A free metronome app that vibrates the phone

I'm looking for an app to facilitate piano practice. Currently, most free metronome apps only provide sound or light indication of the beat, which is understandable because they want to sell hardware or software to make money. But what if I just want to attach my iphone to my arms and let the vibration guide my playing? I've tried multiple apps and only found "Pulse" works, although not great (it vibrates too lightly). Does anyone know any app that vibrates properly as a metronome?