Arm Extension Kits for DJI Frames

by Bosnianbill, published

Arm Extension Kits for DJI Frames by Bosnianbill Oct 15, 2014
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This kit upgrades ANY multirotor that uses the DJI-style arms. It is a drop-in swap for the stock arms and you’ll notice many improvements. Specifically, the kit:

  1. Allows you to install ANY length or diameter arms. The kits are available for 8,10,12 and 16mm CF tubing.
  2. Can use ANY motor – even large, low KV pancake motors.
  3. Can mount ANY size propeller, simply by varying the arm length.
  4. Stabilizes your multirotors – Better aerial photography!
  5. Lightens your quad (the kit is less than 100g), which increases your flight time significantly.
  6. Is stiffer than the stock DJI arms.
  7. Is tougher than the stock arms.
  8. Is cheaper & easier to repair after “gravity-related incidents” than stock DJI arms.


Here is what you’ll need:

  1. Arm adapters
  2. Motor mounts
  3. Carbon Fiber tube
  4. M4 x 20mm screws (4 for [email protected] Home Depot)
  5. M4 Lock Nuts (the one with the nylon bushing), (4 for $0.64 @ Home Depot)
    Here’s how to upgrade your multirotor.
  6. Remove the propellers.
  7. Unplug the ESCs.
  8. Remove the two bottom screws from each of the arms.
  9. Remove the four top screws from each arm and set it aside. Repeat on all arms.
  10. Cut your carbon tubes to the desired length.
  11. Insert the M4 screw and nuts into the arm adapters and motor mounts but don’t torque them down yet.
  12. Slip fit the arm adapters and motor mounts onto the tubes. They will be snug.
  13. Tighten the arm adapter screw until you cannot turn the tube. DO NOT OVERTIGHTEN!!! It just needs to be snug. There is NO rotational stress on this part so PLEASE don’t go crazy with tightening this screw. Nothing good will come of it. You’ll either crush the carbon tube or break the motor mount. There is a wide variation in CF tube dimensions but the gap is wide enough to accommodate them all. Please do NOT tighten the screw until the gap is closed. If you cannot spin the arm adapter, then it is tight enough.
  14. Slip on the motor mount but don’t tighten it yet.
  15. Lay the arm on a flat surface to make sure the motor mount is level with the top of the arm adapter. Once it is, snug the screw until the mount will not turn. Again, please DO NOT OVERTIGHTEN. Snug is good. (Too tight and you’ll get pissed at yourself for stripping or breaking something. Just avoid the pain and don’t gorilla-tighten the screws…).
  16. Install the arm into your frame. Use the original SHORT screws on the top and they will cut their own threads into the ABS and it will act like a lock nut. In almost 120 hours of flying these I have NEVER had a screw back out of the ABS. Again, just snug the screws up and DO NOT OVERTIGHTEN. Yes, you are strong and I am SURE you can strip the plastic, but please don’t do it. Snug is good.
  17. Use original LONG screws on the bottom two legs. The long screws will add additional strength to the legs and, like the top screws, will never back out or vibrate loose. Snug is your friend. Over-tightening will end up something broken or stripped.
  18. As you install each arm, zip tie your ESC to the arm to keep it from flopping around and stressing the soldered joints.
  19. Once all the arms are mounted, double check that the motor mounts are still perfectly level. This is critical on all CF-armed multirotors. Close is not good enough. If they are not nearly perfect then your yaw control will suffer and it will not give you a stable hover.
  20. Mount the motors. The mounts accommodate everything from 18mm to 50mm motors. Cheap insurance: Use Loctite on the motor mounting screws. (I have not tried every single motor on the market but have tried KDE Direct, Tiger, Multistar, Sunnysky and DJI and they all mount while the motor mount is attached to the CF arm. If you have a motor that cannot mount with the tube installed it is no big deal and usually happens with the 16mm tubes and a motor with a narrow mount. Just slide off the motor mount, install the motor, re-install the motor mount and re-level it. Easy). (Pictured is the Sunnysky “Angel” line which has a tiny mounting screw pattern.)
  21. TIP: The motor mounts have a provision to allow in-tubing wiring. Aesthetically, this is nice and I’ve designed in a “bumper” on the end to run your wires through and protect them. From a practical point of view it is not optimal. Thru-tubing wiring requires more wire, which can add up to 100g of unnecessary weight. It is also harder to maintain and inspect. Finally, it is MUCH more difficult to repair if you have a “gravity-related incident”. With outside wiring zip tied to the arms, you simply unplug everything, replace the broken part (usually the arm), and zip tie it all back together. Five minutes and you are back flying. (Pictured is the Turnigy 16-Pole MultiStar 4225-390Kv pancake motor, which has a wide mounting screw pattern).
  22. Plug in your ESCs and check the motor rotation direction (STILL with NO PROPELLERS). Once correct, mount your props.
  23. Go fly. Depending on how wild you got with the arm lengths you may need to adjust your gains. I have 92 hours on 1855 propellers on my 1.7kg 700mm MTM F450 and my gains are: 180 180 130 150 80 80.
    When things go wrong…
    OK, we all strip screws. It happens & all is not lost. Here are some workarounds I’ve used.
  24. Instead of the short screw, use the long one in the stripped hole. If you already stripped a long one….
  25. Put a drop of CA in the stripped hole. Let it sit overnight & it will soften and expand the ABS. The next day, insert a new screw. If that doesn’t work,
  26. Put a SINGLE drop of acetone in the stripped hole. Be very careful since acetone melts ABS. Let it evaporate and dry overnight. The next day, insert the new screw. If THAT doesn’t work,
  27. Fill the hole with epoxy. Let it set. Drill a small hole (about 3mm) and thread in the new screw. If THAT doesn’t work,
  28. Drill the hole deeper, about 15mm. Insert a 12-15mm M4 screw & thread it in. If THAT doesn’t work…
  29. You are out of luck.

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As a followup to earlier comments...These are undersized. The height is short by a few millimeters, and also the screw holes are out of line. I dont think my printer made the error, I think they were designed too small, but I have no way to compare.

Is there a version of the motor mounts that have a longer landing gear?

I printed a set of 4 for 16mm. My first attempt was to have it lay flat on the top surface. That resulted in a clamp failure because of the layering. The finished 4 were reset to print on the face of the clamp end, with full support. The legs have to be facing left or right because there is a weird thing with the supports. The ones for the legs only want to point along the Y axis when viewing in layers, no matter which way the mount is pointing on the platform. So I made one today for 12mm (which btw is way too small...11.4mm) for a 13mm tube. So I am trying one in 14mm. I have UM2 using the Olson block 0.6 nozzle, 25% infill, 0.6 shell, 1.2mm bottom and top. I acetone finished them and they look nice. The only cosmetic issue is the infill printed before the shell, so its ribs show through, but that will just make a stronger unit.
Thanks Thing creator.
Ok, the 14mm is too big for a 13mm tube. I was hoping it would be undersized like the 12mm was. It is, but only 13.5mm. Cant rescale it because then all the dimensions would change. Oh well.

Do you have the models that you can post? That way these can be modified?

These boom clamps look real nice. Did you finish them in an acetone vapor? Is it possible to alter the total overall height in order to have a taller interior space (of course oversleeving the aluminum spacers with a longer tube) so as to be able to carry a larger battery? Anything over 4500mah 4S or 4000 6S wont fit inside. Thanks

Really cool.
What is the weight of one piece?
Do you know, out of what material the TBS adapters are made? Does any one know how much the TBS ones weigh?

Got these printed. I really like these so far! Only problem I can see is that since I use a TBS Discovery, it's shorter than regular arms. Going to cover it with layers of plastidip until it's thick enough and I'll report back!

Yeah weird question. The 14mm ones are 37mm tall, which is the TBS and DJI 450 frame plate spacing. Whoops..my bad...I just looked at the scale of one in Cura, and it shows up as only 35mm!!! So that combined with the fact that they print out .5mm too small on the diameter of the tube hole may be resolved by setting the Z scale to 37, although youll end up with an oval, unless the screw holes for the top and bottom plates are also off..(I havent fit tested mine, since the tubes dont fit, wasnt much point) in which case keep the XYZ scale locked and the whole thing might end up closer to the correct dimensions.
Ill hope for better luck with the 8mm or 10mm...since Im building a 1/2 scale Alien 680.

What do you mean by "Shorter" than the arms? these can be as long as required with the booms?

Finished printing, turns out sanding was not best way to go. Patience+a mallet was far more successful (1/4 broke, my fault I think). Took it out for a spin, and it flew beautifully. Accidentally caught a tree, and wrecked hard. What I love about these it that they perform very well in flight but are the first things to break in a crash rather than expensive gear. Will upload the custom 16x19 motor mounts I made for the 16mm tube soon! Reprinted and repaired super fast, since only the 3d printed stuff broke, and flew again today. I really like it!. One idea: I really like the brass/other nut insert idea. Just saw the perfect solution on youtube: Make the hole slightly smaller, and push the insert in with a hot soldering iron that melts the plastic the insert is touching. I'll try to make a remix for that if I find the time+inserts. Also want to make this foldable, but not sure how..Great job!

Printed all 6 16mm arm-frame mounts, made my own motors mounts for my setup. (Will upload:) i printed slowly and without supports, but my CF tube which is exactly 16, wouldnt fit, i got 3 in my drilling it out a bit, but broke the other 3 mounts :/ reprinting today now that Ive got the drilling proccess down. Might actually tweak the piece with a tiny bit more space
Also going to make a remix with captive nuts for the plate mounting. Great start!

Comments deleted.

What about using this brass insert for making a more durable threaded holes? Would it be a good idea?

Awesome! I might just have to try out the 16mm :)

also, since it is cheaper and much easier to cut, what do you think about wood beams? Bad idea?

Wood would work, lots of people use it on their quads. It IS cheap and easy to work with. Unfortunately, it is not very strong so being extra to the flying field for on-the-spot repairs. CF can take a hit & keep flying. Wood cannot. Still, it IS cheap and available everywhere...

What fill density do you recommend?

Nice design, thanks for sharing! I tried printing a set, but I'm having an issue with the dimensions of the arm blocks. Measuring my DJI Flamewheel arms, the height of them where they are mounted to the frame is ~36.9mm, whereas the height of your arm blocks (16mm version) is around 35.2mm. I tried mounting one to my TBS Discovery frame, but there was some flex on the frame plates because of the thinner mount. First I thought it might a calibration issue with my printer, but the STL model seems to measure to 35.2mm as well. Are there other DJI arm models that these are modeled as a replacement for? Any chance you could make available a version of the arm blocks 36.9mm tall? Thanks!

you can always edit it with sketchup and use the push pull tools to make it thicker

thanks for sharing this design

printed the 16mm model with abs, seems the center duct shrinks and it becomes hard to insert the tube.
so ppl just need to accommodate for that

it will be cool if you can make the same kit but for square arms

I had the same issue. However patience and a mallet worked perfectly. Even made such a good fit the M4 screws were really necessary to hold it in place!