Mountain bike suspension poses extreme test for bearings
11 February 2013
Contrail MGN full-suspension bike from Bergamont is a top mountain bike model. It uses innovative technology, such as butted frame tubes with various wall thicknesses, as well as carbon fibre frame components to deliver Olympic winning performances. No less important, light polymer plain bearings also handle the severe stresses of the bike’s rear suspension.
Full suspension bikes – also known as ‘fully’ – with a spring-mounted rear wheel are found to be the best by anyone who rides mountain bikes, either professionally or simply for the fun of it. They give a far more comfortable ride and are ultimately faster, because the rider has to respond less to uneven ground.
The adjustment of the suspension must be good and tight in order to provide the desired comfort and performance. They must be set up to absorb sudden jolts so as not to endanger the stability of the rear suspension, and the whole system must also be ‘drive-neutral’ with motion of the suspension not being transferred to the pedals.
The Contrail MGN model, designed by Bergamont’s product manager Thomas Marquardt, features supported single linkage rear suspension with a bell crank on the frame. Known as a ‘Top Fully’ in the trail bike category, these are for athletes who win marathons and enjoy cycling for fun. Bergamont originally used ball bearings for all the bearing points of its ‘Fullys’, as Mr. Marquardt explains:
“At the two front pivot points, at the connection between the swingarm and the frame, we used deep groove ball bearings, because the bearings here have to execute a relatively large swivel movement.”
However, Bergamont has taken a new approach for the Contrail MGN and is using a plain bearing at the rear joint, on the rear axle. There are good reasons for this, the most important being lateral stiffness. A plain bearing offers a much larger contact area than a roller bearing. Mr. Marquardt again:
“Even with very severe stresses and large lateral forces, nothing wobbles. The plain bearings will perform very well with the high pressure loading at this bearing point, and the pressure is also very well distributed over the larger area”.
Bergamont needed advice in selecting the optimal bearing material and its tests with plain bearings on the suspension joint were subsequently supported by bearings specialist igus, a company that offers up to 35 different tribologically optimised polymer materials.
The first tests were performed with igus’ iglidur G because this universal material serves the broadest range of applications. However, as the wheels often run through streams and puddles, and they are often cleaned after use with a garden hose or even a pressure washer, iglidur J proved the better choice as it offers greater dimensional stability when wet.
Self-lubricating iglidur polymer plain bearings provide a long and reliable service life and the feedback from bike owners has been encouragingly positive. The torsional stiffness of the wheel is good and suspension response is fast, thanks to careful design of the bearing point, which has a major influence on performance.
The use of plain bearings is one of many factors that proves Bergamont’s high-tech claim for its mountain bikes and trail bikes. Other design features include butted tubes of varying wall thickness, which contribute to their light weight (10.8kg), a fork fitted with a carbon crown, and 3D forged dropouts.
Beijing 2008 Olympic gold medallist, Sabine Spitz’s bike, had iglidur plain bearings in a sensitive suspension fork. The polymer components stood up to repeated shocks, edge loads, and the dusty conditions of the difficult and spectacular course at the Chinese Laoshan Velodrome. iglidur bearings were also built into derailleur guide rollers of the winning bikes at the 2004 Athens Olympics.
Plain bearings and the art of food production
In the food and beverage industry, bearings need to endure extremes of temperature, continuous high speed operation, excessive moisture, regular washdowns and chemical cleaning. In addition, they must also adhere to strict hygiene standards and regulations as they cannot be allowed to contaminate any part of the process.
One of the biggest issues is not the contamination of the food or drink by the bearing itself, but by the lubricant. Food grade grease is available, which is both colourless and odour-free and also resistant to washdown, but it comes at a high price. Removing these lubrication barriers completely, plastic bearings have entered this market.
FDA-compliant iglidur A500 from igus is a high-temperature plastic bearing material suitable for use at up to 250ºC. It has excellent resistance to cleaning agents, oils, alkalis and acids and because of its FDA approval, specifiers can be assured that it will meet newly proposed EU regulations as and when they come into force.
But while FDA approved materials are not expected to pose any particular concerns in adhering to these new regulations, they will nonetheless need to undergo a process of testing, and that requires time, investment and expertise.
igus has significant R&D facilities and expertise that it can rely upon to prepare any information needed to ensure such conformity when the time comes.
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