Adapter for bicycle forks

This article came to life in a form of a written account of all the work that had to be done in order to successfully manufacture an adapter for either a rigid or forks a suspension forks on my bike. The purpose of the adapter was be to raise the bike frame. The bike frame and the headset carrier (which is located on the adapter) would be assembled together with the help of aforementioned adapter…

If you need this schematic in the form of vector graphics, please contact me.

The adapter schematic.

This whole project started out as a low cost way for me to get an awesome bike. I already had a decent bike frame but I couldn’t save enough money to buy a good suspension forks, so I had to do some re-designing.

Finished adapter, two mounting screws and a spacer.

Finished adapter, two mounting screws and a spacer.

An opportunity came up so I bought a Cannondale P-bone – rigid forks. The only problem with this forks is the fact that it was made to be used on a completely rigid bike. If you mount it on a bike which was designed with suspension forks in mind, it will change the bike geometry in such a way that it will greatly influence the performance. A bike with ruined geometry does retain its function, although just barely. You won’t get any comfort or any performance when using such a bike.

In order to avoid ruining the geometry I had to invent an adapter which would be used to elevate the frame to a desired height. (Updated 2006. You need to use a specific forks for each bike (each forks has a prescribed length) in order to successfully mount it on the bike. Follow the specifications given for you bike and don’t mess with the geometry. Each company has a team of experts working on bike design for a reason. If you feel dissatisfied with your bike bike frame, it is much better solution better to buy a new one if you can afford it.)

Picture of an adapter when successfully mounted on a bike.

The adapter itself is really just a simple actor (workaround) made of steel. Its sole purpose is to mimic a headset crown race located at the crown of the forks as to elevate the mounting ring (located on the headset) for an additional 6 cm.

This is how it should look like.

I encountered a problem during the manufacturing process. The steering tube had a different diameter from the forks, starting right at the supporter ring. Steering tube has 28.50 mm in diameter extending for 99% of its length, but near its handle its diameter gets big larger; 30.00 mm. I found out later that this was normal for all the steering tubes which are of 28,50 mm in diameter.

Another picture, this time with exact measures.

I spent a lot of time figuring out a workaround for this problem. One of the first ideas that came to me was to simply forget about using a support ring. I wanted to make an adapter which had a fixated hole on it. Smiley The upper portion of the adapter body would serve as a main headset ring. But how was I to accomplish this? Worst of all, where was I supposed to get high quality steel? I didn’t want to spend too much time on this project, nor did I want to travel far to find an alloy that would suit my needs. It turns out that almost all lathe machine operators use low quality steel (which is easier to work with). I decided not to ruin my fine FSA Orbit Xtreme Pro headset (read review) with rust coming from the adapter made from low quality material.

Turning the spacer on a lathe machine.

Lathe machine operator Antun Bender.

Unfortunately I found no alternative but to go ahead and manufacture a turbo complicated custom adapter. The upper body of the adapter (where the bike geometry is identical to a F600 model) needed to be resized to match the lower body, which was thicker. In other words the upper body had to be exactly 30.00 mm in diameter, while the lower body had to be 28.50 mm in diameter. The outer wall has to be 0.75 mm thick. The height of the adapter had to be no less than 7.5 mm.

Now… Since I am neither a metalworker nor a machine operator, I had to do a lot of reading and had to talk to many people who knew more.

I found out that it was possible to find a quality piece of steel, which would conform to what I need (precision, safety) and then work on it using a precise lathe machine.

Luckily for me, the manufacturing process was relatively simple because it involves nothing more than turning (in a lathe machine) and then a bit of lathing. There was nothing special about the manufacturing process. Smiley I visited several lathing facilities in Dugo Selo and upon visiting the first one, the people working there tried to make me look like a fool so I moved on and found another one: “BADIM-COP – Lathing services”. The name of the lathe machine operator was Antun Bender, Tel.: +385 (1) 276 0420, address: “TRSTENIK, 73 Zagrebačka street”.

The adapter, viewed from below. Notice the narrowing at the base of the adapter.

I spoke to the owner, explained what needed to be done, gave him the blueprints, the forks, the headset main ring and everything else he needed. We finally struck a deal, I had to pay 200 kn (about 35 dollars, or 27 euros) and he would use his own materials. He used a part of some wicked looking axle, which he took from a tank I think. I went home to grab some lunch and to bring the bike frame so we can put the adapter into actual use.

Drilling holes which would later be used to fasten the adapter to the forks.

By the time I got back the work had already been done, we both were happy. We started testing the adapter and noticed that it had a serious flaw. The work was done without a flaw but unfortunately the whole piece was up side down. The small ring did make a tight fit but unfortunately it was up side down… Smiley What can one say when something like this happens? We decided to ignore this temporary setback. We started from scratch but this time under my direct supervision.

A 3 cm diameter drill bore.

We had to invest a lot of effort into making this adapter, it wasn’t easy at all, but the lathe machine operator was real good at his job.

On this picture, the adapter is mounted on to the bike forks.

For the first time in my life I came to realize that there indeed was a difference between one micron (1/1000 of 1 mm) and one millimeter. Smiley When all was done we drilled 2 holes on the adapter body (for the screw threads). Using these, we could tightly fasten the adapter on to the forks.

A schematic for a small improvement to the existing design.

When all was done I gave 300 kn (about 50 dollars, or 40 euros) to the owner. We washed our hands and parted our ways.

You wouldn’t believe all the things I had experienced with that forks mounted on my bike. For a time I was an owner of a unique bike and I have to admit that I often gloated over other people when they started staring at my self-made contraption Smiley You had to be there to believe it.

Beside all the work we did on the adapter we also did some work on a ring 12 mm in height, which we used as a spacer.

The adapter can be used on any rigid or suspension forks as a distancer for achieving the desired height (of the frame), the only thing that may vary from bike to bike is the length of the adapter body. I think that any potential reader should further customize my design to suit his own needs, I didn’t need any further customization myself so I haven’t made one but I was thinking about using a three piece system that would make a tight fit – in that case you would be able to reduce the weight of the adapter by 50 to 70%. The core should be made of aluminum and the rest should be made of steel. You could also make it out of one piece of steel, with cavities, but that would look awful. You can find out more about P-bone adapter in a. The adapter simply had to be as thick as it was, because its length is dictated by the Cannondale head tube (which is 5,5 cm thick).

By the way, I also considered making a custom headset for my bike, so I drew another plan (have a look at the picture above). In the end I concluded that it isn’t any good so I stopped developing it.

I saved the best for the last: the whole adapter thing I made weighs 980 g , or 2 kg together with the forks…

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