Fiberglass Frame Experiment

FWIW. My understanding of fiberglass is that it is NOT structurally stable enough to use to build a bicycle frame, a bicycle fork, bicycle seatpost, bicycle stem, or bicycle rims ...

After you have fabricated the mold(s), fiberglass would probably be "okay" to test your lay up technique before using Carbon Fibers + appropriate resins ...

Basically, impacts on the fiberglass will more-than-likely result in a catastrophic disintegration.

With enough glass cloth and enough resin...Hell yes he can build a fiberglass bike. Glass boat hulls take all kinds of crazy pounding and stresses, from hard hits on docks to the vibration and stress of motors to multiple mounting hardpoints for equipment that induce strain on the glass to the constant high-speed pounding against the water.

Doubt the strength of glass? Go watch some ocean-going cigarette boats running 100+ MPH for hours on end.

The Bowden Spacelander utilized a fiberglass frame.

Although the frame is often described as "fragile", that is coming from doofusses used to the recycled battleship cannons of the 1960's called Schwinn's, Murrays, Columbia's and Huffy's. Rideable Spacelander examples can be found if you have the financial means to procure one.

Given a sufficient amount of material (the OP appears not to be concerned with weight) a glass bike is an achievable goal. However, much like some carbon frames have aluminum BB inserts, dropouts, headset cups or what have you, design consideration must be given to any hardpoint or contact with componentry. Fiberglass is plenty strong enough for bicycle construction, but NOT in .030" wall thickness.

The OP mentions a mold. Good luck manufacturing and/or paying for THAT. I would ballpark you around $7,500-$12,000 for the build in 6061-T6 and that's working from your fully developed 3-D model files. If you want me to design the frame from your sketches I would add another $5K to that.

You could possible do a wooden mold, but again...good luck with that and the cost of your new mill, polymer coating, etc. Then there are the bladders, inserts, mandrels and other assorted tooling you may or may not want to incorporate into your build.

OP, may I ask how old you are and how much experience you have with fiberglass fabrication?

Not sure I understand what you mean with "clear". FG thin enough will be transparent, with the natural tint being yellowish. But whether you use polyester or epoxy resin, there'll be plenty of sections of the frame that have to be too thick to remain transparent. And since polyester in particular don't like UV light very much, leaving the frame naked isn't good for durability.

A chopper fork opens its very own can of worms. It'll have both structural and "steering" geometry challenges to deal with. Foremost, getting the transition from fork blades to steerer tube strong enough would be a challenge. If it was me, I'd stick with a metal fork. Since you pretty much have to have a metal steerer, going fo FG blades is a lot of added engineering for very little purpose.

Clincher rims are difficult to build. You need a very complicated mold/plug to create the raised lip on the inside for the bead to hook against, and still be able to remove the mold/plug.
Another option is of course to make the brake track section extra thick, and then machine out the excess to create the raised lip.
Doesn't sound any easier, just another kind of difficult.

I'm actually going into this project cold, but in doing my research, I intend to primarily use foam molds, and I may stiffen some of the tubes with EMT conduit. For the rims, I may use a combination of the molds for the outside, and a tire for the lip, or at least the intertube to apply clamping pressure. While I'm not using 3D CAD, I do know enough of what I want to begin the build.

Make certain you use foam that is not going to react with the resin. Good luck.

Is EMT the right material to add stiffness, isn't it a realtively soft, low tensile strength steel?

Depends. I sure wouldn't ride a 1/2" EMT frame to the end of the room and back. A 2"3" EMT frame might last awhile if wrapped with enough glass cloth! 😄

We are almost all riding 1/16" thin carbon [censored] nowadays with NO steel inside it. A little steel of any type wrapped in 1/4" or more of glass and resin formed into that "Choppah" look (every time I hear the word I hear Ahnuld screaming, "Get to da choppah!"😉 IF the thing were designed with one hellava safety factor thrown in.

lectraplayer said:

I'm actually going into this project cold, but in doing my research, I intend to primarily use foam molds, and I may stiffen some of the tubes with EMT conduit. For the rims, I may use a combination of the molds for the outside, and a tire for the lip, or at least the intertube to apply clamping pressure. While I'm not using 3D CAD, I do know enough of what I want to begin the build.

Have you actually done ANY FG work before?

The most common sequence is that first you build a plug, basically non-functional mock-up of the item.
Then you build your molds up against this plug.
Then you remove the plug, and lay up the FG structure you intend to keep in the molds.
If you're only gonna do a one-off, it'd make more sense to simply sculpt the frame in foam, and then wrap in in FG.
Overall it'll save you work for an one-off, and if you want to make copies you can always use the finished item as the plug for mold making.

But sticking EMT tubing into the structure for "stiffening" sounds like a really questionable idea.
One of the more difficult things when working with laminates is how to transition from one material to another.
And with that approach, you're adding several of these transitions where it isn't necessary.
It's extra difficult if you do the budget thing and use polyester resin, since that isn't particularly prone to stick to metal.
Epoxy resin is a little easier.
Still, if I thought I needed steel for strength in something comparable to a 'bent, I'd build it in steel to start with.(exhaust pipe is quite common to use, is availabe pre-bent, welds well and is cheap) Then I'd just build a bolt-on, non-structural body to cover it up.

As for the rims, and using a tire as part of the mold, it just sounds like you haven't really looked at a tire.
As potential mold parts they're floppy, flimsy things.
Hugely unlikely to let themselves be positioned accurately enough to be of any use for the degree of precision needed to cast the lip in a rim.
If it was me, I'd much rather take my chances with building a fairly complicated, separable plug. With a jig, you should be able to do machining within tenths of millimeters using a hand-held router.

And as CB mentions, make sure the resin of your choice doesn't react with your foam.

alfeng said:

FWIW. My understanding of fiberglass is that it is NOT structurally stable enough to use to build a bicycle frame, a bicycle fork, bicycle seatpost, bicycle stem, or bicycle rims ...

After you have fabricated the mold(s), fiberglass would probably be "okay" to test your lay up technique before using Carbon Fibers + appropriate resins ...

Basically, impacts on the fiberglass will more-than-likely result in a catastrophic disintegration.

Not a problem as such.

What would be more appropriate to say is that FG hasn't got the strength-to-weight ratio to build a competitive, or a "sensible" bike.

But once you chuck that concern out the window, building a FG bike is no big deal.

If I was ever asked to build such a thing, there'd only be one part that would have me worried - the fork crown. Anything else I'd be happy to tackle.

Well, OK. Plenty of small parts. Chain, chainring, sprockets, axles, brake assemblies, saddle clamp etc.

But frame - not a problem.

The fork crown though - tricky.

Can't see many ways forward there. Either building a kinda-sorta traditional looking fork, but with a very tall fork crown. 3" of overlap between steerer and fork legs. Or a dual crown setup. Or maybe an extremely oversized steerer tube.

Only option I can see to build a normal looking fork entirely out of FG would be to get it weaved.

Meaning you don't start with sheets or strips of FG, but with the separate strands, which then are woven into the finished shape, placed in the mold, and injected with resin.

Expensive process. Done in CF every now and then, so it should be doable in FG too - but fairly pointless.

Pearls unto swine kind of thing.

I wouldn't worry about impact and "catastrophic desintegration either" - well, not as long as I'm free to achieve whatever weight of the finished product that I see suitable.

Failure mode will still be different, but it's not a problem as such making a FG structure crumble catastrophically at the same kind of load where a

"sensibly sized" steel structure would fold catastrophically.

It wouldn't make engineering sense from any other angle than "must use FG", and if you ramp the forces up high enough, eventually steel would win out.