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How transfer cases work

18 April 2017

The old-school secondary lever for the transfer case has largely disappeared from modern SUVs. These days, buttons are far more common.

A case of transference

Most serious 4×4s have transfer cases, but what are they, and how do they actually work?

Once upon a time, serious 4×4s had two shifters. There was your normal shifter for navigating through your vehicle’s gears, and there was the second, smaller one, which you used to put your vehicle in four-wheel drive and engage low range.  These days, well, things are different. That second little lever is pretty much gone. Hardly any vehicles still come with one. For the most part, you select 4WD and low range with the push of a button. Permanent 4WD vehicles only have a button for low range, and sometimes one that will allow you to lock 4WD. But something that hasn’t disappeared is a transfer case. It’s still essential. Back when you selected 4WD and low range manually and mechanically, though, it was a far more noticeable cog in the 4×4 machine. You felt that 4WD engage with an audible ‘clunk’.

A more modern (and far more complex) setup from an AWD vehicle.

Selectable 4WD
Let’s start by looking at an old-school transfer case that takes a vehicle from 2WD to 4WD. As the name suggests, it transfers power from just the rear wheels to the front ones, too. You have a transmission input shaft that sends power into the transfer case. When you’re in 2WD, all power is sent to a rear output shaft, which ultimately sends power to the rear wheels.

When you engage 4WD, a chain drive inside the transfer case is activated that now also sends power to the front wheels via a front output shaft. When you further engage low range, a set of gears in the case is engaged that reduces the gear ratios, which multiplies the torque, giving you more power to work with when crawling over rocks in first or second. A good example of this kind of set-up is your typical bakkie. It’s rear-wheel drive only until you put it in 4WD. The transfer case is used to send power to the front wheels, and to reduce gearing, if needed.

Permanent 4WD
Permanent 4WD is different from selectable 4WD in that it doesn’t have a transfer case that allows you to manually send power to all four wheels. Instead, the case permanently sends power to all the wheels. As we’ve mentioned before, though, you don’t want equal power to be sent to all the wheels when driving on tarmac. You want your wheels to spin at different speeds when your turn in order to prevent damage to the vehicle and the tyres. So vehicles with permanent 4WD have some sort of system that allows the wheels to turn at different speeds, usually differential gears or a viscous coupling. Because of this, there’s usually some way to lock the wheels and send equal power in all directions when venturing off-road. A good example here is the Land Rover Defender. It’s permanent 4WD, but has a manually lockable centre differential for off-road driving. Once you engage that, equal power is sent to all wheels. The transfer case then also allows you to engage low-range.

AWD systems
The problem with a transfer case is the fact that it is a big and heavy item. This is okay in a large SUV, but it’s a problem in smaller vehicles. Because of this, many smaller SUVs (and even some cars) have a gearbox/differential/transfer case combination that is very different from your traditional system. A good example is Audi’s quattro system. Instead of having a gearbox, transfer case and differentials on both axles, quattro makes use of a system that is embedded in the gearbox and uses a centre differential to split torque between the front and rear.

A traditional part-time Borg Warner transfer case that you’d find in a bakkie.

How long-range gearing works
A transfer case allows you to put your 4×4 into low range, which multiplies torque for slow off-road driving. But what exactly does this mean? If you’re into cycling, this whole process is much easier to understand. You’ll know what it feels like when you try to climb a steep hill in the wrong gear. It’s almost impossible. Put a small gear with less teeth behind a larger one with more teeth, and it has to turn faster to keep up. This is how you can get your bicycle to move much faster than the pedals. Through this transfer, you increase speed. Put a bigger gear behind a smaller one, though, and you do the opposite. The second gear is moving slower than the first one, but it is turning with more force. So you’re reducing speed, but increasing force. This is how low range in a vehicle works. The input speed is being dropped way down through the reduction gears, but torque is being multiplied. This is ideal for serious off-road situations, where you want to move very slowly but have lots of torque.

Text: GG van Rooyen