The Spyder drive belt is a tensioned string system that transmits the drive pinion torque to the rear sprocket via the tension of the upper part of the belt. The natural response of a string system is to vibrate when stretched and released at different speeds and strengths. The accumulated energy of these movements in the belt will be dissipated throughout the entire vehicle through the drive pinion and the rear sprocket.
A momentary belt vibration may be felt when the vehicle is driven under increased load/torque conditions (acceleration and/or going uphill). When the belt tension is within specification, the phenomenon may occur between 3800rpm and 4300rpm in 6th gear on 1330 engines (or between 4700rpm and 5500rpm in 5th gear on 991 engines). This vibration is felt through the handle bars, the foot rests and the seat. The harmonic vibration described here is considered "normal behavior" for such a belt drive system. In no cases is there any damage produced to the unit's mechanical components by this harmonic vibration.
Although belt vibration cannot be entirely eliminated, changing the belt tension will change the RPM/Speed at which the vibration occurs and the amplitude of its occurrences. Adjusting the drive belt tension lower will reduce the vibration level. Under high load/torque conditions, the vibration levels are diminished in amplitude and the RPM which the vibration occurs is lowered by roughly 400rpm.
For someone driving regularly between 68 and 75mph having a lower belt tension means they will feel a belt vibration when going through speeds of 62 to 65mph but at a lower amplitude, and feel less vibrations under acceleration and cruising speed. For someone driving regularly between 62 to 65mph having a belt tension set at specification means they will not feel as much the belt vibration since it occurs at higher RPM/Speed 68 and 75mph.
Statically, the belt tension is based on the distance between the drive pinion and the rear sprocket. The belt can be preset to a specific value using the tension adjustment procedure. While standing on its wheels, the belt tension of the Spyder increases due to the rotation motion of the swing arm. Further additional weight on the vehicle (driver, passenger and luggage) further increases the static tension of the belt.
Dynamically, the big picture is that the belt tension continuously varies with the road surface irregularities. Furthermore, to transmit torque from the drive pinion to the rear sprocket, the given amount of tension is decreased from the lower portion of the belt and transmitted to the upper portion of the belt. The inverse is also true when the vehicle is under deceleration. The amount of tension transferred depends on the acceleration or deceleration of the vehicle and slope of the road.
The belt tension is the factor which determines the belt vibration. The engine speed at which vibration would occur depends solely on the tension of the belt since the length and mass of the belt remains relatively constant. More vibration is transmitted to the vehicle when the tension is high. This effect can be explained by the fact that higher tension implies higher stiffness and higher stiffness implies higher transmissibility. This also explains why even though more vibrations are observed on the lower portion of the belt, only the vibration of the upper portion of the belt is strongly felt on the vehicle due to its higher transmissibility (because it has a higher tension in the majority of driving conditions).
At constant speed and zero road slope, the upper part of the belt has a higher tension than the lower part of the belt due to aerodynamic resistance. Under acceleration and/or hill climb conditions, the upper part of the belt further increases in tension. The degree of acceleration or hill climb proportionally dictates the engine speed at which occurs the belt vibration and the amplitudes of vibration transmitted to the vehicle.
By reducing the tension of the belt, the transmitted vibration at very low torque conditions can be completely removed. The transmitted vibration at higher torque is similar to that of the specification belt tension but occurs at slightly lower RPM/speed.
So basically, if you experience unpleasant vibrations have your drive belt tension checked at three different points of rotation of the rear wheel/sprocket and see how it varies, and compares to the vehicle's specification. If the belt tension changes more than 250N between 2 readings this would indicate an out of round rear sprocket which should be replaced. Otherwise, you can adjust the tension to change the speed and amplitude at which it occurs.
