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Saturday 4 January 2020

Bike steerer tube bung

This was requested by Dean to stop the grub and water filling up the steerer tube while cleaning the bike. He's used various things in the past but with the power of a 3D printer and some design work using Fusion 360, I came up with something that fits tightly.

This is not intended to stay in place while riding. In fact I would think that keeping it in place would trap moisture and encourage corrosion. If you are looking for something to keep the mud out while riding, I would suggest some air filter foam stuffed up the tube. That will keep the mud out and allow any moisture to evaporate.

The bung is designed to be pushed in to a tapered fork stem. It just pushes up, just inside the steerer tube, and holds in place while the bike is being cleaned. Best to clean out the tube with a rag before inserting the bung.

Despite taking lots of measurements from three bikes, the end result needed trial and error to get the bung to sit just inside the tube. If it goes too far in, it leaves too much tube to fill up with water and it is difficult to reach to pull back out.

They need a loop of strong string to help get it back out.

The differences in the diameter between an SR Suntour, Fox and Rockshox forks were only about a millimetre in total between the three. I still had to make them to fit specific forks. Only 0.5mm diameter difference between each!

Of the forks I have tried, the smaller fits the Suntour, the medium fits the Rockshox and the larger fits the Fox. I modelled them with a dot in a different location on the handle, to help identify which is which.

They all use two metric rubber o-rings. An o-ring with an inside diameter (ID) of 25mm and cross sectional diameter of 3.5mm fits all of them. I would guess that imperial o-rings with ID 1" and cross section of 1/8" would work just as well.

They 3D print as two halves with two alignment keys. I printed it all without any supports and just a skirt. It needed no cleaning up and can be pushed together. It's a tight fit. I also used a dab of superglued (CA) but that is not essential.


Download 3D models:
Steerer bungs, Fusion 360, STEP and STL (Zip)
Licence attribution - small business exception


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