my crank arm is broken

[krautboy]

Hey guys I am new to this. I have a massive problem, when I was cycling to work this morning the pedal came off my mountin bike. Now the main issue is that the crank arm is ruined. The screw thread is worn out, and I need to get a new one.
I have seen a couple of videos of how to remove the crank arm, but i dont have a professional crank arm remover, are there other ways to remove the arm?
Where is the best place to by crank arms, relativly cheap.
It is only the left crank arm which is broken, so no need to buy a full set.
I measured the arm, it is 20 cm in total (from one side to the other), 18 cm from the center of the screw to the end of the arm, 17cm from the end of the screw to the end of the arm. I am not sure, which measurement is correct. So help would be much apprechiated.
I bought this bicycle at empirestores.co.uk for 170 £, seems that this bike is not really good.
I have attached some pictures, hope that helps.
Couldnt attach the pictures, so I just copy them in here.
http://tinypic.com/view.php?pic=bgwj6v&s=6 (Crank)
http://tinypic.com/view.php?pic=jr6877&s=6 (View of 20 cm crank arm)
http://tinypic.com/view.php?pic=33k6l9f&s=6 (View of 18 cm crank arm)
http://tinypic.com/view.php?pic=11w54bn&s=6 (view of 17 cm crank arm)
http://tinypic.com/view.php?pic=2s7f5v4&s=6 (bike full view)
http://tinypic.com/view.php?pic=35n7xnc&s=6 (view of crank arm from top)

 

[OldGoat]

Crank arm length is usually stamped on the back side of the arm.
I wonder if the pedal hole threads being stripped has to do with someone installing the pedals on the wrong side. The left (non-driveside) pedal has a left-hand thread, and screws counterclockwise to install and clockwise to remove. It is possible to screw a left-side pedal into the right-side crank arm, and the reverse, by crossthreading the screw threads; however, doing so will destroy the threads in the crank arm. Additionally, precession is then likely to cause the pedal to fall off the crank, as happened to you.

"'Precession" refers to the tendency of a part subject to rotating stresses to rotate in the opposite direction of the stress rotation. In bicycle applications this becomes a particular issue with the threads that hold pedals into the crank, and the threads that hold the bottom bracket assembly into the frame." [Sheldon Brown]

"On bicycles, left hand threads are used mainly in three places, on left pedals, right bottom bracket (BB) bearing cups, and freewheel cones, to prevent unscrewing under operating loads. Unscrewing occurs from precession, in which a round object rolling in a circular ring in one direction will itself turn in the opposite direction." [Jobst Brandt]

I'm willing to bet that each of your pedals was installed on the side opposite its intended location.....if true, you will need an entire new crankset, and not just a left-side arm.

 

[swampy1970]

Hey guys I am new to this. I have a massive problem, when I was cycling to work this morning the pedal came off my mountin bike. Now the main issue is that the crank arm is ruined. The screw thread is worn out, and I need to get a new one.
I have seen a couple of videos of how to remove the crank arm, but i dont have a professional crank arm remover, are there other ways to remove the arm?
Where is the best place to by crank arms, relativly cheap.
It is only the left crank arm which is broken, so no need to buy a full set.
I measured the arm, it is 20 cm in total (from one side to the other), 18 cm from the center of the screw to the end of the arm, 17cm from the end of the screw to the end of the arm. I am not sure, which measurement is correct. So help would be much apprechiated.
I bought this bicycle at empirestores.co.uk for 170 £, seems that this bike is not really good.
I have attached some pictures, hope that helps.
Couldnt attach the pictures, so I just copy them in here.
bike 6 Pictures, bike 6 Images, bike 6 Photos, bike 6 Videos - Image - TinyPic - Free Image Hosting, Photo Sharing & Video Hosting (Crank)
bike Pictures, bike Images, bike Photos, bike Videos - Image - TinyPic - Free Image Hosting, Photo Sharing & Video Hosting (View of 20 cm crank arm)
bike 2 Pictures, bike 2 Images, bike 2 Photos, bike 2 Videos - Image - TinyPic - Free Image Hosting, Photo Sharing & Video Hosting (View of 18 cm crank arm)
bike 3 Pictures, bike 3 Images, bike 3 Photos, bike 3 Videos - Image - TinyPic - Free Image Hosting, Photo Sharing & Video Hosting (view of 17 cm crank arm)
bike 5 Pictures, bike 5 Images, bike 5 Photos, bike 5 Videos - Image - TinyPic - Free Image Hosting, Photo Sharing & Video Hosting (bike full view)
bike 4 Pictures, bike 4 Images, bike 4 Photos, bike 4 Videos - Image - TinyPic - Free Image Hosting, Photo Sharing & Video Hosting (view of crank arm from top)

Cranks are measured from the center of the bottom bracket and pedal spindles. So, just measure from the center of the two holes and you'll be fine.

Like a previous poster said it should be posted on the back - in mm. Failing that a quick look at the bike specs online should tell you.

A bottom bracket remover isn't that much - but it'll be cheaper just to wheel it into your local bike shop. It could be that under the endcap on the crank you have a bolt with a built in crank remover. I hate those things and use a trusty all steel unit but YMMV.

 

[krautboy]

Hey thanks guys for your reply.
I checked the right crank arm, that one is obsolutly fine, the pedal is the right one on that side.
I checked the inside of the crank arm, it gives me these numberes and letters, but I have no clue what they mean.
FC-GSP312
EN-14766
You think this is kind of a serial number for a crank arm?

 

[OldGoat]

EN 14766:2005 refers to the European Safety Standard for Mountain bicycles. See this link in interested, though it doesn't bear on your problem:
http://www.baltikvairas.lt/filemanager/download/29/EN 14766 .pdf

FC no doubt stands for Front Crank (as in Shimano FC-7700), and GP312 is a model number. Any idea who made the crank? Is mfr. name stamped or decaled on the opposite crankarm? re you sure you're reading the GSP312 right?