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 glass/CF steerer sandwich
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SpiderMonkey
recumbent guru

USA
690 Posts

Posted - 06/09/2019 :  09:47:07  Show Profile  Reply with Quote
I cut the steerer on the CF kidbike fork today using my off-brand fork-cutting jig (spaced further with extra washers to fit a park abrasive hacksaw bit). Worked well, and I immediately noticed that the steerer isn't all CF, but a glass-cf-glass sandwich. At least that's what I suspected. Testing with multimeter confirmed it. Gray layer doesn't conduct. Black layer does. (Rest of fork is CF as tested a while back).



No surprise, as this is an el-cheapo from China.

I can see the sense in having a couple thin layers of glass around the CF where aluminum parts are likely to come into contact (plug, stem). This seems a bit more than that. (OTOH, if they were really trying to rip you off good, they wouldn't even bother with a layer of CF, I guess. The el-cheapo "carbon fiber" stem spacers you get are all glass inside with a thin layer of CF on the outside, also confirmed by multi-meter.)

Would anyone bother trying to stiffen this up with another layer of CF? I know even e-glass is stronger by area than regular CF (just not as stiff). I guess it could always be done later if for some reason it feels flexy.

My guess is that if this fails, it's not going to be a result of this steerer material sandwich, but something like a lousy join down at the fork crown or a flaw in the build of the legs, or something like that. I ended up having to remove the finish from the fork legs, and they look pretty well done. No strange joins or voids or anything.

--SpiderMonkey

Edited by - SpiderMonkey on 06/09/2019 09:49:23

SpiderMonkey
recumbent guru

USA
690 Posts

Posted - 06/11/2019 :  05:02:33  Show Profile  Reply with Quote
I was doing more prep on this fork and a little more research. The glass-carbon-glass sandwich appears to be pretty common on other forks, higher priced ones included. I've seen them on videos where an ultrasound dude is critiquing really minor delamination as a result of bad stem assembly, and he never bothers mentioning the glass layers.

I also see that in the fork under consideration, further down, there are areas where it appears that the CF underneath is exposed. Testing with multimeter confirmed it, so it looks like when these come out of the mold there's more CF toward the bottom, and the glass is put on last, and final machining down to the correct steerer diameter sometimes goes through the glass layer.

--SpiderMonkey
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