They may have the slowest solo lap times, but they’re some of the trickest motorcycles you can find at the Isle of Man TT. The Supertwins may only be sub-700cc twins based on regular road bikes, but they are heavily modified to increase power to around 100bhp, reduce weight to a minimum of just 150kg and to make them as aerodynamic as possible for maximum speed down the Mountain Course’s long, fast straights.
As a class, Supertwins have been around the club racing scene for quite a few years. Until 2022, the Isle of Man ran its own, slightly different regulations, and called the class lightweight – paying homage to the historic class which was first run in 1922 and which had last been run using 125cc Grand Prix bikes, when it was phased out after the 2009 event.
Last year the class was given a revamp, to reflect the new parallel twin motorcycles being sold today. Machines like the Aprilia RS 660 and Yamaha MT-07/R7 were not eligible for the Lightweight TT, owing to engine capacities just over the 650cc maximum mandated by the previous regulations.
Which bikes are used for Supertwin racing?
Kawasaki-powered machines have dominated the Supertwins since the class was introduced, as the Lightweight TT, in 2012. Ryan Farquhar won the first Lightweight TT under the new rules using a race bike based upon a humble Kawasaki ER-6. Kawasakis won the first five ‘new’ Lightweights, while the past four events have seen the Italian Paton dominate. The Paton is a high-spec sports bike, which uses the ubiquitous Kawasaki motor at its heart, but has unique retro styling, which pays homage to the Italian’s Grand Prix racers from the 1960s.
Last year saw the Aprilia RS 660 join the grid for the first time, with Lee Johnston finishing second to Paton-mounted Peter Hickman, despite a lack of development time with the bike. It was expected that the Italian bikes would provide a sterner challenge to older Kawasaki-based designs but, with Johnston missing TT’23 through injury, there has been a lack of big name riders on Aprilias this year.
Yamaha’s new R7 makes its TT debut in this year’s Carole Nash Supertwin TTs. Michael Rutter and last year’s Supertwin winner, Peter Hickman, are running the 689cc Yamahas, in what is commonly accepted as a developmental year for the machine. Much of the engine tuning knowledge for the Aprilias and Yamahas is coming from the United States, where the ‘Twins Cup’ class is proving popular as part of the MotoAmerica race series, and the late arrival of Stateside development parts means that the R7 is not quite able to match the Kawasakis for top speed this time around.
As well as the top runners, there are still a few privateers running Suzuki SV650s in the TT Supertwins class. These remain the mainstay of Supertwin racing on the club scene and, although lacking the competitiveness of the newer designs, add a pinch of V-twin flavour to this intriguing class.
What modifications are made to Supertwins?
One-offs and prototype machines are not allowed in Supertwins, meaning that the racers blasting around the TT course must be based upon motorcycles you can buy in a showroom.
Beyond that, the number of modifications which can be made are staggering. You name it, and you can probably do it – Supertwins has the freest set of regulations of any Isle of Man TT class. The standard frame and engine must be used, but can be heavily modified. The engine’s bore and stroke must remain as homologated but, beyond that, more or less anything goes. Gearboxes and clutches can be changed for racing type items, while the cylinder heads and camshafts are tuned. All engines have a rev limit of 11,000rpm, but the sub 651cc machines (namely the Kawasakis) benefit from being able to change or modify the throttle bodies, while the bigger capacity Aprilias and Yamahas must run stock items.
On the chassis front, brakes, suspension and steering dampers can be changed, with teams generally utilising items from the superbikes like the Kawasaki ZX-10R or fitting pure racing components. Swingarms can be changed, as long as they are made by the same manufacturer (for example a Kawasaki ZX-6R swingarm could theoretically be modified and fitted to a Z650 Supertwin), while lightweight wheels and racing tyres are used.
Where modern sportsbike designs are moving towards smaller frontal areas and aerodynamics designed to generate downforce, Supertwin teams fit big fairings aimed at maximising top speed down the long TT straights. Fairings can be changed, or added in the case of naked bikes like the Kawasaki Z650 and Yamaha MT-07, with most of the teams fitting bodywork originally developed for 250cc Grand Prix bikes. As one crew chief told us ‘those were the last bikes where they really designed fairing for being in the slipstream, and that’s exactly what we want here at the TT.” Fuel tanks are also changed, with a 20-litre capacity allowed – up from the 13-15 litre items found on the homologated base models.
Of course, these modifications don’t come cheap. A brand-new Paton S1-R road bike costs in excess of £40,000, while a race prepared Kawasaki or Yamaha will typically have in excess of £20,000 of work spent on it in order to challenge for TT honours.
The results, however, are staggering, with power rising from just under 70bhp to around 100bhp – with weight dropping from around 190kg for the street bike to a waif-like 150kg for the racer. It all adds up to a motorcycle capable of lapping the iconic Isle of Man TT course at an average of over 122mph. Not bad for a motorcycle which started life as a humble road bike.
What’s a Supertwin video
