Rock Crawling

4x4 Africa - rock-crawlingWhile our aspiring off-road driver Bud has spent a considerable amount of time planning his epic off-road road trip through Southern Africa, he had never seen a rock crawling 4×4, and being convinced that much of Africa consisted of rocks, he felt that he needed to know how to get over and through them, so, he booked a rock crawling training session with the Instructor at the off-road club.
Bud had also read that rock crawling is one of the toughest off-road disciplines so with this in mind, he arrived at the club with his first 4×4- the one with the solid front axle and no electronic driving aids. He was sure that if any 4WD vehicle could cope with rocks, it was this one. However, he was mistaken, as the Instructor was quick to point out.

Rock Crawling Suspension


• Leaf Springs:

articulation“Look, Bud, I am sorry to tell you this but not even this 4×4 will survive more than a few minutes on the rocks we have here. You may be one of the best off-road vehicles ever built but it is meant to be used for “normal” off-road driving- not rock crawling. For one thing, you need a purpose-built suspension for rock crawling, and even with the solid front axle, you will not get the amount of suspension articulation you need to get over some obstacles.”
“Leaf springs, such as the ones you have, only allow around 450 mm or so of suspension travel; what you need is as much as you can get, which is about 600 mm or thereabouts. Standard leaf springs and shocks also have a problem in that they cannot always support the entire weight of the 4×4 on sometimes only one corner. Leaf springs are more fragile than you might think- one over extension can either break a leaf spring or cause it to lose its tension, or temper, as it is also known.”
“Then there is the problem of “axle bounce”; under severe torque leaf springs can cause an axle to start bouncing. This can break prop shafts, side shafts, as well as crown wheels and pinions because instead of transmitting the torque to the wheels, the torque is absorbed by the leaf springs, which causes them to compress and relax in quick succession as the tyres lose and regain traction. These shocks to the driveline usually end in prop shafts buckling, side shafts breaking and/or crown wheels and pinions losing teeth.”
“The only way to prevent this happening is to spend a fortune on having your suspension rebuilt or converted to a link-based system, such as you find on Jeeps and some Mitsubishi 4WD’s, or to buy a 4WD with a link-based suspension as standard. These designs rely on very strong links, or control arms, to hold the differentials in place, with extra transversely mounted sway bars to prevent sideways movement.”

• Shock Absorbers:

coil-sprung-suspension“Then you have the problem of shock absorbers. Even with a link-based suspension, you will find that standard, or even the heaviest duty shock absorbers do not have sufficient travel on the one hand, and that they cannot cope with the loads of rock crawling because of the speeds with which an axle can move up or down on the other hand.”

“Under “normal” off-road driving conditions a shock absorber only has to deal with relatively short compressions and extensions. A normal shock absorber very rarely extends its full length, and when it does, it does not usually happen in quick succession. In rock crawling, the shock absorbers might have to deal with long compressions and extensions all the time, and at the kind of axle articulation speeds no normal shock absorber can handle.”

“Of course, rock crawling shock absorbers come in different capacities and ranges of travel but the one thing the best brands have in common is the fact that they are coil-overs, which means they incorporate a coil spring in the basic design. This extra spring is in addition to the main coil spring to help reduce the load on the shock absorber and by juggling the capacities of both springs, you can set up the 4×4 for different rock trails. No two rock crawling trails are the same so most rock crawlers have their suspensions set up so that adjustments to the extension and rebound rates of the shock absorbers can be altered to suit a particular trail.”

Rock Crawling Tyres


mud-terrain-tyres“Then there is the matter of which tyres to use for rock crawling: there are probably as many opinions on this issue as there are off-road vehicles. However, it turns out some basic guidelines seem to work in most cases, so, we can look at some of the underlying principles that make a good rock crawling tyre:”

• Tread pattern

“Some rock crawlers swear by mud tyres but the problem is that they mostly do not have added tread to the shoulders to help with lateral traction. The tread on normal mud tyres is often narrower that the casing of the tyre, which means that the sidewalls often have the added task of providing traction, which is not what they were designed to do. A proper rock crawling tyre will have distinctive tread blocks right on its shoulders to alleviate the pressure on the sidewalls should it be necessary.”

Apart from the tread pattern that should have large voids between the tread blocks to allow for deformation of the tread to accommodate uneven surfaces, the compound of which the tread is mage should be soft enough to provide traction without spinning, but at the same time hard enough to withstand gouging by very rough and sometimes very sharp edges of rocks. There are dozens of different compounds so, the thing to do would be to do some research on which tyre/rim combination works best on any particular vehicle.”

“Nonetheless, if a rock crawling tyre is rated for on-road use, it is not as good for rock crawling as a tyre that is not. The reason for this is because a particular tread pattern might not be able to drain off water on paved roads, or it might not offer sufficient stability under hard braking or cornering conditions.”

“The main requirement of a rock crawling tyre, beyond having to provide traction, is that it must be able to deform to a point beyond anything that is required of an on-road tyre without blowing; therefore anything that qualifies a tyre to be used on-road detracts from its ability to work in the rock crawling context.

“But apart from all that, the sidewalls of a rock crawling tyre need to be strong yet flexible enough to be able to withstand extreme deformation. There are radial rock crawling tyres on the market but the best rock crawling tyres are bias-ply and by far the most rock crawlers use them because the added strength of the sidewall offers more protection against punctures and cuts.”

• Bead Locks:

“But there is more, Bud, for instance, you must have a way to prevent the tyres coming off the rims when you are running at reduced inflations. The way to do that is to have beadlocks fitted to clamp the tyre beads to the rim. Air pressure is normally enough to hold the beads on the rims but with the amount of deformation on rock crawling tyres combined with lower than normal pressures, the outside bead is usually the first to peel over the rim edge, something that can destroy both tyre and rim in some situations.”

“You get two types of beadlock- one that bolts onto the outside of the rim and one that fits inside the tyre, sort of like an extra inner tube. Both types have advantages and disadvantages, but both work well enough in most cases. Therefore, if you do not want to spend the extra money on specially designed rims which have steel inserts into which the clamping bolts are screwed, you can use your standard rims with the internal tube-type beadlock.”

Gearing


rock-crawling-and-wheel-articulation“OK, Bud, so now you have a basic rock crawling 4×4, and if you took this one here and performed all the modifications you need, you might have enough torque and power to tackle most rock crawling trails but some trails are more demanding than others, and these are usually the one used for rock crawling competitions. There are quite a few of them in South Africa, and you will be amazed at what this type of specialised 4×4 can do on rocks.”

“Building a completion standard rock crawling 4×4 is very expensive but it depends on what you want, but one thing you cannot do without is enough power. Again, it depends on what you want and how much money you are willing to spend but one of the cheapest ways to increase the torque on the wheels is to change the gearing in the transfer case.”

“Rock crawling is all about power, but the downside of too much power is the fact that you can break prop shafts, universal and CV joints, drive shafts, differentials, and clutches. Standard clutches were not designed for rock crawling so you have to think about having a heavy duty, button type clutch fitted as well. However, while you are at it, think about having a heavy duty prop shaft made as well, in addition to skid plates to protect as much of the underside of the 4WD as you can.”

Then when you are done with that, increase the gear ratios in the differentials: a smaller pinion and a larger crown wheel is another way to increase torque to the wheels, if you do not want to change the gear ratios in the transfer case. Some rock crawlers do both but with the amount of torque you have on your Land Cruiser, you should not have to do both, that is to say if you want to convert it to a rock crawler, of course. If you decide to convert it, just remember that you cannot then use it for anything else, so it is up to you whether you want to do this or not. However, I would not do it if I were you; I would rather buy another 4WD with a link-type suspension as a standard feature. If you converted this 4×4, you will only spend a fortune on something that will never be as effective as a vehicle that has a suitable suspension already fitted as standard.”

Electronic Driving Aids


“It is difficult to say which electronic driving aids work for rock crawling: some people swear by traction control and HDC systems, but traction control can sometimes cut engine power, which is never a good thing when you are rock crawling, because it cause you to fall off a rock.”

“Hill Descent Control might work but to my mind you are better off by manually controlling the brakes, because it gives you a “feel” for what the 4×4 is doing. ABS might work for you but it might not; most rock crawlers who regularly win competitions have brake systems that allow them to apply full braking force to individual wheels.”

“Rock crawling sometimes require very tight turning and it helps a lot to reduce your turning circle if you are able to apply full braking force to only one rear wheel, once you have disengaged the diff locks of course.”

“Also, remember that you will be in 4WD mode in low range almost all the time so you will have a massive amount of engine braking force when you are getting down from steep obstacles. Rock crawling is ultimately all about vehicle control and if you have to rely on electronic driving aids to control the 4×4, you are not qualified to be on the rocks in the first place, but there are rock crawlers who might feel differently, so go with your gut feeling; if you feel you need some sort of electronic driving aid to help you, try it and see if it does anything for you: if it works, fine- if it does not, just disengage it.”

Diff locks vs. Limited-slip diffs


“Which brings me to another point: you need all the traction you can get to get over some obstacles and the only way to get this is to use the diff locks. It is not easy to get wheel spin on rocks, especially if you are using good tyres and the suspension is setup properly. However, it can happen and if it does, you might fall off a rock, so use the diff locks to eliminate the possibility of wheel spin. Rock crawling is inherently dangerous; therefore, you want to reduce the likelihood of any sort of slippage on the rocks.”
“Limited slip differentials allow a certain amount of slippage; they do not positively lock the two side shafts together in the way a proper diff lock does, hence the name “limited slip differential”. They may be of some value in getting you out of a mud obstacle but the sort of torque you need to get over the rocks in even a moderate rock crawling trail will just overcome the limited locking action of this type of differential.”

“However, the problem with diff locks is learning how and when to use them: you cannot steer on hard surfaces when the diff locks are engaged, so, you might find that a traction control system that uses torque distribution, like the system on your other 4×4, and not the ABS system could be of some use, but as I said before, the traction control could suddenly decide to cut the power if it should sense a dangerous situation. Unexpectedly losing power is the very last thing you want when you are rock crawling, so to my mind, you will be much better off by being able to control your 4×4 without any sort of electronic driving aid.”

Approach and Departure Angles


“OK, Bud, one last thing about rock crawling 4×4’s: you want to have a 4WD with the wheels placed as close to the corners as you can get them, because long body overhangs can get snagged on an obstacle before the wheels can reach them. On the front of the 4×4, this is called the approach angle, and is a line drawn between the contact point of the front tyres with ground and the point on the bumper that will make contact with an obstacle before the tyres can. The same goes for the back end; also a line drawn between the contact point of the rear wheels and the point of the bumper that will make contact with an obstacle before the tyres can.”

“The thing is though that people do not generally reverse up obstacles, so the “departure angle” is the angle at which the bodywork will be snagged on an obstacle before the rear wheels are on the ground. Many rock crawlers cut the back end off trucks just like this one, at a point where there is no more body overhang, effectively placing the wheels right at the rear corners of the 4×4. This is however not so easy on the front end, but big wheels and body lifts can increase the approach angle to a point where you should have no problem getting up even very steep obstacles.”

To go rock crawling – or not


rock-crawling“Look, Bud, rock crawling is all about power and vehicle control, so my advice to you would be to learn to control a 4WD in more normal conditions first. Rock crawling requires a very different skill set than normal off-road driving no matter how tough the “normal” conditions might be, and even if you decided to get a rock crawling vehicle, I would not feel comfortable allowing you onto the rocks with it, at least not until you have completed at least the intermediate off-road driving course; you are now doing the basic course, but hang in there, I will allow you to do some rock crawling as soon as I am sure you can control a 4×4 in most of the off-road conditions you are likely to encounter in South Africa, and with a ton on the back of your 4WD. Off-road driving, and especially rock crawling is also about safety, not about seeing how quickly we can get you to a hospital, so for the moment, be safe, and stay away from the rocks.”

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