If the motorcycle and rider are traveling in a straight path on level ground, the total weight on the tires is constant (neglecting the weight of the burned fuel). Weight is transferred between front and rear tires with acceleration and braking but the total weight is constant.
The total weight or force on the tires will only change if the the motorcycle as a whole is accelerating upwards or downwards (i.e. climbing a rise or falling into a dip) or experiencing aerodynamic downforce or lift.
Weight Transfer is defined as the 'increase in rear axle downward load (and corresponding upward increase in the road reaction on the axle) or the corresponding decrease in front axle load' (Race Car Vehicle Dynamics, Milliken) that occurs with forward longitudinal acceleration.
MotoSPEC provides Data Equations and Data Tables to calculate chassis math channels in many data systems, including AiM Race Studio and MoTeC i2 Pro.
There is more information about MotoSPEC Data Equations and Data Tables at the following link:
CofG Height and Wheelbase math channels can be referenced in a Weight Transfer math channel, according to the following formula:
In the equation above, Mass is the combined mass of the bike and rider, in kg, and can be specified as a constant, or by another math channel that will calculate the change with fuel load. The CofG Height from the Data Equations and DataTables is only a function of Front and Rear Potentiometers and is not affected by fuel load.
The CofG Height is the distance from the ground to the Center of Gravity, considering compression of the front and rear suspensions. The CofG Height can be found in MotoSPEC by choosing CHASSIS as one of the graph series and clicking the TABLE button. In the Table of CHASSIS coordinates below, the same motorcycle is shown with three different combinations of front and rear potentiometer readings:
With the Longitudinal Acceleration being positive with forward acceleration, the Weight Transfer is the 'increase in rear axle downward load (and corresponding upward increase in the road reaction on the axle) or the corresponding decrease in front axle load' (Race Car Vehicle Dynamics, Milliken).
In braking, with a negative Longitudinal Acceleration, the Weight Transfer is also negative, being the decrease in rear axle load and corresponding increase in front axle load.
Below is a screen grab of the Weight Transfer vs Speed, Pot Readings and Long Accel from both IMU and GPS. The Weight Transfer is calculated using the IMU long acceleration, having been filtered and flipped to provide a positive value with forward acceleration.
The Weight Transfer can be seen to decrease as acceleration reduces over the length of the straight.
GPS Speed = black
FORK (front pot) = green
MotoSPEC RrWhlTrvl (math channel) = red
Weight Transfer = blue
GPS LonAcc = green
IMU INLINE ACC Filtered Flipped = drak green
To calculate the Weight Transfer, it is important to have accurate measurements for the CofG coordinates in MotoSPEC and also the longitudinal acceleration.
MotoSPEC provides a CofG Calculator with guidelines in the Help Manual for measuring and specifying the CofG.
The best Long Accel channel should be determined, and may come from the IMU, an accelerometer or derivative of the GPS speed. The Long Accel channel can be zeroed and filtered if necessary, to eliminate noise and ensure that it is zero when stationary.