WARNING: This post is purely for demonstration purposes only. If you decide to follow its instructions to service your own Dahon folding bicycle, proceed at your own risk. I will not be liable for any injury or warranty loss that may arise from you following these instructions.
Previously I wrote about how loose Bino had gotten at his Achilles’ heel, his main frame hinge latch. The last time this happened, I brought him to Junni Industries in Quezon City for repairs. This time, with more mechanical know-how and hoping to save time, I decided to do it by myself.
Noting the many similarities of the Dahon Vitesse to cheaper variants of Tern’s Link folding bikes, and their shared manufacturer, Mobility Holdings Inc., I got Tern’s FBL Hinge Parts set for the 2013 Link C7 from Thorusa.
TOOLS AND SUPPLIES NEEDED
- Tern Link C7 FBL Hinge Parts set (2013-up)
- Hex keys: 2 mm, 5 mm
- Torque wrench
- Pliers or Philips screwdriver
- Adjustable wrench or 6 mm open wrench
- Medium-strength thread locker, e.g. Loctite 242
Open the frame hinge as normal. Fold the main frame tube 90 degrees to expose all the inner parts, as folding it all the way hides them.
Ideally, you would strip the frame first of all its components, especially the cranks and cabling. If that’s not possible, you will need to find a way of supporting the bike even with its main frame tube folded up. I used my Minoura DS-30AL display stand to support the heavier rear half of the bike.
Tension adjustment bar attached to the tension pin.
The first thing that has to go is the hexagonal bar that serves as the main latch lever’s tension adjustment. I call this the tension adjustment bar. Like a typical barrel adjuster, you increase tension by backing it out and turning it counter-clockwise. Back it all the way out with your adjustable wrench or 6 mm open wrench until its threaded end separates from the main latch lever’s security pin.
Main latch lever security pin.
You can pull the main latch lever security pin up and out once free.
Once disengaged from the main latch lever, remove its other end, by either undoing a screw or removing a circlip with pliers, and pull it out. This also releases the tension pin that sits on the front half of the frame tube. Pull and slide it downwards; it should still have some grease on it.
Removing the top hinge pin bolt.
Removing the bottom hinge pin bolt.
The two hinge pin bolts.
At this point, the only thing remaining of the hinge assembly that connects to the front frame tube is the hinge pin bolts – there are two of them. Take your 5 mm hex key and undo them both. Note that the lower of the two hinge pin bolts has a plastic bushing which the front frame tube rotates on; remove this as well. Removing the hinge pin bolts results in the frame separating into its front and rear halves.
The separated front and rear halves of the Vitesse’s main frame tube.
Once the two halves of the frame are separated, you can access the plastic bushing, shown here held by my pliers. It fits into a recess in the lower pivot.
The final component that may need replacement is the main latch lever itself. It’s held in place by a grub screw (i.e. a screw with no obvious head) on the drive side. You should be able to undo both of these with your 2 mm hex key.
Removing the grub screw that secures the main latch lever and its pin axle.
Grub screw removed.
The main latch lever spins on a really long axle, so slide this downward to free it. You can use a 3.5 mm bolt to screw into the axle to make it easier to extract. I didn’t have one, and my main latch lever was in good condition, so I left it alone.
Clockwise from left: latch tension pin, grub screw, main lever security pin, tension adjustment rod, tension adjustment rod securing screw, hinge pin bolts, 3 mm hex wrench, plastic bushing
At this point the Vitesse’s main frame hinge assembly is fully disassembled. Take the Tern FBL Joint Repair Kit and replace any parts as necessary. In my experience, I’ve had the tension pin bend and the main latch lever crack before.
When reassembling, take note that the following parts should have medium-strength blue thread locker applied to them from the factory:
- Hinge pin bolt x 2
- Grub screw
- Tension adjustment bar
If this is missing from their threads, reapply. This will prevent them from walking out of thread engagement due to vibration while riding.
Once fully reassembled, make sure to readjust the tension adjustment bar with your open wrench. As per Tern’s Owner Briefing video, when properly adjusted, the main frame latch should open with two or three fingers, and close with the palm of your hand.
My main concern prior to the repair was that the hexagonal tension adjustment bar had run out of usable screw thread, and it may have been due to the hinge pins becoming bad. Tightening any more resulted in the bar separating.
As it turns out, the adjustment bar used when it was last repaired as a smidge too short, by about 5 cm. Not only that, it was secured to the tension pin by a screw, instead of a circlip on both the original hinge parts and the Tern FBL hinge parts set. This allows a bit more slack, and ultimately more scope for adjustment, without the tension adjustment bar releasing from the main latch lever due to running out of screw thread.
A photo of Bino upon acquisition. Note the tension adjustment bar. It’s anchored to the tension pin via a circlip. The replacement part that Junni Industries used was 5 cm shorter overall and anchored by a screw.
With more scope for tension adjustment, the whole bike becomes much stiffer. Now it actually takes a bit of effort to shut the main latch lever flush with the main frame tube, which is very reassuring. Of course, it will never be quite as solid as Hyro, my cyclocross bike with its traditional diamond frame, but it’s sufficiently stiff and no longer worrisome when riding.
Adjust the tension of the main frame hinge lever with an adjustable wrench or 6 mm open wrench. As per Tern’s instructions, it should require the force of the palm of your hand to close, and three fingers to open.
As with many DIY repairs, I have a newfound appreciation of the engineering that went here – the original reason why folding bicycles appealed to me. However, I am also more cognizant of the limitations of the design, and so Bino will most likely lead a more genteel riding life with me going forward.