Thanks it is quite usefull . I meant the chainline might be too long at the front compared with the back ( or not long enough ) .
If I need to correct the chainline , I do it by adjusting the bottom bracket or changing it , right ?
I took the bottom bracket from my GT i drive to put on the Zaskar , that is why i may have a problem . The cranks are retro TNT Performance
I am not fit enough to use the small one anyway
Not 100% sure what you mean by too long or too short.
To check chainline visually put the bike in a stand or hold it up right. Stand behind the bike by the back wheel and check the alignment between the casette at the rear and the chainrings at the front. If you look parrallel to the sprockets the middle chainring should line up with the middle of the rear cassette (5 sprocket on a 9 speed, between 4 and 5 on 8 speed, 4 on a 7 speed).
Using trick cranks is a classic way to throw your chainline out since they tend to have a different 'q' factor to standard shimano cranks.
The normal way to sort this is to switch bottom brackets. This is assuming chainline is your problem rather than something else.....