The VLSD was quite a new concept when Mazda took it on board for the development of the MX5, Mazda has a habit of taking quirky and new ideas (see Wankel Engine) and running with them, making them their own. In this case the VLSD’s remit was to improve the vehicles ability to get un-stuck and to give the car better drive ability all year round.
This is an excellent business strategy, in a car with so little in the way of driver aids anything to help the average middle class customer keep the car moving and not succumbing to the issues of an open diff while at the same time not fully committing to the aggressive nature of a proper locking differential.
However, the VLSD was never engineered as a Motorsport solution, or even realistically hard road or trackday use. The reason for this is simply in the design, the VLSD operates its locking action by the viscous fluid’s shear force, that is to say that it has clutch packs on either side with fluid suspended between the plates and depending on how the fluid resists the twisting force, is what causes the differential to transmit power.
The units themselves are sealed, meaning you cannot replace or renew the silicone based fluid held between these clutch plates. The misconception here is changing to a different weight or more premium brand of gear oil can help these units – with testing we can see that it really has very little effect on the unit at all in this respect.
So, as these units were made for the original 1.6 cars, they are all getting on for over 25 years old now and we can imagine that the fluid is past its best. Considering that the specification for the VLSD brand new could allow 10-20% of lock, this really doesn’t come close to a Torsen or ATB style differential at up to 50/50 or a 1.5/2 way Clutch differential.
Keep in mind that these units, being 25 years old, will have backlash issues as the preload on the bearings changes with age and wear. That’s what gives you the characteristic whine and judder on and off power.
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