How to Choose a Carbon Fiber Tube for Your Drone Build
Share
What a drone arm actually has to do
Hold a motor out at the end of a lever, keep the propeller in a flat plane while the motor pulls, and not vibrate.
That's it. That's the whole job description. Once you frame it that way, the tube spec mostly picks itself, because every number in the spec traces back to one of those three things.
The motor mounts somewhere near the tip. The prop pulls upward. The arm is a cantilever beam carrying that load over its length. Everything else is detail.
The number that decides almost everything
If you only take one thing from this page, take this one: the outside diameter.
Stiffness in bending scales very steeply with diameter — roughly the fourth power, which is engineering speak for "a lot". Going from 16mm to 20mm outside, same wall, more than doubles the stiffness. Not two percent. Not twenty. More than double.
Wall thickness helps too, but linearly. Double the wall and you roughly double the stiffness while roughly doubling the weight. Diameter gives you stiffness without a proportional weight penalty, because you're spreading the same material further from the bending axis. That's why every serious airframe you've ever seen has fat, short arms and skinny, long ones don't exist.
Practically, for arms up to about 400mm:
| Arm length | Outside diameter | Wall | Typical use |
|---|---|---|---|
| Under 250mm | 12mm to 16mm | 1.5mm to 2mm | Small quads, 3 to 5 inch props |
| 250mm to 350mm | 16mm to 20mm | 2mm | The bulk of what we sell |
| 350mm to 500mm | 20mm to 25mm | 2mm | Larger builds, heavier payloads |
| 500mm and up | 25mm and above | 2mm to 3mm | Heavy lift, long endurance, industrial |
Read the wall column and notice it barely changes. That's not an accident and it's not laziness in the table. Once you're at a 2mm wall in a roll wrapped tube, adding more wall buys stiffness inefficiently and adds weight right where you don't want it. If an arm is flexing, the fix is nearly always diameter, not wall.
Why the wall stops around 2mm
Two reasons, and they pull in the same direction.
The first is weight. Arm weight sits at the end of a lever just like the motor does, so it costs you more inertia and more current to move it than the same grams would in the centre plates. Every gram outboard is worth several inboard. Thin walls are worth chasing out there.
The second is that thick walls in a roll wrapped tube are harder to make well. More layers means more chance of a resin-rich spot between them, and a resin-rich spot is a delamination waiting to happen. A clean 1.5mm wall is a better part than a sloppy 3mm one, and it'll survive a crash that splits the thick one along a layer boundary.
The counter-argument is damage tolerance, and it's real. Thin walls dent and crack more readily when you land badly. If you're flying something expensive or flying somewhere you can't afford a failure, a thicker wall is cheap insurance. Just be honest that you're buying crash resistance, not stiffness.
Plain or twill, glossy or matte
Doesn't matter for performance. Pick what you like looking at.
That's the honest answer and I'd rather give it than invent a mechanical difference. Plain weave and twill weave differ in how the fabric drapes and how the surface looks. Twill has the diagonal pattern most people picture when they think carbon fibre. Plain has a tighter checkerboard look.
Where the weave can matter a little: twill drapes more easily over complex shapes, which is why you see it on curved parts. On a straight tube, both are straight. Buy the one you want to look at.
Gloss or matte is cosmetic, full stop. Matte hides scratches better. Gloss shows off the weave more. Neither has any effect on how the tube performs.
Length, and where you cut it
Tube comes in 500mm and 1000mm from us. You cut it to your arm length.
Two things worth knowing before you cut.
Cut on the long side and trim. It's a lot easier to take 5mm off than to add it back. Measure your motor mount, your frame plate, and the gap between them, then add a few millimetres of safety and cut there.
Support both sides of the cut. This is the mistake that ruins tubes. A carbon tube is stiff along its length and thin across its wall, so a blade that's pushing through the middle with nothing supporting the far side will pinch the wall shut at the cut. Two blocks of wood with a gap between them, tube laid across, cut in the gap. Slow. Fine-tooth blade or an abrasive disc. And a mask, because carbon dust is genuinely nasty stuff to breathe.
How to mount it
Clamps, and one at each end of the span.
A single clamp in the middle lets the arm pivot around it. Two clamps at the two mounting points hold it rigid, and they also let you adjust the angle before you tighten anything. Put the bolts up properly — a clamp that isn't tight will let the arm move under load, and movement is what turns into vibration and eventually into a crack.
One thing that catches people out. Tightening a clamp onto a thin wall crushes it. The clamp squeezes, the wall has nothing behind it, and it deforms inwards. You'll feel it go soft as you tighten, which is your warning. If your wall is under about 1.5mm, put a reinforced sleeve inside the end first, so the clamp is squeezing aluminium rather than carbon. It costs a couple of dollars and saves the arm.
Vibration, briefly
Every arm has a natural frequency, and if a motor's rotation or the prop blade-pass frequency lines up with it, the arm will ring. You'll see it as jello in the camera footage and feel it as a hot motor.
Stiffer arms have higher natural frequencies and are less likely to be excited by a spinning prop. This is another argument for diameter over wall, and another reason short arms are easier to get right than long ones.
If you're fighting vibration and the hardware all checks out, a shorter arm or a bigger diameter will do more than anything you can change in the tune.
What we'd spec at WHABEST for a few common builds
5 inch freestyle quad, 220mm arms. 16mm outside, 1.5mm wall, plain weave, matte. Light, stiff enough, and cheap enough that you won't cry when you break one. You will break one.
7 inch long range, 300mm arms. 20mm outside, 2mm wall. The extra diameter keeps the arm out of the frequency range where the larger props like to excite it.
Cinema or survey rig, 400mm arms, heavy payload. 25mm outside, 2mm wall, and clamp at both ends with sleeves in the tube ends. Weight is less of a concern at this size than stiffness, so favour diameter.
Micro build, 100mm arms. 10mm outside, 1mm wall. Nothing here is stiff in absolute terms, it just needs to be light and roughly straight.
Frequently asked
Square tube or round for arms?
Round, most of the time. It's stiffer per gram, it's easy to clamp, and it doesn't care which way up it is. Square tube has its uses — it sits flat against a plate and gives you faces to bolt through — but for a cantilever arm, round wins.
Can I use one long tube through the frame instead of two arms?
Yes, and a lot of racing frames do exactly that. One piece through the body, motors at both ends. You get more stiffness because there's no joint at the frame, at the cost of a much bigger part to replace when one end breaks.
Should I glue or bolt the motor mount?
Bolt it. Glue on a smooth tube needs surface prep to work properly, and it doesn't come apart for repair. Clamp or bolt, and check the fasteners occasionally.
How much does an arm tube weigh?
A 20mm outside, 2mm wall tube at 300mm in roll wrapped carbon is around 25 to 30 grams. A 16mm at 1.5mm wall over 250mm is nearer 12 grams. You can feel the difference in flight, which is why people chase the small sizes.
Does a tube fail suddenly or gradually?
Depends how it's loaded, and it's worth knowing. In tension the fibres either hold or they don't, so it's sudden. In bending you'll usually get warning first — a creak, a visible whitening at the clamp, a soft spot you can feel with a thumbnail. If you find a soft spot, retire the tube. It's a few dollars and the alternative is a motor departing at altitude.
Tubes are grouped by outside diameter in the carbon fiber tube sections, and the fittings are in tube parts. Still working out the bore? The sizing guide covers OD, ID and wall together. Cutting and mounting is in the workshop guide. Building something unusual? Send us the arm length, the payload and the prop size and we'll suggest a spec.