Supermono History

Supermonos are the original modern-day single cylinder race bikes. They were born out of the Sound of Singles racing formula of the 1990s, and have constantly evolved since then. Along with all the safety requirements of the FIM and the ACU, the rules for British Supermono are simple; they stipulate that each bike must be powered by a naturally aspirated single cylinder four-stroke engine, and... that's it. There are two types of bikes used in the Supermono class; the traditional Supermono, and the newer Minimono. Both have various strengths and weaknesses and this leads to a significant amount of technical variety, innovation, and very competitive and interesting racing, as each type of machine performs differently under different circumstance.

What is a Supermono?

The traditional Supermono is based around 250 Grand Prix bike geometry and proportions. They are typically specified as follows:

• Approximately 22° to 24° steering head angle
• Wheelbase in the vicinity of 1310mm to 1380mm
• 3.5” x 17” front wheel and 5.5” x 17” rear wheel, running on 250GP slicks (or intermediates or full wets in the event of rain)
• A seat height of approximately of 775mm to 795mm

Supermonos run larger displacement engines than the majority of Minimonos and this means that they have higher power and torque outputs. This allows the Supermonos to get out of the corners faster; particularly slower, tighter ones. It also means that they generally have a higher top speed and consequently an advantage at tracks with longer straights or with long uphill sections.

However, the extra power and greater weight means that larger wheels and tyres are required. Although this maximises the traction out of corners, wider tyre profiles combined with their greater mass and rotating inertia make a Supermono less manoeuvrable through technical or fast flowing sections of track. Braking distances into corners are also increased. However, being based on 250GP dimensions means that a Supermono can more easily accommodate a larger rider with minimal changes to handlebars, seat height and foot peg position.

What is a Minimono?

As the name suggests, a Minimono is a smaller race bike, and they are generally based around 125 Grand Prix bike geometry and proportions. They are typically specified as follows:

• Approximately 22° to 24° steering head angle
• Wheelbase in the vicinity of 1215mm to 1270mm
• 2.5” x 17” front wheel and 3.5” x 17” rear wheel, running on 125GP slicks (or intermediates or full wets in the event of rain) from any of the tyre companies
• A seat height of approximately of 700mm to 730mm

In most ways, the Minimono is the antithesis of the Supermono. They run smaller, higher speed/lower torque engines, and they have less mass, meaning that smaller 125GP wheels and tyres can be used. The combination of these characteristics enables them to brake later into turns and, particularly through fast corners, carry more speed. They are more manoeuvrable and the lower weight allows for faster acceleration than the Supermonos out of faster, flowing turns.

However, the relatively low engine output power means that tracks with long straights, preceded by tighter corners and with uphill sections, put Minimonos at a disadvantage. Being based around 125 proportions also means that the bike may be physically too small for some riders to fit on. Larger riders also contribute more to the all-up weight of the package, and therefore influence the power to weight ratio more than if they were riding a Supermono.

What are the possible combinations and specifications and costs of each?

With little restriction from rules, there are many possible combinations of components that can be used to build a Supermono! However, there are obviously some combinations that will work better than others, and when looking to build a Supermono there are a few things to consider:
The first thing is the choice of engine, and the later generation of Supermonos have been built around a range of engines. These include the BMW F650 series, Yamaha 660 engines from XTZs and SZRs, and the Suzuki DR 650/750 series. Depending on their state of tune these produce between 65 and 85BHP. Many of these engines have had their capacity increased, and in race form range from approximately 670 to 800cc. Other engines that could be used are KTM’s 690 or LC4 series, Honda XR series, and the many Rotax single cylinder engines that are available. There are others, such as Husqvarna, Husaberg, and TM, which have been used. However, thorough research is recommended before selecting an engine for your project.

The engine choice will obviously have significant influence on the frame choice and/or design. Initially the engine will govern the chain run and provision for cooling i.e. radiators. For Supermonos, both custom-built and modified production frames can be used, and there have been many custom frames built for Supermonos over the years by a number of companies. These include Tigcraft, Harris, Spondon, GRC, Over, NWS, MHD, and Dieterman. However, not all of these companies continue to build frames, but existing frames, if they can be found, are a good basis for the beginnings of a bike. The other option is to modify an existing GP bike frame, and common GP frames that have been used in the past have come from 1995-present Yamaha TZ250 and Honda RS250 1994-present bikes. 1994-2000 Hondas have a single-sided swingarm, and 2001-present have a conventional double-sided swingarm. The TZs are generally used for left-hand chain drives and the Honda RSs are generally used for right-hand chain drives (which are more prevalent in European OEM engines). This is not to say that this has always been the case, as a few bikes have been assembled with the drive line swapped over through the swingarm. A more exotic and expensive alternative to the TZ and RS would be to source an Aprilia RSW250 GP frame, which has a right hand chain drive swingarm.

Production frames (and in a few cases entire bikes) that can also be used as the basis for the chassis include, but are not limited to:

• Aprilia RS250, RS125, AF1
• Suzuki RGV250
• Yamaha TZR 125 & 250, SZR660 (entire bike is possible)
• Cagiva Mito
• MZ Skorpion (entire bike is possible)
• Bimota BB1 (entire bike)
• Ducati Supermono (rare and relatively expensive)

As stated, this list is not exhaustive, and there are many other frames that could be used, possibly those from 400cc production machines, for example. However, some measuring and offering-up of the engine to the donor frame is required with any proposed frame/engine combination before work should commence.

Once the frame has been determined, the selection of running gear may, due to selection of a production frame, easily present itself. The major running gear components include the wheels, brakes, swingarm and linkage (if applicable), shock absorber, front forks and rear sub frame. In addition to this there are the smaller details of the handlebars and controls, foot rest hangers, foot pegs and foot controls, fairing mounts and the bodywork to wrap the whole package. If possible, using running gear from 250GP bikes is a good option as high quality, light weight components are typically employed in these types of machines. Alternatively, components used on recent Supersport bikes are of good quality and similar proportions to 250GP items, albeit slightly heavier in most cases, and there is a wide range of aftermarket race replacement or upgrade parts for both 250 and Supersport machinery. Of course, there is no reason that older or simpler components cannot be used. A well prepared and reliable bike using older technology can still be competitive and a large investment is not the only way to achieve success and satisfaction in Supermono. The half dry weight of a Supermono (i.e. with all fluids except fuel) ranges from approximately 120kg to 140kg depending on the quality of components used. This gives a potential power to weight ratio ranging from 0.46 to 0.71 BHP/kg (not including rider). The cost of a Supermono can range from £1500 to £18000 for a top line bike, though there is no real upper limit as that is down to the individual, team or company assembling or designing the bike.

Constructing a Minimono requires a similar approach, but before embarking on a build it is probably a good idea to look at the types of Minimonos that have so far been constructed. First, there is the original Minimono; this consists of a 450cc motocross engine, sometimes bored and/or stroked slightly, with power outputs ranging from approximately 47 to 65BHP depending on the amount of tuning. The second type is sometimes referred to as a ‘Micromono’; these use 250cc motocross engines, which have outputs from 37 to 45BHP. The potential advantage of this configuration is that as the engine typically makes up such a large proportion of the bike mass, a significantly lower weight can be more easily achieved by employing such a small engine. The third option is sometimes referred to as a ‘Midimono’. This is a ‘Mini’ rolling chassis with a 450cc to approximately 660cc engine, such as the Rotax 604, Honda XR650, KTM’s 610 and the new KTM 690 series (653cc), having power outputs from approximately 55 to 75BHP. Sometimes the chassis is slightly ‘stretched’ to give the correct weight distribution once the bigger, heavier engine has been fitted. Consequently this gives a larger rider more room to fit on the bike, whilst still maintaining the agility that is a characteristic of the narrower wheels and tyres.

The main builders of these smaller singles have been Tigcraft (who pioneered the concept in 2001), and GRC, and there have also been a few examples built by NWS and Dieterman. In addition, a few conversions of 125GP chassis have been undertaken, though this is a more difficult alternative due to the nature of the modifications needed to fit the engine into the frame. However, both Honda RS and Yamaha TZ125 conversions have been done with some success. Despite having many technical and economical benefits associated with the approach, to date there have been no production 125 conversions. Hence, most Minimonos have been constructed using 125GP running gear, with Honda RS125 components being by far the most abundantly available. There are also many chassis and spare parts available, and being GP bikes there are many upgrade chassis parts to be had. This is not to say that Yamaha TZs are unsuitable, but that they are not as common.

With any of these options it is important to pay attention to the rubber cush drives in the rear wheel hubs as they can fail prematurely as they are subjected to the higher torque of a four-stroke engine compared to that of the 125cc two-stroke for which they were originally designed. In addition, the engine braking is also much greater in a four-stroke, though this can be mitigated by the inclusion of a slipper clutch in the drivetrain, and is a secondary benefit to the handling improvement it also delivers. Standard 125 wheels can be strengthened but the preferred option is to use a wheel with a greater number of cush drive rubbers, and PVM can supply such a wheel due to their modular design approach. Other than that it is possible to use the majority of the running gear from a 125GP bike with little or no modifications. Like the Supermono, more money spent doesn’t mean a faster bike, and a Minimono could be built for as little as £3500, going up to, say, £10,000. Again this is only a guide and, as with everything in racing, much, much more can be spent if the whim takes you.

In summary, it is entirely possible (and encouraged) for people to build their own bikes as well as purchase an existing Super/Minimono. It is a class not restricted by production rules and is therefore one of the few occasions where new ideas can be pioneered and developed. It is an excellent opportunity for companies, independent engineers and students to develop a new concept, or indeed an entire motorcycle, within the safety limits of the FIM and ACU, the (very unrestrictive) rules of the class, your imagination, and your technical and financial resources. It is a good place to learn about engine development, suspension development and design (particularly alternatives to existing systems), electronics development (e.g. control systems such as EFI and traction/engine braking control, and data acquisition) and aerodynamics (provided it meets FIM rules, any shapes are permitted). It also allows development of materials and their construction techniques.