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Old 05-28-2012, 08:42 AM   #76
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DaBit

Now that your off aluminium and looking at the composite path why not ditch the subframe and luggage racks with their troublesome tubes?
Maybe even ditch the luggage itself and make a monocoq body to replace all three.

Then you can hang your remaining bits like lights & exhaust off the structure.

You seem to know what your doing with resin.
How much temperature resistance can you build in?
Enough to intergrate the muffler(s) into the body work?

Calculate the weight savings of getting rid of almost all the metal including the heavy fastners.
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Old 05-28-2012, 03:15 PM   #77
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For max strength composite parts, the important thing is not really the choice of materials, but avoiding resin rich parts, which is pretty difficult unless you are using compression or vacuum moulding techniques/

A very clever composite guy that I have used in the past, made fuel tanks and rear subframes for Yamaha works MX team, and these were made using pre-preg carbon and an autoclave.
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Old 05-28-2012, 03:47 PM   #78
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Quote:
Originally Posted by Twin-shocker View Post
PU foam is an inexpensive core material, but a combination of this and aluminuium and polypropylene honeycomb would make for a much more durable moulding
Sure. Now, I don't have any idea where to get that PP honeycomb (might drop by some boat builders and see if they have a leftover), but I can get stuff like Airex, Depron (which is the 'hamburger/coffeemug-foam'), aramid honeycomb, 3M Lantor coremat, etc.

For the supporting members from luggage-loop to frame mounting points it makes sense to use a good core material, and then I would probably choose the aramid/paper/phenolic resin honeycomb because of it's superior properties. I would only need a little, so using such an expensive material would not increase total cost that much.

For the actual loop itself: well, that won't be under a lot of stress and only needs to be tough enough to resist a direct impact and stiff enough to use it as a tiedown-point. Easily archievable goal. So why use something exotic there?

When playing with carbon, kevlar, hightech core materials, various methods to improve fiber weight %, etc. it is very, very easy to let cost rise to insane heights. It's all about 10 bucks here, 20 bucks there, etcetera, but in the end it adds up quickly.

Quote:
Originally Posted by Rucksta View Post
Maybe even ditch the luggage itself and make a monocoq body to replace all three.
On the LC8, I won't.
If a luggage rack breaks at 100mph, OK, a lot of hassle with tiedowns, tieraps and ducttape, but it will probably end up as a nice story at the campfire. If the entire monocoque subframe breaks at 100mph, well, uhm...

I like to divide a total function of an assembly into different, separable parts with dedicated functionality anyway, and I like the versatility of the classic luggage rack.

As a sidenote: on the SV650 project bike I am very tempted to integrate subframe and underseat fuel tank into one piece. Not the luggage rack though. But that bike is my 'playground'.

About temperature resistance: up to about 120C/250F is easily archievable, up to 200C/400F is possible. For us nutty motorcycle weirdo's that is; if your business is composites and you have access to the right stuff you could probably go higher. But this is resistance only, it doesn't mean that strength is the same as it is at room temperature.
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Old 05-28-2012, 04:05 PM   #79
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Originally Posted by Twin-shocker View Post
For max strength composite parts, the important thing is not really the choice of materials, but avoiding resin rich parts, which is pretty difficult unless you are using compression or vacuum moulding techniques/
In this case: compression, yes. Vacuum, no.

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A very clever composite guy that I have used in the past, made fuel tanks and rear subframes for Yamaha works MX team, and these were made using pre-preg carbon and an autoclave.
Way beyond my abilities. I'm shooting for 35% fiber density using hand wetting/layup and a little compression.
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Old 05-29-2012, 01:38 AM   #80
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Unless you are using proper compression/vacuum moulding there is no real point in using kevlar or carbon on something like a luggage rack, as this will tend to greatly increase costs, but wont provide much of an increase in performance over something like CSM/Diolen which would only be 25% of the cost.

I would suggest you look carefully at how to avoid resin rich parts, before making a start on this project, as I get the feeling that resin rich (a lot weaker) laminate may mean lots of problems here.
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Old 05-29-2012, 02:26 AM   #81
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Simple.

- Use shapes where one could compress the fabric externally and evenly without bridges. Thus round/oval/hexagon: yes. Square/rectangular: only if required. I-beam/U-profile: no.
- Hand-laminate the core, absorp quite a bit of excess resin by patting the laminate with cotton cloth.
- Add PE foil with holes or peel ply and absorber.
- Compress absorber. Maybe that shrink tape stuff is the ticket (never used it before), otherwise wrapping packing tape very tightly around the structure will do the job too. The tricky part is to wrap tightly and not disturb the laminate under the absorber.
- Add heat. The epoxy will thin out tremendously and flow into the absorber a lot easier.

Any composite professional is probably shaking his head right now, but these 'alternate methods' work fine in a 'once or twice a year we do a one-off composite part in the shed'-setting.

BTW: IMHO a little resin-rich does not immediately mean very weak parts. Just a little weaker and a little heavier than optimal. Of course, one try to avoid that condition, but if there is an area which is not as good as the rest, well, don't bother.

The test piece proved this; this piece was extremely resin-rich, the resin was made in two small batches (small batch = mixing ratio inaccuracies) and made out of mainly leftover pieces. It really takes effort to do worse than that. Still, the test piece performed well.

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Old 05-30-2012, 01:37 AM   #82
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Resin rich parts will actually prove to be weak and unreliable.............which I think is something you are going to find out for yourself first hand.
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Old 05-30-2012, 04:25 AM   #83
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I don't think I will have any resin-rich parts, but when I do first hand experience is the best way of learning I guess.
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Old 05-30-2012, 03:07 PM   #84
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Good luck.............
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Old 05-30-2012, 03:59 PM   #85
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Zega's are gone, money is in the pocket, sot he next thing to do is order some extra fabric.
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Old 06-07-2012, 03:44 PM   #86
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The first steps tonight. Just toying around basically.

Bending the loops from 5/8" impact resistant PVC tubing. Was far harder than I guessed it would be; the pipe doesn't like to be bent in a 30mm (~1 1/4") radius. Applying heat was a delicate balance. Not enough and the PVC would not retain it's shape, too much and the pipe would wrinkle, crack open or lock the bending spring.



They are copies of the originals, except that the inside dimension is slightly larger to accomodate for the composite fabric.



Toying around with mounting the loops and positioning the loops. I intend to use more carbon and less aramid in these mounts because I can well use the good compressive and tensile strength of carbon and the risk of a heavy direct impact is not that high.

A form like this, reinforced with steel at the subframe mounting points and tapering from loop pipe thickness down to 6mm (1/4") thickness at the subframe would provide a lot of strength and stiffness with only a few layers of carbon. I intend to use wide mounts like that on the top of the racks also. They give extra stiffness and strength.

This form was made using solid 3mm (0.12") PVC sheet, heated a little with the heat gun to soften it. Because solid PVC sheet was what I had available.
Not sure if I retain the solid PVC as-is; it is fairly heavy. Since I need to build up thickness using foam I might lighten the PVC by drilling holes in it before glueing the foam on, or replace it with a sheet of Depron alltogether.

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Old 06-20-2012, 04:48 PM   #87
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Slight progress, about halfway done.



I ended up tearing a piece of carbon/aramid cloth into separate fibers. Used the carbon fibers and some aramid in the length-wise direction of the pipes, and used the remaining aramid to tightly compress the lengthwise carbon/aramid fibers to the PVC pipe.

Comparing the amount of cloth used and the laminate thickness vs fiber content in the datasheet of the cloth, I managed to reach close to 60% by volume of fiber around the pipes.

Should do the trick, I hope.
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Old 06-21-2012, 11:39 AM   #88
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It is interesting to note that high end bicycle frames all weigh roughly the same no matter what they are made out.

Building composite structures is easy, but building composite structures that are actually lighter than metal is very difficult and requires lots of engineering and quality control work.

Twin-shocker, fiberglass is very heavy material and CSM is about the worst of fiberglass on a strength/weight ration. Its purpose is to build bulk in applications like boat hulls. Despite common perception fiberglass does not often result in lightweight structures. When was the last time you saw anything that needed to be lightweight built out Chopped mat?
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Old 06-21-2012, 12:33 PM   #89
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It is interesting to note that high end bicycle frames all weigh roughly the same no matter what they are made out.
When I compare bicycle frame weights, I see carbon frames weighting in at about half the weight of steel, and aluminium/titanium are in between. Let's say 2kg for a steel frame, 1.5kg for an aluminium frame, 1kg for a carbon frame.

Now, I am not a bicycle frame specialist, so maybe this is outdated and I am wrong.

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.

Quote:
Building composite structures is easy, but building composite structures that are actually lighter than metal is very difficult and requires lots of engineering and quality control work.
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. Same with high strength aluminium alloys, and with those alloys joining becomes a problem too.

But yes, one must find out how to join the different parts of an assembly and how to arrange fibers to obtain the required strength in the direction needed.

I am fairly certain that the strength and stiffness of the rack pictured above is OK. I did orient the fibers in the expected direction of force applied, made sure they were straight and not buckled, turned the aramid/carbon distribution towards carbon on the areas that are expected to be loaded in compression, and I did what I could to create a laminate with high fiber/low resin/little air content.

But I am not so certain about the pipe/'mounting thing' joints, steel inserts and impact resistance. Time will tell.



Twin-shocker, fiberglass is very heavy material and CSM is about the worst of fiberglass on a strength/weight ration. Its purpose is to build bulk in applications like boat hulls. Despite common perception fiberglass does not often result in lightweight structures. When was the last time you saw anything that needed to be lightweight built out Chopped mat?[/QUOTE]
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Old 06-21-2012, 01:36 PM   #90
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I am a bit out of touch with bicycles as well, only know mountain bikes from a few years ago when pretty much everything weighed right around 3.5-4 pounds no matter what it was made of. Limiting factor was generally flex or beer canning with thin wall tube, not ultimate strength.

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.

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. The early Y-frames where well known for punching the shock though the carbon frame, and woe unto anyone who dragged a carbon bike across the rocks clearing an obstacle, many would wrap in the chainstays in various materials to protect the carbon from this.

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.

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.

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.

There has been lots of work with composites in the boat world and results are generally a bit disappointing unless it is a mega-buck build with good engineering support.

All this is not to say building things and trying new materials isn't worth while, I still have a pretty cool Carbon-Kevlar foam core skateboard I built when I was a kid. More an argument against expecting huge gains from exotic material.
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