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Author Topic: Swinging Arm issues  (Read 204 times)

banquo

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Swinging Arm issues
« on: March 04, 2020, 05:46:46 pm »
I noticed significant play in the swinging arm following the Mighty Falcone's trip up the Stella Alpina, and although I'm sure that the short but brutal trip wasn't the cause, the play was bad enough that there was no point in putting off the repair.
Having read of Mark's problems in removing the swinging arm bolt, I was ready for a fight, but in fact the through bolt came out easily, after removing the mudguard, rear wheel, sprocket and chain, and the bolt was still shiny from its original cadmium or zinc plating.

Any optimism from this success was short lived though, once I worked out how the pivot had been designed.
They say that the NF frame was designed by Tonti, and maybe it was, but he must have either had an off day, or given the design for the swinging arm pivot to his apprentice.
Unusually, the through bolt does not act as the axle; the axle is a hollow thick-walled tube of about 26mm outside diameter x about 18mm bore through which the through bolt is a sliding fit. This steel axle pivots on a pair of STEEL bushes, which are flanged, and the axle is slightly longer than the dimension between the outer faces of the bush flanges, to give slight end-float. The axle is compressed by two cup washers, that are in turn compressed by the flanges on the frame, under tension from the through bolt.
Cup Washers

Swinging arm bush flanges- note flanges of bushes are the same diameter as pivot tube, making drifting out extremely difficult

Swinging arm bush drawing - note the diameter is wrong; it's actually the outside diameter of the bush, not the internal

All of that is fine except for the following issues:
  • There is no lubrication or sealing provided for the steel axle or the steel bushes, so they both rust and wear
  • As the bush flanges are the same diameter as the tube on the swinging arm, it's not possible to drive them out easily, because there's nothing to support the swinging arm on except for the trailing arms
  • As water and road salt make their way past the bushes, the axle rusts between the bushes, and of course the rusted section increases in diameter, so it's impossible to get the axle out without applying brutal force
I have never seen steel bushes in a swinging arm, but they could have worked well, if the designer had decreased the diameter of the axle between the bushes, to allow for rust, and also provided a grease nipple, so that the axle and bushes could be both protected and lubricated. If they had even greased everything during assembly, that would have helped, but it's clear they didn't..
Anyway, I had to get the axle out, so, after soaking everything for two days in Plus-Gas releasing fluid, I supported the swinging arm on top of a tube, and started to drift the axle out. I tried a soft hammer and a soft drift, but neither had any effect, so eventually I had to resort to a large club hammer and a steel drift. Still the axle would not move, so I removed the tube that was bearing on the flange of the bush, supported the axle so it was resting on a trailing arm instead, and resumed drifting, whereupon the axle came out, taking the lower bush with it. I mananged to get the opposite bush out with a steel drift, but I cannot overemphasise just how much brutal force was required, despite all the prelubrication. The main problem was the rust in the centre of the axle, but the steel bushes were an interference fit in the swinging arm, and of course were 'welded' into place with rust.
Once removed, the wear on the bearing face of the axle is obvious:

This is after cleaning, so rust has been removed to expose the serious pitting, and you can see the wear step between the journal and the un-worn axle.
Here you can see the rust pitting of the centre of the axle, between the bushes. Again, this is after cleaning all the rust off with wet and dry paper, but I estimate that the axle had increased in diameter by at least 1mm, and there was no way it could be removed through the bushes

My friend Tony had kindly offered to make some new bronze bushes for the repair, but as the axle had been completely destroyed by mushrooming the ends during drifting out, the problem was bigger than that.
Initially, we had hoped he could turn a completely new axle, but his lathe was not capable of boring the axle at 18mm for the through bolt, and we could not find any hollow bar stock of the correct internal diameter. On that basis, I resorted to asking him to cut the bad ends off the axle, cut the remains in two, and make a short section for the centre, to make up the difference. At the same time, the central section was reduced in diameter from 26mm to 25mm to give clearance for later withdrawl. Normally a 3-piece axle would not be an option, but as the only thing that stops it from turning is the clamping force, and all 3 pieces will be centrally located on the through bolt, I don't see a problem.


The bronze bushes should be better than steel, and we have added a grease nipple to the underside of the swinging arm tube, so that everything will be protected and lubricated. The bronze bushes are a sliding fit in the swinging arm tube, and will be retained by Loctite 641.
The only worry we have is the end float, as I stupidly didn't measure the axle length before drifting it out, and Tony has had to guess how much longer to make the axle than the outer flanges dimension to give some end float.
I'll find out if any adjustments are required when I put it back together, and won't Loctite the bushes until I'm sure it all fits, but that day will have to wait until I receive the powder for powder coating it.
If only they had made it like this in the first place...
I'd strongly recommend that any owner adds a grease nipple. In retrospect, I'd prefer risking getting some swarf in there, and drill and tap it in situ (using grease to minimise the chance of swarf entry), rather than do nothing, but if there's any chance to dismantle it and fix it properly, I'd take that chance. Mark's axle had seized into the bushes, and instead of the axle swinging in the bushes, the through bolt was swinging in the frame flanges, causing serious wear, and an even bigger nightmare getting it dismantled.
You have been warned....

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monohorizontal

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Re: Swinging Arm issues
« Reply #1 on: March 06, 2020, 09:16:14 pm »
salut c'est un des points faibles du falcone un graisseur serait le bienvenu , moi je graisse bien et je règle le serrage de l'axe du bras oscillant pour un jeux correcte et après mes falcones ne roules jamais sous la pluie alors tout va bien

hi this is one of the weak points of the falcone a greaser would be welcome, I grease well and I adjust the tightening of the axis of the swinging arm for a correct game and after my falcones never roll in the rain so everything is fine
« Last Edit: March 06, 2020, 10:52:56 pm by banquo »
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Vusette

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Re: Swinging Arm issues
« Reply #2 on: March 11, 2020, 04:50:39 pm »
Hello,

Interesting story!
Where will you drill the holes for the grease nipples?
As far as I can see, if you put them on the upper side of the swingarm they will be difficult to reach with the grease gun.
On the underside, it's also difficult. And whats more, the grease will have trouble to move around the shaft.

Philip

banquo

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Re: Swinging Arm issues
« Reply #3 on: March 11, 2020, 06:52:38 pm »
Nipple is on the underside facing slightly backwards. I have an extended one in for now, but may change to an angled one once I've checked the assembly.
Orientation isn't important, as you pump the whole cavity full, until it squeezes out the ends; all surfaces should be pretty much covered I think. I'll also grease the through bolt on assembly.
« Last Edit: March 11, 2020, 10:59:20 pm by banquo »

monohorizontal

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Re: Swinging Arm issues
« Reply #4 on: March 11, 2020, 07:09:45 pm »
salut sur mon falcone rouge le bras oscillant était déjà percé au dessus vers l'arrière et dans le milieu je n'ai jamais installé de graisseur c'est la graisse qui bouche le trou mais il serait bien avec un graisseur aussi

hi on my red falcone the swinging arm was already pierced above backwards and in the middle I never installed greaser it's the fat that clogs the hole but it would be nice with a greaser too
« Last Edit: March 11, 2020, 10:57:57 pm by banquo »

banquo

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Re: Swinging Arm issues
« Reply #5 on: March 11, 2020, 11:06:46 pm »
This is the finished arm after powder coating in RAL6014. The powder was matt, but came out satin, which suits me, and matches the rest of the bike pretty well.






monohorizontal

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Re: Swinging Arm issues
« Reply #6 on: March 12, 2020, 05:52:24 am »
très bien comme sur ma rouge avec le graisseur en plus c'est parfait joli travail banquo
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