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Belt twisted twice

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Re: Belt twisted twice

Postby Bietrume on Wed May 16, 2012 8:01 am

Blackstridaaustria wrote:Bietrume's explanation and the mowerforum post are meeting the point most exactly, I think.
Just the last phrase of the post I don't understand:
"The point that starts the twist can easily be damaged and may continue to twist, even after the original problem is corrected.."


What they say, I think, is that once the belt twists, some internal damages appear (some cords break). So it creates a permanent distortion of the belt when loaded, which will trigger a new twist even if the root cause has been identifed and solved. This suggests that twisted belts should always be replaced.
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Re: Belt twisted twice

Postby Blackstridaaustria on Wed May 16, 2012 1:03 pm

Bietrume wrote:What they say, I think, is that once the belt twists, some internal damages appear (some cords break). So it creates a permanent distortion of the belt when loaded, which will trigger a new twist even if the root cause has been identifed and solved. This suggests that twisted belts should always be replaced.


Thank you Bietrume, that's a logical explanation.
These belts seem to be very durable, but they don't withstand twisting or buckling.
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Re: Belt twisted twice

Postby stridahd on Sun Jun 03, 2012 1:58 pm

I am back. It took me a while to answer for several reasons. The one reason is that the twisting happened two more times. The last time 5 minutes ago, when going to the office...

The cause for the twist appears most likely to be the fact, that the rear wheel twists towards the frame from time to time. It seems that the wheel has some kind of freedom as the frame has a u-shaped scrap mark from the rubber of tire on the lower/horizontal pipe of the frame.

The question is how I can fix the movement of the wheel. I assume that some axle-parts and/or ball-bearings have to be exchanged?

However as I am a bit below the 100kg payload maximum of the Strida and the roads I have to take are not the best, I assume that I will have to switch to another bike :(
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Re: Belt twisted twice

Postby stridahd on Sun Jul 29, 2012 12:06 pm

It has been a while and the good news is that the belt twist did not happen again - thanks to Blackstridaaustria and his friend! At the beginning of July I had to chance to to visit them and bring my Strida all the way from Germany to Vienna, where they fixed it.

Most likely the problem was part no. 338, where the "disc part" was bent. This allowed the rear wheel to twist several degrees under heavy load (=myself). The other part exchanged by Blackstridaaustria was the Freewheel, which was partially worn and the belt guard was beaten up a bit. From my point of view the main fault was part no. 338. Together with the worn Freewheel and a heavy rider the belt could twist.

In the meanwhile have been driving at least 200 km (11 km per working day for three weeks plus visiting friends on the weekend) without any problem.
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Re: Belt twisted twice

Postby Blackstridaaustria on Sun Jul 29, 2012 8:17 pm

Mike and me are indeed very glad that the problem is solved. :D

Pic below for demonstration only, these are not Stridahd's original parts:
Image

At the first sight I was not fully convinced that such a small part is able to cause a twisted belt.
Now I am sure, thank you for posting!
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Re: Belt twisted twice

Postby one_half_3544 on Sun Sep 09, 2012 8:40 pm

My 5 cents to put in. Belt twisted during starting on a green light:
Image

Caught between snubber bolt and brake disc:
Image

Some facts:
The belt is (relatively) new.
The freewheel is almost worn out, but there was no slipping yet.
The freewheel is contaminated with dirt, it is possible to rotate the rear wheel in both directions using pedals when the bike is upside down (and the pedals rotate when I walk the bike).
The snubber bearing is turning freely.
Belt tension was rather low (after belt snapping I have returned to "normal" tension).
I am far from weight limit for strida - my weight is ~75kg.

Part #338 is bent indeed (both the disc part and the stem - it now has partial internal thread from the bolt =))):
Image

Since the belt is now doomed(?) anyway I think of replacing just the part #338 - to see if it is the only culprit or not.

Or I could replace part #338 and the freewheel and see if the once twisted belt is prone to subsequent twisting even if the culprit is removed.

What do you think?
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Re: Belt twisted twice

Postby Blackstridaaustria on Sun Sep 09, 2012 9:58 pm

I'm sure that Bietrume's meaning is absolutely trustworthy:

Bietrume wrote:What they say, I think, is that once the belt twists, some internal damages appear (some cords break). So it creates a permanent distortion of the belt when loaded, which will trigger a new twist even if the root cause has been identifed and solved. This suggests that twisted belts should always be replaced.


Of course, we all would like to find out what's the initial reason for the twisting of the belt - if you change part nr. 338 and leave the old belt it couldn't be verified if the initial reason was 338, because the belt might fail due to the above quoted reason.

If you replace 338 and freewheel it's the same situation.

To find out if the culprit is really the 338 part you'll need to replace the belt - and keep the old 338 part to see if the twisting happens again.
But doing that means also to damage a new belt (very probably), that's an ugly double windmill (translated with different words: "to be caught between devil and blue sea" or "catch-22").

I'm really sorry but I can't help to decide...

However, one thing should be mentioned: The bolt 379 must not be overtightened, but it must be secured with bolt adhesive.
I mean the overtightening is the initial reason for the bending of the "washer section" (the narrow piece with the bigger diameter) of 338.
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Re: Belt twisted twice

Postby one_half_3544 on Mon Sep 10, 2012 1:01 pm

Blackstridaaustria wrote:
To find out if the culprit is really the 338 part you'll need to replace the belt - and keep the old 338 part to see if the twisting happens again.

This belt is (was =) relatively new, so I think I'll:
1. replace #338 and leave the current belt - to see if it will continue to twist. After it snaps/twists/exhibits some other abnormal behavior, I'll:
2. replace the belt and see if it twists (with new part #338, but old freewheel).

That sequence should give more data about the problem.

However, one thing should be mentioned: The bolt 379 must not be overtightened, but it must be secured with bolt adhesive.


What is the recommended torque for bolt #379? I was unable to find it in the manual.

I mean the overtightening is the initial reason for the bending of the "washer section" (the narrow piece with the bigger diameter) of 338.


You are right - it might have happened during rear wheel removal and installation in the field, when torque wrench is unavailable.
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Re: Belt twisted twice

Postby Blackstridaaustria on Mon Sep 10, 2012 1:22 pm

one_half_3544 wrote:What is the recommended torque for bolt #379? I was unable to find it in the manual


Sorry, no idea, usually I tighten the bolt very gently by hand, meanwhile I've damaged two of the 338 parts :roll:

Missing 379 torque was added to the improvement wishlist draft...
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Re: Belt twisted twice

Postby one_half_3544 on Sun Sep 16, 2012 8:42 pm

Blackstridaaustria wrote:Sorry, no idea, usually I tighten the bolt very gently by hand, meanwhile I've damaged two of the 338 parts :roll:


Did you check it immediately after tightening or after riding?
Could it be that is it damaged not by tightening, but by the wheel movement?

I've replaced #338 part and tightened #379 bolt with 5Nm torque - #338 part still looks good after unscrewing =) Will see what happens after riding though..
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Re: Belt twisted twice

Postby Blackstridaaustria on Sun Sep 16, 2012 9:26 pm

one_half_3544 wrote:Did you check it immediately after tightening or after riding?
Could it be that is it damaged not by tightening, but by the wheel movement?


On the black Strida I've destryoed the 338 part definitely by too much force -
I've mounted the wheel and wondered, while tightening the bolt, that it didn't become firm.
Therefore the bolt was unscrewed again and I saw the deformed 338 part...

On the silver Strida I'm not 100% sure, but I believe it was the same reason.
I guess that any force, applied to the wheel via the tyre in axle direction, should be absorbed by the wheels flexibility.

one_half_3544 wrote:I've replaced #338 part and tightened #379 bolt with 5Nm torque - #338 part still looks good after unscrewing =) Will see what happens after riding though..


Hmmm, here I'm also unsure, sorry, but I think that the very gently tightening will be below 2 or 3Nm.
2Nm are the lowest amount which can be set on the BBB torque wrench, I do not trust the wrench at this range limit...
I'll try to figure out a safe torque amount asap...
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Re: Belt twisted twice

Postby Blackstridaaustria on Mon Sep 17, 2012 9:52 am

According to Ming's customer support the torque should be 6-8Nm.

Many thanks to Sara for the fast reply!
Image
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Re: Belt twisted twice

Postby one_half_3544 on Wed Oct 17, 2012 7:23 pm

one_half_3544 wrote:1. replace #338 and leave the current belt - to see if it will continue to twist. After it snaps/twists/exhibits some other abnormal behavior, I'll:
2. replace the belt and see if it twists (with new part #338, but old freewheel).


Ok, the belt still twists with new #338 part (3 times in a row today!), so now I'll replace the belt with a new one.
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Re: Belt twisted twice

Postby Blackstridaaustria on Wed Oct 17, 2012 8:36 pm

...and you're sure that the 338 part (magnet spacer) is still OK?
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Re: Belt twisted twice

Postby one_half_3544 on Thu Oct 18, 2012 4:16 pm

Yes, #338 part is ok.
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