Frame Reinforcement Issue

[userix]

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.

 

[wheelbender6]

I think theses frame crack because the steel is not flexible. Earlier steel frames were heavier, but made from a mild steel with more elasticity.
To fix your frame, you could attach your reinforcements in such a way that they form a second set of chain stays. Mount them halfway between the seat stays and chain stays.
I would use steel instead of aluminum. It is much easier to weld.
In the long run, I would find a used frame from the 90s or older on craigslist or something.

 

[biknut]

I don't think its going to be worth the trouble to fix your frame. Get a new bicycle and start over. Why your frame is cracking is a mystery. My guess is the steel tubing it's made out of is defective, or the shape of the frame doesn't work well for motorizing, but it's probably the metal.

When you let out the clutch to start the bike, don't stop peddling. See if that helps with the lock up problem. How much do you weigh?

 

[userix]

I weigh 160lb

 

[biknut]

I weigh 160lb

That's not enough to cause any problems. I weigh 215.

In that case, as long as the motor mounts were all tighten properly, then the frame is probably at fault imo. That's assumeing you weren't jumping it, or running WFO for long periods of time.

Considering how cheap low end bicycles have gotten, I would buy a new bike, and use your old one for a parts bike rather than fool around trying to save the one you've got.

I recommend a OP Roller from wall mart as a good replacement for your bicycle. It's cheap, looks about the same as your bike, and is Aluminum.

 

[2door]

I'm not familiar with the Legacy Cruiser but the frame looks very much like the Jaguar and Southport cruisers which are very popular foundations for motorized bicycles. I have two Jaguars both of which have hundreds of miles on them with absolutely no evidence of metal fatigue or weld failures. Your weight is just a little over mine so rider weight isn't the problem. Neither is the shock of clutch engagement.

If I understand your original post you have a new Legacy and are looking for ways to reinforce it. I don't feel reinforcing the frame is necessary. I have to think you had a defective frame on your first bike. I have no idea where or why but that Schwinn frame design is more or less trouble free...in most cases.
By the way, your chain tensioner bracket will prevent a lot of problems. Good work.
Please keep us informed on your efforts and their results.
Tom

 

[userix]

Just for prevention's sake and hoping the existing crack won't propagate all the way through, I fabbed some reinforcements using the leftover aluminum flat stock from my custom tensioner and some adel clamps (big fan of them as you can tell) from Ace. I don't know how to weld or have the equipment and the local muffler shop won't weld metal to anything that has any gas in/near it. I was too lazy to take off my gas tank, as it was a bitach to get it on the curved top tube without it splitting at the mounting studs and causing a leak (like my previous two tanks). Not exactly sure if these are considered "gussets" (not even sure if I am using the right term), but it is very sturdy and would seem to prevent any forward horizontal compressive forces exerted by the motor and drive chain.