Amp Research Amp-Link
AMP RESEARCH AMP-LINK
CW EVALUATION
An equal and opposite reaction that bolts on
WHAT YOU CAN'T SEE CAN'T HURT you, right? Well, if you believe that, you shouldn’t apply for a job at a nuclear powerplant. You probably shouldn’t ride dirtbikes, either. There are, you see, lots of invisible forces that have negative effects on the way dirtbikes handle. And one of the most-unseen (and least-understood) of all is drive-chain torque reaction.
Simply put, drive-chain torque reaction causes a bike’s swingarm to try to extend downward during acceleration and drive the rear wheel underneath the rest of the bike. The reasons why are not so simple, though, for they involve some fairly complex geometric relationships such as the extreme downward angularity of the swingarm (at or near full suspension extension) on most dirtbikes, and the angle at which the top run of the drive chain pulls on the rear sprocket during acceleration. But for our purposes, what’s more important than the mechanics of this phenomenon is its net effect: Any time drive-chain torque reaction occurs, it effectively makes the rear suspension overly stiff and relatively ineffective by not allowing the swingarm to move up and down freely.
Several years ago, Austrian engineer Horst Leitner invented a gadget called the a-trak to solve this problem. Although it worked rather well in some applications, the a-trak needed a few refinements that it never got, thanks partly to Leitner’s decision to sell his company. But he now has formed a new company (AMP Research, 1855 Laguna Canyon Rd., Laguna Beach, CA 92651; 714/497-7525) and has redesigned his original concept. He calls the end result the AMP-Link.
Much like the a-trak, the AMPLink causes the top run of the chain to run roughly parallel to the swingarm by looping it over an additional sprocket above and just behind the swingarm pivot. This changes the angle at which the chain pulls on the rear sprocket during acceleration, which, in turn, reduces the tendency for the swingarm to be forced downward. The AMP-Link also has another sprocket below the swingarm pivot that helps eliminate the effect of engine braking on rear-suspension action.
Unlike the a-trak, the AMP-Link attaches its sprockets to the frame rather than to the swingarm. So, as the rear wheel moves upward, the chain is gradually lifted off the upper sprocket until, near full compression, the AMP-Link is completely out of play. This is important, says Leitner, because when a motorcycle’s swingarm goes over-center (when the rear axle is above a straight line drawn between the centers of the swingarm pivot and the countershaft sprocket), torque reaction reverses and tries to compress the suspension and pull the swingarm upward. The old design did nothing to correct this reversal, and, in fact, made it worse. (The exceptions are a-trak-equipped ATK motorcycles, also designed by Leitner, which have steeply angled swingarms that hardly go over-center, even at full compression.) While the new design also does nothing to correct this, at least it doesn’t make the reversal any worse.
To test the AMP-Link, we took it and an otherwise stock 1990 Honda CR500R to DeAnza Cycle Park, a motocross track near Riverside, California, that has an extremely hard surface and big whoops at the exit of most of its turns. First, we rode the bike in stock form, then installed the AMP-Link (a 15-minute job that required no permanent modification). The difference was immediately noticeable. Harsh bumps in and after the turns felt like they had been reduced in size, and, more important, the brutal 500 became much easier to ride. It was as if the suspension had suddenly been made softer, yet the rear end had not gained any tendency to bottom or wallow. Our only complaint was that the spinning of the device’s two sprockets in their needle bearings was fairly noisy.
There’s no denying that a good suspension tuner can largely compensate for drive-chain torque reaction through skillful shock re-valving; but that’s a case of curing the symptoms and not the disease. The $ 129 AMPLink represents a bolt-on improvement that deals more directly with the problem, and that doesn’t require you to send your shock away for revalving. which can put your bike out of commission for weeks.
Unfortunately, the AMP-Link currently is available only for 1988 through 1990 Kawasaki and Honda motocrossers. But for those bikes, it’s a clear solution to an invisible problem.
