Frame Reinforcement Issue
I have a Schwinn Legacy Cruiser (Steel frame) from Target and had problems with the frame snapping completely at the area marked with a red dot. It splits right behind the weld spot between the seat stay and the top tube only on the left side of the frame (same side as chain tensioner). After getting another one, I am thinking of ways to reinforce the frame so it doesn't happen again. Now on the new frame, a hairline crack is starting to form on the right side seat stay right behind the weld spot between seat stay and seat tube. The left side (chain tensioner side) is perfectly fine. I am not 100% sure of where the force is coming from and its direction. The 2nd and 3rd pictures are my guesses as the force vectors causing the frame split.
2nd pic, I hypothesize that the engine, when popstarting or pulling hard, is generating a force vector that is horizontal and parallel to the floor and thus resulting in compression of the seat stay tubes, eventually leading to stress fracture at the point right behind the weld.
3rd pic, I hypothesize that the engine, when popstarting or pulling hard, is generating a force vector that is downward and parallel to my custom chain tensioner mount, thus pulling on the seat stay tube toward the chain stay and causing a stress fracture at the aforementioned point.
Perhaps it is a combination of both forces that torques the seat stay tube to fracture.
The 4th pic is what I believe is a good way to reinforce the seat stay tube to the seat tube by using flat stock aluminum and some heavy duty adel clamps bilaterally. Again, I am not 100% sure if this is the best way to reinforce my particular frame and prevent future stress fractures.
The 5th pic is an untouched photo so that anyone that wishes to help me can doodle what they believe is a good way to reinforce my frame. Thanks in advance for any help.
P.S. I notice sometimes when I attempt to start the bike, it is so tough that when I release the clutch at moderate to fast speeds, I actually lock the rear tire and skid, instead of turning the motor to start. The clutch is set correctly and it runs fine without problems. The bike rolls freely when the clutch is disengaged and in the locked position on the lever. This doesn't happen all the time, but occasionally, but I am thinking that this hard lock up on starting attempts is generating massive amounts of force to the seat stay tube and snapping it like a twig. But then again, the new fracture is starting to form on the opposite side seat tube (pedal chain side), so I am not sure anymore.
Last edited by userix; 07-04-2011 at 04:05 AM.