Photography by Mark Wernham
Many of us have now seen the slow-motion video of Casey Stoner’s run-off into the gravel at Twin Ring Motegi this past Sunday. He was leading the Japanese Grand Prix, but so hard-edged has MotoGP become that the old happy ending of brilliantly making up lost ground is no more.
What we saw was Stoner’s bike going light over a rise then giving the expected wobble. The resulting side loads on the front wheel then knocked the brake pads back into the calipers so that when he braked for the turn immediately ahead, the lever came to the bar. He instantly stroked the lever a second time but it was now too late—he ran off, losing time that would relegate him to third behind zero-defects Dani Pedrosa and the hard-working Jorge Lorenzo.
A motorcycle’s straightline stability comes ultimately from the damping effect of its tires against the pavement. But anything that tends to lift or lighten the bike on its tires reduces this damping—maybe enough that a wobble arises.
Every racer makes his own compromise between speed of steer response and vehicle stability. If he dislikes the little shivers that a very responsive setup produces, he can reduce the fork offset to put in more trail and the result will be both more stable and a bit slower-steering. If his style calls for the very fastest response, he may push the setup closer to instability.
Have a look at videos of bikes cresting “a slight rise” at the Isle of Man and you will see some really big gollywobbles. And we have all seen bikes give a shake as they pass through the vertical while quickly changing direction—as just after the bottom of the Corkscrew at Mazda Raceway Laguna Seca.
What’s interesting here is that today’s bikes, with their big-tube inverted forks, have no way to provide a fork brace. When Yamaha TZ750s in my care had problems with pad knock-back, I could make a strong beam joining the two fork sliders just above the tire, held in place by two clamps per side. The strength of this beam killed the knock-back.
But in an inverted fork, there is no place for such a beam. All that holds the front wheel in plane is the combined stiffness of the axle and its attachments to the fork bottoms. This may be another aspect of the necessary compromise between stiffness, which is essential to stability, and flexibility, which is so important for “feel.”
Here’s a link to the MotoGP Motegi Video:
http://www.youtube.com/watch?v=A_zLOqg2GaE
