Before cutting the steerer tube, it is important to calculate and measure how much to cut. The ideal length is when the steerer tube rests just below the top edge of the handlepost base. This will maximise the clamping length for best stiffness and strength.
The tricky thing about this modification is that the starnut is already in the steerer tube. As the starnut cannot be removed without damaging it, the only way to continue using the starnut is to push it further down into the steerer tube, before cutting the steerer tube to the required length.
Starnut comes pre-set in the steerer tube of the fork. Standard depth is about 10-15mm.
My plan is to cut about 12mm off the steerer tube, as shown by the marking here.
Marking on the inside to show the initial position of the starnut.
Special starnut setting tool by Park Tool. Used to set starnuts to a depth of 15mm.
As I needed to set the starnut even deeper than 15mm, I swapped out the pin at the end of the tool for a longer pin. This pin will not thread into the starnut, but will be inserted through the hole to help centre the tool on the starnut. Washers are also added to control the depth of the starnut during the hammering of the tool.
Washers are used to control the depth of the starnut, and also to ensure that the thread of the starnut is not damaged.
The gap between the tool and the steerer tube before pushing the starnut further in. Upon proper setting of the starnut, the tool will rest fully on the edge of the steerer tube.
Before hammering the tool, the fork is set on a sturdy workstand for proper support. This method prevents damage to the fork ends.
After pushing in the starnut. It has been pushed deeper by about 10mm.
Now that the starnut has been pushed down, away from the area that will be cut, it is now time to shorten the steerer tube. My preferred way of cutting a tube is to use a proper tube cutter as shown. This is the same tube cutter I used when I shortened the handlebar of my Avanti Inc 3.
To cut the steerer tube, tighten the cutting blade onto the marking (12mm) made earlier.
Turn the tool around the cutting line, and tighten the tool after every 1-2 turns. Eventually the cutting blade will cut through the thickness of the steerer tube.
After cutting, the steerer tube is now 12mm shorter than the original length of 140mm.
New crown race from the LitePro headset for the new Tern fork.
Installed the crown race onto the Tern fork. For more info on installing headset for Dahon/Tern bikes, click here.
Now that the Tern fork has been modified, it is ready to be installed onto the Dahon Boardwalk! Before that, I double checked the instruction manual that came with the Tern handlepost to make sure that I am installing the Tern handlepost and fork correctly.
Instruction manual for installing the Tern handlepost
Before installing the new Tern handlepost and fork, the old handlepost and fork first needs to be removed from the Dahon Boardwalk.
Amount of steerer tube that protrudes from the top of the headset.
From my measurement, the length of steerer tube that protrudes above the headset is about 36mm. This is 3 times more than the 12mm of steerer tube with a Dahon handlepost and fork! With this large increase in clamping length, the interface between the handlepost and the fork will be a lot stronger and stiffer.
After installing the Tern handlepost on the Tern fork, this is how it looks. The steerer tube could have been about 1-2mm longer, but it does not matter too much.
Using the top cap to pre-load the headset bearings. The bolt of the top cap threads into the centre of the starnut in the steerer tube.
Two clamp bolts at the base of the handlepost tightens the handlepost securely to the steerer tube.
Tern handlepost and Tern fork installed onto the Dahon Boardwalk
In order to install the Controltech Stem, it needs to be slid on from one side of the drop bar. This means removing the bar tape, shifter and accessories from one side of the drop bar.
Installing the Controltech Stem onto the drop bar
As the stem and handlepost design is different from the previous Fnhon handlepost, the Di2 wires will need to be adjusted to account for the different length and location.
Controltech Stem installed onto the T-bar of the Tern handlepost. Note that the gap between the handlebar and the handlepost is very small!
Found a new place for the Di2 Digital Gear display! Also, the stem area is much neater with only 5 visible bolts, as compared to 10 previously.
With all the accessories mounted on the handlebar again
Finally, the Tern handlepost and fork have been installed successfully onto the Dahon Boardwalk. However, this project is not over yet. As I found out later on during test riding, there is some problem with the steering, which makes the bike quite difficult to ride.
In the final part (Part 5) of this project, I will reveal more details about the steering problem and describe how I finally managed to solve it. Also, I managed to find space to install a mount for the Shimano Sport Camera!
Part 5 is now up! Click here to continue reading.