I hired a bike last week to take the route from Myrdal to Flam. The cost for the bike was 350 NOK (Approx £35) for around a 1-2 hour journey depending on if you stopped off.
The bike hire process was easy. I reserved the bike online and paid at the cafe when I collected it. I didn't need to pre book to be honest as there were plenty of bikes and it was absolutely raining, but can imagine on a good day they could be in high demand.
The bikes themselves are good quality mountain bikes with sturdy frames and large thick wheels for the gravel paths.
The route was easy and mostly gravel but moved onto roads. Road traffic was minimal and I felt pretty safe throughout (they provide helmets too). Some comments mention that the route isn;t for beginners but I would disagree...just don't go too fast and you'll be fine.
The views along the way were something else. Great waterfalls, views of the valley, and rivers, as well as tunnels. It was pretty cool. There are plenty of places to stop along the way if you wish. The cafe does provide a takeaway bag of food if you need one, however it does cost and arm and a leg (typical high Norwegian prices).
Once at the bottom I left the bike in the allocated space and got a train back up to Myrdal to continue my journey. Be aware the train back up is almost as expensive as the train back up as once in Flam you are in the tourist trap and they can charge what they like for trains!!!
If in the area I would recommend the bike ride.