John in CR's monocoque composite frame build log

[otas]

John, have you considered the old aircraft building technique - making your item with all the shapes as a positive form and laying on it fiberglass/carbon? Best example of this is Burt Rutans' Long-Eze, google it, you will see what I am talking about. You can use a foam as you positive mould, polystyrene is good enaugh, you can also insert your tubes, bearings and laminate them in with strenghteneing in the stress points. You may dig out the foam where required - for your batt. wells etc. There is slightly more work to make a nice finish, as opposed to laminating in negative mould, but this technique is excellent for one-offs. Where are you in Czecho? I may put you in touch with a composite expert.
cheers otto

 

[John in CR]

Sorry guys no progress to show. I'll claim the excuse of no answer for the structural support for the rear suspension to keep it compact, light, and narrow enough that my heels don't clip the swingarm pivot (using one of my motorcycle pivots is out), but the real reason is that living in paradise and getting a week of good weather at the tail end of rainy season makes me want to ride instead of build.

Otto,

Using a styrofoam male mold is the easiest way to get a nice shape, but boat building experience tells me that's the only benefit and that shaping is the easiest part. Multiple days being covered in fiberglass dust turns me off, and I'm unlikely to save any weight. Plus I can't think of an easy way to go that route and use a steel skeleton without making the steel strong enough to stand on it's own and the fiberglass pieces more like body panels than an integral part of the structure. If I was experienced enough in composites to get the strength I want without the steel, then sure I'd do it, or if I had experience with planes.

I have no issue building a 5m catamaran with a target weight of 70kg using almost no metal, but it won't go fast enough for water to become hard in a crash and with life vests I'd never be out of swimming range from shore. What is essentially a DIY emoto with pedals capable of well over 100kph introduces risks where I believe steel is essential for my build.

John

 

[otas]

OK John, I did not realise your ambition to do a ton on your ebike with pedals. My light plane is all composite made out of kevlar prepreg honeycomb sandwich, it was vacuum bag moulded in autoclave and it seems to be very strong and light. I have not personally made any structural part, it was all factory made in form of a kit. I did not know about your experience in boat buiding.

I had (perhaps) a silly idea to combine Sinclair C5 with a 2KW scooter motor and battery. The C5 is quite aerodynamic, unfortunately it does not have any suspension, as it was in those days limited to 15mph. If it could stand up to go over 100kmh and still retain contact with the ground would have to be seen. I believe a guy from a Lotus designed the chassis.
otto

 

[John in CR]

Otto,

I'm with you and want to graduate eventually to some type of cabin bike, but for different reasons. I just want rain protection and an at least somewhat protective barrier between me and the ground. I'm not all that interested in efficiency, since I already can do 100kph on about 3kw. A tailbox that has double duty as a cargo trunk, and some aero shapeliness in the front should allow a more comfortable riding position than a tuck and more speed at the same consumption. Electricity and batteries are cheap enough that 30wh/km is fine for the around town range for my needs. I'll leave the super efficiency chase to others and pursue practicality and versatility, because I believe I can best serve the cause chasing cheap and easy bikes or trikes that can carry useful cargo and passengers matching traffic speeds.

These current bikes are a step toward that end by putting a sizable battery pack inside the frame.

John

 

[otas]

John,
I am a newbie, at the first step of the electric biking with Jeremy's folder. He is an excellent and patient tutor. I like it for its convenience and miniscule dimensions, especially when folded. I intend to use it as a runaround, as I am getting older and inpatient (realising I have not that much time left!) when I have to pop in the village, I find the queing up to park and queing up to go out very frustrating. Not to mention to lug my 2 ton intimidator car just for some menial task. So the max speed and longevity does not warry me too much. Of course the rain protection here in sticks of North London could be usefull, but a compromise is a compromise.
With your aerodynamic ambition you will have to grow in length, reduce the front cross section and lower the C of G. With so much nearly horizontal surface you will be able to stick on some solar cells to recharge your batteries. Just joking. I will watch your development with an interest. Good luck.
otto

 

[John in CR]

Otto,

Please none of that negative thinking. Ebikers live forever. I gotta stay positive hitting the big 50 in a few weeks that hasn't sunk in yet, and I'm committed to to positive life lengthening changes to go with the half century mark.

BTW, solar isn't a joke for me. We're coming out of rainy season and I want to put my 500W of solar cells to work with some DIY fold-out panels to charge my bike mostly while parked and some trickle while riding. That will be the tailbox bike with the taper going to horizontal instead of the usual vertical. I'm with you on the long and low bike, since that's my daily rider for almost 2 years.

John

 

[otas]

John,
preji vse nejlepsi do Noveho Roku 2011 a hodne stesti v novem projektu.
Kde v CR se vyskytujes? Ja jsem obcas v JC na Taborskem letisti. Bylo by prijemne se treba setkat.
ahoj ota

 

[Jeremy Harris]

John is in Costa Rica, Central America, rather than the Czech Republic, otas.

Jeremy

 

[otas]

Ouch, that explains he did not react to my czech gobbledy gook!
Thanks for the explanation Jeremy.
You see I am new to this community and just about finding my way around and the abbreviations and assumptions can be misleading.
otto

 

[Gordo]

Ouch, that explains he did not react to my czech gobbledy gook!
Thanks for the explanation Jeremy.
You see I am new to this community and just about finding my way around and the abbreviations and assumptions can be misleading.
otto

Otto;
A few of us old duffers still think CZ is your place. Google still does too.

 

[John in CR]

After a 2 month hiatus, time to get back to this project, since I need good homes ready to receive the mountain of batteries that should be here around the end of the month. Time to make some critical command decisions about swingarm, suspension, headset, etc.

 

[deVries]

Sorry guys no progress to show. I'll claim the excuse of no answer for the structural support for the rear suspension to keep it compact, light, and narrow enough that my heels don't clip the swingarm pivot (using one of my motorcycle pivots is out)

Hi John, just came across this thread. Is this project a high-priority that you're going to "punch through" soon, or is it going on the back-burner with more pressing projects?

Much of my background is in reinforced plastics. Do you have a drawing or design you can post of the specific problems & part(s) in question?

 

[deVries]

Even with epoxy and fiberglass I'd worry about high stress connections like the headset set in it, because I believe the very short grains near the connection would start to compress in use and it would become loose. I think of it like setting a basketball pole in concrete in a 4ft thick block of styrofoam. At first everything would be fine, but once the little styrofoam balls around the concrete start to compress from small vibrations, it would become loose.

What is the moisture content of your raw balsa wood? It may need to be dried before impregnating with a low viscosity resin or epoxy. This will prevent your basketball pole scenario. Also, use tubing, thin sheet metal, or fiberglass/resin bonded onto/into "the matrix" for the reinforcement you need to solidify & harden the surface as necessary.

I think the way I plan to use the balsa on these bikes will add a lot of structural integrity to the frame. The steel will be strong in line with the bike, but laterally that thin steel will be like spaghetti. With thick balsa set in between the parallel strips of steel to about 3/8" deep and epoxied in place with one interior glass cloth layer for insurance, I believe the frame will become very stiff. The balsa and plywood will keep the steel in line with the bike, so I can take advantage of the steel's strength in its strongest plane. As long as I don't get carried away adding more material than necessary, especially steel at the critical connections, I think I should end up with a pretty light but strong bikes with cavernous space for batteries.

You need to think "sandwich" construction. Sandwich the balsa with fiberglass layers on both sides. Mate the metal cage indenting it into the balsa composite where possible.

Check out that thread of wood bikes. There is a video where the guy assembles a "wood plate" sided bike using metal structural parts that mate from right side to left side. This is the type of design you need to be thinking more along those lines, rather than the necessity of a welded & unified steel cage or skeleton. The composite balsa with more than one layer of glass *sandwiched* (Oreo cream center is balsa with chocolate outsides fiberglass) combined with that wood-metal video build I referenced will make an awesome kit bike.

This is on my "to build" list within two years or less... :wink:

Who has the cheap waterjet cnc cutting equipment on ES? :wink: 8)

Seriously, does anyone know of low cost waterjet availability for cnc cutting?

 

[John in CR]

This is going to the front burner. My main decision is which motor to use with the first fram....A ventilated 9C as a mid-drive at high power via high voltage, OR a variety of much heavier hub motors. The 9C deserves the lightest build, and a built from scratch swingarm, so I should fine tune the construction techniques first. Weight within reason isn't a worry with the others, so I can throw extra steel at the areas of highest stress. Plus I already have swingarms with reinforced clamping dropouts ready for those motors. That pretty much makes my mind up for me, and go with the quickest to ride frame first. With all that battery space, big scooter hubbie here I come.

WRT the scratch build, I've never constructed anything with a suspension that I didn't just rob from something else. My thought for the pivot was to either pivot on bearings, or use metal bushings like on the 2 lightweight motorcycle frames I have. In either case, the left and right portions of the actual pivot will be more spread apart than on a typical swingarm pivot for a bike to reduce the potential for play in the pivot and allow me to use the pivot axle also as the axle for a jackshaft. That gives me a place to combine pedal and motor power, and avoids changing chain length issues.

I need to do up some drawings, because I'm all ears about those rear pivot and suspension connections.

John

 

[John in CR]

deVries,

Good point about the balsa. I've modified the plan somewhat over the past 2 months. I'm treating the balsa lumber as mostly a means for easy shaping and finish. Once I finish the steel work, which will include steel cross pieces, then I'll inlay the permanently attached balsa, like the down tube. I plan to bond the ladder shaped steel and balsa lumber together using epoxy and fiberglass cloth/matt laid on the interior. That should all but eliminate the balsa from mushing like the cream of an Oreo, and the balsa will still add good rigidity to the overall length.

I want to use the top tube balsa piece at the entry cover, so the structure will need enough cross support to make the balsa hatch irrelevant to the strength. With a good fit it will at most provide some extra lateral rigidity.

Let me get the metal work done and wood started. If you guys convince me that I need more strength, then adding some fiberglass on the interior will be a matter of simplicity with little in the way of added weight or space used.

John

 

[John in CR]

After taking a hard look at how to handle the high stress connections (rear suspension, swingarm pivot, BB, and head set), I was going to just rob those parts from a cheapie dual suspension steel bike I have, but then I thought I'd just end up with the same crappy ride with lots of battery space. Instead of reinvent the wheel and spend a bunch of time trying to make that work when I know next to nothing about bike or motorcycle frame geometry, I decided to go a route where all that is done for me. This will end up much lighter too, and a much quicker build.

What I believe to be a perfect frame solution presented itself, so I pounced. I picked up a downhill bike, a Cannondale SuperV700, and will modify it with a large battery compartment that doubles as the seat. The first order of business is where to hack into this thing, so I'm soliciting input.


Basically I want to cut the saddle and its supports off and build a monocoque battery box directly onto the downtube. The top of the box will run from the headset back to behind and below the saddle (the new motorcycle like seat needs to be much lower than the bike saddle).

The purple lines are where I will cut the 2 V supports. I will tie the 2 together and create a horizontal platform that will be the lower support inside the battery box.

I can go 2 routes with the other frame cut. The easy route is the light blue cut, keeping the downtube intact. The other route is make the cut in red, and use carbon fiber on the remaining half pipe to more than make up for the loss in strength and rigidity. I like picking up that over 1.5" of extra space forward and down. I would add carbon fiber inside and outside of the remaining 1/2 round, as well as use carbon fiber to close it back up as a half pipe, since I want that flat section anyway.

Complicating the decision is a reinforcement I need to make. 2 years ago the frame developed a crack at the attachment for the rear shock. Just that front edge at the point of the hot pink arrow in the pic. He repaired it with a weld, which has held up, but heat of the weld definitely weakened the alloy in that area. Even though I won't be hitting any big drops like the previous owner did downhilling, I'll have a lot more weight, so reinforcement is essential. My plan is to do it from the inside with a plug of some high strength impact resistant castable ceramic that I have, so there will be no way the tube can give and fail again. That repair will be easy if I go the route of cutting the tube above in half, and I'd go ahead and do the same as a preventative measure up near the headset too.

If I go the less intrusive route and just cut the top tube off near its weld to the downtube leaving the weld mostly intact, I would access the interior to make the plug by cutting a hole within the circular weld of the existing top tube. I believe the extra material of the weld, along with the fact that my new construction will be supported via a bond with the entire length of the downtube instead of just the 3 points as original, will ensure the hole isn't a problematic weak point.

If I went with the half round approach, I think the way to go would be to use a strip of balsa as a form to create a centerline rib in the half round using unidirectional carbon along the length, and then wrap it all with several exterior layers of multi-directional cloth. I've never worked with carbon fiber. Does anyone have any input on how what thickness I should go to for an end result assured to be at least as strong and stiff as the original full tube of alloy of that big diameter?

I do realize that I will have to do extra reinforcement around the transition points of the complete tube to the half round to avoid creating a failure point there.

 

[Jeremy Harris]

John,

I think it's a good plan to start off with a known good geometry like this, it'll save heaps of work and experimentation with getting things to work OK.

If that's an alloy frame, then I'd avoid carbon fibre in direct contact with it, especially in the humid climate where you are. You would need to insulate the carbon from the frame well, with a layer of glass, to prevent corrosion, and the massive difference in Youngs Modulus (stiffness) between the carbon fibre and the alloy would create significant challenges around the high stress joint with the front tube. The box itself could be CF, but I'd be inclined to make the connection to the frame from plain epoxy glass, as it'll be more than strong enough and the reduced stiffness over CF will make for a more reliable joint.

I'd be inclined to keep the front tube intact and fit your seat box/battery compartment up to and around it. Add in strong connections to the two compression tubes that take the vertical seat loads and I think you'd have a pretty strong frame, probably stronger than a home-made frame a heck of a lot less work.

Jeremy

 

[deVries]

My suggestions:

1) Cut about 2" to 2.5" up above the turquoise line to make an open collar on that heavy seat tube, so you can attach your battery-seat structure to the collar & glue/bond and/or mechanically attach to it. Cut with something that won't do heat damage and/or provide cooling too, so you don't heat-damage the heavy duty down tube.

2) To reinforce the rear suspension shock attachment point wrap a 25mm to 1.5" wide ring/collar of fiberglass strand or cloth fiberglass with epoxy around the tubing next to it, so it's built-up into a collar about 6mm to 3/8" thick where that rear suspension weld is. This will strengthen the down tube from the outside, so you don't have to mess with the inside. If there is room, empty space, beneath the shock attachment & the down tube, then put some layers of cloth between the two metal supports that the shock attaches to. (Or, alternatively, lap some cloth strips into right angles along the sides of the welds on the inside/outside too, but that is probably not necessary.)

Of course, before any bonding, the surfaces should be sand blasted (or wire wheeled) to bare metal & cleaned with a good solvent. Use epoxy that is designed to bond to metal (aluminum ideal in your case).

I would not risk cutting into the down tube period or cut even close to the down tube at the seat tube joint.

Look forward to your progress, and I agree with Jeremy about not using carbon fiber. Fiberglass (or Kevlar for less layers) will be more than sufficient, and fiberglass is the low cost solution.

 

[John in CR]

Using today's lingo (or maybe it's passe already), you guys are da bomb. Having an experienced sounding board of almost infinite size really helps fine tune ideas and avoid so much trial and error. If the internet disappeared tomorrow, I would be in a world of hurt.

Jeremy,

I was planning glass and carbon sandwich for the 2 side panels, which will actually be a single piece wrapped around and bonded to the bottom of the downtube (as well as bonded across the top of tube on the inside of the U). I did forget all about no carbon in direct contact with alloy, so thanks for that reminder. The less stiff glass helping create a better bond between the alloy and carbon never even crossed my mind, but I was worried about that bond for any interior layers due to the difference in stiffness of the alloy and carbon, so the insulating layer(s) of glass is perfect.

Let me cut the seat off and make that repair access hole through top tube joint area first, along with some mock ups of the battery box. The bike is so tall that I have less room than it might seem, but I may be able to get away with a front hump like a motorcycle gas tank. At the end I'll need to be able to swing my leg over the seat and put my feet on the ground when stopped, but right now I have to stand on a pedal to get on the saddle even in its lowest position. I do have a lot of clearance under the downtube, so if I don't go with the controller there, it opens up the possibility of a second removable battery box for long range. Then I can keep it a lightweight build with a smallish 1kwh or so pack for everyday riding, and don't need so much room.

While cutting the tube might seem like a lot of extra work, and yes I would have some extra epoxy cures to wait on, but doing that reinforcement plug through the small access hole is going to be a real time eater because I have to back fill the bottom space and seal it before doing the plug above the fill.


deVries,

I like the collar idea, and may retain a wedge of that top tube to accomplish something similar. If the 2 seat supports formed a V shape instead of an inverted V, then I'd be more inclined, but the space is too limited between them and anything higher than the minimum needed for rear suspension clearance is wasted space. If I had width to work with like on a motorcycle, I wouldn't be so concerned, but like any bike space is a premium.

Regarding the reinforcing repair, I don't see a way around doing something on the inside, whether a ring, a donut, or a solid plug. To me my real risk is that tube deforming ever so slightly under the forces of the suspension. It happened already with a lighter rider, and now the area is far weaker due to the loss of the heat treatment's effect because of the weld. With the swingarm leveraging the forces so high, I don't see how I could count on an solution from the outside. Reliance would be on just the epoxy bond between strong exterior ring and the alloy tube. Let's say I reinforce half of a beer can in that manner. Once I start applying force to the aluminum near the reinforcing ring, wouldn't it easily deform and separate?

If I cast a plug or thick ring inside the alloy tube using this stuff, the shock resistant 750 is what I have, http://www.cotronics.com/vo/cotr/pdf/onepg700.pdf. With shrinkage of "nil" since I won't bake it, I think I can lay to rest any possibility of failure due to tube deformation or deflection there. That is, as long as I avoid something stupid like casting the plug out in the hot sun, only to put the alloy tube under stress at normal cooler temps because of the quite different coefficients of thermal expansion.

John

 

[Jeremy Harris]

John,

If you want to cast a solid plug in that area (which sounds like a good idea given the repair) then you could first squirt some PU foam in there from a can, via an access hole to make a dam that would stop too much of the resin getting down into the rest of the tube. With a bit of ingenuity, maybe with an extension to the plastic tube that comes on cans of spray foam, you could probably get the foam just where you want it. A bit of practice on some scrap tube, to get an idea of expansion ratio etc, would be a good idea. I did something similar years ago to fix a rattling cable in a long alloy tube, I fitted a length of clear fuel hose on to the can of spray foam, poked it right up the length of tube (maybe 8ft) and squirted a plug of the stuff right up inside to hold the errant cable in place.

Jeremy

 

[John in CR]

True, the foam would be the perfect solution if I can get the right amount. It really makes me want to hack that downtube open, so I can get in there and see what I'm doing to make sure I get it done correctly. Then could either make a thick carbon fiber and glass ring as the reinforcement and save my ceramic for another time, or use a lot less and easily cast a ceramic donut instead a full plug.

Another advantage of doing the big cut is that I feel like it forces me to use carbon fiber, so I can do a stronger and lighter structure.

Here's a drawing of a cutaway view for what I had in mind, not to scale on the layer thicknesses. Note that I will use epoxy resin, not poly.


My biggest question how thick to go with the carbon cloth. My thought is to just use 4oz fiberglass cloth, which I already have, for the insulating layer before the carbon. It looks like I need to use something like a 5.7oz carbon cloth. I'd err on the side of caution inside the trough and go thicker than the alloy with the carbon cloth and again with the uni-directional carbon, but I can't find any guidance online for the battery box itself that will have a bolt on top cover for access and a slight curve of the sides.

The big force on the box will be the rider weight. It would be bonded to almost the full length of the downtube, and I would tie it into the those shortened seat supports and enclose it at the rear. From the side the overall shape would be like the drawing below, with the long dimension along the top of about 70cm and height of about 20cm above the platform supported at the rear by the 2 seat supports. That platform will be bonded to the sides:


With the only cross structure for the 2 sides being at the outside edges and the platform, am I way off base thinking that constructing the sides with just a layer the 5.7oz carbon cloth and a layer of 18oz glass cloth is sufficient? Should I plan on another layer of the carbon inside the thick glass, and maybe a final interior layer of 4oz glass as insulation plus a bit of strength?

One last question (yeah right the last one LOL). I'm interested in ease of a good finish. Would molding the composite structure into a thin sheet of aluminum held to the curve I want with a temporary frame be a bad idea, leaving the aluminum as the exterior skin? Or should I just use sheet metal as a mold by polishing it smooth and coating the inside with mold release, and then give it a good gelcoat before starting with the epoxy and cloth layers on the inside?

John

 

[John in CR]

Ooops, I just had to ask another. If I use an angle grinder to cut this heat treated alloy, will I destroy the heat treatment if I pay attention to the heat and cool it often and just short duration cuts?

 

[deVries]

Let's say I reinforce half of a beer can in that manner. Once I start applying force to the aluminum near the reinforcing ring, wouldn't it easily deform and separate?

My suggestions:
2) To reinforce the rear suspension shock attachment point wrap a 25mm to 1.5" wide ring/collar of fiberglass strand or cloth fiberglass with epoxy around the tubing next to it, so it's built-up into a collar about 6mm to 3/8" thick where that rear suspension weld is. This will strengthen the down tube from the outside, so you don't have to mess with the inside. If there is room, empty space, beneath the shock attachment & the down tube, then put some layers of cloth between the two metal supports that the shock attaches to. Also, lap some cloth strips into right angles along the sides of the welds on the inside/outside of the two attachment plates. You could build-up several layers on the outside of both attachment plates to thicken the attachment plates wrapping some of the layers around the tube. Then, re-drill the holes through the fiberglass to mount the shock through metal/fiberglass hole.

Note bold lettering above that you should reinforce that entire area. The collar/ring will be bonded making a second tube that will also slightly SHRINK onto the aluminum tubing. Then add the additional repairs noted in bold above. Assuming proper bonding, which is easily doable, then I just don't see a need to plug the inside. The tubing is not weak like a beer can, & you're adding the necessary reinforcement to support the tubing (around the tube) preventing delamination & compression or fatigue failure, imo.

Regarding cutting into the down tube to make it U-shaped before adding reinforcements... I would just consider it a mold then. I would not count on it to support anything, and I personally wouldn't want to make extra work when the down tube is a perfect structural member already proven. To beef-up the U would cost more time, weight & $ than leaving it be :wink:

If you go with the U, then I would shape the balsa half-moon fitting tubing diameter for wider fiberglass sandwich above/below balsa. I don't think you need to close off the top of the U, and I would be using several layers of fiberglass & skip the carbon fiber. Maybe use a layer or two of Kevlar if it's cheap enough. :wink: Also, I would use enough layers so each fiberglass sandwich side (w/balsa in middle) is at least 5mm to 6mm thick. Balsa, Oreo center, should be 12mm to 20mm thick with grain aligned vertically, imo. All this reinforcement will have to lap up into both sides of the tubing beyond the U & into the round perhaps 3-4 inches beyond the U at both sides.

Maybe you could add more batteries beneath the down tube in an enclosed case too, since you don't have to worry about fork travel bumping the case. It's for road use, so adjust for less travel. It's not for thrashing/crashing DH off-road on rough bumpy trails?

Ooops, I just had to ask another. If I use an angle grinder to cut this heat treated alloy, will I destroy the heat treatment if I pay attention to the heat and cool it often and just short duration cuts?

Wrap wet towels around tubing just beneath cuts & trickle water onto the towels and/or tubing if need be.

 

[John in CR]

Thanks deVries,

My concern about glass only is wrt rigidity. The battery box structure will add plenty of rigidity back so I'm not worried about the downtube as a whole, especially since it spreads the rider load along it's length. I mostly concerned about rigidity around the transition points of original tube to new construction and going from the very rigid alloy tube over 3" in diameter to a far more flexible glass epoxy composite. I have balsa lumber, so I could cut pieces to fit loosely into the tubing, impregnate it and wrap several layers of wetted glass around it, and wedge it up into the tubing as well as attach it well to the half pipe. That should eliminate my worries about the transition points. A few ounces of carbon and epoxy seems easier and would definitely be lighter.

Also regarding weight, I have no doubt that my reinforcement mods will weigh less that the metal I'm cutting off (including the current seat support. Plus I would incur most of the half tube reinforcement work and weight anyway, because with the downtube intact I would want to create a flat space for batteries. It just wouldn't have to be as strong.

Because I plan to take the battery compartment width out to about 10cm, I'm figuring that I will pick up almost a liter more battery space by cutting into that tube. That's about 300wh of RC lipo or 150wh of LiFePo4, more than 2 packs of 5ah6s lipo or 20 extra A123 M1 cells.

Yes, I'm looking hard at the area under the downtube, but only for a range extender pack. The bike is made to handle 200mm travel forks, and the forks I have are 180mm shocks so I could easily handle 4L of batteries under there, so I will definitely include attachment points and hardware to later be able to attach a pack under the downtube. Once I get the bike up and running, I'll build a housing for such an add-on pack.

I must be missing something about an exterior reinforcement of that weakened areas around the rear shock attachment, but I'll just leave it at that.

John

 

[deVries]

I must be missing something about an exterior reinforcement of that weakened areas around the rear shock attachment, but I'll just leave it at that.

If you're going to open-up the down tube along that red line you posted, then there's no reason not to cast a solid epoxy plug with micro-bubbles & micro-fiberglass fibers... or, whatever you wish to use. The tubing isn't perfectly round, but a smaller diameter round metal collar/tube might still be bonded inside & underneath that pressure point? You might be able to cast the metal ring/tube into position by filling the air gaps with epoxy to bond it to the inside of the down tube.

Anyway, you're going to have to improvise as you go, since this is a one-off without a complete design or drawing to refer to.

I'm interested in ease of a good finish. Would molding the composite structure into a thin sheet of aluminum held to the curve I want with a temporary frame be a bad idea, leaving the aluminum as the exterior skin? Or should I just use sheet metal as a mold by polishing it smooth and coating the inside with mold release, and then give it a good gelcoat before starting with the epoxy and cloth layers on the inside?

I don't think coating with gelcoat will be very easy to do in a narrow/deep box with lots of corners? You're probably going to have to do some bondo work & painting it or use some stick-on material like that 3M simulated carbon fiber.