guy153

Anyone like to guess where this will fail when I tighten down the bolt:




The CF tube is 30mm OD, 1.5mm wall. It's glued into two offcuts of CrMo head tube, 32.4mm diameter, 1.2mm wall (after sanding), welded onto some 3mm mild steel plate at either end. There's a piece of 1-inch 1.6mm wall tube inside with a 3mm cap welded on one end for the bolt to press against. The upper plate is tapped and also has a nut TIG brazed above it so there are more threads engaging the bolt. The bolt is M8 8.8 with probably 1.25mm thread pitch. The CF tubes are sanded just a bit to get a key and glued into the metal tubes with Methyl Methylacrylate glue. When I torque that bolt up, what will fail first? (well, after my back obviously)

unterhausen

The bolt. What do I win?

I suspect the glue line will fail and then possibly carbon due to the bolt buckling.

guy153

You win a lifetime supply of facemasks. The first time I tried it the glue failed and it just came apart. After a bit of experimenting and peering at things I figured out the problem was the fit was too tight so basically all the glue was getting pushed out. With a slightly looser fit and putting the glue on both surfaces (the instructions said just to put it on one, I should never have read them) I got a good joint. After about 4.5 turns the bolt itself sheared and broke off. So I got it out with a mole wrench and tried again. After breaking three bolts, including one that was cut to the minimum length it needed to be (about an inch) I called that a pass. You're right that if the bolt went all the way to the bottom of that thing it would buckle in the middle (I also tried that, with a longer piece of M8 threaded rod with a nut welded on the back). The maximum load on an M8 8.8 bolt is supposed to be about 2 tonnes so since I was breaking them the force must have been somewhere north of that. The 3mm plate inside that the bolt was pressing on was buckled in quite a bit and the end of the bolt had started to cut into it like a tap.

I then tried bashing it with a 2.5lb hammer (in the compression direction, but I wanted to see if the shock would break the glue) and the CF tube broke before anything else did, after some fairly severe punishment.

I was originally going to try with a much bigger 16mm threaded rod but I think the 8mm bolt was quite instructive because it's easier to estimate the force.

So I guess I now have some sort of confidence that a frame held together like this will stay together. Attempting some math, I reckon the theoretical breaking load of a Reynolds 631 tube, 1 inch diameter, 0.5mm wall (which it is in the middle) is probably about 3 tonnes, so this ought to be strong enough.

unterhausen

carbon doesn't work well in compression. You can't push a rope. 3rd law of engineering

Canaboo

Fortunately we use resin too.

squirtdad

nice experiment... .but does it really show a frame will not fall apart? How is the build envisioned? custom extra long lugs? it appears the mount of insertion and glue surface areas is much greater than there would be on a standard lug.

guy153

I'm actually planning on inserting them more than in this test, about 50mm on each tube. In the test it's about 40. But this may change if it proves impossible to assemble the frame like that. Of course no experiment is perfect... but if the tubes had come out really easily (as they alarmingly did before I figured out a couple of simple things about how to do the gluing) then it would be back to the drawing board. The challenge now is to get it all prepped and then rehearse how I'm going to get it all together and aligned in about the 10 minutes you have before the glue starts to set.

The lugs aren't standard-- they're sections of EC30 head tube welded onto the main tubes, longer than they need to be for now, so I will be cutting them down. The bike is only going to have CF TT and DT. The ST and rear triangle are all CrMo (Toyo make some bikes like this). I wouldn't have bothered with the CF but the guy it's for is keen on it and it's an interesting project!