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January 14, 2005
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By: R. Cunningham

Sliding tracks replace swinging links

Chris Conroy, co-owner of Yeti Cycles stopped by to talk about “303”–Yeti’s latest suspension system that utilizes a pair of sliding tracks to control the axle path and leverage ratio of their new 303-DH downhill racer. The design was the brainchild of engineer/co-owner Steve Hoogendoorn–the result of a two-year search for a method to make a suspension that could respond with equal sensitivity when the bike was riding half-way into its travel as it did when it was at full-extension.

?One track is perpendicular to the chain line,? says Chris, ‘so chain tension doesn?t effect the suspension. Both the upper and lower tracks are opposed to braking forces, so the suspension can cycle freely under braking.?

The first prototypes used round tracks, but the latest renditions?the ones that the Yeti Team will be racing this year?use a narrow stainless steel track with multiple Vee-grooves that eliminate lateral flex more effectively, and are more compact. The latest prototype was out being thrashed by Team Yeti’s Duncan Riffle (so no pohoto was available). We did get this snazzy computer graphic of the new configuration to show you the basic suspension design

A triangulated monoshock swingarm is attached to two ?cars? that slide on a pair of tracks. One track is roughly parallel with the top tube and the other traces the seat tube. The cars have special ?linear bearings? with balls that recirculate through tubes after they have rolled the length of the car. Linear bearings are new to bicycles, but commonly used for precision machine tools. They are well suited for keeping the cars running stiction free on the tracks so that the suspension will remain sensitive to the slightest hit. The beefy aluminum swingarm pivots on a pair of ball bearings on each car.

Varying the angle of the tracks a few degrees alters the shock rate and axle path. The advantage of the tracks over swinging links is that, by nature, linkage geometry changes dramatically as the links reach the extremes of their travel. The tracks can be tuned to deliver almost any suspension rate the designer wants. Yeti chose a falling rate at the beginning of the stroke, which turns almost perfectly linear through the greater part of the downhill chassis? nine inches of travel. Most of the falling rate occurs behind the normal sagged position that only comes into use in mid air when the suspension is un-sagged and ready to soften a hard landing. Most riding is done while the suspension is working in the linear part of the curve.

?Our racing team is pretty excited about the 303 Linear Rail Technology Suspension [Yeti’s working title for the design],? smiles Chris. ?They say that the bike stays level and about mid-way into its travel through the turns and accelerates very well. Acceleration is really the key benefit to the design. It can jump out of the corners like a high-pivot suspension, but without the rocking horse sensation that happens when the single-pivot rear suspension unloads.?

One of the secrets of the Yeti’s custom suspension rate is that the latest rendition of the design uses a curved upper track. Notice that there are adjustable plates on the track mounts that can be used to customize both rate and travel.

?We plan to adapt the Linear Rail suspension to all of our dual-suspension bikes, except for the ASR,? said Chris. ?The ASR is pretty dialed for cross-country racing and we feel that it represents the highest performance attainable at that weight level. The present design is a perfect match for our ASX, and we are working on that project as we speak.?

?Complicated? is the first reaction that most first-timers have when viewing the Yeti invention, but the tracks and cars are actually a simple setup. The key to Yeti’s success will be properly sealing the sliding surfaces and concocting a method to minimize the weight of the dual tracks and sliding cars. Sealing the tracks has been addressed with a yet-to-be-seen telescoping bellows assembly that Chris said, ?cost a small fortune to produce.? The weight issue will not be a huge factor as long as the system is limited to Yeti’s downhill rigs.
So far, Yeti’s 303 suspension seems like a winner. We shall see.


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