Loni Hull - Oct 4, 2018
The Evolution of Linkage Bike Design
photo: Dan Kirkpatrick
It might come as a surprise, but when engineers first applied front suspension to a bicycle, widespread adoption of telescoping forks was not a foregone conclusion. Linkage front suspension systems have been applied to bicycles since the advent of mountain biking, and a number of such systems are still sold and work exceptionally well.
The number of linkage front suspension designs that have made their way to market - including Bridgestone, Lawwill, Look, Amp Research (developed by Horst Leitner, famed for Horst rear suspension), Girvin/Pro-Flex/Noleen (purchased by K2), Hurrycat, Parafork, Whyte (PRST-1 and PRST-4), German A, Lauf (they aren’t telescoping forks, so we’ll note them here), and more recently Scurra, Motion Ride, and Structure Cycleworks - demonstrates that enthusiasm for linkage front suspension is alive and well. Although widespread market adoption has eluded linkage systems until now, it has more to do with simplicity, aesthetics, and cost savings than outright performance.
For Structure Cycleworks, when approaching the question of how to address key kinematic and handling goals, there was only one imperative: start with as few preconceptions as possible, plotting geometry for a dedicated chassis instead of designing a replacement fork. It wasn’t about criticizing brilliant designers and engineers who tackled the same goals before we did, but about treating riders to the best of what is possible while taking lessons from what worked (and didn’t) in previous designs.
That flexibility in thinking about how to make a bike move, how to orient frame members and package components - and about what standards to adopt, what sort of hardware to use, and what materials to use for the frame - has proven to be an advantage and a constant challenge.
A few decisions were straightforward. Enduro was our category because it’s how we ride local trails. Boost and 27.5” are standards that we don’t regret adopting. All bearings and components had to be standard bike fare that can be replaced at a local bike shop, even if the frame itself seems exotic. Basic geometry and fit had to be what riders expect, although we made a number of decisions to get ahead of trends and take best advantage of the stability-enhancing kinematics of our linkage chassis.
Other decisions have been more challenging. Where should the front shock be located? Should it be actuated directly or by link / rocker? How long should the front arms be? How much brake anti-dive is too much, negatively impacting bump compliance? How do we minimize stack (fork + travel + upper steering head) while keeping the entire system stiff and strong? How steep should the seat tube be, and should we prioritize actual or effective seat tube angle? How far should we go to keep mud and water out of bearings? (We went to extremes, with x-ring seals at each main pivot).
Drawings - from early sketches to recent eDrawings - illustrate how the design of the SCW1 evolved as decisions became firm. For example, packaging and actuating two shocks influences the shape of everything around them, as you can see here:
Locating the upper steering link above the top tube seemed to offer additional clearance for suspension travel, but did nothing to minimize stack or the length and visual impact of the steering links, my least favourite aspect of early drawings. So we adjusted the shape of the upper steerer and moved the upper steering link back to the bottom of the steerer, where it ought to be.
The elevated chainstay - which I liked aesthetically - did not resist twisting at the axle as effectively as a midline stay no matter how we beefed up its proportions, so it had to go. Function must always drive form.
But form matters too. As much as we might like to think that we can fight human nature, beauty gets attention, and one reason for bike market rejection of past front linkage suspension systems has been that no matter how well they function, they haven’t always been pleasing to the eye.
Aesthetically, not everyone loves our work, although it’s gratifying that many do. Some see a leaping gazelle or cheetah; others a praying mantis.
What matters is that after we confirmed through simulation and countless hours of saddle time that the design functions incredibly, we made a concerted effort to make it visually striking. Even beautiful, in its own way.
In 1992, the Gary Fisher RS-1 introduced a revolution in cycling by proving that full suspension mountain bikes could really work. In the decades since, we’ve seen amazing advances in materials, manufacturing methods, geometry, shocks, multi-link rear suspensions of tremendous variety, and yes, telescoping forks -- which do a better job than anyone in the early 1990’s imagined they might.
However, there has not been a design revolution that addresses the limitations of telescoping forks while offering the kind of massive improvement in performance that will assure adoption by hardcore enthusiasts everywhere.