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TECH: Axle binding

13 September 2016

Axle binding poses a very real threat to part-time 4×4 drivetrains. We look at this mechanical phenomenon that often has a twist in the tale.

So you just bought your first 4×4 and you’re thinking of taking it on that epic adventure you’ve always dreamed about. Since you’re a careful person, you might ask some questions with regard to the trip and your vehicle’s abilities. So you join an online forum of some sort, on which you post the question: “When should I engage 4H on an off-road trip?”

Brave move that. Prepare to be astonished to what lengths some people will go to in order to subtly brag about their driving prowess. If the internet is to be believed, there are thousands of Sebastien Loebs out there, able to control a drifting bakkie at 120km/h. There seem to be two schools of thought on this topic. There are the guys who say ‘never’ and those who say ‘always’. The conversation inevitably leads to terms like axle binding, driveline windup and rotational speeds. So you sell the 4×4 and buy a 3 Series, because oversteer is at least easy to understand.

What’s axle binding?
Axle binding, also known as driveline windup (and a few other things that sound vaguely similar) is a real thing and it can cause massive damage. To understand axle binding, you first need to understand the basics of part-time four-wheel drive. As the name suggests, it can be used only part-time due to the nature of its set-up. In 2H you can drive as fast as you like without causing damage, as only the rear wheels are engaged and the front wheels are allowed to spin freely. It’s the 4H that causes the problems.

Unlike a full-time 4×4, which has a centre differential and allows some slippage, a transfer case locks. This means that both front and rear driveshaft are forced to revolve at the same speed. Some people might tell you that this isn’t a problem and that their car is tough enough to take it. Don’t listen to them. Their 4×4 hasn’t broken… yet.  At this point it’s best to use your imagination. For the purposes of this article, let’s use the Suzuki Jimny, which is a good example of a 4×4 with part-time four-wheel drive.

Let’s say this Jimny is parked on an even piece of tar and the wheel is turned 45 degrees to the right. The problem is immediately apparent from the intended path of travel. The front right wheel is going to travel a shorter distance than the front left wheel. Due to both drive shafts rotating at the same rpm, the front wheel will have a slowing effect, which is going to put pressure on every single part between the front wheels and the transfer case. In this situation the parts are stressed instead of moving freely and the resulting effect is axle binding. Luckily, it’s easily noticeable. The steering will feel a lot heavier than usual and in severe cases it might even jolt from the driver’s hands.

The obvious solution would be to put it back in 2H, but once you’ve reached this stage, the various parts are too stressed and compressed to even try. The best solution is the straighten the car out as quickly as possible and then engage 2H, or phone a towing truck if it’s too late for that. In these scenarios it’s usually the weakest parts in the system that shatter first and the truth of the matter is that there is no cheap part involved. We’re talking differentials, gears, chains and drive shafts, none of which are cheap to replace.

Now, we have seen a few online comments suggesting that it’s perfectly fine to use 4H on tar when driving in a straight line, which makes a certain amount of sense. We would not recommend it, however, for two reasons. The first, and most obvious, is the fact that no road is perfectly straight all the time, and there are corners, unless you commute between Vryburg and Upington. In addition to that, one has to consider the difference in tyre sizes. If any one of the four wheels is over- or underinflated, it’s going to put additional stress on the drivetrain. It may take longer, but it will eventually break. As a rule of thumb, 4H on a high traction surface is just not worth it.

Off the tar
A different set of rules applies to gravel roads and off-roading, but there’s still a massive risk of axle binding if you’re not careful. It’s perfectly fine to use 2H on a well-maintained gravel road and even more so when you’re carrying a load on the back. When the rear end starts to go light, it would obviously be a good idea to switch to 4H. This does not, however, mean that you can drive like a spanner.A loose gravel road allows for more slippage, which means there’s less stress on the components, but there’s still a huge risk when cornering hard. In many ways it’s even scarier, because axle binding at speed will lead to understeer.

In addition to damaging the components, you’ll likely have to replace the entire car after the previously mentioned understeer pointed you directly at the nearest tree. As for technical off-road driving, it’s worth remembering to always tackle an obstacle in the straightest line possible. The combination of a steering wheel on full lock and a throttle pedal nailed to the floor can result in the same kind of damage as on tar.

The gravel will still allow some slippage, but the continued stress will weaken the parts until they eventually break. And you definitely don’t want one axle down when you’re halfway up the Road to Hell. It’s also worth mentioning to look out for axle binding when driving in dunes. The speeds are relatively high and sharp turns are often necessary.

Text: Gerhard Horn