|
oekmont
|
|
|
Group: Forum Members
Posts: 550,
Visits: 27K
|
What weight of carbon => usual 200-240g/am
What are you hoping for with the Kevlar? Kevlar provides crash safety. That means 2 things:
-in case of a crash there will be no big parts flying around
-it is harder for an object to penetrate your structure
It will not prevent your part from being damaged and useless.
The second benefit obviously doesn't do you any good here. And for the first... If it's a high speed crash you have bigger problems like flying luggage. If it's a low speed crash your luggage will fall to the ground from less than a metre, instead of hanging from the aramid threads and potentially get stuck in your back wheel. If you think you need that, a single rowing aramid around the part, placed onto the core, would do just fine.
You would better off if you exchange the aramid layer with carbon, as this will make it less likely for the part to fail, instead of keeping it together if it fails.
|
|
|
|
|
bkadv
|
|
|
Group: Forum Members
Posts: 5,
Visits: 37
|
+xI am talking about fundamental material characteristics. Things aren't getting more practical than this. The people who are most into that topic choosed this measurements to be the most universal facts. Anyways, here is how I would do it in composites: -spot the functional faces. Bolt/mounting points etc... -forget about the rest of the existing part -remember all physical limits. Everything that might collide with the new part. -imagine the most flowing, harmonic looking surface between your fixed functional faces, that stays within the limitations. - make a model of that shape. -take a mould Now the part itself: The shape I have in my head is just a single shell like structure. To get enough stiffness, I would estimate 2-3 layers carbon on each side of the core material. As the shape has a compound curvature, I would likely take soric lrc as core material. Varying thickness between 3 and 5mm should do the job. Don't use core material, where you are going to bolt the part to your bike. Instead build up wall thickness with carbon. I would then go for infusion. Vacuum compression will work too. a tube structure might have it's advantages, but because of all the shaping, glueing and the usual difficulties of wrapping complex foam cores the shell solution will be lighter in the end. I would not take any Kevlar. As long as you are not dealing with abrasion, the benefits of Kevlar kick in, when your part fails. And not in a way that keeps the function of the part going. And structural you can't take the Kevlar into account, as your part has to deal with bending forces, and Kevlar can't take these, because it has very bad propertys when it comes to compression. you are just weakening the carbon structure, because you are concentrating the forces onto the carbon on the side that takes pressure. Hey Oekmont, Thank you for the insightful answer, yes since i am new I was trying to avoid vacuum/infusion but it can certainly be doable. I been watching lots of tutorials. I can see how hard it would be to wrap tubular shape core. I will make 1 side first and do lots of testing by man handling the rack. Obviously i will not drop my bike its not a KLR haha Your idea I can see it working.. Im good at imagining a model in my head. I can create a simpler shape and mold it. What gram carbon layer should i be using per layer? I believe, once i start ordering the materials, I will start off with gaining experience making molds of the body panels and making replacement panels but will use kevlar carbon blend since these panels will definitely touch ground and have impact. My non ABS plastic have survived this trip even with mild low sides and falls. I didnt know how to ride a motorcycle 1 month before I did my trip, now i can off road and have lots of mountain riding experience..through rivers..sand/deserts... Riding on iced up roads where cars are in ditches etc and lots of snows and blizzards I didnt know how to camp either, first time i camped in my life was in Japan..with my bike. I learn everything from Youtube I do have a BS in computer engineering and AAS in electro mech tech.
|
|
|
|
|
oekmont
|
|
|
Group: Forum Members
Posts: 550,
Visits: 27K
|
I am talking about fundamental material characteristics. Things aren't getting more practical than this. The people who are most into that topic choosed this measurements to be the most universal facts.
Anyways, here is how I would do it in composites:
-spot the functional faces. Bolt/mounting points etc...
-forget about the rest of the existing part
-remember all physical limits. Everything that might collide with the new part.
-imagine the most flowing, harmonic looking surface between your fixed functional faces, that stays within the limitations.
- make a model of that shape.
-take a mould
Now the part itself:
The shape I have in my head is just a single shell like structure. To get enough stiffness, I would estimate 2-3 layers carbon on each side of the core material. As the shape has a compound curvature, I would likely take soric lrc as core material. Varying thickness between 3 and 5mm should do the job. Don't use core material, where you are going to bolt the part to your bike. Instead build up wall thickness with carbon. I would then go for infusion. Vacuum compression will work too.
a tube structure might have it's advantages, but because of all the shaping, glueing and the usual difficulties of wrapping complex foam cores the shell solution will be lighter in the end.
I would not take any Kevlar. As long as you are not dealing with abrasion, the benefits of Kevlar kick in, when your part fails. And not in a way that keeps the function of the part going. And structural you can't take the Kevlar into account, as your part has to deal with bending forces, and Kevlar can't take these, because it has very bad propertys when it comes to compression. you are just weakening the carbon structure, because you are concentrating the forces onto the carbon on the side that takes pressure.
|
|
|
|
|
Steve Broad
|
|
|
Group: Forum Members
Posts: 408,
Visits: 4.1K
|
+xBecause the benefit of an aramid is its resistance to break, not necessarily its flexibility. Aramids are still stiff, they just aren't as stiff as carbon. Generally in a hybrid layup, you would use the carbon on the outer most layers and the area aramids internally. Bkadv, I completely agree with Oekmont. If you do go ahead with it, lose the PET core - your core is the PVC, you wouldn't then wrap that with another core (PET core is no where near pliable enough for this anyway). Your layup is otherwise solid. I wouldn't make the skins any thicker than that as it will become a bit of a danger in an accident. That layup is plenty stiff enough for everyday handling etc. Steve, I'd be interested to know what you are basing your opinion of composites and fasteners on? My experience is mostly in motorsport, and some in boats and gliders - all of which have quite demanding fastener applications. They do take special consideration, but you just do it properly, there's no reason I would look at something and say that cant be done with composites because it needs to be bolted somewhere. The aramid won't stop the carbon from braking once it exceeds its elastic limit (not the right term but, hopefully, you know what I mean :-) ) I had originally planned to build the front clip of my car using aramid and carbon but, after talking to various experts, the only advantage was that in the event of a crash it would be easier to pick up the pieces compared to a wholly carbon clip :-) I needed the stiffness so went with carbon only. AFAIAW the OP is new to composites. Adding fasteners to composites has to be done properly otherwise they will simply pop off at some time in the future, especially if under stress which they will be on a bike. I am looking at this from a practical perspective as opposed the expert/ideal one as I am just someone mudding along on his own in the real world and can only comment on my own experiences and successes/failures.
|
|
|
|
|
Steve Broad
|
|
|
Group: Forum Members
Posts: 408,
Visits: 4.1K
|
+xSteve: of course we can agree to disagree. But then you should think about why you are disagreeing with state of the art knowledge, like it is tough at university. Of course you can build it in titan. but in the very end every structure made out of titan or aluminium will fail. It might take virtually forever. But the question isn't if, but when it will happen. Light metals got no stall line in their whöler curve. And there are steels which are much stronger than any titan alloys. This is a common misconception. State of the art knowledge is great, but doesn't always transfer into the real world very well, especially amateur back garden sheds. Stuff made in perfect conditions in a lab will have different strength characteristics to the same part made in someone's garage. My concern was that I assumed that the OP wanted the part to survive another world trip or using the dirt bike for what it was designed for. Parts made by a novice will be nowhere near as strong as stuff made by experts under controlled conditions. Personally I can't see the point in trying to make a lightweight rack but each to his own. Obviously I don't know what your real world composite/titanium experience is, but I have been playing around with both for around 10 years so have a lot of failures to base my comments on :-) Yes, there are steels stronger than titanium, but we were discussing making a lightweight rack so they are irrelevant. If he wasn't interested in the weight there would be no point in making a new one out of either carbon or titanium :-)
|
|
|
|
|
oekmont
|
|
|
Group: Forum Members
Posts: 550,
Visits: 27K
|
Steve: of course we can agree to disagree. But then you should think about why you are disagreeing with state of the art knowledge, like it is tough at university.
Of course you can build it in titan. but in the very end every structure made out of titan or aluminium will fail. It might take virtually forever. But the question isn't if, but when it will happen. Light metals got no stall line in their whöler curve.
And there are steels which are much stronger than any titan alloys. This is a common misconception.
|
|
|
|
|
Hanaldo
|
|
|
Group: Forum Members
Posts: 2.5K,
Visits: 28K
|
Apologies; that sounds quite confrontational... I didn't mean it to be, I'm genuinely interested.
|
|
|
|
|
Hanaldo
|
|
|
Group: Forum Members
Posts: 2.5K,
Visits: 28K
|
Because the benefit of an aramid is its resistance to break, not necessarily its flexibility. Aramids are still stiff, they just aren't as stiff as carbon. Generally in a hybrid layup, you would use the carbon on the outer most layers and the area aramids internally.
Bkadv, I completely agree with Oekmont. If you do go ahead with it, lose the PET core - your core is the PVC, you wouldn't then wrap that with another core (PET core is no where near pliable enough for this anyway).
Your layup is otherwise solid. I wouldn't make the skins any thicker than that as it will become a bit of a danger in an accident. That layup is plenty stiff enough for everyday handling etc.
Steve, I'd be interested to know what you are basing your opinion of composites and fasteners on? My experience is mostly in motorsport, and some in boats and gliders - all of which have quite demanding fastener applications. They do take special consideration, but you just do it properly, there's no reason I would look at something and say that cant be done with composites because it needs to be bolted somewhere.
|
|
|
|
|
bkadv
|
|
|
Group: Forum Members
Posts: 5,
Visits: 37
|
+xWhy are you mixing aramid with carbon? By sandwiching the aramid within carbon you lose the flexibility of the aramid. I didnt now that, thank you. I am just learning  If i replace kevlar with Carbon Black Twaron. So Actually, should all layers be replace with this material while retaining the foam core and 3D foam in between the black Twaron? For balance in impact using this with XCR epoxy for the UV stability and durability? as its always exposed to sunlight only thing im worried is, will it be strong enough for me to pick up the rear of the motorcycle from the composite rack. like lifting the composite rack, will the bolt areas snap. how would i go upon reinforcing the bolting areas? Is it somehow possible to introduce some other kind of core in around the bolt areas to secure it to the motorcycle body frame? thanks
|
|
|
|
|
Steve Broad
|
|
|
Group: Forum Members
Posts: 408,
Visits: 4.1K
|
Why are you mixing aramid with carbon? By sandwiching the aramid within carbon you lose the flexibility of the aramid.
|
|
|
|