Essential differential

Written by: Tom Shelley | Published:

Some striking developments in one of the main types of differential used in motor sport have caught the eye of Tom Shelley

Some striking developments in one of the main types of differential used in motor sport have caught the eye of Tom Shelley

A limited slip differential for chain-driven racing cars in the US - which resemble Formula 1 cars, but are derived from motorcycles - is winning races by taking an idea from sports cars of yesteryear and improving on this.

The basic idea is called the ‘Salisbury differential’, which uses clutch packs to create friction between the rear wheel axles when they rotate at different speeds. The problem with the simple system is that, if the clutches are pre-loaded, they introduce friction and heat generation in the differential when the car is taking a corner - even if there is no risk of wheel spin.

The new product was conceived and manufactured by Lee Williams, a former British race car engineer now running Williams Racing Developments in California. “What I have done is to modify the standard Salisbury design to control the separating forces of the gears and allow us to run the locking portion of the differential independently of the differential gears themselves,” states Williams. “This allows us to run the differential with zero preload and hence completely open when there is no torque applied to the unit. As torque is applied, the ramp angles, which can be changed, apply a load to the clutches in proportion to the torque applied by the engine, with no interference from any other factors.”

Williams’ offering is very much an improvement on the Salisbury differential, aimed at two classes popular in the US - called D Sports Racer (DSR) and F1000 - both of which use 1000 cc motorcycle engines to give extreme performance at relatively low cost. When DSR started, drive was to a solid axle, with no differential at all. One company then modified a Quaife-style differential, an idea invented and patented by Quaife Engineering in Otford, Kent. This has no clutches, but progressively locks the two axles together as torque increases.

Williams says: “We saw the need last year for a better performing differential than the Quaife, lighter in weight than the competition, capable of putting more power to the ground and with a greater range of adjustability to give the competitors even more tuning aids when going to different circuits.”

The fact that he has come up with a solution where the differential is completely open, with no torque applied, means that cars fitted with the WRD unit can turn in and roll through the mid corner a lot more freely than with other preloaded or fixed bias differentials. “For example, a standard preloaded Salisbury may only be able to provide torque bias ratios from four to effectively infinity, whereas the WRD modified differential allows bias ratios from effectively zero to infinity,” he explains. “Couple this with the fact that the locking portion of the differential is now independent of the differential portion itself and the wheel spin control on the inside tyre is much greater.”

This, says Williams, allows the drivers to get on the throttle a lot sooner, without the risk of a lot of inside wheel spin, all of which contributes to much lower lap times. “With the same driver in the same car, we have seen reductions in lap time of 1 second per lap at most circuits.”

The unit that Eureka saw had been analysed and designed by Billy Wight, who runs Luxon Engineering - based in San Diego, California - which working very closely with Williams. Wight used SolidWorks, CosmosWorks and Altair Hypermesh to design the housing and sprocket mount, to produce a product that, in William’s words, is “strong enough for the job”, that offers safety factors of 2.5 in the sprocket mount area and is considerably lighter than the competition.

Pointers

A version of the Salisbury limited slip differential has been developed that, in a class of low-cost race cars, outperforms the Quaife design

Text: The new design runs with zero preload and is thus completely open when there is no torque applied to the unit

Text: As torque is applied, the ramp angles, which can be changed, apply a load to the clutches in proportion to the torque applied by the engine, with no interference from any other factors


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