I have to repair a nice sized hole in the top tube of my frame. The frame manufacturer used all unidirectional fibre in the original construction, but I was wondering whether I could just as well use twill weave carbon fibre instead?

The first ply down should definitely be a woven ply. Ideally the last ply too but as this will be the visible ply you might want it to match the frame.
It’s not ideal to bond to UD fibres so make sure you prepare the surface well and get it as clean as possible.
If you put UD down as your last ply it might be a good idea to put a woven ply down before the last ply.

Clean the hole up, make sure it’s nice and rounded. Scarf it back and do the repair. Twill should be fine. Presumably as a DIY job you’ll be wet laying?

I currently only have twill. Happy to order some unidirectional if I will get a more optimal repair.

I have vacuum bagging equipment so am planning on employing that for the repair.

The way I am planning on doing the repair is as follows.

1) damaged area removed and I have created a nicely scarfed rectangle down the top tube.

2) I have created a lightweight foam insert to fit inside the cut out section. Was planning on sandwiching 2 twill layers 2 the inside of the frame and then fitting the foam core into the frame using thickened epoxy and then wrapping the twill layers over the top of the foam core. This will then be vacuumed down.

3) Once the above has cured I was then going to finish and fair the hole and then apply the n number of layers under vacuum.

The foam insert is solely so that I have a surface, otherwise, I was unsure how you fix the frame when you have a nice large 4″ hole in it.

Sounds like a reasonable plan.

Obviously I don’t know the shape of the area you’re fixing but if it’s a suitable (single curvature) shape I’d use a former/mould of some kind on the outside, e.g. a piece of released plasticard to vacuum down over the top of the repair to avoid just crushing the foam. Should also help reduce the amount of making good to do.

For my my roadie repair last year I got a bit of advice over on talkcomposites.

The replies seemed to suggest that variations on UD at 0* and +/-30* was about all you need, woven fabric is much easier to work with so I ended up using both, but probably didn’t need the woven layers…

I’d use a former/mould of some kind on the outside, e.g. a piece of released plasticard to vacuum down over the top of the repair to avoid just crushing the foam

Good idea, but the foam I have used does not collapse under vacuum so just sticking peel ply should suffice.

For my my roadie repair last year I got a bit of advice over on talkcomposites.

The replies seemed to suggest that variations on UD at 0* and +/-30* was about all you need, woven fabric is much easier to work with so I ended up using both, but probably didn’t need the woven layers…Thanks for the link. Was hoping I could just use woven fabric just so I could avoid having to buy UD.

You can use whatever you want, as long as its strong enough for the repair.
You only need to use UD for aesthetics, if you’re not that bothered, then no need to buy any.

You only need to use UD for aesthetics,

Other way round UD means all the fibres are aligned to carry load in a single Plain, hence offsetting some UD layering ~30* for torsional loads, using a woven fabric generally means half your fibres are at 90* to the primary load path and doing relatively little structurally.

It does no harm so long as you add enough material aligned to the primary load axis, you can use woven fabric certainly (do you know what fabric weight/type you have already?) but there’s a reason frame manufacturers use mostly UD now and it’s not aesthetics…

Bonded new epoxy to the cured epoxy on the frame is (I think) never going to be as strong wet to wet, so I don’t think the repair will totally return the frame to pre damage levels of stiffness etc, so use whatever fibre you have.

Happy to purchase UD fibre if it means the repair will be better than if I use just woven carbon fibre.

As it is the tob tube do I assume that the orientation of +/-30* and aligning down the length of the top tube is because of how a top tube is loaded. eg side ways stresses are much less than those at the angles stated?

cookeaa, the reason I said to only use UD for aesthetics was because flanagaj’s frame has a UD finish, so it would match.
That’s why I also said you can use what ever you want as long as its strong enough for the repair.

Manufacturers know which angle to lay the cloth (the general public usually don’t), that’s why if it were me, I would just use the cloth which gives the most room for error/strength from ever direction. Personally, I would use plain weave for the repair and then a layer on top to match the finish of the bike or whatever you’re repairing.

Also, a carbon repair is (if done correctly) stronger than the area its replacing, so it will NOT be weaker.

Manufacturers know which angle to lay the cloth (the general public usually don’t), that’s why if it were me, I would just use the cloth which gives the most room for error/strength from ever direction. Personally, I would use plain weave for the repair and then a layer on top to match the finish of the bike or whatever you’re repairing.

Thanks. That does sound like a good idea. Not being a composite engineer who understands the stresses and strains placed on a top tube mid way along I would struggle to understand the best way of laying UD. Using a weave with a top layer of UD should give me a frame repair that is adequate.

Also, a carbon repair is (if done correctly) stronger than the area its replacing, so it will NOT be weaker.

I am vacuum bagging the repair and have created a good 3″ scarf either side of the hole. So I am hoping that the repair will be sound.

I am also hoping that a top tube is not subjected to such forces as other frame sections, so not so critical to get it 100% spot on?

Your plan seems good to me.
With the scarf as large as you have prepared you should be able to see the orientation of each UD ply. Unless of course they are all at 0 degrees.
Laying up UD plies at +\- 30 over such a small area will have next to no effect. You’d be better off with woven plies at +/-45. Most of your plies (if not all) should be at 0/90.

You sound like you have it all in hand now, before you know it, you’ll be like me trawling ebay for broken stuff to buy and fix!
I got a pretty good nick, P/X Dirty Harry 29er frame for £60 at the beginning of the year because it had a light crack in the seat stay 🙂

Post up some pictures, its always interesting (for me anyway!) to see the before,during and after pics 🙂

Oh, if you flick the carbon with your finger, thats one way to hear how thick the carbon is and you’ll find, the centre of the top tube is usually very thin as its not a high load point, but its still strong enough to sit on.

Here is some photos of the repair. I vacuum bagged the carbon fibre and put 5 layers of 200g 2/2 twill. I am going to put 3 layers of UD so as to match the frame.

1) Section of damaged carbon top tube removed from frame.

2) I bonded in a foam core with epoxy thickened with micro balloons. Once it was dry I sanded the foam. The photo below shoes the foam ready to be bonded.

3) Photo after 5 layers of 2/2 twill vacuum bonded onto the frame using epoxy

Good job, nice and neat.