Superprestigio Tire Bounce

Date :

December 12, 2014

Superprestigio Tire Bounce

Watching last January’s Superprestigio, I was struck by something I saw the motocross-based bikes doing only when they were upright on the so-called straightaway. They appeared to be bouncing, often rocking, on just the springiness of their 17-inch Michelin rain tires. I wondered at the time whether this was just a detail or if riders noticed it. The bouncing took place at about three or four cycles per second.

If real, one thing this bouncing suggests is that the bikes are “hard topped” when upright (all the action takes place in the corners, with the suspension loaded). Hard topping is a situation in which the suspension spring is strong enough to hold the suspension at full extension.

In 1976, my rider, Ron Pierce, was bothered by rear-wheel bouncing on the Kawasaki KR750 braking to approach the old Loudon turn 10 (a banked right-hand “salad bowl”). When hard topping takes place, the suspension is held fully extended by the spring and so cannot damp out the bouncing because the suspension damper piston cannot move, leaving the tire as the only “suspension.”

I built a set of shocks (yes, the KRs were antique twin-shockers) with negative springs (springs that are compressed only as the shock approaches full extension), hoping this would allow the damper pistons to move and quell the bounce. Usually in racing you don’t expect much from such changes because a lot of ideas just don’t work, or at least not first try. But this did, so I was very pleased.

I will be watching closely during this weekend’s Superprestigio in Barcelona, Spain, to see if I imagined last January’s tire bouncing or if there’s something going on that I wish I knew more about.