pic: Carbon fiber monocoque drivetrain Top view

[Edxu]

Any clue on how expensive this would be compared to if it was made out of standard drivetrain steel or aluminum?

[Tom Line]

Originally Posted by Edxu

Any clue on how expensive this would be compared to if it was made out of standard drivetrain steel or aluminum?

Extremely expensive.

There are a number of ways to fabricate carbon fiber structures.

The first is to use dry cloth and saturate with epoxy. Generally speaking this is what home-builders use. It's going to be heavier than a well made vacuum bag or pre-empregnated setup, but doesn't require expensive materials or specialized building techniques.

Your next choice is how to form the parts. Will you make a male mold? A female mold? Use a spacer material like polystyrene that can be chemically removed?

Fastening things in carbon fiber can be tricky. The strength in a fiber structure comes from the fiber itself. The epoxy is merely there to hold the fiber in the shape you need. Fastening to the carbon fiber incorrectly will crack the epoxy and destroy the structure's strength. You also tend to need backing plates to spread the pressure out.

Then there's cure times, cure temperatures, mixing ratios, substrate materials......

Starting around 2:10:
https://www.youtube.com/watch?v=504I_hJDFck

Wet Layup - what many FRC teams would do in their shops:
https://www.youtube.com/watch?v=-sOU3zEigt0

Vacuum Infusion:
https://www.youtube.com/watch?v=hEyF5KOkhUY

The last two months of my life finally coming to a close (and why I know so much about fiber construction):

[cbale2000]

Originally Posted by Edxu

Any clue on how expensive this would be compared to if it was made out of standard drivetrain steel or aluminum?

Assuming they'd be doing their own layup, and depending on the number of layers it could be anywhere from $100 to $500+ (only for the carbon fiber, not counting the other supplies like epoxy and such). You can get 50" wide carbon fiber cloth for as little as $20 a yard if you look around a bit.

Personally if I were to build something like this, I'd use a high compression strength foam core material rather than building it monocoque (plus it's a bit easier to do the layup). Then simply use a lightweight wood for the center material in areas you need to drill holes (sitka spruce is a good option) and then use just 1-2 layers of carbon fiber cloth. You could also make the walls of this design much thinner using such a method.

Originally Posted by Tom Line

The first is to use dry cloth and saturate with epoxy. Generally speaking this is what home-builders use. It's going to be heavier than a well made vacuum bag or pre-empregnated setup, but doesn't require expensive materials or specialized building techniques.

IMO, vacuum bagging gets a bad rap for being too complicated for the average user. Our team managed to build a very simple vacuum bagging table using some non-stick plastic bag material, PVC, sticky tack, and a basic shop air compressor with a vacuum valve.

Originally Posted by Tom Line

Your next choice is how to form the parts. Will you make a male mold? A female mold? Use a spacer material like polystyrene that can be chemically removed?

None of the parts we made used a mold, we simply applied the cloth directly to the core material, added epoxy, vacuum bagged, and repeated. This won't work for all shapes, obviously, but for most FRC uses it works fine.

Originally Posted by Tom Line

Fastening things in carbon fiber can be tricky. The strength in a fiber structure comes from the fiber itself. The epoxy is merely there to hold the fiber in the shape you need. Fastening to the carbon fiber incorrectly will crack the epoxy and destroy the structure's strength. You also tend to need backing plates to spread the pressure out.

As long as you're smart with what kind of core materials you use and where you use it, backing plates are largely unnecessary, when they are, you can usually get away with some 1/8" polycarbonate. Again, for most FRC uses.

Examples:
www.chiefdelphi.com/media/photos/27823
www.chiefdelphi.com/media/photos/31319

[Bryce2471]

Originally Posted by Tom Line

Your next choice is how to form the parts. Will you make a male mold? A female mold? Use a spacer material like polystyrene that can be chemically removed?

I'm not the expert, but frame parts would likely be vacuum bagged in a female mold.
The gearbox plates would be cut from stock plate on a CNC router.

Fastening things in carbon fiber can be tricky. The strength in a fiber structure comes from the fiber itself. The epoxy is merely there to hold the fiber in the shape you need. Fastening to the carbon fiber incorrectly will crack the epoxy and destroy the structure's strength. You also tend to need backing plates to spread the pressure out.

The axles in this design are supported by aluminum plates that would distribute the load to the carbon structure.

Then there's cure times, cure temperatures, mixing ratios, substrate materials......

I don't know anything about what would be used for these. That might be intentional on the part of our sponsor.

Originally Posted by cbale2000

Assuming they'd be doing their own layup, and depending on the number of layers it could be anywhere from $100 to $500+ (only for the carbon fiber, not counting the other supplies like epoxy and such). You can get 50" wide carbon fiber cloth for as little as $20 a yard if you look around a bit.

This could be done in house with the coaching of our sponsor, although that's not what I was imagining when I designed it. I was surprised at how affordably our sponsor could create carbon components.

Personally if I were to build something like this, I'd use a high compression strength foam core material rather than building it monocoque (plus it's a bit easier to do the layup). Then simply use a lightweight wood for the center material in areas you need to drill holes (sitka spruce is a good option) and then use just 1-2 layers of carbon fiber cloth. You could also make the walls of this design much thinner using such a method.

I'm not able to picture how this construction method would be used here, but maybe I'm missing something.

[cbale2000]

Originally Posted by Bryce2471

I'm not able to picture how this construction method would be used here, but maybe I'm missing something.

So, for example, if you were using a foam center material, the process would look something like this:

1. Cut/sand foam to desired shape (for this example, a simple square box)
2. Find a clean flat surface (the cleaner and flatter the better), this will be the vacuum table
3. Lay down a layer of disposable absorbent material on the table (there are lots of options for this, you can even use common paper towels if you're working on the cheap)
4. Lay foam part flat on the table (on top of the aforementioned absorbent material)
5. Lay carbon fiber cloth on top of foam part, preferably cut so that it hangs just over half way down the sides of the part
6. Lay down a layer of "breather cloth" (basically a non-stick cloth material to prevent the absorbent material from sticking to the part and wicks away excess epoxy)
7. Add more absorbent material (paper towels) to the top of the part
8. Cover with plastic
9. Insert vacuum hose under plastic
10. Seal plastic completely to table using sticky tack (or similar), ensure the area around the hose is also sealed
11. Turn on air pump, run overnight
12. In the morning, remove part from bag, peel off breather cloth and absorbent material
13. Flip the part over and repeat steps 3-12 again
14. ???
15. Profit!

You can also use a space heater and half of a cardboard box to speed up the epoxy setting process if you want (don't leave it unattended though, for obvious reasons).

In the case of your design, you could use this method to basically make the whole thing with 5 pieces (or as one big piece if you were feeling really ambitious) and you wouldn't have to make the frame walls so wide (leaving more space for electronics and such).

And now for some related old pictures...

1, 2, 3, 4

Edit:

After re-reading your previous comment a few times, I'm thinking you might have been referring to how the construction method would be used in this situation, and not so much how it works (correct me if I'm wrong)... >_>
Answering that question might require some CAD work to explain unfortunately.

[jkelleyrtp]

This is how I've personally done a few of our parts, it uses really only 4 +-2 layers of carbon fiber and is much easier to shape your parts. We just get the desired shape we want and make it out of foam core, plan around hole placement and how much carbon fiber needs to be in the pockets, and then lay the carbon fiber. I'm not sure how it would work on a frame of your size, but it can help with rigidity and cut costs.

[gblake]

Like some of the other posters, I'm no expert; but I immediately wondered why the design includes so many 90 degree corners and edges.

Aren't sharp corners/edges the exact places where structures made from carbon fiber, or similar materials, are most likely to fail, when the entire structure flexes?

Isn't the key to success allowing many fibers to share a load instead of letting the load get concentrated onto a few fibers (at corners/edges)?

Color me curious.

Blake

[Bryce2471]

Originally Posted by gblake

Like some of the other posters, I'm no expert; but I immediately wondered why the design includes so many 90 degree corners and edges.

Aren't sharp corners/edges the exact places where structures made from carbon fiber, or similar materials, are most likely to fail, when the entire structure flexes?

Isn't the key to success allowing many fibers to share a load instead of letting the load get concentrated onto a few fibers (at corners/edges)?

Color me curious.

Blake

From what I understand of this material. It is important to give the structure three dimensional aspects so that it can't have strength and rigidity in the important directions. This can be achieved in a number of ways. The two ways that have been discussed in this thread are using a filler material, and forming a structure with flanges, corners, and box shapes. In this design, I have gone the flanges and corners route.

Imagine a flat piece of carbon fiber that is 4" wide. In some directions it is strong, but in others it is very flexible and not very strong. Now imagine that same amount of carbon fiber is instead a 2" by 1" C channel. In that configuration, it might break along its corners if enough force is applied, but it would be a lot stronger and more rigid for the same weight.

[gblake]

Originally Posted by Bryce2471

From what I understand of this material. It is important to give the structure three dimensional aspects so that it can't have strength and rigidity in the important directions. This can be achieved in a number of ways. The two ways that have been discussed in this thread are using a filler material, and forming a structure with flanges, corners, and box shapes. In this design, I have gone the flanges and corners route.

Tubes/cylinders are strong/rigid three dimensional shapes.

Carbon fiber bicycle frames are made of roundish tubes, not box shapes.

Antenna towers, crane booms and frames, car roll cages, are generally made from (round) tubes, not boxes.

Rounded shapes might be worth investigating.

Definitely ket us all know how the eventual frame holds up, if one gets built.

Blake

[Bryce2471]

Originally Posted by gblake

Carbon fiber bicycle frames are made of roundish tubes, not box shapes.

This is for a number of reasons. Chief among which is probably aerodynamics.

Antenna towers, crane booms and frames, car roll cages, are generally made from (round) tubes, not boxes.
Rounded shapes might be worth investigating.

I agree. There are definitely merits although making sure that there are still adequate mounting options may be difficult.

Definitely ket us all know how the eventual frame holds up, if one gets built.

Because this was mostly an exploratory design exercise, I doubt that it will be built. If it is, I be sure to post about the experience.

[gblake]

Originally Posted by gblake

...
Carbon fiber bicycle frames are made of roundish tubes, not boxes.

Originally Posted by Bryce2471

... Chief among which is probably aerodynamics.

Maybe. Maybe not. I think the jury is out unless/until we talk to someone who has crawled through all the trade-offs (including buyers' psychologies) in the real world.

[JesseK]

Originally Posted by gblake

Maybe. Maybe not. I think the jury is out unless/until we talk to someone who has crawled through all the trade-offs (including buyers' psychologies) in the real world.

If we presume there are bumpers on the sides, then we can presume an impact will spread pretty evenly across the outer edge vertical face. If that were rounded, the force of the impact would be more concentrated.

I forget the VA/NC team who usually does carbon fiber. They've been next to us in the pits a few times, so I got to talk to them. IIRC, a lot of their super structure was rounded tubing. The edges of the drive train frame were filleted with a small (0.25"-0.5") radius but otherwise the faces were straight.

On the plus side, this stuff is so lightweight you can disassemble the vast majority of the superstructure into COTS parts and the carbon fiber frame while still being under the withholding limit. The downside to that is that you're spending all day on Thursday re-assembling...

[Electronica1]

Originally Posted by JesseK

I forget the VA/NC team who usually does carbon fiber. They've been next to us in the pits a few times, so I got to talk to them. IIRC, a lot of their super structure was rounded tubing. The edges of the drive train frame were filleted with a small (0.25"-0.5") radius but otherwise the faces were straight.

The team you are looking for is 1829 "Carbonauts".

[gblake]

Originally Posted by JesseK

... The edges of the drive train frame were filleted with a small (0.25"-0.5") radius but otherwise the faces were straight. ...

That's what I was imagining, not 100% round. Also not 100% sharp(ish) corners.

[EricH]

Originally Posted by gblake

Maybe. Maybe not. I think the jury is out unless/until we talk to someone who has crawled through all the trade-offs (including buyers' psychologies) in the real world.

I would put "stress concentrators" as a (singular, but definitely not the only) reason that sharp angles aren't used with carbon fiber bike frames. Sharp angles tend to try to take all the stress they can, in general... and in the case of carbon fiber, it'll be very not pretty when (if) it breaks, particularly if somebody happens to hit the break. There are ways around this--like more material on the sharp corners--but it's generally better to not have the corner quite so sharp in the first place.