From Mountainbike.com today by Bill C.:
HPL poised to introduce new shock technology
Oh it's a wee lucky day 'ere in Burbank, for today, we're letting you in on a little pre-production, insider, dare-we-pull-out-the-term "prototype," scrimmage we have going on down here in lovely SoCal. The name of this game; Torsional shocks. That's right, and a company called Rotational Dynamics is leading the charge with the bike at right. A collaboration with frame builder HPL, this 100mm devil has already been featured in the magazine, but until recently, most were left in the dark on what she's all about. It's time we get to the nitty gritty.
The focal point of the whole kit and kaboodle is a rubber torsion spring developed decades ago by BF Goodrich. Like an eloquent day laborer, it's simple and really easy to understand an inner shaft (in this case the shaft is keyed with a hexagon in the center) is bonded (sprayed with an "adhesion promoter" and then injection molded, to be exact) to a piece of rubber, which is bonded to an outer shaft. The outer shaft is bolted or welded to the frame and a keyed axle goes through the center of the shaft. When the axle is twisted, the rubber stretches and wants to return to its original position and voila; you have a shock. On the HPL, the compression is fairly linear until the point where the rubber begins to strain harden and ramp up creating a natural lockout when the shock is rotated approximately 60 degrees (the angle is dictated by the cross section of the rubber). A bolt on the top tube adjusts the shock's preload.
"What about damping?" you say (Canadians may substitute the word "aboot"). The answer comes in the form of a surface effect damper positioned at the outer edges of the assembly. Both the compression and the damping units can be swapped out with firmer/slower, or softer/faster combinations to tune the ride, though initial production bikes will not feature in the field damping adjustments.
In addition to the unique suspension, HPL has developed "Hollowform" stays each half of the square stay is speed machined (pictured) to spec then the shells are "vacuum brazed" together to form a weld-free final product that HPL claims is stronger than a TIG welded stay. Since there are no drawn tubes or jigs involved, HPL can easily change the stay's design while keeping its costs down. On the horizon is a completely Hollowform-ed front triangle, and the company is looking into a stainless steel hardtail made from Hydroformed stainless sheets.
So, I got a chance to visit the HPL offices and take their bike out for a bit of a jaunt. Of course, at first I was a little skeptical of the technology, but came away impressed with my ride. As far as drivetrain induced bob is concerned, HPL did themselves a favor by licensing the Horst link from Specialized right from the start. The 100mm of travel flat out silenced small bump noise (vs. a hydraulic system) since there is no stiction or spikes in the travel. Though a prototype, the rear felt laterally stiff and vertically plush when set up with over 1.25" of sag...but that's enough for now. Look for more on the HPL in an upcoming issue of the magazine (the final look of the bike might be somewhat different), or take a look at www.humanpropulsionlaboratory.com for more.
