The frame is designed for a front wheel of size 50-355 (18" Any wider 355 tire would fit, but currently there are none on the market...) or any 406 (20") front wheel and a rear wheel of size 60-559 (26") or 40-622 (28") in the rear. Any combination of front/rear wheels will work out without affecting steering noticeably. We suggest using 20" front wheels if racing tires are desired, to maintain sufficient ground clearance. If you want to combine 20" front wheels with the big box, the box needs to be lifted above the frame by spacers to allow for space for the front wheel. Schwalbe Marathon racer tires in 40-355/40-559 are a good compromize between sportiness and everyday use.