3900. Trek 3900 is considered to be the top among commuter bikes. It has good frame quality and good riding comfort. The 3900 is an outstanding representative of mid- to low-end models. It has some good sales among TREK and most of the riders who buy it like it. Its super strong body frame is made of TREK's unique patented Alpha aluminum alloy. This Alpha aluminum alloy has extremely light weight and super hardness, which further reduces the weight of the entire vehicle.