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Bike Friday use SKS Velo 55's. They're cheap and plasticy but have one killer feature: they pop off. So when you are packing for travel they are ridiculously easy to take off. Bike Friday use them without the stays at the back, which I think looks ridiculous as it hovers a great distance above the wheel. I found you can use the stays and just set them a little loose at the nut of the black plastic insert. You can then pop the fender and the metal part of the stay off when packing but leave the plastic insert on the bike. This not only works well and looks good, but it allows the rack to be installed and keeps the rear fender from rattling against it. It also means that I can happily keep the fenders off whenever I don't want to use them.