Frame cracks, carbon fiber


New member
Local time
4:11 PM
Oct 11, 2023
I have developed cracks in the horizontal tube of my carbon fiber e road bike. The bike store says I had a crash. That is untrue. Anyone else seen spontaneous cracking in the horizontal tube?
my friend has one of those carbon fibre analog bikes and his cracked by the head tube ....
Stress fractures/cracks from the vibration from the road he told me (shrugs)
Post a few pictures.
Specify which frame you have, how old is the frame, how much have you ridden it, has the frame been exposed to extreme temperature?
Personally I have not been a fan of China Carbon Fiber either eBike or Bike frames. There is too much "hand work" as compared to Steel-Aluminum-Titanium.

Yes - what you describe can be reasonably caused by Impact Damage (where the fiber's shear) or by a "Hidden" Manufacturing Defect - by Hidden meaning not visible during production visual QC.
Unfortunately, carbon fiber frame failures happen often enough on DIY mid drive builds that its common practice to warn people away from them when someone asks whether to go that route. Even if it involves a bespoke-to-the-motor frame and not something hanging off the bottom bracket like a BBSHD.

C/F also often fails spectacularly. An old adage when it comes to frame construction: "Steel bends. Aluminum snaps. Carbon fiber shatters." That may be a bit dramatic insofar as c/f is concerned, but not by much. Myself personally I've had a c/f seatpost split on me, which a) made me think in horror about the kind of bodily injury I could suffer if I came down on splinters and b) caused me to examine my priorities and remove c/f handlebars and stems on various bikes and switch back to alloy.

My favorite meme on the subject of bicycle frame material:

Carbon Fiber had limited high end application in Road Bikes for Rigidity - Design Capability for AeroDynamic “Edge” Profile - and properly done can be Lighter than Titanium. Like anything knock off material is “Carbon Fiber Like” and just is a gimmick
The fact is this - "Carbon Fiber eBikes" - here in the U.S. Enthusiasts Carbon Fiber eBikes under $1500/as low as $899 have ZERO Genuine Carbon Fiber (think if it as a Plastic Frame with decorative fabric embedded in it) the Urtopia Carbon 1 does have non-functional "Carbon Fiber" matt material in it (where the embedded fabric is "a Carbon Fiber kleenex matrerial) - you have to be way beyound USD$6000 and up to have a Genuine Structural Carbon Fiber eBike Frame. ie: Trek Fuel Carbon - Cannondale etc. where the frame is contructed of structural cabon fiber matting - hand contructed One-Frame-At-A-Time
Two possibilities:
1. Frame quality issues: A real carbon fiber frame is formed using carbon layers cut into different angles, and closely attached to the mold according different directions;
some factories with insufficient technology use long strips of carbon layers that are conveniently cut to wrap, and it looks like The appearance is the same, but the internal strength is not enough;
or the worker made a mistake when fitting the carbon layers, causing the carbon layers to not fit tightly, and the quality was not strict and there was no inspection problem;
or the inner mold used by the manufacturer was an airtight expansion bag, it's a good technology named EPS, but the manufacturer’s technology is insufficient causing the carbon layers to Uneven stress on the layers insufficient stress, etc. result in insufficient strength of the carbon fiber tube.
EPS technology can improve the strength of carbon fiber products. The inner mold of the inflatable bag is a primary EPS technology. Most high-quality carbon fiber manufacturers are using this technology, but the way it is used may not be correct.
The new latex inner mold technology is an improved version of EPS technology. The use of 3D latex inner molds instead of inflatable bags allows the carbon fiber layer to be pressed more tightly. The strength of the same material is increased by 30%, but the cost increases by nearly 30%.
In addition, some frame are not the one-piece carbon fiber frame technology, they use different carbon fiber tubes, connected through adapters, which will result in insufficient overall strength of the frame..
2. It was hit by an external force, and it was not reflected when the impact, but the damage was actually done inside the carbon fiber.
Normally, the top tube is a part that is less likely to cause problems.