When watching motorcycle racing on TV you may have noticed the riders dragging their knees on the racetrack whilst cornering. The reasons for this are fairly complex and are explained in this article. Motorcycle racers drag their knee for a number of different reasons the combined effect of which results in considerably faster cornering speeds by increasing the natural limits of the motorcycle and tyres. From the beginning of motorcycle racing in the early 1900's through to around 1970 every rider raced with their knees tucked in firmly to either side of the petrol tank. In the early 70's Jarno Saarinen - previously a motorcycle ice racer - moved into the world of Motorcycle Grand Prix and brought with him an innovative style of racing which became know as the "hang-off" style. Through the early 1970's several of the best racers of the era including Kenny Roberts, Barry Sheene and Paul Smart developed this riding style - in the highest level of motorcycle racing - which has became common-place today.
This riding technique makes use of 3/4 inch thick blocks of plastic or leather - known as knee sliders - which are connected by Velcro to the upper shin area of the motorcyclist's leathers. Although all sports bike leathers are equipped with the Velcro which can have knee sliders attached, it is advised not to use this technique on a public road as it would be constitute a serious driving offence.
The main reason motorcycle racers drag their knees whilst cornering is to achieve a greater cornering speed; this is achieved due to the following factors.
Shifting the Centre of Gravity
To achieve the correct seating position the motorcycle rider slides their body off the seat in the direction of the inside of the corner by hooking the underside of their other knee on the edge of the seat. This riding position shifts the combined centre of gravity of the motorbike and rider in the direction of the inside of the corner, enabling an increased cornering speed compared with the speed which could be achieved with the same angle of lean with the rider sitting upright.
More Speed for No Extra Lean
Another of the factors which results in increased cornering speed is due to there being a limit of just how low the motorcycle can lean due to the limitations of grip of the motorcycle tyre, and as there is a direct correlation between corner speed and the lean angle this ensures a limit with regard to the cornering speed. Whilst riding through the corner with a knee in contact with the racetrack, the motorcycle rider can accelerate gently while supporting some of the weight of the motorcycle preventing the angle of lean from exceeding the tyres limit. This results in even greater cornering speed and increased in-corner acceleration and exit speeds. This technique is used mainly on longer sweeping corners where the knee is in contact with the tarmac for a fairly long length of time.
The reason above applies to riders of a high level in competitive motorcycle racing. Other motorcyclists of a lower level may have other reasons for using the knee-down cornering technique during track day events - when the race track is available for members of the public to ride on. Some motorcyclists drag their knee to gauge the angle of lean to make sure that they are cornering within their abilities or within the limits of the Motorcycle Tyres and the bike itself.
By using the "hang-off" seating position the perception of speed is increased as the rider's viewpoint is significantly closer to the surface of the racetrack. This together with the fact that riding through a corner with the bike leant over (somewhere near its limits) with the knee-slider in contact with the tarmac is an exhilarating experience.
Taking It to Extremes
Some highly talented Moto GP (Modern day Motorcycle Grand Prix) riders such as Ruben Xaus and Randy de Puniet have taken dragging their knee one step further by also dragging their elbow at the same time. There is little evidence that this results in an increase in cornering speed although it indubitably thrills the crowd.
So for high level motorcycle racers the advantages of this technique are an in increase in cornering and corner exit speeds; whilst motorcycle riders on track-days gain less of an advantage in increased speed but do achieve a more enjoyable and thrilling track-day experience.
source:: Why Do Motorcyclists Drag Their Knee?
Edited by razorBlades, 17 February 2011 - 12:26 AM.