In a bobsled competition, there are two runs that each last less than 60 seconds. The first five seconds are decisive as they determine how much energy the bobsled has for its journey through the ice channel. In addition to the gradient of the track, only the muscle power of the pushers provides propulsion at the start of the race. After that, it is all down to the pilot, who steers the bobsled using ropes, which move the front runners via spherical plain bearings and levers. It is important to find the ideal line, which ensures that the bobsled loses as little energy as possible during the short run. Even the smallest steering movement at the wrong place on the track can decide the outcome of a race; if the runners dig into the ice and generate too much friction or if the bobsled swerves sideways and slides transversely to the direction of travel.
“We don’t have a motor and can’t regain any speed lost through braking,” explains Nico Walther, team leader and pilot for the German national team in the two-man and four-man bobsled. “To steer a bobsled, you need a lot of intuition and feel. I must be able to feel what is happening underneath me.”
Brute force and sensitivity
Bobsled pilots not only have to perform at their best at precisely the right moments, they must also combine two very different qualities. During the first 50 metres, only the brute force and speed of the pilot and his pushers count, as they bring the bobsled up to speed. To reduce air resistance to a minimum, the pushers jump into the sled and duck down behind the pilot.
“From that point onwards, it’s all about sensitivity and not force,” says Walther. During the high-speed run, in order to steer the vehicle, he moves the steering ropes by no more than two or three centimetres and therefore the runners by 10 to 15 degrees from the neutral position. The boundary conditions for a steady hand are not exactly ideal because the irregularities in the track often generate vibrations and shocks in the bobsled. In addition, the centrifugal forces in the curves are enormous and reach up to 5g.
The Formula One of winter sport
In the sport of bobsledding, the bobsleds are as important as the athletes themselves. “The ratio is 50:50,” suggests Walther. “The best pilot cannot perform well in a bad bobsled, and the best bobsled will not help a bad pilot win.”
Only a handful of companies manufacture the torpedo-shaped bobsleds worldwide, which Walther describes as the Formula One cars of winter sport. In the world cups and at the Winter Olympics, Walther drives bobsleds that are developed by the Institute for the Research and Development of Sports Equipment (FES), with support from Schaeffler. The bobsleds comprise two parts: a front cowling and a rear cowling, which are connected to each other by an articulation joint. The front cowling can rotate relative to the rear cowling about the longitudinal axis of the bobsled with the help of a bearing journal. The steering unit on the front cowling also rotates about the same axis.
“This is important for traveling around corners because otherwise the rear cowling could lift in the corners and the bobsled could fly off the track,” explains Enrico Zinn, in charge of the technical development of bobsleds at FES. Zinn and his team receive feedback from the pilot after every run – for example, whether the steering was adjusted directly enough – which is then used to further improve the bobsled. “The technical development depends on our feedback system, with which we precisely adjust the components and the entire vehicle to suit the athletes,” emphasises Zinn.
High static load rating and rigidity
The ice surface of the bobsled track is never perfectly flat, but is formed into different radii by the ice master, which generates high loads at high speeds. Due to these loads, the individual components of the bobsled must meet high requirements. To provide the articulation joint with the required strength, the two Schaeffler tapered roller bearings used in this joint must have a high static load rating and must be resistant to shocks.
In addition, the bearings and the joint must have high rigidity in order to hold the front cowling and rear cowling in the correct position relative to each other and to prevent flexing due to the high transverse acceleration. However, the most important factor is to minimise the bearing clearance using the correct preload for both bearings.
“As soon as there is clearance in a bearing position, performance is reduced and the bobsled is slower,” says Zinn. The plain bearings that Schaeffler develops for the steering system must also have a high static load carrying capacity in order to achieve a sufficient operating life despite the enormous vibrations. The steering bearings must also operate smoothly to ensure that as little friction as possible is generated when steering forces are transmitted to the runners.
Zinn explains further: “This is important to ensure that the pilot can feel the steering directly, therefore enabling him to develop the required sensitivity to help win the race.”