The purpose of this thread is to shed a little light on some of the principal differences between how Carbon Clincher rims are produced in Chinese factories vs Taiwanese factories.
A few caveats:
1. There will be some generalities here because it would be impossible to know how EVERY factory does what they do and it would be false to claim otherwise.
2. There will be some omissions because some information is not mine to discuss and if I did so I would have to close up shop and sell fruit by the side of the road.
A few other facts:
1. What I am describing I have seen in person and held in my hands. This is 100% first hand knowledge and this is what I do to feed my family and buy my beer.
2. Cats don't always land on their feet.
Carbon rims are made in a mold that has a top half and a bottom half. Carbon prepreg is wrapped around a bladder much like paper machete is wrapped around a balloon. In most cases the rims are made in one piece but not always. The rim is then placed in the bottom of the steel mold.
Here is where things can diverge:
The issue becomes how to make the shape that will be the part of the rim where the clincher tire attaches to the rim. There are some VERY specific dimensions set by ETRTO that need to be followed to ensure tires will mount (and stay mounted) on the rims. See: DRAWING A
DRAWING A: The ETRTO specifications for a clincher rim
The Taiwanese method:
Taiwanese mold makers (in any industry) have a solid reputation around the world for being good mold makers. And it is not just enough the be able to make a good mold, the mold must also interface with the machines that will inject stuff into it, whether it is a gas, plastic on anything else or any other piece of equipment that comes into play during the manufacturing process.
Chinese mold makers...not so much...but they are cheaper.
With molds it is basically a cavity inside a machined steel box. Some of the cavity is cut into the box and some of it must be achieved by pieces that are slid into place usually after the mold is closed called, funnily enough, "sliders". Sliders are also machined from specialty steels.
This is the method used in Taiwan (I can't show a picture of a rim mold).
The advantage of using sliders is tolerance and pressure. I will come to that in a bit.
The Chinese method is a bit different. See: DRAWING B
DRAWING B: Sliderless Clincher rim production
The Chinese method is to make a cavity in the mold but NOT to use sliders. Instead a silicone rubber section is inserted in the top of the rim and a layer of carbon is placed over top.
Both methods then inflate the bladders to pressure and bake until golden brown. Rims then cool and are removed from the molds.
Removing the rims too early (to speed up production) can result in rims that look ok but are not completely straight. When building these rims it is tough to get good spoke tension balance and they will often go out of true over time.
The Chinese rims then undergo an additional step where the carbon layer and silicone filler are MACHINE out of the rim.
Which is better and why?
Using a mold with a slider is much more expensive for the factory but the advantages are tolerances that never change (as the pressures involved will not impact the steels) and less voids in the rim specifically in the rim bed and brake track areas because, unlike steel, silicone rubber will compress under pressure.
The end result is that rims manufactured in molds with sliders will be stronger due to better compaction and less voids that those produced using the insert method.
Stay tuned for:
Prepreg choices: Is T-700 right for you?
10 signs your polyester resin in cheating on you.