20" BMX Tires - Yes they are small enough that, depending on the size and geometry of the front fork, the fork can come in contact with the tire hoop they are secured to. If this is of concern, we recommend purchasing some of our extra straps to place and wrap on the tire basket to add a layer of rubber between the bike and rack. We also have Small Tire Baskets to accommodate 20" tires. 20" Mountain Bikes - These have larger tires than most BMX bikes and so typically fit great. 16" Kids Bikes - The rear wheel on smaller bikes aren't quite big enough to reach the lower cross bar but many rack owners have tied the back tire down to the lower bar or placed it in the center of the rack and tied the rear tire to the main post.