generally you can go up 2-5mm on a bike that is designed for fenders (if you don't use fenders).


It depends on the frame, but I have been limited by:
- front derailer motion
- brake mounting point (I have dremmeled brakes, or just taken them off for a FG
- on my road bikes, the width of the brake calipers usually can't be 28mm or the tire will have to be deflated to get them inside the calipers.
- often a slightly bigger tire can be put on the front fork than on the rear - this will give you a little bit more cush up front.


Also, most people should be riding their tires at the minimum pressure on the sidewall. According to detailed testing, there is very very little difference in rolling resistance until you get below a critical point. You can also go 10% less pressure in the front tire since the back tire takes about 50% more weight than the rear wheel