Lightweight and durable: Plastic bearings in a racing motorcycle

Motorsport on two wheels is characterised by a whole range of different vehicle classes – right down to single-cylinder machines. The limited engine power of these bikes is not a disadvantage if the weight – i.e. the mass to be accelerated – is also greatly reduced. Plastic bearings instead of metal alternatives make an important contribution here. And they offer further advantages in the tough world of racing.
 
At Krämer Motorcycles (KMC) there is just one product to buy: Riding pleasure on two wheels. More precisely, a Supermono racing bike, which by virtue of its extreme riding characteristics not only won the title in the single-cylinder championship, but is keeping up well in the current season in the Super Twin race. The bike, offered under the model name HKR-EVO2, reaches top speeds of up to 230km/h. At the Tandragee 100 road race in Northern Ireland, Shaun Anderson achieved an average speed of 100.485 miles/hour (161.715km/h), making him the first and only racer ever to break the magical 100 miles/hour barrier on a single-cylinder motorcycle.
 
Although the company is still relatively young, a lot of development work has already gone into the machine. It began in 2009 as a private project between three friends, Christof Henco, Markus Krämer and Nico Rothe – whose initial letters (HKR) denoted the type designation – to develop their own motorcycle purely for motorsport purposes. This led to the founding of Krämer Motorcycles in 2014 by Markus Krämer, who previously worked as a development engineer at KTM. Accordingly, the decision for the central component was not difficult: The single-cylinder KTM 690 LC4 engine served as the drive unit right from the start. The rest of the bike was designed around this, partly using standard parts and partly with components developed in-house.

In motorsport, every gram counts. Metallic bearings cause the weight of the vehicle to increase and therefore affect the overall performance of the vehicle. The bearing points are often subjected to heavy loads due to the tough racing conditions and must therefore be adequately lubricated. This in turn ensures that dirt collects at the bearing points, which can impair the components.

Krämer Motorcycles relies on iglidur Q and J plain bearings and igubal rod ends, as these offer a lubrication-free and maintenance-free solution. They ensure constant functionality as no dirt accumulates at the bearing points. As the plain bearings are made of plastic, their weight is only around a third of the metal bearings previously used. Krämer Motorcycles uses iglidur plain bearings in the link, brake and gearshift system as well as in the footrests. In addition, two igubal rod ends are used in the foot brake lever. Another advantage of all igus components: Their service life can be calculated precisely with the help of online tools.

KMC looks at each and every component and tries to come up with in-house developments that meet their own requirements instead of the standard products. Reduced weight is the top priority here – the less, the better. Since the maximum power of a single-cylinder engine is limited, the only way to gain an advantage is to reduce the mass.
 
But simplicity of design is also an issue, because it also determines how easily – and therefore how quickly – a repair can be carried out. In the world of racing, this is crucial to be fit and ready for the next race. And a certain degree of flexibility is also required. Krämer points out the wheel suspension as an example: "If you want to change the weight distribution, then you have to use a different length here. Such configuration options must be taken into account right from the start."
 
Another issue is the price. Krämer wants to appeal to a clientele below the fully professional segment. Depending on the configuration, the bikes, which are used exclusively for racing, cost between 15,000 and 25,000 euros. The manufacturer must therefore also ensure that high-quality components are available at a favourable price.

The latest development from the KMC developers is a new link system for the rear swingarm, which connects the rear wheel to the frame via a damper. The shaft of the previous link system contains interior workings consisting of nine metal parts, which had to be laboriously assembled and lubricated. The new design, which will now be installed in future machines after a year of development, only requires two lubrication-free plastic bearings, which are pressed in and then operated maintenance-free. In addition, the weight is reduced by around a third – from 535 to 355 grams. And last but not least, the price of the components – including the iglidur Q flanged bearing – is significantly lower than that of the previous design.
 
This application is predestined for the use of bearings from igus, as large forces occur, but only small rotation angles of up to 10°. This is where the fact that the plastic does not require lubrication pays off. With a similarly simple metal design, you would have to ensure that a complete rotation is achieved, as otherwise there would be no complete surface lubrication. The use of igus products is also in line with the principle of simplicity: "Sometimes you just have to press-fit a plastic bushing where several parts would have to be joined together in the metal version," explains the KMC boss.

Markus Krämer already had his first contact with igus at his previous employer, KTM. There, the products had been used in a few niche areas, for the gearshift system and steering. The final decision in favour of using the plastic components was made at the suggestion of the design engineer, who had already worked intensively with igus in bicycle construction. It quickly became apparent that the planned applications could be covered by the igus standard range. This made the search for an alternative supplier superfluous.
 
It was easy to start using plastic bearings, as the company was not reliant on customised products. However, Krämer reports that the development of the link system that has now been finalised was time-consuming, as a whole series of tests had to be carried out. The KMC boss explains: "The standard tolerance specifications from igus were not suitable for our application. A margin of a tenth of a millimetre multiplies here, and then you suddenly feel a movement of one to two millimetres on the rear wheel – unacceptable for a racing machine." In the search for the right tolerances between the bore and shaft, tests had to be carried out to find the right combination. With igus, the collaborative work went very well.

Of course, the weight of the machine could be reduced even further, for example by using carbon fibre parts. However, KMC refrains from such extremes in small series production: "That would make the machines almost twice as expensive," explains Krämer. Other racing teams do not have to take this into account when putting together prototypes. However, KMC is not only concerned with racing itself, but also with selling machines. And to do this, an attractive price point must be achieved. Even without these design tricks, he sees himself well ahead in his class in terms of weight: "Definitely in the top 3". Krämer prefers to focus on smaller improvements that are nevertheless noticeable overall. And part of these improvements, in addition to the self-constructed steel frame and a powerful aluminium-based braking system, are the components from igus.
 
The effect of this combination of lightweight and extreme braking power surprises and delights riders in equal measure. While other 80-BHP machines are considered to be rather underpowered, the HKR-EVO2, which weighs just 12 kg, cuts a fine figure with this drive, even in a field of significantly more powerful but heavier motorbikes. Strong braking deceleration and precise tracking despite high cornering speeds with extreme inclined positions put the racing machine ahead, especially on tracks with tight bends. The result can be seen in Töging: Numerous winners' trophies adorn the shelves in the production hall.