I’m working on a frame that will incorporate a carbon ST and have some questions. I’ll have a sleeve at the BB and ST/TT junction and will be using a seat post for this build.
I assume a carbon specific drill bit is needed for the bottle bosses. What size and where to get one?
do you use riv nuts for the bosses? If so, what material? I assume aluminum.
what adhesive is used for the sleeves? I was thinking 3M DP420.
is it a good idea to also glue in the riv nuts at installation?
I’ve got some experience with bonding carbon seat tubes into Ti frames as part of my day job at Curve Cycling. We’ve also done fully bonded carbon/Ti lug construction in the same style as Atherton bikes.
Since we’re only doing a frame or two at a time, we’ve simply been using (and re-sharpening/discarding) HSS drill bits. I’ve been told carbide bits will last longer, but they’re also a lot more expensive.
To get the holes nicely aligned & spaced, I 3D printed a tube block with integrated seats for some drill bushings. Works really well.
As for size, 6.9 to 7mm will likely be the most appropriate. But it will depend on the body diameter of the rivnut you choose. For best results, you also need to choose the correct length rivnut based on the wall thickness of the tube.
Yes. But I’d like to move away from them. We’ve been using stainless steel rivnuts with a large (flat) flange. Stainless steel is harder to crimp, but it will last longer than aluminium. For one-off production, crimping can be done with a quality hand tool. But your hands will get wrecked if crimping more than a few in a row. This depends on hand-strength of course.
Another thing to remember is that you need to make sure that the crimping tool will fit in the frame. If the bottom rivnut is low on the ST, there’s a risk that the tool will hit the DT. Our workaround has been to crimp the bottom rivnut with the ST only partially inserted.
3M DP420 should work. It only has a 20 minute working life. But that should suffice for a seat tube. I believe Atherton uses Loctite EA 9466, and we used this on our first prototype road frame with a carbon ST. This frame was subsequently brought to EFBE and went through, and passed, all the fatigue tests with the “weight” ramped up to 130kg as opposed to the standard 120kg equivalent of a standard ISO test.
My colleague has also been riding a gravel frame that we made at around the same time all over the world, including loaded up with bags and in gravel races over the past 3 years and it’s still going strong. He must have done well over 30,000km on that thing by now.
Yes. Absolutely. It helps to keep them from spinning and also isolates the carbon from the metal to help avoid galvanic corrosion.
Other tips:
The carbon should be lightly sanded. Do not bond to a painted or coated carbon tube. It should be ‘raw’ carbon.
Make sure that everything is clean, clean, clean before bonding. Acetone & nitrile gloves are recommended - you may need more than one pair as you go along, since you will inevitably get adhesive all over the gloves.
Do not adjust or move the tube after the working life of the adhesive has passed. Use baby wipes to wipe off excess adhesive when you’re done bonding. Get the kind that’s not overly wet. When you think you’ve wiped off all excess adhesive, wipe again. Even in places you’re sure you didn’t touch during the bonding process. It’s a real pain to remove globs of adhesive or adhesive fingerprints once it has cured.
Huge thanks to you! Lots of good info and tips here. I really appreciate this.
I’ve been looking for a 3D print file for a tube block but haven’t found one yet. I just recently picked up a 3D printer and am slowly learning to draw in Fusion. Any chance I could pay you for the file?