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identiti66
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Hi all.. sorry if this makes no sense but I'm clueless. I have a completely knackered elbow and need to immobilise it and then protect it from the rigours of mountain biking. Have kind of worked out making the mould but laying the carbon fibre (or similar) is way out of my scope. It needs to be impact and abrasion resistant. From a quick read, perhaps a few layers of Diolen with Carbon top layers? Help keep the cost sensible. I'll probably line it with either sheepskin or foam Is that enough to get started?
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Hanaldo
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Why not kevlar with a diolen outer layer? If you're after black that is. Both kevlar and diolen are generally cheaper than carbon and suit your needs more?
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identiti66
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That's the kind of info I need.. best options for material and then how to get the thing made. End colour doesn't really matter to be honest.. although if the end result looked cooler than a very cool thing that would be a bonus 
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Hanaldo
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Well aramid fabrics like kevlar and diolen are impact and abrasion resistant. Not as stiff as carbon fibre, so if you need a very rigid part then you might use some carbon fibre in the layup as well. Exactly how much protection do you need? Are you looking to be able to come off your bike at speed and land elbow-first into a pile of rocks, or do you just need something to protect your elbow from tree branches and perhaps handle a fall but nothing too extreme? If you're looking to make something that is going to give you really good protection, I would be using mostly kevlar, ie. 2 layers of 240gsm, 3 layers of 400gsm, 2 layers of 240gsm. All layed at +/- 0/45/90 degrees to the previous layer. Diolen is a bit softer, so if looks aren't a factor then I'd probably leave it out, maybe just paint the final product. It's hard to say exactly how many layers you should use without knowing your exact requirements, but really your layup is totally up to you and what you want, you could throw in a layer of carbon in there to stiffen it up, you could put a layer of diolen on each side for looks/abrasion resistance, you could do more layers to make it stronger, you could do less layers if it's not too necessary. You could even do a layer of hybrid carbon/kevlar, which will give you some of the advantages of each. What is important for impact resistance is that you orientate the fibres in as many directions as possible. So in terms of material, kevlar is probably your best bet. Cutting it is a nightmare if you don't have the tools, but if impact and abrasion resistance are the priorities... Carbon to stiffen the thing up if needed, but it isn't good with impact resistance so don't make it your outer layer or use too much of it unless it's carbon/kevlar. One layer of pure carbon fibre in the middle of the layup would be fine. In terms of how to make it, I think vacuum bag it at a minimum, infusion as the best solution. If you don't really have the equipment for proper vacuum bagging or infusion, when I first started I used to just use the vacuum pack clothing bags and a vacuum cleaner. Worked ok, better than simple wet-lay for sure.
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identiti66
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Thanks for that.. plenty of good stuff to think about..  I need to be able to come off, at speed (not racing speed but a reasonable lick), and protect my elbow from whatever happens to be at the point I hit.. hopefully not rocks but I guess I won't know until I'm heading their way. Needs to be prevented from bending or straightening, that is critical. Think a drawing might help.. I'll get some paper out tomorrow. If I'm not confident about doing this and getting it right, are there people out there who might be willing to help a chap out if I bought all the materials?
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macgtech
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Slightly off topic, but noticed the comment above about cutting Kevlar - I am having exactly that issue - just can't cut the cloth (it is the Kevlar/Carbon weave), as the shears just don't want to know. Any tips? I thought Diolen would be the answer but it frays badly when it is sanded (or at least it has in my experience) so it looks tatty unless you cut it perfectly. There is no doubt a solution to that too though.
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carbonfibreworks
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macgtech (31/03/2014)
Slightly off topic, but noticed the comment above about cutting Kevlar - I am having exactly that issue - just can't cut the cloth (it is the Kevlar/Carbon weave), as the shears just don't want to know. Any tips? I thought Diolen would be the answer but it frays badly when it is sanded (or at least it has in my experience) so it looks tatty unless you cut it perfectly. There is no doubt a solution to that too though. Kevlar shears from EC . Chris Carbon fibre works LTD
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carbonfibreworks
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I made a lower leg support for a client, one layer of 200g carbon then two layers of Kevlar another 200g carbon then a layer of 450g carbon on the outside, made with epoxy and infusion and to say it is strong is an understatement  |
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identiti66
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Wow that looks really nice.. and cool too.. Got a rough sketch, it needs to be super strong along the A - B line.. if I land hand first, that can't break - or my elbow will.  I could also probably do with it coming round a bit further so it 'pops' on.. but I'm not sure whether it's possible to make it flexible in that direction while remaining strong A - B. Plus I'll have to look into securing it somehow.. velcro straps or something like that?
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Hanaldo
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If it doesn't need to be strong along it's circumference then you could use a couple layers of uni-directional carbon fibre, orientated along your A-B line. Possibly even just in strategic areas, such as the elbow bend. That would ensure it's nice and stiff, whilst the kevlar spreads the impact load. Should also remain fairly flexible along it's circumference, depending on how many layers you use and the type of fabric you go with. Having thought about it, you should probably use a layup similar to Chris'. One layer of 200gsm carbon fibre, two layers kevlar, 2 layers of uni carbon along the elbow bend, and then one final layer of carbon fibre, also probably a heavier fabric like Chris used, ie. 450gsm. I was probably over-thinking the impact strength of the part initially, 2 layers of kevlar would do well for what you need, and the carbon fibre will prevent any flex. What you want to consider though, is will this thing become dangerous in itself in the event of a crash? Ie. if you make it so strong that it will not flex and it will not break, you want to be sure that it is not going to cause you harm that may otherwise have been avoided, for example lacerations from the edges of the part. One possible way to avoid that would be to flare the edges away from your arm ever-so-slightly, would only take a millimeter or two. Or if the foam liner that you mentioned earlier was thick enough, that might do the job too.
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