What makes a car good on dirt roads?
Dirt-road driving is still, thankfully, something we Aussies need to do a lot of. But what sort of car is a good dirt-road cruiser?
Any time we discuss driving techniques or vehicle setup for a terrain it’s a good idea to start with understanding exactly what that terrain is and the challenges it throws up.
We’re not talking here of 4WD tracks that require high clearance, slow speeds and four-wheel-drive, just dirt roads you could get a normal car along if you wanted to. That still leaves a huge variety of roads – some are so hard and smooth you can squeal tyres and leave rubber, others are rough with inches-deep ruts, and some are covered with loose gravel or dirt. There’s also potholes, dust bowls, leaf litter, small branches and rocks. That’s just in the dry – when wet traction will decrease, almost to ice levels if it’s smooth clay, and there may be large puddles or even streams. Dirt roads often have corrugations, and might well be steep, narrow, have no lane markings and no signposts. They could be dusty, have camber one way or the other, and you might find rocks, animals or trees in the middle of the road for no reason at all.
The consistent theme with all of the above is the roughness of the terrain, and that leads us to the first and most important criteria for a dirt-road car which is a good suspension setup and tune. Power? Doesn’t matter how much you have if you can’t put it to the ground. All-wheel-drive? Important, but again a good suspension tune helps with traction.
So what are we looking for in dirt-road suspension? A certain kind of softness, for a start. If the suspension isn’t soft then it won’t absorb the small bumps. And it has to be long travel so it can absorb bigger bumps, which means the wheel can move a long way relative to the body. That means undulations can be absorbed without disturbing the vehicle.
The best way to understand what dirt-road suspension goes through is to watch this video:
Now you may be thinking something like one of the readers on my Facebook page:
Does that flexing of suspension mean that the vehicle is necessarily barge like?
Not necessarily. A barge-like feeling from a car comes from several factors; vague and slow steering, slow response to power, soft and wallowy suspension. A good dirt-road car has direct steering, albeit well-damped, immediate response to the throttle, and while the suspension may be soft it’s soft in a way which allows bumps to be absorbed for traction, not necessarily for comfort. The difference?
Imagine a wheel rising up over a bump. For occupant comfort you want that wheel to move up slowly, and rebound back down again smoothly. This gentle action will smooth out the bump effectively…but slowly. The problem is that by the time the wheel has dealt with that bump the next has arrived, and then you get the problem of excessive pitch, yaw and roll as the car is bounced all over the place.
Good dirt-road suspension follows the offroad race/rally tune of quickly dealing with undulations. Again we have the wheel rising over a bump, but while the movement is initially allowed – to absorb the shock – it is quickly damped and the wheel returns to normal. That’s not as comfortable as a occupant-tuned suspension, but it is more effective for traction and actually more comfortable at speed. It is this tuning that the aftermarket 4WD suspension companies tend to focus on. As an example, the photo sequence below shows an LC200 on a moderately rough road – see how far the suspension is moving up and down? That’s why you need long travel.
Another way to think of dirt-road suspension vs ride-soft suspension is how you catch a high-flying cricket ball. It’s less painful if you catch the ball and slow it down gently, taking your time. But the professionals don’t do it that way; they catch and return very quickly. Less comfortable, but quicker.
The summary is that soft suspension capable of dealing with dirt roads doesn’t need to equal barge-like handling, as proven by any rally or offroad race car, and by the better dirt-road cars like this LC200.
However, there is a trap, as described by this little story. It was a few years back when the LC200 was new, and I was in Cape York driving one fitted with a bar and winch, but no suspension upgrades. The other car I spent time driving was a LC100 poverty-pack, yes the one with no turbo and no power, but it had aftermarket suspension tuned for dirt roads.
The LC200 had too much power for its suspension; the car could, and did easily get into trouble where it was unable to recover from a bounce before the next one hit it. I will admit I nearly lost it at one point; driver error, but also indicative of a poor suspension tune. The problem was that the bar and winch added significant weight over the front axle, and that created a pronounced fore/aft pitching effect the original suspension was never designed to cope with.
In contrast, the LC100 had too little power for its suspension; it felt assured and comfortable at all times, even when you were driving with your foot welded to the floor.
The moral of the story is that the stock suspension is designed for a stock car; as you upgrade the car or add weight to it, and you must in turn modify the suspension. As a side effect, aftermarket suspension tends to be very good at heat dissipation so the dampers don’t overheat, fade and then fail.
The other major feature of a good dirt-road car is an effective all-wheel-drive system. Modern cars can easily overwhelm two driven wheels, so you need all four working for you. And not one of those on-demand systems that only drive the rear axle when the front spins, no matter how many milliseconds are claimed before torque is sent to the back. The most effective all-wheel-drive systems are those that split torque 50/50, or close to it front and rear at all times. Examples are any Subaru (their claims are not just marketing bumph), the Discovery, Patrol Y62, the LC200, Everest, Prado and the Suzuki Grand Vitara. Vehicles that are part-time 4WD such as Fortuner, most utes and the MU-X are better handlers in the dirt once 4WD is engaged, and you go a bit easier on the track and your own tyres too.
Other features that help in the dirt; a stability control system that permits a little slip without getting over-excited then comes in smoothly, responsive but well-damped steering, a powerful engine, powerful headlights to deal with the dark, ABS that can handle dirt roads, and a smart gearbox that can be manually controlled when the driver wants. Tyres are important too; relatively narrow tyres that are high-profile, so have a lot of rubber between the rim and the ground as the tyre itself is part of the suspension. In practice, that means 17-inch rims for 4WDs, not 19 or 20-inch.
If a car has all that, then it’s likely to get good marks on our dirt road testing.