Bicycle Mechanics - Crank Nightmare!

First post on this forum so hello.... I am getting back into mountain bike riding now that I have moved to the Scottish highlands and am cycling to and from work.

I am having major problems with the left crank of my bike, well two bikes, the left crank keeps coming loose, my dealer says that I have just been unlucky but it is driving me potty.

The first bike was just some crap one from tesco that I picked up just to ride to and from my local shop, after two days, the left crank came loose so I just chucked it in the scrapheap.

I went down halfords and purchased a saracen rufftrack, cost me £199, I thought it was a decent bike and would be good enough for getting around town, up and down hills, nothing too intense.

After two weeks (today), the left crank went, same as what happened with the first crap bike, so I have taken it back to halfords and they are going to put a new crank on.

Now, am I just unlucky or is it possible that I am putting too much pressure on the crank when I am going up hill, when I ride up hill I stand & pedal otherwise I find it too tough to get up the hills.

I am not overweight, I am in good shape so I feel I should be able to stand and pedal when needed without worrying about if the crank will be under too much pressure......or should I be sitting down when going up hills if I want the crank not to become loose (will take some practice though!).

Years back, I had a Raleigh bike which lasted me around 5 years, no issues with the cranks, even my brother used it now and again and he was overweight yet the bike was strong enough to take him so I am confused to why I am having these problems....does anyone have any suggestions...advice or tips?

Thanks - John

I've never heard fo this problem.. i don't think your crank should be causing you a problem unless you weight 300+ pounds

I've never heard fo this problem.. i don't think your crank should be causing you a problem unless you weight 300+ pounds

No problems with my weight, i am a keen sportsman, do running and football on a regular basis, i wish it was something to do with my weight, because then i would know what the cause of the problem was......there must be a reason why i am having problems with the crank or am i just very unlucky?

here is a link to the bike i have
http://www.discountbicycles.co.uk/biz/product.php?xProd=2284&xSec=302

Just one more point, it is the left crank arm which is coming loose and needing replacement.

These are square taper cranks/axles? Two thoughts, lowend bikes are more likely to have lowend crank fixing bolts. If the fixing bolts are really torqued they can stretch and not tighten adequately or worst case break. Check the end of the axle relative to the inside of the crank by torquing the bolt to suggested torque and then removing the bolt. The end of the axle should be a mm. or so short of the bearing surface inside the crank where the fixing bolt washer sits. If the axle is flush with the inside of the crank there is no way to tighten the crank. The taper square is off. Perhaps the end of the axle could be shortened 1-2mm to fix this. Replacement L cranks should be cheap and easy to get as another option. If the bolt won't stay tight Loktite or similar should be used. Steve

Thanks sch, I am picking up the bike on Wednesday so I will find out soon enough if it was just a badly fitted crank or if I need to think about buying better quality cranks.

I will also mention what you have said to my dealer if the crank does break again.

Had a neighbor kid with the same problem. I reinstalled three times. Finally cleaned out all the grease from the threads of the crank bolt (there was lots for some reason), used a new bolt and applied loctite. Then he moved away and took the problem with him. Solved.