Car Advice

What is a locking differential and what is in the front of the Golf GTi 40 Years Edition?

A “locking differential” is something we’re seeing more and more in the front axle of front-driven cars. But what is it, and why do we need one?

Here’s the summary, or the TL;DR version – the Golf GTi 40 Years Edition has magic in the front axle that helps traction and handling. It works. If you want to know how it drives, click this. If you want to know how it works, keep reading.

OK, so it’s only the car nerds left. Welcome. Let’s cover the basic problem first.

The engine drives a car’s wheels through a series of gears. In a straight line, that’s pretty simple. But when you turn a corner the outside wheel travels further than the inside wheel, and therefore the outside wheels speed up relative to the inside wheel. Yet somehow, both wheels still need to be driven.

The solution is a differential, a complex series of gears that allows both wheels on an axle to be driven, but at differing speeds. Without a differential cars would be hard to turn around corners, scrubbing the inside tyre and understeering like a…well, insert your favourite understeer metaphor of choice.

So differentials are wondrous devices, but suffer a huge problem when performance driving; they’re lazy, sending drive to the wheel that’s easiest to turn. They will readily spin an inside wheel when there’s not much weight on it, such as when you’re cornering hard. This is the “one tyre fire” you see from time to time, and it’s pretty hopeless for traction as the outside wheel, the one with traction, gets no love and the engine’s torque is largely wasted.

This differential problem has become more and more acute as power increases, and with the Golf’s 195kW (213kW on overboost) it’s a real problem. VW’s solution is not new and is representative of other manufacturers, so let’s take a look at what they’ve done with the Golf and translate a bit of over-zealous marketese into reality:

Electronic Differential Lock (EDL)

VW claim that the Golf has an Electronic Differential Lock. It doesn’t. This so-called EDL is nothing more than brake traction control; the car monitors the relative wheel speeds of the two front wheels, and when one of them is spinning more than it should – allowing for the turning angle, throttle position, yaw rate and so on – then the computers lightly apply the brakes to that spinning wheel, which has the effect of increasing torque to the other wheel which has better traction. The “EDL” system works in a straight line and on corners. It cannot lock the front differential, which is why it is mis-named. An electronic limited-slip differential would be closer to the mark, but even better would be “brake traction control” which is the proper name for it.

Extended Electronic Differential Lock (XDL)

The marketing people got carried away here. It’s the same basic mechanism as the EDL, but more focused on pre-emptive wheelspin or wheel slip when cornering. To use VW’s words: “When cornering, XDL responds to the load relief [ lighter weight ] at the driven wheel on the inside of a corner. The ESP hydraulics are used for the XDL to apply pressure to the wheel on the inside of the corner in order to prevent wheel spin. This improves traction and reduces the tendency to understeer. As a direct result of the one-sided and precise braking pressure, cornering is sportier and more accurate.”

The XDL would certainly help traction by reducing inside-wheel spin, but I’m not convinced it’d be all that great for understeer, based on the description of how the EDL system works and experience with similar systems. The reason is that differentials balance torque across both wheels, even when something like XDL intervenes, and what helps with understeer is asymmetric torque; more on the outside wheel, less on the inside wheel. Certainly the XDL would help with traction by reducing inside-wheel spin, but that’s not the same as reducing understeer. So it looks like XDL is a special programme of brake traction control for cornering.

Front Differential Lock

Wow, those VW marketing people sure love the term “differential lock”! On the third try we’re actually close to a proper differential lock. VW’s description is: “the front differential lock is an electronically controlled mechanical differential lock. The drive torque is distributed via an electronically operated clutch in accordance with the driving situation. The differential lock actively counteracts understeer when cornering by increasing the drive force to the wheel on the outside of the bend. This provides more agility and precision on curves, and a more linear driving response. The front differential lock also provides maximum traction for front-wheel-drive vehicles.”

So what’s happening here is true torque distribution, if we take “drive force” literally, not something you always trust press releases to get right. That means we now have what XDL didn’t deliver, which is more torque (turning force) going to the outside wheel than the inside wheel. This will definitely help with understeer, and traction. Now is it a locking differential? It could be, if it’s capable of entirely locking out the front differential so both front wheels on the axle are forced to turn at the same speed. However, the chances of needing to lock up that far are remote and it wouldn’t be for very long, so it’s more of a theoretical feature than a realistic one.

The Front Differential Lock also appears to be computer-controlled, so it’s not a purely mechanical device like a helical limited-slip differential you find in the front of the Evo and, we think, the AMG A45. As this “front differential lock” is computer controlled I’d say it could fairly be called a torque vectoring system which you would have thought is a cool enough name for marketing types. Either way, it’s a very good bit of kit and would definitely help with everything VW claims about agility and precision – read more about how it drives in the link below.

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Robert Pepper

Robert Pepper