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Monday 8 January 2018

Fixing a wobbly Dahon frame hinge

How to fix a steel Dahon frame hinge that wobbles, even when tightened
The closed and clamped hinge. Grab this and push it up and down to see if it wobbles
Between our family and our friends, we use Dahon folding bikes a lot. My heavily modified (including electric hub motor) Dahon Boardwalk has done around 20,000km of heavy riding, carrying my 80kg self plus panniers on our steep (and some rough) mountain roads (here's my page about my electrified Dahon Boardwalk, here's my post on the Xiongda 2-speed motor I'm using on my Dahon, here's my post about fixing a broken Dahon handle post). Our friend Bradley also does heaps of distance on his Dahons, mostly 2 Speed TRs (one is also electrified). It is tremendously useful to us to have folding bikes, especially living on a mountain where getting a lift in a friend’s car can save a lot of time and effort.
On the other hand, Dahons have also caused us a lot of bike mechanic grief. It seems to me that the culture of marketing and “innovation”, with almost annual “new models”, means that the designs aren’t refined, but are simply shuffled. All of the Dahons I’ve known have had serious design flaws, providing fairly major bush engineering challenges to keep them on the road: splitting seat tubes, frame and steerer hinge problems, headset problems (especially with threadless). We’ve only bought steel-framed Dahons, very conscious of the repairability of steel, as well as how fragile and unreliable aluminium frames are in diamond frames, let alone under the greater stresses of folding bike geometry.
We’ve had to repair nearly all the frame hinges that have done reasonable distances. This is what happens:
Even with regular tightening of the hinge adjusting bolt, frame hinges tend to eventually get a wobble. This is an up and down wobble, found by grasping the hinge of an unfolded bike, and lifting it up and down while holding the rest of the bike down.
What seems to be the problem is that the mating faces of the hinge are manufactured flat, so any tiny wear at top and bottom allow a small rocking movement, regardless of how tight the hinge is. A little rocking brings huge pressures on the surface at top and bottom and leads to more wear, making more wobble even when tightly adjusted. Once worn slightly convex, clamping pressure only acts on the middle of the hinge faces. What is needed is to spread the clamping pressure back to the top and bottom of the faces.
On our steel Dahon frames, we have found a remarkably simple remedy: laying a very small pad of stick welding at the top and bottom of the male mating face, filed smooth (I use a 2.6mm mild steel rod, at 130A). This creates pressure points at the very top and bottom of the hinge mating face, and leaves a slight gap in the middle - where the faces were rocking. The raised pads are perhaps a little more than 1mm high after filing.
The welded pads are on the left side hinge face, top and bottom. At the top of the right side face you can see the worn area which has caused the wobble
Before welding, the hinge clamp is undone by removing a small circlip from a vertical pin and removing the pin so that the clamp can be folded out of the way. 
What this achieves is to distribute the hinge clamping pressure onto 4 discreet points on the perimeter of the hinge faces: top and bottom welded pads, hinge pin on one side, clamp on the other.
I’ve done this repair on (I think) 4 Dahon frames, with excellent results.


15 comments:

  1. Thank you very much for sharing this solution.
    I also have a foldable Dahon that I electrified myself and I got the wobbly hinge problem due to abusing the bike by carrying myself, the electric kit and a child on it. Also due to the age of the bike.
    The wear on the inside of the hinge is very visible but your solution should fix that.
    However, I suspect that the hinge pin is also bent. I could not get it out because the safety pin is stuck and its head is too worn. Any advice on how to get that out? WD-40 did nothing. It thought about ripping it off, drilling it out or superglueing the allen hex key to the worn head and trying to unscrew it...

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    Replies
    1. Hello Mirjana, good to hear you've been trying similar modifications to your Dahon. I've never been aware of having a bent hinge pin on our Dahons. My Dahon Boardwalk has the hinge pin permanently welded in - removal would be ugly. The Speed TR in the shed has a grub screw with hex socket holding the pin in - I take it that this is your problem? We have problems with hex socket grub screws often enough (dealt with one last night!). First stage is to try and find a hex key (imperial or metric) or torx key (sometimes they fit into a worn hex socket) that fits it tight. If that doesn't work, we've had successes with techniques like this:
      - drill the hex socket deeper with a drill
      - make a tapered, square section tool that can be driven into the drilled socket - the sharp corners bite into the sides of the hole
      - a broken HSS drill bit can be ground to shape; drive the square tool into the hole in the hex socket of the grub screw
      - once you get the tool well jammed in (may require re-grinding) then grip it with a chuck or tap wrench and turn it, and hope it spins the grub screw.
      Good luck!

      Delete
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  2. I have what looks to be the same Dahon Bike. I wondered where the frame of your bike is slightly crooked if looking down a the frame. My bike from the part where is folds upward toward the handle bars goes crooked on a slight angle to the left. I am also have the issue of the clamp not closing tightly.

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    Replies
    1. Hello, the link in Mirjana's post will help if your hinge is just loose. After you tighten the lock, if the frame is still bent I would learn to ignore it. It shouldn't affect your using the bike. I had a look at both the Dahons here, and they both look very slightly bent, but I hadn't noticed before.

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  3. The frame should not be crooked. Since it's a steel frame, you should be able to simply straighten it up without loosing structural integrity.
    About the clamp, here is how to adjust it properly:
    https://www.youtube.com/watch?v=zhCU25fLGZM

    ReplyDelete
  4. Hello,

    I have Dahon Speed P8 which is chromoly frame and the same latch mechanism as you. When the bike is folded or the latch is not fully engaged but the frame is in almost straight position, I can see and feel the movement by few millimeters of each part of the frame. Is this normal? When the frame is unfolded there is not play.

    Thanks

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    Replies
    1. Hello Mark, from my understanding of your description, your hinge sounds normal. The hinge pin will always have a little play in its holes in the 2 parts of the frame, so there will be some movement between the frame parts. Tightening the latch should take out the play, so it doesn't move and wear while you ride - as you describe.

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    2. That's great to hear. This frame is quite old, I think from 2006. A lot of scratches and paint chips but I like the idea of having chromoly frame and something to work on.
      I've inspected the two contact points where frame parts join that you mentioned above and fixed, and in the case of this frame there is almost no wear, so I guess from that perspective frame is really solid.

      First thing I wanted to upgrade is actually handlebar. I like relatively upright sitting position and with telescopic handlepost extended almost to maximum it's quite flimsy. I'm thinking of replacing existing handlebar with riser one, similar to what's on the BMX but less pronounced. This way telescopic handlepost could be less extended and much more rigid. What do you think?

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    3. Some Dahons, including our Speed TR in this post, give quite a stretched-out riding position, with the handlepost leaning forward. Conversely, my old Boardwalk gives a more relaxed upright position. I encourage you to try whatever might work for you. As it would be very difficult to modify the handlepost itself (being aluminium), trying different handlebars is an easy way to adjust riding position. The main question will be whether the bike will fold well. We've put bars with some sweep-back on our TR, and it works well.

      Delete
  5. Hello I have a problem with the main frame hinge in DAHON SUV D6. iT is hard therefore is not as foldable as a new one. I do not know it there is a way to extract it and give it service. Any ideas? Thanks

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  6. Hello Sergio, it looks like Dahon SUVD6 can have a steel or aluminium frame. Steel frames are generally repairable but aluminium not. Some Dahon hinges have the hinge pin welded in, some have a little grub screw that can be unscrewed to remove the pin. I imagine you've tried oiling the hinge, but if that doesn't help, you'll have to look into the hinge and lock and work it out. Good luck!

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  7. Another repair concept: untested.... I have a pair of Dahon Speed 7s. I ran one into the ground; worn groupset but terribly worn frame hinge. Now the second is going the same way. Tonight I just used a slotting disc and cut through the hinge pin and drilled and knocked out the bits. Only now have I found your repair method which is brilliant; I shall try it. I still feel that it is a weak, soft 8mm hinge that allows the flat faces of the joint to wear. My solution: buy a length of 10mm silver steel on Ebay. Get a machine shop to drill and ream a length of 20mm steel bar out to a perfect internal 10mm bearing clearance to your silver steel. Now cut the old hinges off your frame and grind out where they used to be to have a shallow groove to fit the 20mm bar. The 20mm bar will be cut into three to form the hinge parts, but only later; to preserve alignment, for now assemble the frame in riding position with its own clamp (maybe even tack weld a bar to hold the frame together). Weld the 20mm in place, top and bottom to the front of frame, middle bit to rear of frame, as per original. Now use a 1mm wide grinder cutting disk to cut the 20mm into the three hinge parts, disassemble the frame halves and weld up properly and they will all be in perfect alignment. I might tack weld with MIG and then braze the joint as its is hard to get any weld metal where the joint closes. I guess I'd run webs away from each part of the hinge down the frame. Might have to re-ream for heat distortion and add grub screws or something to stop the hinge pin falling out, but you'd have a massive hinge pin bearing area compared to the factory one. Whilst just drilling mine out it was as soft as butter, and you could harden your silver steel. Excuse brain dump.

    ReplyDelete
    Replies
    1. Hello Unknown (spooky identity!),
      I think your idea is really good - the full bush engineering solution. It would be great if you could let us know how it goes.

      Delete
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