How to Drive on Rocks
More than any other type of four wheel driving, picking and climbing through rocks requires real feel for the vehicle. For some people ‘Rock Crawling’ is a competitive sport and an opportunity to customize a vehicle specifically to challenge the toughest of terrain.
For most people driving over large sections of good size rocks will simply be about with getting from point A to B and many people would prefer to take a longer route than subject their bodies and vehicles to the rigours of boondi-bashing. With a little patience driving over rock doesn’t have to be an experience that leaves your passengers regretting the trip or your car battered and bruised.
Thrashing It and Control
This is precisely the time a little self control and planning needs to be employed. The take a run-up and thrash-it-over-the-hill approach just doesn’t cut it on the rocks. Most likely the car will end up stuck, with at least one flat tyre, probably some panel damage and possibly a fair bit of damage to the undercarriage.
Select low range 4x4, first or second gear, and choose a driving line. Think constantly about where each individual tyre will be placed and where the highest ground is so as to keep the car from bottoming out. Avoid having the car wedged atop a mound and see-sawing unable to move on. Likewise deep gullies are great places to lodge your front and rear bumpers and leave all four wheels swinging in the breeze.
Avoid dropping both front wheels over ledges together and approach them at an angle allowing each wheel to enter and depart the obstacle individually.
Focus and Rhythm
Don’t focus your eyes on the two metres at the front of your car. Because of your slow speed you have plenty of time to scan the route and modify it as you go. With practise you will always have a solid path picked out and the actual driving is just a matter of going through the motions.
Torque and rhythm are the keys and by selecting a low enough gear small throttle adjustments are all that is required to maintain smooth movement. The brake will really only be used to wash off momentum after surging over a larger obstacle while the low gearing nearly makes the clutch redundant. You certainly don’t want to be coasting over rocks with the clutch depressed letting the car drive itself. You need to propel the vehicle.
The aim is to maintain a forward rocking motion, ‘stepping’ from one rock to another and letting each individual wheel find traction. This is the 'feel' component. This combination of gearing, throttle control and wheel placement results in what can be a quite boring and hypnotic drive – albeit one that can involve quite a bit of rolling around from the occupants. It is about constant weight transfer and changes in traction while allowing the car to do its job.
Even the steepest of hills can be ascended and descended safely providing the car is not allowed to achieve such extreme angles as to roll. Because each wheel is involved in the climbing process it virtually wedges itself between each rock providing a sort of anchor for the next wheel to take over. Water, mud and loose rocks all change the dynamics of this situation and care and good judgement should be used before tackling any large slope.
Drive vertically up and down steep hills. Much like driving in sand dunes the constant weight shifting involved can present problems if the vehicle is driven across the face of hills.
Available ground clearance is vital when driving over rocks and if you are attempting to climb boulders larger than the gap under your car you are probably going to end up in trouble.
Tyre pressure for sand has been written about here, however, it also plays an important role in climbing rocks. We are trying to achieve a soft, flexible reaction from the tyres and by releasing some air we can get them to mould and grip the rocks. Reduced pressure also has the effect of making the tyres act like shock absorbers rather than a fully inflated tyre that bounces off the rocks. The image above illustrates how a fully inflated tyre bounces over rocks compared to a tyre deflated by about 25%.
Care should be taken not to pinch the vulnerable sidewalls between rocks and to not slam wheels into rocks and damage rims.
Driving through rocks is not the most comfortable way to travel but sometimes it is the only route to a destination. The key is care and control - bashing your way through eventually leaves bodies sore and 4WD's broken.
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