R. Cunningham
Every time you think you have mastered the latest monkey-motion, a new way to make the rear of a mountainbike go up or down is discovered by some genius inventor. There are seven designs currently in use by the top brands of mountain bikes: Monoshock, cantilever beam and Mac-Shock. Parallel link, GTs iDrive, flexible-frame softtail, and a host other four-bar-link spinoffs. The proliferation of plush raises the question, « Do we really need a million inventions to accomplish a single task? » This is part one of a series of two that will explain the best, worst and most ugly aspects of some of the most popular rear suspension designs.
THE PARALLELOGRAM SWINGARM PIVOT
A OTHER PAUL TURNER IDEAS
Volvo/ were the most well-known versions of the dual link swingarm pivot.
Cannondales 1999 World Cup Downhill Rigs. A swingarm that is conventionally designed arcs around one pivot on the frame. Designers can make the rear axle arc around a point in the space by using a pair or more of links. Paul Turner, RockShoxs founder, and the Outland VPP machine were the first to utilize the system. The rear wheel was able to trace a path using a pair of links that were unevenly spaced between the swingarm and the front section. This allowed the swingarm to track a path that would have been impossible with a conventional swingarm.
PLUS COLUMN
It is possible to create a mountain bike with long travel that can be pedaled easily. The parallelogram linkage can be easily adjusted to make the rear suspension cancel out the bobbing caused by power in the granny chains, while still being active through the rest of the gearing range. As in Schwinns’ case, the suspension can be set up to track the fork while maintaining the same wheelbase as suspension cycles.
MINUS COLUMN
Any suspension that relies on highly-stressed links is susceptible to weight and complexity. Each pivot point on the swingarm and frame must be strengthened. Close proximity to pivot bearings can lead to accelerated wear and a loose-feeling tail end.
EXAMPLES:
Both Karpiel and KHS sell downhill bikes that use this system today. The parallelogram swingarm pivot is only available in cross-country production by the very clever Schwinn Rocket 88 series.
HOW IT WORKS
Designers can adjust the path of the rear wheels by using a pair or links between the frame and swingarm. Tony Ellsworth uses the linkage to make the swingarm track the angle of his chain to maintain his suspension active. Cannondale designed the linkage to mimic a high-pivot swingarm, and then slowly move to a more conventional position at full compression. There are no limits to what you can do. 1. Draw a pair imaginary lines through the middle of the lower (A) and upper (B) links to find the point where the rear axle curves around. The pivot point of the swingarms is located where the lines intersect (C).
