days driving on rough roads, to look for fresh markings of something that's come undone, or is working its way loose. Wobble things back and forth as you do your inspection to ensure everything is tight as it should be.