pic: Wood Coast Drive



Just an afternoon CAD project to make a West Coast style drivetrain using the wood construction techniques we learned about this season.

  • About 31 lbs including belts, 2/3 of which is the gearbox and motors. The wood frame weighs ~6.5 lbs.

-Simple to assemble; contains only 17 wood parts, compared with more than 40 in our 2015 drivetrain.

-15mm HTD belts on 24t pulleys inside tube; assembled as Joey Milia described here

-WCP SS gearbox and 4x1.5" Colson wheels. Easy to adapt to most other gearboxes and wheels.

It’s lightened more aggressively and uses smaller tubes than most of what we’ve built in the past, but certainly “probably should work”. Areas I’m most concerned about are the joint in the belly pan, which could be reinforced with some 1/8" plywood and epoxy, if necessary, and the weight of the gearboxes warping the tubes. That could be fixed by adding a cross-brace between the gearboxes.

Bumper mounts, if necessary, could easily be added by extending the top plates and belly pan out between the wheels and adding a perpendicular plate to brace them.

I heard at World’s that you were limited in the size of the pieces that you could cut by the size of your laser cutter. Can you handle larger pieces now, or is this more of a theoretical design?

All the pieces fit within our laser.

I would remove all the diamond pocketing in the mock “rails”. It cuts the fiber up so much, and it’d be hard to justify the weight it saves for the decrease in strength for a more traditional FRC game with contact.

Yeah, taking a second look at it, the belly pan definitely wouldn’t survive a contact game. I think you could design the gearbox crossbrace to handle that though.

That makes sense; will do. Edit: Removing the holes only cost ~.5lbs; not that big a deal, given how light the chassis is anyway.

The intent is to epoxy 1/8" plywood plates to the center of the belly pan, making it one piece. I’m planning on adding a brace between the gearboxes.

Very cool. 31lbs with a 6-cim drive is very impressive, too.

When you take a thin layer of fiberglass and epoxy it over wood the strength and stiffness goes off the chart… you’d have to figure out a way to fillet the corners a bit to allow the glass to flow smoothly over the edges.

Actually, once you got good at glassing you could replace the plywood with balsa and go for…

Oh, wait. You said this weighs 6.5 lbs? Yeah, I don’t think the weight savings will be worth it. Wood is pretty amazing on it’s own.

Jason

I’m new to the idea of wood construction, would it be worth it to replace the front and side panels with polycarbonate?

The main advantages of wood (IMO) are that it’s dirt cheap and can be laser cut very quickly and easily. Polycarbonate is neither.

While both are true, I saw that someone was concerned about cutting the fiber up too much. I don’t necessarily understand what that is, so I was guessing that is was something to do with structural integrity. Polycarbonate is (IMO) much more suited to take impact than plywood is, hence me asking the question in the first place.

Polycarbonate cannot be cut on commercial laser cutter. It releases dangerous fumes when melted.

Plywood’s not bad at impacts; it’s springy and returns to shape when metal dents. Until our safety captain forbade it, I demonstrated that by hitting our demo swerve module with a hammer. We also ran over our prototype drivetrain with a car, with only minor damage.

We also use wood since it’s so fast to make; we get turnaround times less than half an hour on our laser, while we’d have to cut Lexan by hand.

Then why NOT use it for the front/side panels? I’ve gotten a lot of the benefits of using plywood, but no answers as to why polycarbonate would or would not be better.

In this case, plywood takes impacts at least as well as polycarb, and is faster by far to cut and install.

Here are some more possible reasons: Wood is lighter than polycarb for the same size of piece (though it’s quite possible that you might need a thicker piece). Wood is actually stiffer than polycarb–take a sheet of birch ply and a sheet of polycarb, hold at one end, shake. Wood is immune to loctite spidering… and it’s a lot easier to drill without cracking if you forgot to CAD the holes for the laser.

Note: The above specifically applies to birch plywood, ideally Baltic birch plywood.

That being said, there ARE teams that build chassis out of polycarb: 1714 has been very hard to see for many years because their primary building material is polycarb (or is it acrylic? think it’s polycarb). But they have to be very creative in terms of material attachment and stiffening.

Polycarb is denser and less rigid

I think your mixing up polycarbonate (lexan) and acrylic (plexiglass). Polycarb is extremely impact resistant but cannot be cut on a commercial laser cutter. Acrylic is very brittle and can very easily be cut on a laser cutter.

In what way? I happen to be comparing WOOD and polycarb, thank you very much. The comment about the laser cutter was in a statement about WOOD.

I believe you are comparing wood and acrylic.

Polycarb is much more impact resistant than baltic birch.

It’s very difficult to crack both polycarbonate and baltic birch while drilling. Out of the two baltic birch is easier to crack.

Again you can not laser cut polycarb with a commercially available machine.

My understanding was that acrylic is easy to drill, polycarb needs a special drill bit or it’ll crack, and birch merely splinters if you make a mistake–not crack.

Again you can not laser cut polycarb with a commercially available machine.

I never, EVER, said you COULD cut polycarb on a laser. If you can show me where EXACTLY I said that, by QUOTING, then I’ll retract this statement. Otherwise, leave any discussion of cutting polycarb by laser out.