Is there any general rule or recommendation to avoid usage of certain gears combinations (for 2xN and 3xN gears systems)? Do they differ for mtb and road bikes? I know that it's recommended not to use biggest front chainring with big chainrings on cassette (and opposite) because parts will wear faster and I will loose a bit of pedaling efficiency, but I'm not sure, if I should avoid 2 or 3, or even more biggest (smallest in opposite case) chainrings on cassette.
This is an old chestnut from the days of 5 speed freewheels and chainrings without shifting ramps. With modern hyperglide cogs and the relatively slinky thin modern chains, cross chaining is simply no longer an issue. While there are very few studies on this, the one I've seen suggests that there is no loss in mechanical efficiency for cross chaining at the angles typical of most bikes.
In general for a given gear ratio, big/big is more mechanically efficient than small/small. So the only science out there suggests that cross chaining on the big chainwheel is actually more efficient.
Use the gears you like, check your chain wear and replace your chain often and the expensive bits of your drive train will last a long time.
The reason to avoid using, say, the larger front chainring with the larger rear sprockets (or vice versa) is nothing to do with loss of efficiency or anything like that, but because of a phenomenon called
If you're in the smaller chainring, which is more inboard, then you should be on the more inboard rear sprockets, the larger ones, and similarly, when your in the more outboard chainring, the larger chainring, you should be on the more outboard, smaller sprockets. Cross chaining happens when the chain has to cross from being inboard at the cassette, to being outboard at the cranks.
Cross chaining puts quite a lot of stress on your chain and gears, can lead to a noisy, squeaky drivetrain (chain rubbing against the inside plate of the front derailer, for example), and causes your chain to wear quicker. Quicker chain wear then leads to quicker cassette wear, and quicker chainring wear.
This is why gears are positioned as they are, to encourage you to use the bottom 7 rear gears with the smaller front chainring, and the top 7 rear gears with the larger front chainring. This means that the middle 4 rear sprockets (on a ten speed bike) can be used on either front gear, while the bottom 3 and top 3 should be used with the corresponding chainring.
A typical change-up (based on a 53-39 front, 11-23 rear setup) will go like this: Start off low, maybe 39-21, change the rear up until you get to maybe 39-15, then shift to the larger chainring and at the same time, do what's known as a
recovery shift, which is shifting the rear gear down 2 or so sprockets so that jumping from the small to the big ring isn't such a massive increase in gear (53-15 compared with 39-15). This way you find yourself in 53-19 with a good, steady cadence. Then simply proceed up the sprockets as far as you need.
A change-down might go in reverse, or you might simply change to the smaller ring when you get down to the middle sprocket if you know you're going to be slowing for some traffic lights, or getting prepared for a climb.
I see too many cyclists simply going into lower gears in their rear sprockets while leaving the front in the big ring. It's a good idea to stick your head between your legs every once in a while to see where your gears are. Without those fancy indicators on your brifters and shifters, you can forget which ring you're in!
I try to avoid the two biggest/smallest rear sprockets when in the large/small chainring, but that's just a rule of thumb. In the flat, I try to stay around the middle sprockets in the cassette to have as wide a gear range as possible without switching chain rings.
Using these combinations will indeed increase wear and reduce efficiency, but how often do you really have the need to ride these combinations?