Click-A, Click-A

Click-a, Click-a

TDC

KEVIN CAMERON

ONCE DECADES OF HARD WORK HAVE dealt with large effects, smaller details shine through. A case in point is motorcycle suspension. As to large effects, what these units do is convert unwanted chassis motion into fluid kinetic energy inside a glorified piston pump. Arrangements of orifices and flexible washers modulate the fluid motion to result in a desired relationship of damper-rod velocity to damping force.

Over the years, the fluid passages have evolved from crude drill holes into streamlined ducts, and a wealth of practical experience has created a cadre of technicians who can usually help with whatever chassis or handling problem a rider is having.

Only a few years ago, shocks were attached to remote oil reservoirs by flexible hoses. The reservoirs were necessary to accommodate the volume of the damper’s piston rod, which moved in and out through a seal. The reservoir, accordingly, was a cylinder with a sliding piston in it, separating damper oil on one side from nitrogen gas at six or so atmospheres (90 psi) on the other.

The more engineers worked with such remote reservoirs, the more small problems cropped up. It had once seemed convenient to use the flow driven by changes in damper-rod volume to control compression damping. But that flow was so small that it was hard to control smoothly. And compression damping deals with the upsetting upward blow to the motorcycle delivered by hitting a bump. It proved easier to dream up optimum damping schemes than to implement them via such a small-volume flow.

Washer stacks for damping control were mounted on the piston, where getting at them required complete disassembly of the damper. After a while, it seemed to make more sense to have a solid piston and put the washer stacks at the ends of the cylinder, from which they could easily be unscrewed and removed for re-stacking. Ah, much better, much more rational.

But details keep interfering. When a race is over, the bike goes back on its stand, and personnel, full of unused raceday tension, futz with it compulsively. Hey, look at all this freeplay in the rear suspension! The crew chief takes hold of the wheel and lifts it. Click-a, click-a, it goes. Freeplay. They investigate. It turns out not to be in any of the pivots. Crikey, it’s in the damper itself, that high-tech paragon. The damper guy is summoned, and he, after investigating, makes a few calls, waking up people in foreign countries.

Turns out that the piston ring used to seal the damper piston to its cylinder needs to have enough up-and-down clearance in its groove to function properly (stuck rings can’t seal). And since the rear linkage ratio (how much more the wheel moves than does the damper) is typically something between two and three, any piston-ring up-anddown clearance is multiplied times that factor at the rear axle. If wear enlarges this clearance, that, too, is multiplied.

Now, the trend may be away from piston rings and their clearance and back to something we might call a “packing”— rather like filling the piston-ring groove with many turns of oakum then forcing it into the cylinder. Now, the click-a click-a is gone.

Another rich source of click-a click-a is what are called valve transients. If we have the damper rod moving in the rebound direction, with fluid smoothly flowing out from under-deflected damping-control washers, and then we reverse direction, there will be zero compression damping during the time it takes for the deflected washer to flatten out and return to its seat. Once it does, the reversed direction of piston motion next has to generate pressure to begin to deflect the compression stack, lifting the compression washer off its seat, bending it and any helper washers stacked on top of it, forcing fluid to flow out from under them.

But how can it lift? A moment before, rebound pressure was pushing that compression washer hard onto its seat, squeezing out all the oil between. The washer is effectively stuck there, like a wet glass to a bar. It’s going to take time for any oil to flow between because the flow path has been squeezed to nothing by rebound pressure. So, the lifting of the washer is delayed, causing a force spike to appear in the damper rod. And force spikes are upsetting to motorbikes. A moment later, the fluid does seep between washer and seat, and the force spike drops. The low pressure in the now fast-flowing fluid sucks the valve closed again, causing another spike. Open, shut. Open, shut. The valve is—what’s the right word?—chattering! Uh-oh. Bad word.

Lately, there’s talk of “texturizing” the valve sealing surfaces to provide tiny pathways through which fluid can enter to “unstuck” the valve more promptly.

Now, the argument becomes complicated, but the bottom line is that when racers talk about “chatter starting at the rear and feeding through to the front,” they may be onto something. Lots of things can upset the smoothness of tire grip. Several MotoGP teams have reported that chatter-like vibration can continue even after all sympathetic chassis oscillations have been quelled by the usual raising or lowering of their frequencies. This is a case of the Cheshire Cat’s grin remaining even after the animal is gone.

How? It turns out that the tire tread by itself—with zero help from in-step chassis oscillations—can slip-and-grip against pavement at a basic 16-20 cycles per second. This seems inherent in the rubber and the tire’s construction. This is what Honda’s Shuhei Nakamoto was talking about at Mazda Raceway Laguna Seca when he said, “This is not chatter in an engineering sense, but the rider is saying, ‘chattering, chattering.’”

As pointed out to me this past summer by former Cycle editor Cook Neilson, heavy racing cars pound ripples into racetrack braking zones and corners. As bike tires run over such ripples, chatter may be induced. This may provide another reason chatter is not observed on Japanese company test tracks. With no heavy race cars hammering them, they are too smooth to upset tire grip.

And who knows? Damper-valve chatter combined with damper piston-ring freeplay can generate cyclic driving forces, too. Uncover and list every tiny problem, attack and solve every one, and your expensive top rider just might be able to give his full attention to catching the man ahead.