Is there a possibility of a cheap frame snapping out of the blue?

[Gavin67]

If you were to put a engine onto a cheap bicycle frame and just ride it normally on roads, is there a possibility of the welds to all of a sudden snap from the vibrations? Or is it only when the bike crashes or has an direct impact to the frame that snaps it.

How cheap the bicycle has to be to most likely do this? Most bicycles in the $140-$200 range are made of hi-tensile steel, the weld quality on the $200 ones are a bit better and they tend to have nicer parts.

Do you necessarily need something like a chromoly frame?

 

[bakaneko]

i think it also depends on your weight and conditions of your roads. i have a $50 walmart bike and it is fine but im a very light rider.

 

[crassius]

cheap or expensive, none were designed for this vibration

I see mostly snapped frames right above or below the motor mounts, but sometimes see cracks around the welds near pedal cranks. For years now, I've been putting some PVC in mounts, but over enough time, any frame is subject to failure.

With good mounting, most frames are good for 3 to 5 years of hard use, with bad mounting, many will fail in first year.

I'm just a repair shop, so much of what I see are the fails.

 

[KCvale]

With good mounting, most frames are good for 3 to 5 years of hard use

I confirm this, and with mild use these bikes can last a decade or more.

 

[gary55]

Like Crassius, most (but not all) I have seen have been by the front mount. All of these were due to concentrating the mount pressure in a small area of the down tube accept the gt2a gasbike/kings frames. I have seen 2 of these break where the pedestal meets the down tube. I have also seen a few down tubes break at the head tube. 2 of these were caused by accidents. I am guessing at this, but I think since I have never seen this happen on mountain bikes and only on cruisers that the geometry of the frame on mountain bikes creates less lateral (shear) stress on the down tube.

 

[butre]

I've seen a lot of cracks on the chainstays as well

 

[Gavin67]

Would there be anyway to tell if the frames integrity is compromised? Or can it really just snap without warning?

 

[Deleted member 25783]

Just do a walk around before every ride, if something shows a crack fix it before you ride again.

 

[6blueoval9]

As a former mountain biker supporting both higher quality bikes as well as department store models there is a difference in the quality of frames.

Quality control being one main issue and the quality of the material itself...6061 aluminum is not all created equal and companies like Pacific cycle will spend less money on material that didn't make the best grade in quality.

6061 tubes on a Trek or a Giant is not the same lesser quality tube used on a Huffy or a mongoose you buy from Walmart.

The thing with fixing a crack is unless you can heat treat the area welded afterwards it's not really stronger than the weld it was before...The heat treating process is pretty much it's strength and of course a quality weld in general...You would be surprised at how many air bubbles you might find in some of those welds.

Walmart bikes are intended mostly for kids or adults for recreational use only and may only "simulate" a particular style of biking like mountain biking or bmx or bmx free style...are they capable of enduring what the high end bikes do...no.

Trek, Giant and Specialized and a few more spend alot of money trying to insure their bikes can handle the rigors of true mountain biking or bmx and sorry to say Walmart just doesn't supply that level of safety or reliability and never will.

What I would personally suggest is looking around on Craig's list for a used quality built bike ,if for nothing else to use the frame for a build.

The material and weld quality is far better than any Walmart bike as is the components themselves all though not really intended for motorized use.

Now I suppose if you have or ride rather smooth streets then a Walmart bike may hold up safely at lower speeds but for me personally my life or health outweighs me spending less money on my safety.

Keep in mind a tiny bump at 10 mph while pedaling is not near as great of a force as doing 25 or 30 mph over that same bump.

So ,my answer to the question is, Yes...any frame is subject to failure even a $5000 frame...How great the chance of failure I would attribute to the quality of frame used.

The strongest most sturdy gas bike frame I've seen today is made by Felt who makes high end bikes.

As far as a mountain bike style frame, the GT triple triangle design is hard to beat and these are made with both steel and aluminum versions.

If you want an older cruiser type of bike look for the older Schwinn's as they used to be assembled in America with quality steel and welds.

The felt is priced right at less than $200 which is rather a shockingly low price for that high end company and seems to be a no brainier as a great foundation for a build...I'm surprised they aren't priced near $400 for a frame or more

 

[KCvale]

gasbike/kings frames.
I have seen 2 of these break where the pedestal meets the down tube.

That is not the least bit supersizing, gasbike is infamous for selling knock-off crap.