Carbon Fiber repair for my S-Works SL7

Prologue

The TopTube of my bike cracked after a crash in a race. High speed (about 52kmph) crash. The Bike did summerassaulting, while doing so, the handlebar bumped back into the TopTube. The Carbon Fibre at that spot cracked due the huge load. After the crash, I was so sad about the Bike.

First race of my this Belgian season, and my race ended in a crash! This S-Works I tell you, is quite an expensive ordeal. I started looking for my repari options here in Belgium. One of the reported one that I saw was on holiday. The one that I was actually able to ring, said it will take two to three weeks. Other few shops that did carbon fibre repair were unreachable as it being weekend. While looking at options I had a word with friend who does Carbon Fibre repairs. He gave me confidence that I can carry out the repair myself. The advice on what epoxy to go for was crucial. Another friend Joel reached out to me. He has studied composites, and has done his carbon fibre repairs(that I know of). He helped me with advice, tips. Crucial was the layup discussion, which then had me take more time for it, pause, gather as much information as possible, then work on it. Going through multiple article, video, the most crucial bits were the information from EasyComposities, and video by PeakTorque which details the carbon layup recipe on a top tube repair.

Material

Easy access to shipping from EasyComposites made things very easy for me. Here is the list of materials required:

Carbon Fibre Sheets: Depending on the the layup that is ideal for the repair work, the Carbon Fibre sheeet is chosen. For case of Top Tube, I went with 3K twill sheet and UD Carbon sheet. Twill sheet provides strength in all directions, while UD (Unidirectional) Carbon sheet provides strength in a single direction. The original layout might just be using UD Carbon sheet. But, we can't be sure or see that. So, there idea here is lay one UD carbon Path, which will give the strength along the direction of top tube. The twill sheet will give the strength in all directions.

Epoxy: For this type of repair, slow curing epoxy is ideal. Slow curing exoxy typically takes about 24 hours to cure. While this is slow time, it is also provides with the felxibity to correct the layup if needed. For things like frame building, or if you can put the frame the in oven, heat curing epoxy can be used.

Prep the frame:Marked about 1.5inch from either way of the cracked carbon. I would like to wrap this much of the frame with new carbon. Tape the frame at those marking. Cover the entire frame, wherever epoxy would go. Sanding:with 180 grit paper, sanded down the frame to reveal the raw carbon. Taking off the paint and primer, with layer of carbon revealing itself is goo enough. If there are frays of carbon fibre, it is better to cut them, as after the repair they can interact and cause erosion while riding. Go through multiple grits as possible. Wet sanding is better to avoid the dust from paint and carbon flying off.

Layup recipe:This step will depend on the what part of the frame one is working on.In case of my frame, the top tube will likely have very tine layer of UD carbon. Now, I can’t ‘see’ that and can’t really follow the same layout.In short, it looks impossible to restore the original layout.So, what we try to do is replicate the original layup as much as possible, while strengthening the section. My idea was to have four layups in total. I think this is too much. But, I went ahead with the idea that I can always sand down the layers.First layer was 3k twill shortest patch to give the strength. The second layer was UD carbon, which would give the strength along the direction of top tube. Ideally, this could be only layer to maintain the original characteristics of the frame.The first and second layer could also be swapper. The third layer was bigger 3k twill to cover. The fourth layer was even bigger 3k twill. It is good to measure the cutouts for the layout using paper. Wrap the paper on the tube section to get the exact dimensions for the layers. Remember that the cutout will not exactly be ‘rectangular’, it will be trapezoidal following the shape of the tube.Design the layup such that the joins for each layup or not exactly on the same spot, potentially avoiding a weak spot.

Laying down the carbon fibre: You will find a lot of information on this.Prepare your epoxy. Important to mix as per manufacturer recommendations. Use gloves and protect the frame so that the epoxy does not end up in unintended places. Paint the epoxy on frame, add the carbon fibre layer, paint over, add, paint again.Or, you you can create a carbon fibre sheet sticker!Use a big plastic sheet, put a layer of epoxy, put the carbon fibre sheet, paint over with epoxy, put anther plastic sheet to cover it. Now, use your cutout stencil. Put the stencil and cut the carbon fibres in desired shape. These will be your stickers. You can just grab them and apply on the frame.Since the carbon fibre sheets were sandwiched in epoxy, it will be nicely soaked.So, paint the frame with epoxy. Put the carbon fibre sticker layer by layer. For a good measure paint over will light epoxy layer.Analyse the layers, there should not be any gaps. If there are gaps, paint over with epoxy.Compression:In order for layer to conform to the frame adequately, we need the carbon fibre layers to be layer without nicely, with optimal epoxy and not excess. Compression will enswurethe carbon fibre layers adhere to the frame, and the excess of the expose gets squeezed out.Vacuum bag work great.Heat compressive tapes work great. Take the tape, and wrap it over the frame tight and evenly, similar to handlebar tape.After the compression tape is secured on the both side, use a heat gun to heat the tape. The tape with contract as your heat it up. Notice the excess epoxy squeezing out.If the epoxy is too much, make pinholes toward the end of the tape layer for the epoxy to escape. Now the process is done. Fingers cross, everything will settle down in 24hours.Sanding, cleaning up:After 24-30hrs, or recommended by the epoxy manufacturer, unwrap the bike.You make keep the initial taping around the carbon fibre layers while you do sanding up and cleaning up. Sand down excess epoxy.

Finishing:You can polish the carbon fibre with carbon fibre specific cutting and polishing compounds. You can use normal paint process of base layer and paint to protect the frame.If you choice neither, do some layer of UV resistant treatment over the epoxy layer. UV can damage the epoxy, so use maybe UV resistant expose layer or UV resistant coast at least.

Aftermath: Two Satifactory 1.12B races, one 1.12B DNF, and for LWU races later, the frame is behaving quite well. Maybe I am not too sensitive, or maybe I am thinking too much, but the bike did initially feels a little more stiff than original. At LWU Kenesselaare race, going over a bump, pedaling hard at about 60kmph, the back wheel went into huge wobble, which settled about like five seconds. Maybe, it was just the technqiue.

What should I do?Sand down a layer of carbon fibre depending on the fee. This will involve some black magic feel o the sensitivity of the Bike.I now know what how the layers are laid, can identify which is which, especially with one UD sheet going in there. For the moment, it is just my speculation, and above my level of understanding.