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How auto lockers work

24 December 2016

  • When one wheel finds itself in the air, a diff lock become a rather essential off-road item.
  • An auto locker replaces the standard gears in a vehicle’s differential housing, allowing the diff to be locked automatically, thereby letting a wheel turn, even if its partner is spinning in the air.

Thinking about installing an aftermarket differential lock on your 4×4? One alternative – which is often quite affordable – is an auto locker. Well, what is an auto locker, and how does it work?

In our November issue (issue 151, page 98), we spoke about differentials and diff locks. We looked at how diff locks work, and why they are so useful in off-road situations. While we spoke about limited-slip differentials, which could arguably be called a kind of automatic diff lock, we didn’t talk about auto lockers in general. So let’s take a moment to discuss them here.

The aftermarket world of diff locks
Not all 4×4s come with diff locks. In fact, they are becoming less common in modern vehicles, but thankfully you can fit an aftermarket model, should you wish to engage in some serious off-road action. A very popular option is an air locker (ARB makes a very good one that you can have a look at online). As its name suggests, an air locker is a diff lock that can be engaged with the help of a compressor. The compressor sends air into a pneumatically operated locking system inside the differential. Once installed, it works almost exactly like a factory-fitted differential lock. You press a button, and the diff lock is engaged. The downside of such a system should be obvious: it is complex, which means that it is expensive to purchase and tricky to install.

If you want an air locker, you need to buy a quality product from a reputable dealer. A cheap alternative will only let you down when you need it most. Another option is an electronic locker. A good example is the Harrop Eaton ELocker, which has an electromagnetic locking mechanism. Much like an OEM locker or air locker, the lock is engaged with the help of a switch. The advantage of an electronic locker is that it doesn’t require airlines or a compressor like an air locker, but it is still a complex unit that’s expensive. Moreover, electronic lockers are not easily found in South Africa. If you want one, you’ll probably need to import it.

Recapping the differential
Now we come to the auto locker, which is a popular alternative to the differential locks mentioned above. What is an auto locker? Well, it is a diff lock that engages and disengages on its own, requiring zero input from the driver. To understand how it works, we need to briefly discuss how a differential works. We discussed this in detail last month, but it’s worth recapping. While driving in a straight line, all your wheels are turning at the same speed, but this changes when you reach a corner.

When turning sharply to the right, for example, your right wheels will have less distance to travel than your left wheels, since the arc made by your right wheels is much tighter than that made by your left. This means that your outside (left) wheels need to turn faster than your right wheels in order to complete a smooth turn, and that’s where the differential comes in. With a solid shaft between your wheels, a wheel will inevitably slip while completing a turn. A differential, however, has something called spider gears, which allow the wheels to turn independently, at different speeds.

A differential always sends the same amount of torque to each wheel, but thanks to how the diff works, the amount of torque that actually gets transferred depends on the resistance. An open differential allows power from the vehicle’s engine to follow the path of least resistance. So when one wheel is in the air, the power from the engine ‘escapes’ via this wheel, with almost nothing being transferred to the wheel that’s on the ground. A diff lock works by locking the two wheels together, forcing the same amount of torque to be transferred to each wheel. So instead of one wheel spinning uselessly and the other not moving at all, an equal amount of torque is transferred to both.

The auto locker
What you should notice from the above is that a differential comes into play only when a vehicle is turning. When a car is travelling in a straight line, the wheels all turn at the same speed, so there’s no need for the differential to do anything. This means that a differential can safely be locked when a vehicle is moving in a straight line (regardless of surface), and needs to be unlocked only when turning. An auto locker takes advantage of this fact.

1. Detroit lockers
The first auto locker we’ll look at is what is commonly referred to as the Detroit Locker. While it’s called an auto locker, a more accurate term would actually be an auto ‘unlocker’. That’s because it keeps the wheels locked together, and disengages only when the vehicle turns. It doesn’t allow a wheel to spin slower than the differential carrier, but it will let it spin faster. So when an outside wheel needs more speed during turning, the locker disengages and allows the wheels to spin at different speeds. So with this auto locker, you are almost always driving with your diff lock on, except when you’re turning. When fitting a Detroit locker, the entire differential carrier assembly is replaced.

2. Lunchbox lockers
A second kind of auto locker is the lunchbox locker. It operates in very similar fashion to a Detroit locker, but is different in that it makes use of a vehicle’s standard differential carrier, and just replaces the gears inside. The name comes from the idea that this process is similar to the way in which a lunchbox is used: you use the same lunchbox every day; all that changes is the content. Because it makes use of the stock differential carrier, a lunchbox locker tends to offer great value for money.

Pros and cons
So what’s not to like about an auto locker? It tends to be cheaper than the alternatives, and requires no input from you, so why opt for anything else? Well, sure, there is a lot to like about an auto locker, but it also has some drawbacks.

The good Auto lockers are quite affordable and don’t need to be engaged (and disengaged) manually, which means you’ll never forget to put it on before tackling an obstacle, or disengage it afterwards.

The bad You’ll probably notice increased tyre wear (we’ll explain why in the point below). You’ll have no control over when the diff lock engages and disengages, which can be problematic when tackling very tricky off-road obstacles.

The ugly An auto locker can have a dramatic effect on a vehicle’s driving behaviour. It’s usually less noticeable in a vehicle with a long wheelbase (LWB), but in short wheelbase (SWB) vehicles it can be downright scary at times. While turning, your outside wheels will spin faster than your inside ones (this, after all, is the entire purpose of a differential). But because of how an auto locker works, when torque is suddenly applied to it in a turn (by, for example, switching gears) it can disengage the locker.

This can cause it to behave quite unpredictably. It might understeer heavily (hence the tyre wear), or jerk to one side. This is the biggest reason to give an auto locker a skip. As mentioned, it isn’t as likely to be a major issue if your drive a LWB 4×4. But in a SWB, you’ll notice it quickly.

Text: GG van Rooyen