The legal limit for tyre tread for motorbikes over 50cc is 1mm across ¾ of the width of the tread pattern and with visible tread on the remaining ¼. Motorcycle tyre manufacturers usually use tread wear indicators to help to notify you when your tyre tread is reaching its legal minimum and tyres need to be replaced.If you are unsure best get the tyres checked by a tyre specialist, after all it's the only part of your macjhine thats in touch with the road surface.