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Old 06-22-2012, 10:43 AM   #91
kpt4321
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Originally Posted by DaBit View Post
But there is another important factor to consider: these frames are made with state of the art materials and tooling, which is a huge difference from what is available to people wanting to build just one piece without the need to sell their kidneys.
If a DIY guy in the shed was building a steel bicycle frame from standard sized tubes, no way he could get the weight down to 2kg and retain an acceptable stiffness. With carbon he could do a lot better. Not as good as the manufacturers with autoclaves etc., but very acceptable.

The strength vs. weight ratio for even simple room temperature cured hand layups with only 35% fiber is much better than that of standard steel or even 4130. So regarding UTS or stiffness it is not that difficult to do better than steel.
You have yet to convince me that the "average guy" is going to be able to build a CF structure with a strength-to-weight ratio better than that of steel, in his garage.

More importantly, you seem to be focusing on strength, but MANY applications are stiffness limited, not strength. The stiffness of woven CF is not all that great, necessarily.
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Old 06-22-2012, 01:30 PM   #92
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I would imagine that bonding ready made carbon tubes into aluminum or steel lugs, would result in a pretty effective bicycle frame that was strong as well as light. However bearing in mind the fact that making the tooling required for compression or vacuum moulded frame parts from scratch, is probably well beyond anyone without many years of composite experience, I doubt very much its viable to build from scratch.
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Old 06-23-2012, 12:06 AM   #93
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I would disagree on the backyard builder aspect, most of the high end metal frames are basically built by guys in sheds, you buy the tubes from Reynolds and after that it is all about being a good welder/fabricator. Even old school lugged frames can be pretty light when built by someone who knows how and has access to good tube.
But when welding you use special tubes which are very thin-walled in the middle and gain wall thickness near the joints. These are not generic tubes. Lugged frames do need lugs that accurately match the tubing otherwise the brazed joint is not that strong. One could make these in the backyard, but it does involve lathes and mills.

If you don't use the special tubing, building a 2kg steel frame would be pretty hard.

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Composite on the other hand took years for even the big boys like Trek to figure out, I saw quite a few mangled Carbon-Fiber wheels and frames in the late 90's and early 2000's.
Of course. These guys live on the edge and use as little material as they think they can get away with. When living on the absolute edge, it is easy to tip over.

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and woe unto anyone who dragged a carbon bike across the rocks clearing an obstacle
That is an undesirable property of thin high carbon laminate indeed. The reason why I wanted to implement more aramid fibre than I actually did.

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The big difficulty with carbon is you have to arrange the fibers to the load path to see big gains, and to do that you need to figure out where the loads actually are.
With some structures common sense helps. For example the loop of the luggage rack: luggage pulls the entire thing toward ground due to it's weight and wants to bend it inside. Vibration and rear suspenion action worsen this and creates forces, but these are mainly the opposite so the same fiber direction would do.
Bending of the tubes themselves is not such an issue; after all the loop is 'braced' bij the luggage.

The 'bending inside' of the loop and the torsion exerted at the mointing points is the biggest problem, so that is why I incorporated extra bundles of carbon fibre on the 'outside' (plane where the luggage sits onto the rack' and 'inside' (opposite side) of the loop. I also ran a lot of extra fiber to the spots where the mounting points meet the loop to cope with the local high stress caused by torsion.

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I bet the rack will be fine and pretty cool looking, I just doubt it will be much tougher/stronger for weight than simply using thicker wall or larger diameter pipe.
Forget about the cool looking; I don't think I can lay up another layer of carbon nice enough to serve as a visible finish. I will try, but if it fails I will just paint the entire thing black.
Tougher for weight, well, the rack as pictured above weights 360 grams/12.7oz. This includes the fairly huge steel inserts around the mouning points.

360 grams with steel would mean very thin wall thicknesses, and the structure would crumble like a beer can when even looking at it.

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The one place you may have trouble is that the pipe will bend in an impact while the carbon won't, so essentially the carbon takes all the load until it fails, then the steel starts working.
This is my biggest fear. A steel rack would bend when overstressed, essentially taking load off the structure. Carbon/aramid composite would not; it would bend a little and then break.

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Originally Posted by kpt4321 View Post
You have yet to convince me that the "average guy" is going to be able to build a CF structure with a strength-to-weight ratio better than that of steel, in his garage.
I am not saying I can do that. I did some destructive testing on sample pieces of laminate I prepared, and it took way more effort to destruct them than a similar steel piece.

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More importantly, you seem to be focusing on strength, but MANY applications are stiffness limited, not strength.
Only compressive strength and impact resistance. It seems you cant have it all in one fiber.

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The stiffness of woven CF is not all that great, necessarily.
Of course not. Maximum strength/stiffness is only at the 0 and 90 degrees direction, and then there is the problem that the fibers are not entirely straight due to the weaving.
But that is where unidirectional tapes and bundles of sereval thousand fibers comes in, as well as slightly prestressing the fibers to make sure that they are absolutely straight and won't buckle during resin application/compression of the laminate.

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Old 06-23-2012, 02:41 AM   #94
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Originally Posted by DaBit View Post
If metal fatigue was such a big problem, then why do they make subframes out of aluminium?
Because they have clever design teams and big computers to get the designs just right.

This:



...is what is special about steel, and it is very very special. It makes lots of things possible and economical which would not otherwise be so.
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Old 06-23-2012, 03:54 AM   #95
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[QUOTE=DaBit;18973968]But when welding you use special tubes which are very thin-walled in the middle and gain wall thickness near the joints. These are not generic tubes. Lugged frames do need lugs that accurately match the tubing otherwise the brazed joint is not that strong. One could make these in the backyard, but it does involve lathes and mills.

If you don't use the special tubing, building a 2kg steel frame would be pretty hard.



Of course. These guys live on the edge and use as little material as they think they can get away with. When living on the absolute edge, it is easy to tip over.



That is an undesirable property of thin high carbon laminate indeed. The reason why I wanted to implement more aramid fibre than I actually did.



With some structures common sense helps. For example the loop of the luggage rack: luggage pulls the entire thing toward ground due to it's weight and wants to bend it inside. Vibration and rear suspenion action worsen this and creates forces, but these are mainly the opposite so the same fiber direction would do.
Bending of the tubes themselves is not such an issue; after all the loop is 'braced' bij the luggage.

The 'bending inside' of the loop and the torsion exerted at the mointing points is the biggest problem, so that is why I incorporated extra bundles of carbon fibre on the 'outside' (plane where the luggage sits onto the rack' and 'inside' (opposite side) of the loop. I also ran a lot of extra fiber to the spots where the mounting points meet the loop to cope with the local high stress caused by torsion.



Forget about the cool looking; I don't think I can lay up another layer of carbon nice enough to serve as a visible finish. I will try, but if it fails I will just paint the entire thing black.
Tougher for weight, well, the rack as pictured above weights 360 grams/12.7oz. This includes the fairly huge steel inserts around the mouning points.

360 grams with steel would mean very thin wall thicknesses, and the structure would crumble like a beer can when even looking at it.



This is my biggest fear. A steel rack would bend when overstressed, essentially taking load off the structure. Carbon/aramid composite would not; it would bend a little and then break.



I am not saying I can do that. I did some destructive testing on sample pieces of laminate I prepared, and it took way more effort to destruct them than a similar steel piece.



Only compressive strength and impact resistance. It seems you cant have it all in one fiber.



Of course not. Maximum strength is only at the 0 and 90 degrees direction, and then there is the problem that the fibers are not entirely straight due to the weaving.
But that is where unidirectional tapes and bundles of sereval thousand fibers comes in, as well as slightly prestressing the fibers to make sure that they are absolutely straight and won't buckle during resin application/compression of the laminate.[/QUO


From your posts on here, you seem to know plenty of theory related to composites, but maybe dont have that much practical experience? This being the case, maybe the end product would have worked a whole lot better if you had made it out of ERW steel, which would have cost a lot less to do, and been more durable, at the expense of maybe being about 1kg heavier?
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Old 06-23-2012, 04:27 AM   #96
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Originally Posted by DaBit View Post
Of course not. Maximum strength/stiffness is only at the 0 and 90 degrees direction, and then there is the problem that the fibers are not entirely straight due to the weaving.
But that is where unidirectional tapes and bundles of sereval thousand fibers comes in, as well as slightly prestressing the fibers to make sure that they are absolutely straight and won't buckle during resin application/compression of the laminate.
You seem to be readily (and often) switching between "what is feasible for your average guy with composites" and "what is theoretically possible with composites." Of course composites can build a structure that is stronger/ stiffer, and lighter than classic metals; I have never denied that. That is why it has become the material of choice for next-generation airframes, jet engine parts, etc. However, don't loose sight of the fact that giant companies like Airbus and Boeing have spend hundreds of millions, if not billions, of dollars developing both the part designs and the manufacturing technology to be able to make some relatively simple parts (u-channels, skins, etc) out of unidirectional composite tape. I know this because I design and build those machines.

However, NONE of this is transferable to the "backyard mechanic," which is what you were talking about, and what I commented on, before you changed the context.
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Old 06-23-2012, 07:12 AM   #97
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Originally Posted by kpt4321 View Post
You seem to be readily (and often) switching between "what is feasible for your average guy with composites" and "what is theoretically possible with composites." Of course composites can build a structure that is stronger/ stiffer, and lighter than classic metals; I have never denied that. That is why it has become the material of choice for next-generation airframes, jet engine parts, etc. However, don't loose sight of the fact that giant companies like Airbus and Boeing have spend hundreds of millions, if not billions, of dollars developing both the part designs and the manufacturing technology to be able to make some relatively simple parts (u-channels, skins, etc) out of unidirectional composite tape. I know this because I design and build those machines.

However, NONE of this is transferable to the "backyard mechanic," which is what you were talking about, and what I commented on, before you changed the context.
I agree 100% with that.............pre-preg carbon parts using proper tooling and production methods are very very good, but simply are not feasible for someone to do at home. I get the feeling that a lot of people think that top end composite work is a bucket and brush affair, and will only find out this isnt the case, when parts fail...................
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Old 06-23-2012, 11:04 AM   #98
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From your posts on here, you seem to know plenty of theory related to composites, but maybe dont have that much practical experience?
I am an engineer, so I tend to figure out what stuff is all about before doing something with it. But indeed, not much practical experience, and especially not in the failure department.

But since I also intend to replace the subframe of the SV Adventure build with a glass/aramid/epoxy composite fuel tank, I am VERY eager to gain some experience with this stuff, especially in a stressed situation. This rack will gain me some insight at least.

I do not intend to use composites there obtain the lightest structure possible. I want to use them because I can shape them in a way I cannot do with metal. Build a plug using foam by adding some and sanding it down again until I have the shape I want, yes, I can do that. Hammering some metal in shape, welding it up and make it look acceptable, no way.

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This being the case, maybe the end product would have worked a whole lot better if you had made it out of ERW steel, which would have cost a lot less to do, and been more durable, at the expense of maybe being about 1kg heavier?
I still don't think so. But we shall see when it's finished and in service. I am not done yet.

Progress will be slow also; I have another more urgent job (at least the misses thinks it is) going on; building roof carriers for the car. Could buy them, but they are on backorder for my model, cannot find them used for my model, blabla. That is something I do not want to fail, so I will build it out of steel.

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You seem to be readily (and often) switching between "what is feasible for your average guy with composites" and "what is theoretically possible with composites."
The unidirectional- fiber part of my previous post had nothing to do with what is theoretically possible, it is what I actually did. As is slightly prestressing them to keep them straight.

But since you really seems to know what you are talking about: what makes a carefully manufactured homebuild part much weaker than a professional part, possibly even weaker than steel? This is not (completely) stubborn-ness, but interest.

The vendor of the composite materials I use (R&G Faserverbundwerkstoffe) also provides a simple calculation tool for their fibers/fabrics and epoxy systems. Here is a quick example with 4 layers of 160g/m2 woven carbon cloth, laid down in 0/45 degrees, using 35 volume-% of fiber, and their 'weakest' epoxy resin system requiring post-cure for maximum mechanical properties and higher Tg.



Stiffness is about half that of aluminium....
(BTW: CSM+polyester is even less stiff than pinewood along the grain)

With a laminate stackup which matches what I did using higher fiber content, stiffness along the grain/length of pipe improves up to about 75 GPa, or only slightly better than aluminium, or 1/3rd to 1/2 that of steel.

This does prove your point that woven cloth will not produce very stiff laminates.
But the low density and freedom of shape allows large diameter structures (such as what I did around the mounting points), fair wall thicknesses, and still a low final part weight. Right? Or wrong?
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Old 06-23-2012, 11:45 AM   #99
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If you want to make tanks, then forget about costly difficult to use materials, make tooling out of CSM, and try to come to terms with basic RTM techniques using CSM/poly. When you are confident with RTM using cheaper materials, try it using carbon/kevlar/epoxy.

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Old 07-13-2012, 11:28 PM   #100
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Don't stop now! I want to hear how it all turns out!
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Old 07-15-2012, 03:43 AM   #101
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I get the feeling that the guy wanting to make a rack using carbon/kevlar/RTM, probably has more enthusiasm than hands on practical experience, and has found out that the job isnt as easy as expected?
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Old 07-16-2012, 06:58 AM   #102
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I am an engineer, so I tend to figure out what stuff is all about before doing something with it. But indeed, not much practical experience, and especially not in the failure department.
Looks like a neat project and all, but the saving of a few ounces is not a good trade for the numerous reasons why steel is a far better material for pannier racks.
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Old 07-21-2012, 12:14 PM   #103
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Interesting thread, I am in the middle of producing a simple soft panier rack out of carbon fibre, carbon fibre/Kevlar and aluminium honeycomb. It's replacing a simple composite bracket that I made from shelf brackets, aluminium plate and gaffer tape! Will post pics when I am done.
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Old 07-23-2012, 04:49 AM   #104
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No guys, I didn't stop. :)

I just went on holiday with the family, and I had numerous more urgent jobs to do first, such as welding together a set of roof carriers for the car, repairing a lot of child toys, fixing a child seat to the bicycle, give the bicycles and car service, blah, blah.

The latest state is this (but it was like this more than 3 weeks ago due to holiday):


Weight is 500 grams. Sand, epoxy, paint, and I am looking at 550 grams for one side of the rack.

Observations so far:
- It is far, far strong enough. I can lift the rear of the bike (and an LC8 is heavy..) easily by attaching a strap on this rack. Pulling the rear aside by attaching a strap is no problem either. and when I step on the tubes with my 95kg/210lbs and jump up and down, nothing breaks.
- It is less stiff than a steel frame. Pretty much less stiff, actually.
- But it can bend a lot without breaking, so I am not yet sure whether this is an advantage or disadvantage.

I'll finish this side, attach 30kg of concrete or so to it, and ride a rough road hard. We shall see what happens, but unless the stiffness proves to be insufficient I think it will work just fine.
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Old 07-25-2012, 12:09 AM   #105
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I think you might find that fails in service pretty quickly, as for composite parts to work well they need to be designed with composite material use in mind, and not just be direct copies of metallic tubular parts.

If your aim is reduce weight of the bike, then maybe replacing all the steel axles with Ti and all not critical fastenings with aluminum, would be a more reliable way of achieving this?
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