RSF - The Off Road Cycling Club

For 'the perfect loop' you should climb up from Lauterbrunnen to Wengen, but to save yourself about 1,500' of vertical dirt road take the bicycle-carrying train up to Wengen and start from there. 

Leaving the station follow the red "bike" signs out of town on the Wengen to Kleine Scheidegg Trail, just follow the railway track. Initially a paved road but soon it turns into dirt very quickly. This, however, is rewarded by spectacular viewings, weather permitting! Steadily climbing now through working pastures with the awesome Eiger, Monch, and Jungfrau all around you. From near the top you just follow the railway track for a while until you get to Kleine Scheidegg, an excellent stop for refreshments. This part of the route is wooded at places but on the whole it is mostly exposed and can be quite hot while climbing.

From Kleine Scheidegg to Grindelwald, follow signs from the hotel area to the red "bike" signs down steep dirt roads through the ski area. The north face of the Eiger is very impressive from here, you'll see Grindelwald in the distance After a couple of miles, the road improves and you can do down-hill very fast, but take care: hikers and farm traffic are there, too! You follow the road through the woods and cycle through the farming village of Grund and so into the lowest part of Grindelwald. 

For the Grindelwald to Zweilutshinen Trail, follow the signs carefully and you'll be off the main road almost entirely. There are side roads, paths, and farm roads almost the entire way - keep you eye out for the red "bike" signs. This connector is easy, and downhill, with some pretty farms. The scenery and nature are pretty limited compared to other trails around here though. 

Finish up the loop on the Zweilutschinen to Lauterbrunnen Trail. Though lacking the grand views of the higher trails, this pleasant path goes right along the river on an easy crushed gravel path. It's mostly forested, with several bridges and a couple tunnels.

This download only contains the GPX route from Wengen to Kleinbe Scheidegg, the other three parts can be downloaded from https://www.mtbproject.com/