High performance diff and differential selection

Find out about "Slip Differentials"

As the car turns on a radius the outside wheel has a greater distance to travel compared with the inner wheel.

If both wheels rotated at the same rate the car would skid or at the very least you would lose traction and continue on a straight line. The diff or differential is the component which manages the amount of slip between the wheels.

In a normal car the diff sends more power to the wheel spinning the fastest. This is fine until you hit a low traction surface such as gravel, ice, mud or even a wet road. In these instances the slipping wheel will be given the full amount of power, which is actually the reverse of what you really need.

The diff needs to determine if a wheel is spinning faster because it needs to or if the faster speed is down to a loss of traction. This is actually pretty hard to determine mechanically at least.

The ideal setup then is to proportionalise the power distribution giving a maximum of 75% to a wheel at any one time. This is where the torque sensing diffs come in. Clever diff setups include the semi locking diff which actually locks up in the event of a wheel spinning giving power to both wheels.

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Torque activated diffs (which use a clutch) and torsion diffs are 2 other popular differential types available. The only time you want a fixed or 1:1 diff is for drifting where a loss of traction is actually beneficial.

Does a diff make much difference? In a competitive environment where you are pushing the maximum power it can make a big difference to your lap times. The advantage can mean a car with 120bhp achieving better times than one with 170bhp. You will notice that in corners the car will pull better and be more predictable and stable.

Modern diffs can also control the amount of power delivered to the rear wheels allowing for impressive cornering ability and great traction.

In cars with computerised 4 wheel drive systems there are complex factors used to determine the power distribution. This is why cars such as the Nissan Skyline, Nissan GTR, Subaru Impreza and Mitsubishi Evo are such capable track day cars.

The EVO traction control and diff setup must rate as one of the most complex around. The new GTR although being a large car can put in very competitive lap times even in inexperienced hands.

We will cover the different types of diff on offer, and compare these in another more detailed article.

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