Saturday, March 14, 2015

Can I Install Tern Handlepost/Fork on Dahon Frame? Part 2: Comparison

In the first part of this series, I introduced the Tern handlepost and fork, and I plan to install them on my Dahon Boardwalk. However, some of the dimensions and designs are different, and so fitting the Tern parts on the Dahon bike is unlikely to be so straightforward.

Before we proceed further to installation, it is important to compare the Dahon/Fnhon and Tern parts, so that we can determine the feasibility and possibility of this upgrade. The best way would be to do a one to one comparison, and take note of the differences. Once the differences are known, it would be much easier to figure out how to make the parts fit each other.

Let's start with the fork. As introduced in the previous post, the general construction of the fork is the same, however there are some key differences which affects the function and compatibility.

To make an accurate weight comparison, we will need to include the weight of the compression bolt (Dahon) or the weight of the top cap (Tern), as it is an integral part of the fork.

Weight of Tern fork plus top cap is 454 grams.

The weight of the Dahon fork plus compression bolt is about 488 grams, as referenced from the fork used during the building of the Mu Ex bike. The weight of the compression bolt alone is 48 grams.

However, do note that the fork for the Mu Ex bike has a steerer tube length of 123mm, as compared to the 111mm for the Dahon Boardwalk. This means that the actual weight of the Dahon fork on the Dahon Boardwalk will be a bit less than 488 grams.

Compression Design:
This is where there is a big difference between the Tern and Dahon fork, although the function is the same. The purpose of this compression function is to preload the headset bearings and to remove any play between the headset parts.

The Dahon fork uses a threaded compression bolt, which screws into the steerer tube that is threaded internally. This means a special compression bolt is required, while the steerer tube also needs to be threaded. Although it works well, it probably means a higher production cost and heavier weight.

Dahon fork's steerer tube is threaded internally, for the compression bolt to tighten into

As for the Tern fork, it uses a more traditional starnut design plus top cap for preloading the headset bearings. This starnut is inserted into the steerer tube, and it is a one way fit which means that it will not come out the same way. This is a lighter weight design, and also means that there is no need for a special compression bolt and a threaded steerer tube. Lower production cost and lower weight, but not sure what is the downside.

Tern fork steerer tube with a pre-inserted starnut

Any standard top cap with bolt can be used to preload the headset bearings

Steerer Tube Length:
This is the most important difference, which is the reason why I needed the Tern fork in the first place. The Tern handlepost requires a longer steerer tube to clamp onto, which is why the Dahon fork is not compatible with the Tern handlepost.

Dahon forks comes with 2 different steerer tube lengths. The 111mm version is for Boardwalk/Speed/Vitesse frames, while the 123mm version is for Mu frames. This is due to the taller head tube on Mu frames.

Dahon fork steerer tube length on the Dahon Boardwalk. 109.5mm, slightly short of the nominal 111mm.

Tern fork steerer tube length of 140mm, much longer than the Dahon fork.

Comparing the steerer tube lengths side by side

Position of Brake Mounts and Magnetix Mount:
Lastly, there are some differences in brake mountings and Magnetix mountings between the Dahon and Tern fork. Due to the different folding method used, the V brake mountings and Magnetix mountings are the total opposite between the Dahon and Tern fork.

Dahon fork has the V brake mountings facing the front, while the Tern fork has the V brake mountings facing the rear.

The Dahon fork has the Magnetix mount on the left side of the fork, while the Tern fork has the mount on the right side. This will affect the folding of the bike.

Now, let us compare the Dahon handlepost with the Tern handlepost. There are many differences here, and some of them contribute to the added strength and stiffness of the Tern handlepost. Let's find out what they are!

The Tern handlepost is quite a lot heavier than the Fnhon handlepost. This is mainly due to the more chunky construction at the joint area and the T-bar at the top. Hopefully the added stiffness is worth the additional weight!

Fnhon 4 bolt handlepost, weighing only 511 grams.

Tern Physis handlepost, 12 degrees tilt angle and 290mm long. Weighs 200 grams more than the Fnhon handlepost, at 710 grams.

Manufacturing Process:
The Fnhon handlepost is made up of a few pieces welded together, which means that weld lines can be seen at the joints. This does not affect the function, although it may not look so nice.

The Tern handlepost is 3D forged into one piece and thus there are no weld lines to be seen. This makes the handlepost look better and is also stronger.

Fnhon handlepost with welding joints at the top and bottom of the handlepost

Tern handlepost with no weld lines to be seen. Nice smooth shapes along the handlepost.

Bottom Design and Clamp Design:
The bottom of the handlepost is quite different between Tern and Dahon/Fnhon. For Dahon/Fnhon handleposts, the bottom of the handlepost is flat, and it rests on the Upper Cover of the headset.

On the other hand, the bottom of the Tern handlepost is specially designed, such that it mimics the shape of the headset Upper Cover. This means that the headset Upper Cover is integrated with the base of the Tern handlepost, eliminating the need for a separate part.

Another difference is the side clamp bolts at the base of the handlepost. Dahon/Fnhon handleposts have one clamp bolt to secure the handlepost to the steerer tube, while the Tern handlepost has two. Two clamp bolts should be more secure and stiffer than one. Also, the two clamp bolt design is more similar to traditional stem designs with two side clamp bolts.

Flat bottom of the Dahon/Fnhon handlepost

Standard design of Dahon/Fnhon handlepost. Compression bolt, single clamp bolt and headset Upper Cover (shown in gold colour).

Tern handlepost with a specially shaped bottom. A rubber seal is also included. Two side clamp bolts to tighten the handlepost to the steerer tube.

Locking Lever Design:
The Dahon locking lever design is really well designed and looks very integrated with the handlepost. With an internal clamping design, the locking mechanism is the most low profile and compact that I have seen across all folding bikes.

Fnhon locking lever design which follows the Dahon design. However, it uses the old type of black sliding clip as an additional safety catch.

Internal clamp mechanism of the Fnhon handlepost. Simple and works well. Best of all, it is hidden inside the handlepost.

The Tern handlepost actually uses an older Dahon locking lever design, with an external over-centre locking type of design. It is more bulky, but it can be easier to adjust and can potentially be stronger if designed well.

Old locking lever design on the Dahon Eco 2 handlepost. Same concept as the Tern locking lever design.

Tern handlepost locking lever with the mechanism all on the outside of the handlepost.

Length of Handlepost:
The length of the handlepost is important as it determines the riding position. One of the main reason I wanted to get the Tern handlepost is because it is short enough for my requirement. However, in order to get an accurate handlebar height, it is necessary to include the stem in the height measurement, as the type and length of stem will affect the overall length/height.

I also realised that for the Fnhon handlepost, I also needed to add the thickness of the headset Upper Cap as it adds to the overall height. This Upper Cap is already integrated with the Tern handlepost.

The Fnhon handlepost uses a 31.8mm stem on top, which adds to the overall height. Same for the Tern handlepost with the new Controltech Stem.

Fnhon handlepost plus 31.8mm stem, with a total height of about 340mm. Note that I included the gold Upper Cap at the bottom as it is part of the total height too.

Tern handlepost with the Controtech Stem, with a total height of about 330mm.

Comparing the two setups side by side. The base of the Tern handlepost can be seen to be much chunkier.

As measured earlier, the overall height of the Tern setup will put the handlebar about 10mm lower than the current Fnhon setup.

Width of Stem:
Lastly, the different stems used on the handleposts will affect the space available on the handlebar. This is due to the different clamp widths of the different stems.

Fnhon handlepost with the old stem has an internal width of about 41mm.

Fnhon handlepost with the old stem has an outside width of about 63mm. Clamp width is about 10mm each.

Tern handlepost with Controltech Stem has an internal width of about 55mm. More clearance between the stem clamps.

Tern handlepost with Controltech Stem has an outside width of about 88mm. This is much wider than the previous setup, which means less space at the sides for other accessories. Clamp width is about 16mm each.

With that, the comparison is complete! We now have a clear picture of the differences between the Fnhon and Tern parts. With this information, we can start to consider the effects and modifications required to fit the Tern handlepost to the Dahon Boardwalk frame. This is the ultimate target which I want to achieve.

In the next part of this series (Part 3), I will write about the considerations required for this project to continue.

Part 3 is  now up! Click here to continue reading.


  1. Are you also good in PF30 bottom bracket ?( I think you do).
    We need your help to post on PF30 topic . Nowadays almost all road bike comes with PF30 and we want to be able to service ourselves. We know it's not difficult but it'd be good if you can guide us. You're good at mechanical stuff.

    1. For press fit 30 bb, there are many video guides online showing how it is done. No need for me to write another guide.

  2. Yep, already checked and we just ordered the tools from CRC, Wiggles and some from FSA and Ebay ;-). ( Not cheap though , but we're 5x teams so it's not expensive for our case , most importantly it's very convenient as we don't need to bring our bikes to bike shop and park there for 1 or 2 weeks or so.