The most deadly terrain in offroading is a hill. That’s because gravity can accelerate the vehicle out of control in an instant. So if there’s one terrain you need caution on, it’s a gradient. And a great way to make hills safer is to use a very low gear to descend. That’s one reason why most 4x4s have low range, a crawler set of gears.
As ever with 4x4s, driving techniques are complex but the basics are; select low range, and then first or second low, then point the car straight downhill, ideally in the ruts and especially so if wet. As the gradient gets steeper the wheels start to turn the engine, and the ensuing resistance provides engine braking and a nice slow controlled descent.
So why bother with engine braking when you could simply use the brakes? Road drivers are taught “brakes are for slowing, engine is for going” and that’s not a bad rule but when you get into advanced driving such as offroad, towing and racing then those simplistic road rules need a bit of modification.
The first reason you might think of is use engine braking rather than the normal brakes is to avoid overheating the brakes. And that can be true – the brakes on anything other than a racecar are designed for relatively short applications punctuated by lots of time to cool down. But there’s another, more important reason to use a low gear and engine braking for offroad driving, which would be vehicle control.
When a 4×4 descends a hill under brakes then those brakes are trying to stop a wheel from turning. If the brakes do manage to stop a the wheel from rotating then your directional control disappears as the wheel skids. This might happen when the tyre rolls over a slippery patch so the braking force is greater than the available traction. And the steeper the hill, the more you need to apply the brakes, and the greater the chance of lockup.
The difference with engine braking is that the wheels are being forced to turn at a slow speed, not being braked. So imagine this scenario; you’re descending a steep hill in neutral, braking. The right front wheel hits a patch of mud. The brakes are trying to stop the wheel turning, so it locks, and steering control is much reduced as the tyre skids. In contrast, when first low is engaged the tyre rotates through the mud, retaining some steering control. The wheel might rotate slower than the car is moving, but the fact it is rotating at all means you’ve got a little control.
Now it is possible to use too low a gear when descending a hill; imagine a very slippery, grassy slope. First low could see you skidding, whereas second low would be a quicker descent but offer more control. And if you are skidding, then increasing speed…slightly…can work to restore control, as well as turning the steering wheels in the desired direction of travel if the car has slewed sideways.
So how slow can a 4×4 descend a slope? That starts with the ‘crawl ratio’ which is the lowest gear the 4×4 has; it’s found by multiplying the first gear ratio by the transfer case by the differential ratio. In the case of our four wagons those ratios are Grenadier 56:1 for the diesel, Defender 51:1, LC300 43:1 and Patrol 44:1, and the lower the better, so a 56:1 ratio is better than 43:1.
The crawl ratio is important, but so is the engine resistance to being turned which is the ‘braking’ part of engine braking. Not all engines resist equally; typically large-displacement, multi-cylinder diesels such as V8s are the best, and small petrol engines with few cylinders such as flat-4s are the worst so the type of engine that affects your descent speed too. There’s also weight – the heavier the car, the quicker it’ll descend – and tyre size. The taller the tyre, the higher the effective gear ratio and the quicker the descent. The degree to which the torque convertor locks up also plays a part, but these days they tend to lock up quickly so there’s less need for override kits.
Then we come to modern driving aids. Every new 4×4 now has some sort of hill descent control system, or HDC. This is effectively cruise control for descents. As always, it works a bit differently from car to car but essentially you enable it with a button, set a descent speed with the cruise control system or maybe just whatever speed you’re doing at the time. The car then monitors its speed as it has sensors on all four wheels, and will apply the brakes to keep you at that speed. Should a wheel slip then its relative speed to the other three wheels is detected and the computers will release the brake pressure slightly and momentarily on that wheel alone, something a human cannot do.
Amazing though HDC is, it works best with the appropriate gear as while clever, it is still braking a slipping wheel. Some driver demonstrators on press launches like to descend in neutral to show HDC off…fine, but that’s a terrible idea from safety and driving performance perspective as it’ll increase the chances of a skid, potentially overheat the brakes and if HDC decides to disengage then you’ll very quickly become acquainted with the bottom of the hill. HDC also works in reverse, and may automatically enable or disable; the first run with we did with the Defender we had to throw away as the driver had switched Terrain Response modes and HDC had come back in without the driver initially noticing. Also, don’t use HDC in sand, there’s sufficient retardation in the soft sand without needing additional braking.
Another descent trick is to use axle lockers if the hill is rutted such that a wheel will leave the ground or be significantly less loaded on its pair on the axle. If you descend in using engine braking with open diffs and diagonal wheels leave the ground or aren’t evenly loaded, then you lose engine braking. Lockers stop that happening.
You can also left-foot-brake downhill using the ‘driving through the brakes’ technique. This is more for older vehicles and today really only has its place in very rutted or rocky conditions where you need to lower the wheels over an obstacle at an absolute snail’s pace. However, some driver trainers still haven’t quite caught up with the times.
So to test the four vehicles we set each one at the same point on a hill, engaged first low, and let the cars idle down the hill, feet off pedals. This was a Slow Race, so last down the hill won. You can see for yourself in the video, but the Grenadier was a clear winner – the other three were pretty close but the Patrol was quicker to the bottom. Time was you’d expect Land Rover and Toyota to win such a comparison, but no. Nissan has historically never had a great crawl ratio, and that Patrol Y62 was running slightly taller tyres than stock but it’d still have performed relatively poorly. Still, all four cars have more than sufficient engine braking for Australian offroading.
The Grenadier’s gearing needs a mention; it’s a ZF 8-speed gearbox so a decent spread of ratios, and they’ve gone for a low first as there’s then seven other gears to play with. That’s intelligent design as if first low is too low just use second, and indeed the car defaults to 2nd low when pulling away.
There is such a thing as too low a low range, and here I think of the Wrangler Rubicon with an 80:1 crawl ratio thanks to a transfer case reduction of 4:1 as opposed to the usual 2-2.5:1. I found driving the Rubicon I never needed first low in Australian conditions (USA is different) and it was annoying not being able to go much above 20km/h in low range.
Now you know about 4×4 descents, have a watch to see the four cars in slow action!