Write an email to your English friend Dave.In your email ,you should

Explain why you need to borrow the bicycle.
Say how long you will need it for
Tell Dave when you will return it.

Hi Sam,
Is everything ok? I am writing to you because I would like to borrow  your mountain bike to go  on  a ride  around the lake this weekend. I will return  it  next Monday .

See you.

Take care

If this is a writing assignment for class it is too short.I think you could add some more details (for example why can't you use your own bike, how you feel about your friend Sam, and say you will be really grateful etc) I think you could say how kind it is of him and how much you appreciate his help.You could also say that you promise to take really good care of it....