Recently, there have been many reports about development of Human Machine Interface (HMI) which used a visual or auditory display to improve driver assist. For a motorcycle, it is suitable to use a haptic display for rider assist, since there are many disturbances in open-air situation such as sunshine and surrounding noise. Therefore, a test vehicle based on a large motorcycle equipped with the haptic throttle grip that is connected to a motor by gear, an original ECU to control the motor, and a lean angle sensor to calculate the lateral force of the motorcycle was developed. To inform surplus driving force warning, the throttle grip, a haptic display outputs additional return torque estimated by the friction circle and ellipse with the lateral force of the motorcycle. One of the purposes of this torque is that it informs the rider to decelerate when the motorcycle state of movement is around the limit of tire grip, and another purpose is that it leads to return the throttle grip to decelerate. For both purposes, the intuitive signal which adds return torque was selected to avoid misunderstanding. As the result of riding tests, all of the riders were able to recognize the signal and understood the request from the motorcycle. It became clear that this system was able to offer the information of surplus driving force warning, and to support the throttle grip operation.