Tuesday, 10 September 2019

Public Service Announcment: Mercane Safety Issue (and what you should do)

UPDATE: I'm revising part of this post as new information has come to light and I've redone some of the videos, so:

There appears to be a number of disc brake calipers that have failed (see this post on Reddit for example) and according to the notification by one of the US resellers the problem may be related to a caliper adjustment being set too far forward (and possibly loading the cam up into a weak place).

To be clear we are talking about this part here:


Mine is adjusted so that the lever is pretty much all the way over to the right before brakes are applied, and only moves a small amount (less than half its throw) in activation.

This is a video I made showing the operation and movement of mine in reply to a discussion on one specific example of failure showing normal operation of the caliper. Take note of how far it moves and from how far over on the right (back) of the bike it moves.



This is an example of a video of a failed caliper:


when you watch that video take note of the starting position, his brake caliper lever is further forward than mine ... I would think this is a key symptom (and it has emerged it is ;-)

So IF you are in a situation where you don't know and don't have a dealer you can discuss this with, I propose that to diagnose this you do the following:

  1. check that the starting position of the lever is more or less all the way back by undoing the bolt securing the cable and it will then fall back to its "un tensioned" position. It should not fall far
  2. if it didn't fall back very far then its probably all good, if it did then you'll need to look more carefully. Either way return the lever to where it was and re-tension that bolt holding the cable.
  3. take the scooter to a place where you can test the brakes without needing them to stop. A wide open grassy area like a park (not filled with people) would be good because then the grass will stop you pretty quickly by just powering off. If you can lock up the back wheel and it does not fail then you know its good to go.
Now, if any of the above seems mechanically too complex then my most sincere advice is :
  • "don't own a scooter" or 
  • "ask or pay someone who has mechanical aptitude to do this for or with you"
So, IF you found that the lever was too far over to the left (the cable side) to actually activate the brake then you'll need to adjust this. There are almost no good guides on the internet for this (probably because until recently these calipers were used on mountain bikes and people usually figure it out themselves in that community or take it to a bike shop). That may be a good option for you here.

However assuming you like me want to do it yourself (because that's how you've learned so far) then I've put together a few videos showing what to do and some "discover as I video" commentary. Each of these is about 3 min long. I'll put them here for your reference.

Taking the caliper off:


Now, with the caliper off lets "cut to the chase scene". The US distributor has paid for an engineers report exploring the problem and offering a way to clarify what the problem actually is: here.





In case that ever vanishes I've made a copy of that on my Google Drive here.

Unfortunately there are no guidelines as to how to be sure if that retention ring is properly installed (I may have a look myself). But the above paragraph makes clear that the symptom of the arm movement discussed in all my videos is pretty much a key indicator.

Lets first look at the components:


I've got an arrow on the left (in red) pointing to a small lubrication port (which I mention in my below video) and an arrow pointing to the thrust bearing where some grease is shown.

In the figure below they've cut away the caliper and shown how the part sit in together.

from this I was then confident to dismantle mine further and look at exactly that retention ring.

The ring is intended to be turned with a two pointed tool which matches holes and allows you to turn it. The spring compresses as the arm is moved and the retention ring forms a point where the arm can not go further because the spring is fully compressed. As shown here:


at this point one simply can not apply more pressure on the brakes because the retention ring is stopping the piston moving out further ... as it should because that's how stuff works. This means that if you need more movement you in fact need to adjust the pads (shown soon).

My caliper shows this normal range of movement in my above video and so if yours is showing that range of movment then you probably do not need to go further. But for the sake of showing how to fix the problem I have. In the video below you can see the retention ring, the brake pads and how caliper moves them.




You can strip it by doing the reverse of my process ... and if you find you need to, you adjust that retention ring (as I show in the video) to the place where the spring does bind at the logical end of caliper lever travel (which I show also).

After that you may need a few small tweaks of adjusting that inside pad adjuster to get things just right and you'll also perhaps need to adjust the way the caliper sits on the swing arm relative to the disc, that's shown also in this video which becomes clearer in what its talking about now that one knows there is an adjuster for the outside pad too...



I talk through some more adjustment bits here, but I'm no professional presenter so you may find it a bit "rambling" ... but I feel (even given that) its worth sharing still. Among other things it shows how to position the caliper (which may surprise people to learn was not a custom job for the Mercane and us used on other bikes) exactly relative to the disc. The important thing is to have the disc in the middle of the caliper and then adjust the inside pad (which otherwise does not move during braking) to be just kissing {but just not} the inside of the brake disc) and then the outside pad (which moves when you squeeze the disc) to  allow the brake to release or operate.


I don't script these videos so sometimes I say the wrong words (or struggle for words)  as I'm not an anchor news reporter reading from an autocue, but just an engineer.

Lastly I do not believe that the gentlemen who explored this in another YouTube video have actually nailed a solution to the problem, but rather like my earlier versions of this post have just shown a way of testing if you do have the problem or not.

Ultimately if you feel that you are uncomfortable with this  you may consider it best to consider a change of caliper the best course of action. I know many discuss the move from a cable activated mechanical one to a cable activated hydraulic one, or even a complete hydraulic system ... myself I don't because I'm of the view that'll just be another different set of problems. The choice is yours

Either way its my view that unless you have one of the faulty calipers that its just best to disassemble your caliper and inspect it as I've done.

1 comment:

Anonymous said...

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