I know your supposed to breath in through the nose, out through the mouth. But the other day when i was riding i realised it was more in the mouth out the mouth so i changed to in through the nose out through the mouth and i couldn't seem to breath fast enough and got very tired very quickly. Any ideas anyone?

I really think a lot of that stuff about breathing through the nose came about when that indoor spinning stuff came about. Really, what you should focus on is diaphragmatic breathing instead- inhaling and expanding the ribcage, taking in as much air into the lungs, filling the lobes at the bottom of the lungs, then exhaling by contracting the abdominals as you exhale. The exhale should be longer than your inhales. You can practice this by sitting quietly in a chair for a few minutes each day, then gradually practice this while riding. If you do practice this, do it with the mouth closed.

It does help to have the mouth closed during exercise... IF you are practicing diaphragmatic breathing. But eventually, when you are doing hard efforts, your mouth is definitely going to swing open! If you look at the pros, especially during the time trials, their mouths are definitely open, they are taking huge gasps of air, and I'm sure the diaphragmatic breathing is out the door. But generally, you want to discourage yourself from shallow breathing, where you are mainly taking air in through the mouth and breathing more through the thorax than through the lungs. To that end, if you shut your mouth and breathe through the nose, I think it helps you to focus on the diaphragmatic breathing. But if you aren't practicing diaphragmatic breathing, then it doesn't matter if you're breathing through the nose or the mouth.

Koffee

I found that I breathe through my nose only when I'm going easy on my bike...once I start pushing it I have no choice but to use my mouth.

It's the whole thing about the mouth having a massively larger opening for air to go through than the nose thing :)