Saw a DIY set of studded 28s on another commuter website. Can't remember the address. I think they were cx tires, though. That's why I had the ? after my inital suggestion...I've never seen a production studded tire in 28mm either. Maybe try some SMPs in 25mm? Depending on what type of rim they could be a much harsher ride than 28mm, though. I've got 2 sets One's on my ss w/Sun cr18 rims(harsh) and the other set's on a 1x8 rb w/high profile aero rims(not so harsh).