Introduction: Silent Knight: a K'nex Compound Crossbow

About: I am a tinkerer who enjoys making almost everything out of Knex, arduino parts, wood, and other materials. I like to discover new uses for things and learn new skills along the way.

This is another Nerf-compatible crossbow I built, shortly after the Magbow. It is a single-shot bow, designed for long range. The name comes from an unexpected aspect of the final trigger design: the crossbow is rather quiet. As a result, it may have potential use in a Nerf war as a stealth weapon. Also, since it is closer to a traditional crossbow in overall design, it should have no problem firing other projectiles as well, although the range and accuracy depend heavily on the characteristics of the projectile itself.

In this instructable, most of the parts are metallic colored, but I find it easier to refer to the parts by their classic colors. Usually, if there is a difference between the classic part color and the part shown in the pictures, it will be referred to in this format: "classic (shown)."

Step 1: Parts Count

Sorting the parts is optional, and some of the parts can be substituted for others. In this particular project, most of the clips except the Y-clips are interchangeable with other connectors. All parts in the list are referred to as their classic colors.


White: 2

Yellow: 49

Green: 26

Red: 18

Gray two-slot: 4

Gray one-slot: 16

Orange: 27

Blue 3D: 3

Purple 3D: 9

Y-clips: 13

Blue clips: 8

Tan clip: 1

Socket clip: 1


Green: 84

White: 50

Blue: 31

Yellow: 10

Red: 5

Gray: 3

Other K'nex parts:

Blue spacers: 14

Silver spacers: 7

Gray gear: 1

Medium solid wheel hubs: 2

Non-K'nex parts:

7 in. rubber bands: 3

4 in. rubber bands: 2

String: about 3 ft-6 in.

Step 2: The Firing Rail and Top Frame

We start with the backbone of the crossbow.

  1. These two frame pieces will form the bottom of the rail.
  2. These frame pieces will form the outer sides of the rail.
  3. The inner rail pieces connect to each other by two blue rods and two yellow (dark gray) rods.
  4. The inner rails go in the middle of the rods, as shown.
  5. This picture shows one outer rail piece connected to the inner rails. The Y-clips connect to the rods.
  6. The firing rail itself is now complete.
  7. For structural stability, an additional pair of frames will be added to the sides, beginning with the yellow (gray) connectors shown.
  8. The connectors that attach to the yellow (dark gray) rods connect to each other as shown, but this is easier after they are attached to the yellow rods.
  9. The orange (brown) connectors go between the yellow (gray) connectors to lock it all together into a single frame.
  10. The frame is now completed.
  11. A view of the completed frame from underneath.

Step 3: The Lower Frame

The lower frame will also help with the structural stability, as well as provide mounting points for the handle, trigger, and bow.

  1. Start by assembling the frame pieces shown.
  2. This attaches to the blue rods in the upside-down upper frame.
  3. Assemble the frame shown.
  4. The red (dark gray) and orange (brown) connectors fit over the blue rods, and the red connectors face down.
  5. The new frame fits over the old one so that the green (black) rods in one frame fit through the corresponding holes in the other.
  6. This is a close-up of one of the connection points described above.
  7. A blue rod fits between the green connectors so that it also fits within the two red (dark gray) connectors. Note that it does not actually snap into place.
  8. The small piece with two white (silver) rods and a red (dark gray) connector will help hold together the connection described above.
  9. This piece connects in the open ends of the green connectors.

Step 4: The Handle

The handle integrates directly into the frame. The actual attachment is a bit awkward, but it is solid when completed.

  1. The handle begins with the frame pieces shown.
  2. The frames connect side by side.
  3. The two frame pieces shown will form the back. Note that they are not identical.
  4. When the green connectors face right and the flat side of the connectors is down, the truss piece with the yellow (light gray, 5-slot) connector attaches to the back.
  5. Under the same conditions as above, the frame with two gray two-slot connectors attaches to the front.
  6. Push the white (silver) rods all the way forward and attach the two green connectors as shown in the next picture.
  7. Prepare the small truss piece shown below the handle.
  8. The truss piece and white (black) rod attach to the green connectors as shown.
  9. The handle is now complete.
  10. The blue rod goes through the hole in the handle to attach to the frame.
  11. The blue rod attaches between the two yellow (light gray) connectors on the lower frame.
  12. The yellow (light gray) connector on the handle connects to the green (black) rods on either side, and the white (black) rod on the handle attaches to the purple (silver) 3D connector
  13. This is a view of the other side. There are no new connections on this side.
  14. These pieces will be needed to form the trigger.
  15. This is the assembled trigger. It attaches in front of the handle.
  16. The trigger attaches to the middle slot of the 3D connector.

Step 5: The Trigger Block and Some Additional Framework

This part will hold back the bow when drawn, and release it quickly. The step also includes some additional framework for later.

  1. We begin with some extra frame pieces for the sides.
  2. These frame pieces attach as shown. They will stabilize the bow later.
  3. These pieces will aid in connecting the trigger to the trigger block.
  4. They assemble to form the guide shown.
  5. The guide attaches to the back rod on the right of the frame, like in the picture.
  6. These are the first parts of the actual trigger block.
  7. They connect to form the double frame shown here.
  8. The gray single-slot connectors attach to the green (black) rods on the back end of the firing rail.
  9. It is now attached to the rail. Note that the white (silver) rods face towards the bottom of the rail.
  10. Loop the two short rubber bands around a yellow (dark gray) rod. This will aid the release mechanism's movement.
  11. The rod connects to the open slot on the red (dark gray) connectors in the back.
  12. This is where the trigger will connect to the block. Note the positions of the two yellow (dark gray) rods.
  13. The gray one-slot connector slides onto the rod that is farther from the 3D slot, as shown.
  14. This structure will attach to the four white (silver) rods on the trigger block.
  15. Here is the connection. Note also that the rubber bands have been moved up onto the connectors before the connection.
  16. These are the trigger hooks. They will hold back the bowstring.
  17. They slide onto the rod in front of the trigger block. Note that the hooks are between the two yellow (dark gray) rods on the frame. When the frame is held right-side-up, the hooks should not fall past those rods.
  18. The yellow (dark gray) rod will hold the hooks together, while the gray clips keep them on the fixed rod.
  19. This is the current state of the bow. Pulling the back end of the trigger block up should also raise the hooks. This is how you know it is correct.

Step 6: The Bow

Finally at the source of power. This is where the crossbow finally begins to take shape.

  1. Here is the initial frame. The blue clip will make sense later.
  2. These pieces will connect the trigger to the block, but they are part of the bow frame.
  3. The trigger connectors connect to each other by the orange (brown) piece between them.
  4. Here is the final connection. Note that the fixed connection is at right angles.
  5. The connector slides onto the bow frame on the side with the blue clip. A silver spacer goes on the rod with the blue clip afterwards.
  6. These are the two ends of the frame.
  7. These pieces connect as shown. Yes, the rods will be slightly bent at the end of this project, but not nearly enough to be permanent or to damage them.
  8. Four of these truss pieces will become the arms of the bow. They are asymmetric: the green end will go to the front and the yellow (light gray) end will go to the back.
  9. The trusses will attach to the ends using blue clips.
  10. Here they are attached to the frame. Note the direction they face relative to the trigger connector and the four yellow (light gray) connectors on the frame.
  11. These two rollers will go on the backs of the trusses.
  12. This is the rollers attached to the trusses and with the string wrapped around them. This is usually easier after they have been attached to the trusses. If the string is not measured and cut beforehand, attach it after the rubber bands are in place (later in this step). When that is done, the rubber bands should not be slack, but they should not be overly tight either.
  13. This is a focused picture on how the pulleys are wound. I will start on the left side in this description, but mirroring the description does not make a difference, as the final bow is symmetrical.
    1. Tie a slipknot around the pulley axle on the left side under the pulley. (The bow in the picture is upside-down so this will be visible.) Pull it tight.
    2. Pull the string around the pulley on the right side. (In these pictures, the string goes clockwise around the pulleys.)
    3. Pull the string around the pulley on the left side so that it wraps in the same direction.
    4. Tie the end of the string around the right axle under the pulley. Pull the loop tight.
  14. The four red (green) rods will attach to the main frame. I know these pictures show the pulleys already strung; that is simply the way this was disassembled and reassembled to take the pictures. You can string the bow at any point you want, even as the very last step.
  15. These rods will add stability to the frame connection.
  16. The stabilizers go on the lower gray rods. (The bow is upside-down.) Note the silver spacer now separates the trigger connector from the green connector.
  17. The bow is now ready to attach to the frame. This is being done before the rubber bands are added because I have the pulleys pre-strung, and the tension from the rubber bands might cause buckling. The attachment points on the frame should take some of the load and reduce this risk.
  18. This is how the frame attaches. The red (green) rods go in the center of the frame, while the two stabilizers go on the additional pieces on the sides.
  19. Finally, it is time to add the rubber bands. They will connect to two rollers like the one shown here.
  20. Three rubber bands are looped over the rollers before the rollers are attached to the front of the bow.
  21. This step is finished. Here is a front view of what we have so far.

Step 7: Finishing Details

These are some miscellaneous details needed to make the crossbow actually fire.

  1. First, a yellow (dark gray) rod will be needed to attach the trigger to the trigger connector.
  2. Here is the connection.
  3. This gray rod will connect to the other end of the trigger connector.
  4. The guide block from earlier should form an opening between itself and the frame of the crossbow. The gray rod should go through this opening.
  5. Here is another view of the gray rod going between the guide block and the frame. The crossbow is upside-down.
  6. The gray rod attaches to the bottom side of the trigger connector.
  7. This is the rest of the linkage between the trigger and the trigger block.
  8. It connects between the gray rod and the trigger block as shown.
  9. Prepare the two small rubber bands as shown in the picture.
  10. Loop a rubber band underneath the frame and over the end of the rod on the bottom of the hook.
  11. Do the same thing with the other rubber band, but thread it through the first one.
  12. Here is a close-up of the starting point. These positions can be adjusted later to increase or reduce the tension in the bands. Too much tension will cause the hook to slip down by itself. Too much slack makes the force from the bands insufficient to pull the hook down. It's a bit of trial and error, but the range at which it works is wide enough that it should not take very long to fix.
  13. Here is the hook in the locked position. As before with the trigger block step, raising the block should also raise the hook. This time, instead of lowering the block by hand, you should be able to unlock it by pulling the trigger. There should be a point at which the block and hook suddenly snap down. This means that the connections and rubber bands should be correct.
  14. The crossbow is now complete!

Step 8: Firing

This works with streamline and non-streamline darts, along with any other sufficiently light object that fits on the firing rail. (Using with actual arrows gives a disappointing but humorously short range though.)

To use the bow:

  1. Pull back the bow with two or more fingers, with the fingers spread so that the bow is over the area where the hooks will be.
  2. Use your thumb to catch the upper rod on the trigger block and pull it forward. This will raise the hooks in front of the bow.
  3. Let the bow rest against the hooks and place the projectile in front of it.
  4. Pull the trigger to fire.

If the doesn't fire the first time, check the rubber bands, as mentioned in the last step. Testing them with the bow drawn is the final say in whether or not they are adjusted right. If it does fire the first time, it should also fire every other time.

In testing, it fired a Nerf dart between 60 and 65 feet. The Nitefinder EX-3, the only real Nerf gun I had to compare it with, shot a little over 70 feet in the same conditions. Both were held approximately 45 degrees and a couple feet off the ground, with the end of the barrel/rail resting on a stool so they would both be the same distance from the ground. The Nitefinder is not modified. For level shots, it also seems to compare nicely with the Nitefinder, although I do not have any numbers for those.

For ammo other than Nerf darts, remember to take safety into account. Sharpened projectiles are fun, but not when pointed in the wrong direction.

One other important thing:

Have fun!