I am a fairly green mountain biker, I am primarily a BMXer in my early days and progressed to a basic GT mountain bike to keep myself in shape now I'm older.
I have a small problem with my GT mountain bike - when I cycle OUT of the saddle e.g.up hills or for speed on flats.....the pedal slams violently from 12 o'clock position to 6 o'clock position occasionally. I had this same problem on a hire-bike also once.
I am unsure wether it is a crank or rear sprocket etc.
I went to my 2 mountain bike shops to ask (without the bike) - one of them said 'ooooo, that could be the crank, you might need a new crank'. The other said it could be rear sprocket - but both gave that impression that they would rack up lots of hours of labour....I am concerned I'm going to be ripped off here as the fault is intermittent and not obvious unless you went for a 15-20 minute bike run pedalling out of the saddle.
Can anyone lend some advice?