Tech talk: Know your gearboxes

Nothing’s worse than a duff gearbox so we caught up with Mick and the team at Nova Racing Transmissions to find out how to keep yours sweet and maybe even make it better…

FB: How good is a standard gearbox out of the crate?

MD: On most modern motorcycles from reputable brands, the standard gearboxes are fine for their intended purpose of road use, but many of them have deficiencies when used on circuits. Similarly, many off-road machines have good gearboxes but when pushed to their limits they have some shortcomings.

FB: How can they be improved?

MD: Gearboxes can be improved in several ways. The grade and quality of the material used can increase strength, while ratios can be optimised to keep the engine in its optimum rev range dependent on power, torque curves and application – circuit racing is different to the TT, and so on. From here, you can also change engagement dog geometry and undercut angles to improve shift quality and rider/throttle connection with the rear wheel. If that isn’t enough, superfinishing and coatings can reduce friction, improve shifting and lower surface temperatures giving longer component life. Oil retention on parts is often increased with the improvement in surface finish.

FB: What is the best way to maintain a gearbox?

MD: Regular inspection of the gearbox will help identify any problems. This is easier with a cassette gearbox, as seen on a ZX-10 or MV F4. Using good quality oils, of appropriate viscosities will also help maintain your gearbox. Following the OEM service schedule should keep the oil in good condition, but this can be improved if changed more regularly. Keeping oil temperatures within a sensible range (ideally less than 110°C) is vital, and if the shafts are disassembled, ALWAYS use new circlips when re-assembling the shafts! Ensure that quick shifters are correctly set with sensible cut times.

FB: What is your most popular product and why?

MD: Surprisingly, the BSA Gold Star five-speed, followed closely by Honda TRX450 Quad. Our BSA adds an extra gear within the original housing and the spread of ratios makes it far better than what was offered back in the day of the original four-speed. The TRX450 is a sport/recreational quad that was never designed to cope with the levels of tuning, impacts and loads now seen in top level quad racing. The Nova parts now give the users a full season of quad racing where the OEM parts were being changed multiple times in a season.

FB: What type of bikes suit an upgraded gearbox most?

MD: Modern or classic bikes that are being raced, or off-road bikes that are being used for heavy duty competition or adventure riding… they take the biggest battering.

FB: How long will a Nova gearbox last?

MD: This is entirely dependent on the specific application and the operating conditions and inputs it is subject to. So multiple manufacturers have used Nova products in World Endurance Racing, and many of these sets saw 3000 to 4000 racing kilometres of use. This mileage is equal to or greater than that specified by the OEM manufacturer. When returned for inspection they looked almost brand new, and we are confident they would be good for many more racing kilometres, but the team decided to move these gearboxes to practice engines and dyno development engines, taking new gearsets for future races. Our Moto3 prototypes have done 12,000 racing kilometres before a component failure, whereas the major manufacturers in Moto3 specify a life of 3000km to 5000km for their components.

FB: What is a close ratio gearbox, and what are the benefits?

MD: A close ratio gearbox has top and bottom gear closer together than a standard road gearbox – ‘close’ is a relative term and we represent this as a percentage. Most standard road and off-road gearboxes have a spread of 55-65% while most close ratio gearboxes are in the range of 45-50%. You can find this percentage by (1- (top gear/bottom gear)) x 100. The ratios in between, for circuit racing are usually staggered with decreasing rpm drops to maintain maximum acceleration. Sometimes ratios are specific for a corner or corners, or to lower peak engine rpm in top gear, giving smoother and better acceleration and deceleration.

FB: What does a Low Inertia Transmission assembly do?

MD: The LITA concept is a way of improving the shifting speed and action, within current Superbike rules. The designs remove weight from the sliding gears (5th and 6th output in most cases) so that they can be moved laterally faster, to engage 1st to 4th in shorter times; we’ve seen shift times approximately halved in several superbike applications. This reduction offers very small improvements to acceleration but more importantly keeps the bike more stable during up and down shifts, particularly when the bike is leant over for cornering, while being easier for the rider to perform.

Our light weight approach is being rolled out to other components within the shifting assemblies and drive train components. Ideally we would like to use a ‘dog ring’ gearbox in all our racing gearboxes but many rules prevent this, although it’s common in car racing and Kawasaki’s H2 range. It is also quite common in classic gearboxes e.g. Suzuki T500 and even all of our Moto3 gearboxes (Mahindra and KTM) as the rules allowed this mechanical solution.

A dog ring box will offer further improvements to shifting speed and action compared to a LITA assembly as the shifting components can be made even lighter with no reduction in strength and durability.

FB: How do you create your gearboxes?

MD: The selection of ideal ratios is decided by the application and approximate engine characteristics, along with any specific goals the customer/team/manufacturer wants to achieve. We establish the constraints of this volume including bearing sizes and positions and any surfaces that may impose limits on ratio selection. Careful attention is also paid to the existing lubrication system. For a ratio change of one gear pair we will then choose a ratio as close to the ideal as possible, considering the loads, tooth sizes and resulting sections when bores, splines and engagement dogs are added. When the CAD models and drawings are finalised (using Autodesk Inventor), the parts are then programmed with CAM software (Hypermill). These programs are loaded into the CNC machines and the parts are machined. Gear cutting and broaching of splines are usually the last processes while the parts are ‘soft’. The parts are then heat treated, super-finished if required, and hard turned to bring bores and shafts to their final dimensions. There are many inspection processes for sizing and batch consistency along the way. The parts are subject to a thorough final inspection and packaged and labelled, awaiting dispatch. For complete gear sets, or those needing new selector drums and forks the processes are similar but with more details and complexity to consider.

As told to Carl Stevens