how wide 28" can be ? or how to beat mountain trails with 28" I have an hybrid bike with 28 in wheels and 40 mm wide tires. The problem is that the recommended pressure is 70 psi. They are quite thin compared to mountain bike tires and that pressure makes me feel every stone on the way. They have terrible grip making the bike slip and the wheels to deviate on any stone bigger that a pingpong ball.

In resume I feel really uncomfortable in that terrain. This problems can be solved if I go with some speed; I can just jump over the obstacles. But this is impossible in group rides as they will be going far slower (also that speed breaks don't work well: my bike just slips on stones). Some of my mates insist that I could mount wider tires without any problem. But I didn't find any wider ones on the internet. Do they even exist ? Also I really doubt that my wheels can handle wider tires as far I am aware they are made for a small variation in wide.

EDIT (looks like there is more that enought room):





Edit2: I got 1.95 tires but the back one is hitting the frame partially it is because it is to big and partially it is because I have to true the wheel but even true they would be millimetres to frame. I cant understand why it is so much bigger!it should be only 10mm wider.
Ok so I presume your tyres are 700x40C. First of all, that pressure sounds high - is it recommended or max pressure? I am sure you could decrease it a fair bit, although if you decrease it excessively you run the risk of pinch puntures.

If you do want to change your tyres, 700c road rims and 29" MTB rims are the same diameter, so you can use, for example, a 29x2.0" tyre - if your rim is a suitable width - see the table at the bottom of this page for recommended rim/tyre width combinations

EDIT: As armb has quite rightly pointed out, you will also need to check for sufficient frame clearance if fitting wider tyres - and remember you'll need to allow some space for mud accumulation if riding off-road