Problem with my mountain bike...can anyone lend some advice?
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.
Possibly the chain is temporarily slipping partly off the front or rear sprocket, just for a single stroke? Or something in the freewheel is broken? I don't think it would be hard to fix.
I would start off by re-adjusting the indexed shifting (it might be very slightly off). If that doesn't fix it, maybe swap out the freewheel...
It should be noted the bike has maybe 60-70 miles in total on it, not a great deal. So I am wondering if the derail-adjustment would be more likely as a worn sprocket or chain would warrant more miles?