pic: Carbon fiber monocoque drivetrain Top view

[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.

[cbale2000]

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?

This depends a lot on how you do the layup. Most of the time, in my opinion, rounded parts are used because it's a bit easier to fabricate, at least for the layup process (in addition to what Bryce2471 said already). Inevitably though, even if you do have 90 angles, the inside angles usually end up having to be slightly beveled just due to the nature of how the fabric connects the parts in most situations. Outside corners are a bit more forgiving but are still nice to round off a bit.

[JackFisher]

Cool concept. Unless your team doesn't have the in-house resources to make a drivertrain, this would likely take a lot of time and delay the rest of the robot. On top of that, unless you design in a standard mounting system, later in season it may be tricky to modify the robot to improve it. How would you repair it at a regional if it broke?

[Gdeaver]

Every year since 2006 I have found some component of the robot that could benefit from a composite material and lead the team on it's construction. Do we have to use composites - NO. We do it to expose the students to them. As far as doing a whole frame or chassis, this would require expertise and considerable experience. If a team does not have this, don't attempt it. Start off with some smaller lay ups. There is always fiber glass pulltrustions to work with. Done right composites can can be wonderful. Done wrong you have some expensive trash.

[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...

[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.

[JesseK]

Ah - found them - 1829, the Carbonauts:
http://www.thebluealliance.com/team/1829/2015

Looks like for 2015 they went to Aluminum. I'll see them this weekend at Rumble in the Roads, anyone have any specific questions for them?

[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.