How To Make a Cobra Punching Bag using the Kewlus PRO Spring

How To Make a Cobra Punching Bag using the Kewlus PRO Spring


Now you can have the ultimate PRO boxing cobra reflex punching bag that is better than any commercial cobra bag on the market at just a fraction of the cost! The key ingredient is the Kewlus Spring. In this DIY we use the Kewlus PRO Spring (KSR12PRO), which is the fastest and able to handle the heaviest punches.

One of the main advantages of the Kewlus Spring boxing cobra bag is the ability to adjust the speed of the bag. Want to make it really, really fast? Just lower the spring on the mast and the action becomes lightning fast!

Note: make sure to leave at least 4 active coils so as not to damage the spring.

Check out this video for how to put it together and to see it in action:

Items You Will Need & Options:

  • Kewlus PRO Spring (KSR12PRO) (US $49.95) - I designed and produced this spring specifically for the purpose, taking out all the guesswork. The PRO version is a heavier gage spring designed to be able to be used with a heavier bag if need be.
  • Base pipe (mast) - couple of options - Option 1: You can either simply use a single galvanised 3/4 inch pipe roughly 42% of your height in length (if you're 6 foot tall, then your pipe should come up roughly 2'6" from the ground level). It will come up to about the middle of your thigh muscle. The outer diameter of the pipe is 1.05 inches or 26.7mm and fits perfectly into the Kewlus Spring. Option 2 - more sturdy: If you want a more sturdy platform, use a 2 foot section of 1 inch pipe at the base and a small section of 3/4 inch pipe to match your height. Going from 1 inch pipe to the 3/4 inch pipe you'll need a reducing coupling. If you want to make a height-adjustable mast, check out this video:
  • Galvanised flange to fit the mast pipe, a set of nuts and bolts to fit the flange and an MDF board for your base - roughly 2 to 3 feel wide by 2 to 3 feet long and 3/4 inch thick - the wider, the better. I used four 40mm (1.5") long M8 (5/16 inch) bolts and nuts to bolt on the flange. See video for preview. For the most stable solution you could bolt the galvanised flange to a concrete slab or concrete floor like in your garage.
  • Suction Cup Base (optional) - I use a suction cup base in all of my latest builds because it's small, rock solid and is easy to setup and put away. You can even set it up on carpet by simply using two laminated chip boards and have the suction cup base connecting the two. Refer to the following page for the instructions on how to make it:
  • Roll of wide duct tape.
  • PVC pipe - same outer diameter as the 3/4 inch galvanised pipe (1.05 inches or 26.7mm). Roughly 2 feet in length (or 61cm) - you can always cut it shorter later on if need be. If you're using a ball and funnel for the top, then you only need 16 inches.
  • Two pool noodles. An alternative to using pool noodles is to make the bag using a rubber ball, a small funnel and the PVC pipe above (about 16 inches long) -
  • Set of butterfly hose clamps (1.3" - 1.7" diameter) - here is a set on Amazon USA:

4-Step Instructions:

  1. Pool noodle option: Measure off and cut 20 inches of 3/4 inch PVC pipe that will form the arm of your punching bag. Cut a section of pool noodle to use as your punching bar base. I recommend about a foot. Slot it onto your PVC pipe until it goes all the way to the end. Take another section of pool noodle (roughly 18 inches) and split it lengthwise so you'll have two halves. Wrap these around the base section. Take a longer split section and wrap that around the base of the top ball. Finish by taping the whole shape from top to bottom until fully covered. It's best explained visually in the video above. This is your punching speed ball.
    Rubber ball option: Follow this video:
    Leather Speed Bag/Ball option: Follow this video: You can buy a suitable genuine leather speedball from the Kewlus store here:
  2. Setup up your base by bolting the flange to the centre of your MDF board. The total length of your mast should reach roughly to the middle of your thigh muscle.
    If you want to make a suction cup base, follow the instructions in this video:
    If you want to make a height-adjustable mast, check out this video:
  3. Insert the Kewlus Spring onto the galvanised pipe so that it overlaps the pipe by one inch. The top of the spring should come up to your hip. Secure it in place using one of the butterfly clamps - you'll need to fully open the clamp to insert it between the coils of the spring.
  4. Insert the PVC pipe with your speed ball into the top of the spring and secure it using another hose clamp. Important: make sure to leave at least 4 active spring coils (coils between the top of the mast and the bottom of the pipe that holds the ball/bag).
Good luck making it and hope you enjoy!

Related Videos

Latest DIY video for how to make the Kewlus PRO Spring Cobra Bag:

Original DIY video for how to make the Kewlus PRO Spring Cobra Bag:

Mounting the Kewlus PRO Spring - finer points:

How to adapt a leather speedball to use with your cobra bag: You can buy a suitable genuine leather speedball here:

How to make a super fast cobra bag using a rubber ball and a funnel for the top section:

How to make a suction cup base for your cobra bag - it's my favourite base:

How to make a height adjustable mast:

For all other related videos check out my channel: and make sure to subscribe for upcoming upgrades and features.

Free 3D Print Files (.STL)

These are my prototype 3D print files that I created in the process of designing and testing various components. If you have access to a 3D printer, you could print these for your DIY project. No guarantees with these.

Spring Mount - This was used to mount the spring onto the 3/4 inch pipe thread. The mount screws onto the thread and the spring screws onto the mount. I tested this mount in PLA but recommend using Nylon for greater strength. For ultimate strength print it on its side. Download File:
Speedball Adapter - This is used for inserting inside a small leather speed ball. Initial tests with PLA held up for a few sessions, but eventually broke after a while. I increased the thickness and recommend printing with Nylon for greater durability. Download File:
Mast Clamp If you want to make a height-adjustable mast, check out this video: The mast clamp screws onto the 1 inch pipe and a hose clamp goes around the top to clamp onto the 3/4 inch pipe that sits inside the 1 inch pipe. Printed in TPU (the flexible material). Download File:

Kewlus PRO Spring Community

Check out the Kewlus PRO Spring Community page to see how others have setup the Kewlus PRO Spring cobra punching bag!

Review & Build by CSquared Boxing

Check out one of the top boxing gear reviewers on YouTube, Carlo Rosauro to see his hands on take on the spring as well as his full build 

Kewlus PRO Spring Build and Review by CSquared Boxing

Get The Fighter DIY Book  

For more detailed and fully illustrated instructions get the Fighter DIY eBook (Third Edition). The Third Edition edition includes this DIY plus 3 other awesome projects like the Spar Bar DIY.