The Quaife ATB (Automatic Torque Biasing) differential is a unique type of limited-slip diff that substantially improves traction for powerful vehicles using only gear mechanisms. This clever design revolutionised the differential market - offering a superior level of performance and warranties compared to its competitors.

In 2007, BMW began welding the crown wheel gear to the open differential instead of bolting them together. This means for models such as the F20 M135i and F22 F23 M235i we must now machine the crown wheel gear off, and drill it so that a Quaife ATB LSD can be bolted on in the open differential's place. This is specialist work that we only offer to be done in house. We offer two methods of upgrading - you can send up your differential for the retrofit to be performed or we offer an exchange program whereby we send you a pre-built differential with the Quaife ATB unit already inside.

An Exchange Quaife Limited Slip Differential is a complete BMW differential equipped with a Quaife ATB LSD. We offer these in exchange for your original differential so that you avoid the lengthy downtime and complications of a Quaife ATB installation. However, this is only offered to you if your vehicle has done less than 60,000kms and if it's done more you will need to send your differential in for conversion. SouthernBM guarantees the Exchange Quaife Limited Slip Differentials for 2 years, and the Quaife ATB itself comes with a limited lifetime warranty. The total cost is inclusive of delivery of the Exchange Quaife Limited Slip Differential and the return of your standard unit.
 
The Exchange Quaife LSD unit is also sold with an additional $500 core charge (not included in the purchase price). We will send you an invoice for this charge once your pre-built exchange differential is prepared and ready to send.  This amount will then be refunded upon the return of your standard differential to us in good working order.

Exchange Quaife Limited Slip Differentials are only available for certain models. If we don't have any exchange differential available to suit your car you will need to send us your differential for retrofitting.

If your BMW has done more than 60,000kms or if you would prefer to have your own differential retrofitted with the Quaife ATB unit you can send it to us for converting. Depending on your location, the process can take 5-15 days including freight transit time. The cost is also fully inclusive of freight to bring your standard differential to us and the return. All you need to do is organise the removal of your differential (at your own cost) and we can take care of the rest.

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All standard BMW models come from the factory fitted with the basic "open differential" - a simple device that allows the driving wheels to turn at different speeds (enabling the car to navigate around corners). The problem with the open diff is that it can only ever apply the same amount of torque to both driving wheels, so when a wheel slips, it will only put through the amount of torque it requires to power that spinning wheel (very little!). The car spins away power in these instances.

Additionally, BMW use various electronic traction systems that "curb" power to the drive wheels during traction loss (indicated by the flashing traction light on your dashboard). The Dynamic Stability Control system (DSC) is designed to "curb the engine output and stop slip on the wheels", a good feature in theory, but is still in its early stages of development and is way too intrusive, often frustrating the driver when trying to push on. If you accelerate hard from slow speeds, or try to power out of turns (especially in slippery conditions), the loss of traction will make the engine sap power away from the wheels. Turbocharged rear-wheel drive cars with high torque outputs suffer the most, such as BMW's M135i, and conditions are worsened by the fact they are equipped with run-flat tyres and stiff suspensions. Of course if you are lucky enough to have sufficiently deep pockets, all BMW M-Powers come from the factory with a mechanical limited-slip differential, demonstrating that even BMW themselves know that powerful cars need these important devices.

Fortunately, there are clever differentials out there on the aftermarket that will transfer torque to the wheel with more traction rather than take it away... 

The Quaife unit uses sets of floating helical cut gear pinions that run in pockets and mesh during normal driving, acting as an open diff. Should one of the driving wheels start to spin however, the helical gears start to generate a torque bias thanks to the axial and radial thrust of the helical gear pinions in their pockets. The result is an instantaneous and progressive transfer of torque away from the spinning wheel to the one with more traction, always varying accordingly and automatically.

With the DSC and traction systems switched off during traction loss, you will feel the Quaife transferring the power to the wheel with more grip in a smooth and predictable manner. Any movement at the rear end is controlled and intuitive for the driver. With the traction systems switched on, you will rarely ever see the light flicker on your dashboard as the Quaife allows you to power through any loss of traction, working in perfect harmony with all of the BMW's standard equipment.

The car ultimately becomes easier to control and is able to apply its power to the tarmac with maximum effect. Acceleration, cornering speeds, handling, steering, and overall dynamics of the car are dramatically improved! The Quaife is capable of varying the torque split from 0-80%, and does this quietly and smoothly in operation. It does not affect any of the traction systems on the car, and does not affect the manufacturer's warranty. All backed by its own worldwide lifetime guarantee.

Watch BBC Top Gear's Chris Harris demonstrate the Quaife ATB in our 335i during his Autocar days.

The Quaife ATB Differential provides significantly improved traction and handling for powerful rear-wheel BMWs.

Once installed, drivers will notice an immediate difference as the Quaife automatically biases torque across the axle to the wheel with most grip. Unwanted wheel-slip is greatly reduced, enabling drivers to exploit the full potential of their machines for the first time, leading to a more exciting, rewarding and rapid driving experience.

Widely regarded as the differential upgrade of choice for road car owners, the Quaife ATB was designed to overcome the flaws of conventional clutch-plate limited-slip diffs. Quaife's patented design;

'Clutch-plate' limited-slip diffs (BMW M Performance, Drexler, OS Giken, etc) operate using an internal spring pack and set of clutches that limit the amount of rotational difference between the driving wheels in low traction conditions. However, the wearing of clutches means that they require special lubricant changes throughout their lives, and will eventually need replacing. Due to their finite life, many of them can't be guaranteed for more than 12 months.

Additionally if high locking percentages are used they can cause abrupt locking of the rear axle and significant "snap-oversteer" out of slippery turns. While these characteristics might be beneficial in motorsport, they aren't ideal for road use.

The Quaife ATB Differential however uses a unique, non-wearing gear design that maximises traction by automatically biasing torque across the axle depending on which wheel is enjoying the most grip. The result in performance is huge amounts of grip, but in comparison to clutch-plate diffs offers an intuitively controllable rear end during any moments of oversteer. 

There are no clutches, ramps, springs or other wear prone components to replace at regular intervals, because the Quaife ATB’s well-proven system of helical gears is extremely durable. This makes the Quaife ATB a genuine 'fit and forget' upgrade that offers compromise-free performance and unbeatable reliability on the street, strip, stage or circuit, but most importantly, it can be accompanied with Quaife's famous limited lifetime warranty.

The Quaife ATB Limited-Slip Differential comes with a limited lifetime warranty. If damage is caused to the Quaife due to faulty installation or overheating, the warranty may be voided and repair costs charged.

Our Exchange Quaife Limited Slip Differentials include an additional 24 months warranty on all other final drive components, such as bearings and seals, or if the car is new we will match your manufacturer's warranty.

The Quaife ATB will not affect your manufacturer's warranty. Over 3,000 Quaife ATB differentials for BMWs have been sold worldwide, and not one of these has resulted in BMW claiming a fault or defect was as a result of the Quaife installation.