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If you put a disc on the front wheel of conventionally raked forks, there is more side area of disc in front of the pivot than behind the pivot. (The pivot is the head tube of the bike.) When the bike is moving forward the air will hit one side or other of the disc wheel. The air pressing on the side will try to flick the wheel all the way round, like a weathervane. This makes the machine difficult to control: you're constantly trying to hold the wheel pointing the way you want it to point. With a vertical headtube (as in the bike illustrated in this post), there's more area of wheel behind the pivot than in front of it, so if the front wheel is a disc it too acts as a weathervane. But now the wheel is always trying to straighten itself, and riding in a wind is safe. I built a low racer recumbent like this oh, about - um - sixteen, seventeen years ago, and it was amazingly fast with discs on both wheels. I believed at the time any side-wind acted as it acts on a dinghy's sail, trying to drive the bike forwards. (To understand the physics you'd need to read up a basic book on dinghy sailing.)
