I'm thinking about getting the Performance 85mm rollers for the winter. Right now I'm running 39x16 because anything more kills my knee on long climbs. I know you need wheel speed to be stable on rollers, but I don't know to what extent. I could always throw a 13t cog on the flop side for the winter for ~80 gear inches. Will this give me enough resistance for a good workout or will I just be spinning me ass off? Never been on rollers.