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Rear hinge – finally the sad truth

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Rear hinge – finally the sad truth

Postby Blackstridaaustria on Fri Sep 13, 2019 10:32 am

Dear reader,

I’m sorry for the long wait but now it’s time to reveal my discoveries.

Maybe uncommon, but I’d like first to express my feelings, I’m
- disappointed – by Ming cycle :evil:
- asshamed – for taking that much time to understand :oops:
- unburdened – because we are now able to fix the rear hinge issues finally :lol:

About 3 or 4 months ago I received feedback about my previous rear hinge kit (the heavy duty version). Some users reported malfunctions (bolt 373 turning loose by itself) while others had no issues at all.

I couldn’t find a quick solution, so I used a few days of my holidays to study once more many articles about cup springs (also named Belleville springs or disc springs) collected again physical data and tried some disc spring calculation software. Instinctively I smelled that the issues must have something in common with our springs, but at this time, about six weeks ago I couldn’t verify that.

However; I’ve learned a lot about those weird cup springs – especially one detail let me think long and hard...I knew that cup springs are able to generate a high force within a very short way of travel.
But the results in real numbers with the data of our springs were highly astonishing and I was in doubt – could that be correct?

According to my calculations the preload at the spring of my Strida SX would be around 1000 kg!!
That is much too high – and it must be nonsense.

Well...and suddenly I noticed something really odd – the protruding lenght of the axle stump (above the frame tube) is in fact different on each Strida! :shock:

Image

That means;
- Ming cycle does not know how to treat a cup spring within its specifications
- They overtension the cup spring during the assembling
- The cup spring can not work as intended because it was damaged while installing

One could also say that the tolerances (of the protruding part) are much too high to allow a reasonable adjustment of the springs pre – tension.

This is also the explanation why the heavy duty kit didn’t work at all Strida bikes – it worked on these with the longer axle stumps, but not on shorter ones.

And we can see that easily even on the real numbers – all we need is just logical thinking!
Look – our cup springs are, unloaded, about 3,0 mm high.
The material thickness is 1,7 mm which is also very important (explanation follows below case 1).

Just check according data sheets; there it says that a heigth reduction of
- one tenth mm will generate a force of XX kg.
- but two tenths create already a force of XXX kg!
- and five tenths produce whopping XXXX kg.

Soo, let’s get back to the real numbers, here’s

Case 1 Martin’s Strida/Germany:
Protruding length 1,4 mm – that means 1,6 mm height reduction of the spring which is a physical impossibility!
(A spring with height 3,0 and thickness 1,7 can be compressed for maximal 1,3 mm, but not for 1,6 mm!)
Effect: Cup spring flattened, washer deformed, frame surface damaged.

Case 2 Luis‘ Strida/Portugal:
Protruding length 1,7 mm – heigth reduction of 1,3 mm (instead of 0,2 - 0,3 mm).
Effect: Cup spring flattened, washer deformed, frame surface damaged.

Case 3 BSA Strida SX/Austria:
Protruding length 2,3 mm – heigth reduction of 0,7 mm (instead of 0,2 - 0,3 mm).
Force in the range of tons – much too much.
Effect as above, just gradually a little less.

I’ve collected more measurings; so far we can’t figure out WHY these are all different – but they are definitely. (Just one suspect; perhaps the paint thickness? Note that one has to count three layers of paint at this point, a tolerance will be tripled here.)

In conclusion, we may claim also that Ming cycle’s pretension of the cup spring, the way of travel, must be totally wrong. (My biggest mistake was to trust Ming’s standards.)

Meanwhile, maybe two weeks later, I’ve received a very friendly and highly informative reply from Mr. Mark Sanders, sharing many details of his own improvement suggestions for the rear hinge.
The, for my meaning, most promising part is an adjustable bolt!
And accidentally just two days later I’ve got the images below, what a coincidence.

Thanks to Christoph, the skilled motorcycle mechanic from Germany at this chance!

Image
Image

I’m convinced that the adjustable bolt will be our final solution to handle all rear hinge issues.
Anyway it will need some time to develop and manufacture this new part, as an interim solution I can offer another system which is, unfortunately, a bit inconvenient.

Please refer to the drawing below for the principle; there was another spacer inside the cup spring added. This spacer (red at the drawing) works as an „axle elongation“, it has to match exactly to the axle stumps’s protruding length.
Image

So, if anybody wants to use this system – I need the mentioned length in advance to create the matching spacer.
Based on the suggestions of an Austrian engineer there was also the top washer replaced by brass to reduce friction.

Image
Image
Image

Questions are welcome as always, please contact me via mail if you in are need of a new rear hinge kit.

Cheers,

Chris
STV@gmx.org
StridaTuningVienna

"Who knows nothing, must believe all." *

[* line from "Aphorismen"; Marie von Ebner-Eschenbach, Austrian poet, 1893]
[* book title; Werner Gruber, Austrian physicist, 2010]
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Re: Rear hinge – finally the sad truth

Postby gabythenewbie on Fri Sep 13, 2019 1:47 pm

Very informative! Wow, it boggles the mind that Ming Cycle could be so careless, especially with keeping their assembly process uniform and setting the preload on the cup spring! It's a great thing you and the dedicated mechanics you've worked with have come together to tackle the issue and find the root cause.

I'm anxious to see what the measurements of my Strida's protruding axle stump are. The one day I don't bring it to the office with me...

Amazing work!
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Re: Rear hinge – finally the sad truth

Postby Neon on Mon Sep 16, 2019 5:53 am

I fully join gabys opinion: You and the other contributors to the solutions for the rear hinge problem are incarnated awesomeness.

You're one of those rare people who doesn't just bitch about problems but solves them.
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Re: Rear hinge – finally the sad truth

Postby flectalux on Thu Sep 19, 2019 3:26 am

Do you think using a POM ring would improve this issue ? or it would brake ?
For information this a short introduction on what is POM material:

https://www.ensingerplastics.com/en-us/ ... pom-acetal
I notice this issue and was thinking to add one ring of POM, maybe 2 mm thick, not sure yet.
Steel against aluminium with friction worried me...i did not open the frame yet :roll:
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Re: Rear hinge – finally the sad truth

Postby Blackstridaaustria on Thu Sep 19, 2019 7:42 am

@gabythenewbie

Many thanks.
Please don't worry (too much) - surprisingly are many hinges working well; even in above mentioned original condition.
(Which is another unsolved mystery...)

@Neon

Also many thanks, solving the problem first makes later bitching much more comprehensible :twisted:

@flectalux

Yes I do know POM, made a few small parts on the lathe back then, nowadays I'm using POM mouthpieces for my electronic cigarettes.

Mark Sanders recommended a force of 100 - 400 N (~ 10 - 40 kgs) for the joint.
I mean that Mark might be a little bit too low here and I'd suggest to use 50 to 100 kgs.
However - I'm pretty sure that a 2 mm thick POM shim can not withstand that force,
the edge of the cup spring will most likely generate at least a deep groove onto the plastic, perhaps it would even perforate the shim completely.

flectalux wrote:Steel against aluminium with friction worried me...


You must know that in the original configuration the spring will rotate mostly on its inner edge.
(Because the outer edge has seized onto the washer and that again on the bolt's underside.)
Here works steel against aluminium, as you say.

But in a suitable configuration with correct pretension will the inner edge of the spring stand still on the steel armour.
It will rotate only at its outer edge and not seize with brass washers.
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Re: Rear hinge – finally the sad truth

Postby flectalux on Fri Sep 20, 2019 3:19 am

Do you have the thickness and diameter of each part ?
Also recently i order some bearing for a Campagnolo hub and discovered that you need to know how precise you need the fabrication and also the hardness of each bearing. I don't know if that would be also the case for this DIY kit ?

Grade defines tolerance and Rockwell HRC method for hardness.
Also because it is so cheap the minimum order is often 100 pieces, so If people would be interested I could order from china and then send it to some people in this forum.
If there is a kit available i would in this case be interested to buy it to save some time...as it is not a cost problem :wink:
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Re: Rear hinge – finally the sad truth

Postby Blackstridaaustria on Fri Sep 20, 2019 5:41 am

Surely I have all dimensions - what exactly do you want to know?

Sorry I didn't understand what you mean with the bearing for your hub; there is no bearing involved in the DIY kit.

I do still offer my repair kit but at the moment as an interim solution only.
The measuring and installing of the so-called "elongation rings" is much too invonvenient for an average Strida user I believe.
Of course I could send you one of these, but shipping to China will cost much more than the material - and I need your actual axle stump length explicitly in advance.

Jfyi - Bill (Strida Canada) and me are working right now on an "adjustable bolt" solution, this will make the rings obsolete.
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Re: Rear hinge – finally the sad truth

Postby flectalux on Fri Sep 20, 2019 5:46 am

Ok, this sounds nice.
I will measure first.
I am an industrial designer by the way if you need some help for this problem i can help.
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Re: Rear hinge – finally the sad truth

Postby Blackstridaaustria on Fri Sep 20, 2019 7:04 am

Many thanks for your generous offer, in fact I'd highly appreciate any help :D

Being an industrial designer in China you may have the right contacts to help yourself first.

Do you know somebody who could manufacture adjustable bolts for us?

Look - I have a clear imagination of what we need, but my time and resources are limited which means: Just yesterday I've purchased here in Vienna some hexagonal metal rods and I'm expecting special thread tools to be delivered today. At the moment I'm not even sure if my (small) machines will be able to handle this challenge.
But once we have a working prototype (which will be anyway very simple!) it would make really sense to manufacture this bolt in China.

I'd suggest you reproduce the kit (and later also the bolt) mainly at home, you save something and maybe we get in return China made bolts and nuts. Moreover that you can perhaps help the Asian Strida community?
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Re: Rear hinge – finally the sad truth

Postby flectalux on Fri Sep 20, 2019 10:02 am

I will try to make a POM ring between the frame and the first ring, but a little smaller than the ring so the side of the ring won't touch the POM. You said it will be too weak, but I still want to try and see :D . My time is also limited as this is free time. Maybe next week i will try to ask a factory to make one piece or two.
For metal parts if i have a clear design then i can ask for some suppliers. But it also depends on the order quantity. Really small better ask a prototype factory, bigger order like 1000 maybe can start to be a factory (even 1000 is a small unit).

EDIT I measured around 1.6 to 1.75 mm between the axle and the frame. The ring has worn out the frame pretty badly as seen on other users. When unmounted this design feels really wrong but I am not an engineer either.
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Re: Rear hinge – finally the sad truth

Postby Blackstridaaustria on Sun Sep 22, 2019 9:15 am

You're right, just one "try and see" is better than hundred theories :D

Ah thanks; I've underestimated the MOQ's of Chinese suppliers due to my European naivity :oops:

Regarding your edit; your feeling can be prooved by simple math:

Note that your cup spring had an unloaded height of ~ 3,00 mm.
The material thickness is equal to ~ 1,7 mm.
On your axle stump with ~ 1,7 mm will the spring be flattened completely because its heigth was reduced for ~ 1,3 mm.
The spring was already totally overloaded and it reacted with a non-reversible deforming while loosing a good portion of its springiness.

With a cup spring like the given ones you need an actual axle stump length of about 2,7 or 2,8 mm.
(Without armour shim but an intact frame surface.)
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Re: Rear hinge – finally the sad truth

Postby Blackstridaaustria on Mon Jun 15, 2020 8:20 pm

I felt forced to report about the latest news on this project:

Mark Sanders developed recently a new version of the rear hinge - the TriLock sytem.
I've hired an Austrian CNC company to manufacture a first, small series for testing purposes.
Right now, the parts are at another company to receive their anodized finish.
Image

You can find images of the installation process here:
https://s769.photobucket.com/user/Blackstridaaustria/library/TriLock?sort=9&page=1

Also, I've developed a new system myself, it looks and works a bit different from Mark's.
https://s769.photobucket.com/user/Blackstridaaustria/library/Rhk3?sort=9&page=1

Both systems are not tested yet.
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Re: Rear hinge – finally the sad truth

Postby Blackfire1967 on Thu Jun 18, 2020 6:53 pm

Hello Chris,

Thanks for the update info. If I can help you for testing this let me know.

BR,

Bert
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Re: Rear hinge – finally the sad truth

Postby Blackstridaaustria on Thu Jun 18, 2020 7:40 pm

Hi Bert,

you're very welcome, I've sent you a pm :wink:

Cheers,

Chris
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Re: Rear hinge – finally the sad truth

Postby SmallStrideBigLeap on Tue Apr 13, 2021 9:34 pm

Hello Striders! I've just joined this forum as a happy owner of a used strida. Unfortunately the way I found it was through googling strida creaking noise... :D

Blackstridaaustria, thank you very much for this explanation! One thing is left unclear to me. I have removed the cup spring and I have the typical damage on the bottom tube. The axle protrudes ~2mm in my case. So if I understood correctly, now that the spring has dug into the tube it is no longer overcompressed? Does that mean the problem took care of itself or do I need to do something more?
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