How to choose a capable off-road car?
Off-road driving means different things to different people, and one of the most common, and potentially most expensive mistakes made by novice drivers is to think that SUV’s and Crossovers are off-road vehicles simply because they have some AWD capability or they look very similar in height and width.
A current post on this forum is good case in point, since it asks if a Renault Duster is a capable off-road vehicle. The answer is simple; No, it is NOT a capable off road vehicle, and no serious off-road driver will ever see it, and other soft-roaders like it as a capable off-road vehicle.
This article is not intended to bash Renault Dusters and other vehicles like it; it is intended to explain why they do not make a capable off-road vehicles. For the purpose of comparison, I will stick to why Renault Duster is not a capable as an off-roader, although the reasons listed here apply to all SUV’s and crossovers from almost all the brands and regions.
It should have a low range
Having a low range is of critical importance in any capable off-road vehicle. It may be true that a Duster has a specially geared first gear that provides a bit more torque to the driving wheels, but the gearing is not low enough to prevent wheel spin, which is an absolute requirement for driving in desert sand.
A proper low range should reduce the gear ratio in first gear to the point where the maximum attainable road speed in first gear does not go over 8 - 10 km/h. The advantages of this should be obvious - the low gearing prevents the wheels from rotating fast enough to start wheel-spinning in sand or mud, so with suitable tyres, all of the rotation of the wheels is converted into traction.
Soft-roaders like SUV’s do not have low enough gearing to prevent wheel spin, which is why they get stuck so easily.
It should have way to disable ABS brakes
ABS brakes can be dangerous in off road driving, since it increases stopping distances. This might sound strange, but the fact is that ABS was designed to work on paved roads, where traction is a lot higher than on even the best off road surfaces.
On tarmac, the ABS allows the brakes to work at maximum efficiency because there is enough traction to slow the vehicle down effectively without skidding on any direction. However, in the off-road environment, and especially with tyres that are designed for on-road use, traction is reduced to only a very small fraction of what is available on a paved road.
Thus, when a driver applies ABS brake when driving on loose surfaces, the wheels lock almost immediately, and before the vehicle has started to slow down. Moreover, as soon as the wheels lock, the ABS system releases the brakes until the wheels lock again, and so on and so forth. In practice, this means that the vehicle effectively has no brakes, and the vehicle could easily drive over a cliff because it cannot stop in time.
Any capable off-road vehicle should not have ABS, or if it has, it must have a means to disable the ABS when it is not being driven on hard, paved roads. The most effective way to stop a vehicle on loose surfaces is to allow the wheels to lock, which causes them to dig into the ground, thereby creating increased rolling resistance, which is what stops a vehicle on any surface.
It should have a proper diff locks
The only differential locks that work reliably are those that lock mechanically. Almost all soft-roaders with some AWD capability have various kinds of viscous couplings that lock only the transfer case to supply torque to the diff that does not normally drive the vehicle.
The problem with this is that viscous couplings can seldom handle high torque values over long periods on the one hand, and that the actual differentials on the driving axles are sometimes not locked. Although the Duster can be retrofitted with a rear differential that locks mechanically, there is no point in locking only the front differential in tricky or difficult driving conditions.
With proper diff locks, all the wheels receive the same amount of torque but with soft-roaders, and especially those that do not have a low range, the torque that is supplied to the wheels is delivered at wheel rotation speeds that is too high for the tyres and suspension to maintain traction in deep sand or mud.
Moreover, soft-roaders are often fitted with traction control systems that are based on the ABS brakes, which means that the system’s stored energy can run out before the vehicle is out of a sand or mud trap. If this happens, the automatic diff-lock control system cannot control wheel spin on an axle, which in turn means that the driver can lose traction on two or more wheels until the traction control system is recharged.
On a proper off-road vehicle with mechanically locked diffs, torque is supplied to all wheels equally all of the time, and since momentum is the off-road driver’s best friend, there are no interruptions in power delivery that can cause the vehicle to sink into soft sand or mud.
No diff- lock system that works with viscous couplings, and/or that depends on a brake-based traction control system can ever be effective in deep sand or mud, since it cannot supply torque to all four wheels for long period of time.
Read more: 5 reasons to not use SUV's for off-road