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Case Study: Superfinishing gear sets
PDJ Vibro explains how its finishing machines are lending a performance advantage to British Superbike teams
Established in 1989, Nova Racing is one of a select few companies to supply racing gearboxes to the British superbike grids. With over 25 years experience in both classic and modern racing, Nova has supplied most major motorcycle manufacturers with prototype race gears, and provided some of the largest WSBK manufacturers’ teams with their sole source of gear kits.
At Nova's facility in West Sussex, UK, a pair of vibratory bowl finishing machines from PDJ Vibro is employed to polish nearly all of the gears and shafts, after they have been machined and hardened, in a process referred to as superfinishing.
For those unfamiliar with two-wheel motorsport, superbike racing employs highly-modified production motorcycles, as opposed to MotoGP in which purpose-built motorcycles are used. For a superbike to be eligible to race, it must maintain the same overall appearance as its road-going counterpart, so the frame cannot be modified although elements within it may be, including the gearbox.
“The major production bike manufacturers have their own racing divisions for upgrading their products for road and track racing,” explains Sean Whittaker, director at Nova Racing. “However they tend to charge a lot, supply can be intermittent and the parts are not necessarily optimised. We are constantly innovating, and are currently introducing a dog ring-type gearbox that meets the superbike championship regulations, yet provides faster and more reliable shifting than conventional gearboxes.”
Before Nova installed its own vibratory finishing bowls, only 30% of the certified motorsport steel components underwent the treatment due to the high costs associated with sending them out for finishing. Raising the proportion of superfinished components to nearly 100% has resulted in a major improvement in the quality, appearance and reliability of the company’s products.
“Superfinishing is becoming very popular in the motorcycle world,” continues Whittaker. “Oil quenching of parts during heat treatment means that the various components come a different colour, ranging from light brown to green-grey to yellowish. So the assembled gearbox tends to look non-uniform, which detracts from its appearance. After superfinishing, all the component parts have the same, highly polished appearance and the engraving shows up better as well.
“There is no doubt that the gearboxes last longer, as the period between our supplying a gearbox and getting it back for refurbishment has been extended,” he adds. “It is because there is less wear on the gear teeth and reduced friction between the bearings and the hard-turned diameters on the input and output shafts.
“Anecdotal feedback from some of our customers suggests that they experience a small increase in power when racing, although this is difficult to verify and impossible to quantify.
“A further benefit of having polished moving gearbox parts is that the time needed for running-in is shorter. Few bikers adhere to the recommended period anyway and some ‘pin it’ on the track immediately, so having a gearbox that requires less running-in is a big advantage.”
Choice of superfinishing process
When Nova’s directors decided to bring superfinishing in-house, a quick search led immediately to PDJ Vibro’s website. Nova was keen to avoid processes that use chemicals, such as the acidic isotropic method used by its previous superfinishing service provider.
One drawback is the expense associated with disposing of the chemicals after use. Another is the tendency of the treatment to erode the metal surfaces and alter component tolerances, which Nova enforces to ± 5 microns. PDJ Vibro recommended the porcelain and paste process, as it polishes surfaces but the amount of material removed is negligible. This has been proved by Nova as, in addition to superfinishing bare metal surfaces, it polishes some components that have been tuftrided rather than induction hardened. Initial testing has confirmed that there is no discernible change to the five-micron thick tuftride layer after they emerge from the vibratory bowl.
Further evidence of the microscopically small amount of metal removed by the porcelain medium is provided by very fine machining marks, such as a single-point turned finish on a shaft, that are still visible on the surface of the metal after superfinishing.
A batch of parts is processed for five hours in one of the two 100-litre capacity bowls, which is filled with two sizes of ceramic medium to optimize finishing over the entire surface of the components. A recirculating water and detergent mixture prevents oxidation of the steel. Afterwards, the components are transferred to the second bowl containing smaller porcelain stones, where they spend 16 hours overnight being polished, again in a water and detergent mixture.
“The whole superfinishing process has proved a great success. It means that all of our manufacture is now in-house, except for heat treatment, allowing us to keep close control over quality and delivery,” Whittaker concludes. “We can react very quickly to race situations and can produce components in two weeks, rather than the six months that most major manufacturers need.”