Tern Link D8 frame shearing in two

April 9, 2014 - 12:50pm
bikerrrguy

Country: USA


First off let me apologize for my improper tone earlier. I was prejudicial against Josh because of some nonsense 10+ yrs ago. Josh I apologize!

I must admit Keith is awesome. He is very level headed and non-prejudicial!

I am back to work making tests and running my biz.

Thanks,

Yan

April 9, 2014 - 1:23pm
thor


Country: USA


now that the thread has turned to a decicive better mood, let me back into the fray, so to speak.



Communication needs to be with the dealer you bought the bike from, OR if he/she is unresponsive, to a Dealer who will take your case. ( In the US that would be me, as I dont shy away from problems, no matter where the bike is originally sold )

Again, its good business and its the way the industry works. Tern Customers have been a little spoiled with the boss posting here and give help, but again thats NOT the standard. There is nobody higher up from Dahon doing that on any boards, You wont find Mr Burke ( Trek) or Michael Sinyard ( Specialized) to post anywhere.

For the non UK folks who are on this forum, let me explain just my personal opinion about the Dealer in question. The UK has Evans, who operate a bunchload of stores all over, its the retail giant. Some ( few) stores have excellent staff, which knows their stuff and can get things done, some other stores ( most ? ) dont fare quite as well ( to say it mildly )

Similar to Performance Stores here in the states ( although they have improved in general ) REI stores here in the US are generally good, but even they have a few bad apples.

Some of the UK Evans store folks are heavily biased about local production. There is absolutely nothing wrong with having pride in domestic product, but that doesnt mean to give second tier service to everybody else. Obviously it doesnt help one bit to talk down a different product either, just because of being an import or domestic product either.

If... anybody with a broken frame, has not been offered a replacement frame, a complete new bike ( if frame is not readily available ), a refund of the original purchase price, or a store credit, pretty much the day they bring in the broken bike, ..... it tells me that the store is not doing anybody a service which should be expected. The Store can make a simple telephone call to the Importer and surely will get a green light to do whatever it takes. This doesnt need rocket science, or exceptional customer service.

Here just how a frame breakage is usually dealt with. Customer brings broken bike to store, Store gets authorisation from manufacturer to replace frame. New frame gets send to dealer. Dealer will put in new frame and send old frame back to manufacturer. Customer pays for the labour the dealer has to convert frames over. Customer picks up old bike with new frame. Some ( fewer than one thinks ) dealers will not charge anything for their labour ( me included )

Obviously that hasnt happened in most of the UK cases ..... than we really dont know what the store offered and when ? I doubt that a coupel guys bought a Brompton from the store without getting any credit for the Tern ?

Lets also think one minute about this. The fact that Tern doesnt take the whole post down, eliminating the pictures, maybe even close the whole forum for good, is a point which clearly states, that they are behind their product and their integrity. It would take all but 5 minutes to delete all of this.

Tern is to blame for one thing...... being very active here, but than when this problem started they seemed to be in hiding. Believe me, that they are working on this problem, its their lifelyhood, passion and so much more.

As a dealer I would be pretty miffed if Tern would go behind my back and doing stuff directly with the customers ( as long as I dont specifically ask for their help to do so )

Maybe this explains a few things, in the meantime I am positive that there will be a solution from Tern.

Thor

April 9, 2014 - 1:32pm
Frank Dippel

Country: Germany

thor wrote:


now that the thread has turned to a decicive better mood, let me back into the fray, so to speak.

Herrschaftszeiten, Thor, jetzt hast Du alles ausgesprochen, was ich schon wochenlang gedacht habe und worüber ich mich täglich geärgert habe, mich nicht adäquat in Englisch äußern zu können. Das erste Jahrzehnt meines Berufes habe ich ein einer geschlossenen psychiatrischen Klinik verbracht, und einiges hier hat mich doch leider sehr an meine Vergangenheit erinnert. Hoffenlich übersetzt das keiner....

Gruß aus Norddeutschland

Frank
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April 9, 2014 - 2:13pm
orbit

Country: UK


Pardon? Oh how I wish I could speak German. LOL

April 9, 2014 - 3:09pm
Rager

Country: France


Thor, good to see you back again with an informative post.

1 further question: how many frame failures were there before the 2013 recall?

Regards.

April 9, 2014 - 3:53pm
thor


Country: USA


I can only speak for the US and only being a dealer, meaning I am not sitting in the US Tern office.

Frame failures in the US to my best knowledge



0



before/after/today/yesterday/   from the beginning





Thor


p.s. vielen Dank Frank :-)

April 9, 2014 - 4:17pm
Rager

Country: France

Thor, I thought you previously stated that you'd dealt with the broken frames that were subject to the 2013 recall?

I see from browsing the net that the broken frames issue is being reported in independent forums, YouTube & Facebook. I fear it's more important that ever for Tern to respond with the findings and put both current and and future customer's minds at rest.

April 9, 2014 - 4:46pm
thor


Country: USA

Rager wrote:


Thor, I thought you previously stated that you'd dealt with the broken frames that were subject to the 2013 recall?

I see from browsing the net that the broken frames issue is being reported in independent forums, YouTube & Facebook. I fear it's more important that ever for Tern to respond with the findings and put both current and and future customer's minds at rest.

NO NO

I have replaced frames/bikes which were recalled but I have not received ONE broken one, saw one, smelled, heard one

again....



0



broken frames

April 10, 2014 - 3:48am
edwinic


Country: Philippines

Keith C. Johns wrote:


Valid points observed. What I note is that the main tube was never welded all the way around; it was tacked to the gusset, and then only the outer perimeter of the two was welded in one step. I woud have thought that the main tube should have been welded fully around and then the gusset welded on top of that--adding to the strength rather than just substituting for its bottom curve.

May not be the whole story, but perhaps it is a start.

I suspect that this is the culprit. Is there a way to know if a full welding was done? by XRAY perhaps?





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April 10, 2014 - 5:57am
akpope

Country: UK

Keith C. Johns wrote:

Valid points observed. What I note is that the main tube was never welded all the way around; it was tacked to the gusset, and then only the outer perimeter of the two was welded in one step. I woud have thought that the main tube should have been welded fully around and then the gusset welded on top of that--adding to the strength rather than just substituting for its bottom curve.

May not be the whole story, but perhaps it is a start.

Just so that you can judge how much weight to put to my comments, I am a Chartered Civil Engineer, with a degree from Cambridge University (the real one, not Harvard) and I have specialised in bridge analysis for 30 years, currently working mainly with metal railway bridges. As well as being known as a pretentious git, I am also regarded as a technical expert in my field  .

I do not work for a bike company.

Having said all that, i am mystified by the photo. A photo end on to the tube would avoid any optical illusions, but looking at the end of the tube on the right, where we would expect the ends to be square and hence be able to scale any dimensions (taking into account the foreshortening from looking down) the top and sides of the main tube appear to be a good third to a half thicker than the bottom of the tube. If we then look at the broken end of the tube (and, to be honest, this is where the photo really could lie if the broken end were angled away from the camera) that also seems to be thinner than the top of the main tube.

When end supported beams bend, they go into tension at the bottom and compression at the top. But they do not always go into the same amounts of tension and compression. The most efficient section is balanced and has the same amount of material at the top and bottom, which is why engineers love I-girders. The stress varies linearly from top to bottom (i.e. if you plot stress against height, the graph will be a straight line) but the zero stress level is at what is called the centroid (like a centre of gravity but with areas not weights) and having a thick area at the top will cause the zero point to move up and the tension stress to increase compared to a balanced section of the same area.

If both of the sections had been welded at the bottom then they would have made the section more balanced and reduced the stresses at the joint. This would not have affected the section at the end of the reinforcement though, and that would be subject to similar levels of load.

I do not know if the top was made thicker to avoid problems with local buckling (doubtful) but the section does not appear to be giving the best ratio of strength (or stiffness) to weight. The details do not look clever.

I would, however, say that it is very difficult to weld one tube fully and then weld another curved shape to fit to the main section AND to the original weld. There is no reason why the weld detail as shown should not work, but it would be better if the thickest tubes (and widest welds) were at the top and bottom.

That one was for the nerds amongst us.

Cheers

Adrian

April 10, 2014 - 7:15am
Steveroot

Country: USA

edwinic wrote:

Adrian,

From a structure like this bike, where do you supposed the break started? On top or bottom side?

This has been covered before, but not as well as Adrian's post. The weld starts to fail at the bottom (center of the loaded beam) where the tensile stress is the greatest. Nice to have comments from people with expertise. Thanks, Adrian!
Steve
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April 10, 2014 - 7:25am
edwinic



Country: Philippines



Yes, I understood where it will come first, the breaking, and that is because of the center tending to move up and giving more stress to tension than to compression.
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Tern P9

April 10, 2014 - 7:49am
edwinic



Country: Philippines


Following the picture (and the analysis), the gusset was teared and it gave way to breaking of the weld. But I thought welds are supposed to be stronger than the member itself. Should we say that we have an inferior welding, in this case?
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Tern P9

April 10, 2014 - 9:19am
vavavoom

Country: Greece


As I already said, I have absolutely nothing to do with the technical field, so please forgive my question if it is irrelevant.

Why do they put that hole on the joint plate? It seems logical to me that if water gets through the hole for some reason (e.g. washing the bike, heavy rain, etc), then it would be trapped inside and mostly in the lower part (see picture). As it has already been noted, in some pictures the lower welds seem to be corroded. In this picture, there is probably some corrosion on the sides (orange arrow).

I don't know, maybe the hole is there for drainage purposes, but is it doing its job correctly?

April 10, 2014 - 10:46am
thor


Country: USA


hole is there... to minimize condensation inside the tube, let welding gases come out, etc etc ... very commom. I would be upset if there would NOT be a hole .. :-)



maybe if Tern would leave the gusset completely away , it would be stronger. With my limited knowledge I think its impossible to weld on top of another weld , or at least counterproductive ..



thor

April 10, 2014 - 11:43am
akpope

Country: UK

thor wrote:

hole is there... to minimize condensation inside the tube, let welding gases come out, etc etc ... very commom. I would be upset if there would NOT be a hole .. :-)



maybe if Tern would leave the gusset completely away , it would be stronger. With my limited knowledge I think its impossible to weld on top of another weld , or at least counterproductive ..



thor

There absolutely needs to be a hole (go Thor !) since, bizarrely, a totally closed welded section will suck in moist air through a weld when the section gets cold (welds are always a bit porous). The water then condenses and cannot, unlike water vapour, get out. I had parapet posts (high strength handrail posts 1.8m high) on my first bridge design, where the Contractor was allowed to use any manufacturer on the approved list, blow themselves off of the base plate because of this. The water sat in the bottom and froze on a cold day, turning the rectangular section into a circle, ripping through the 6mm structural welds and then pushing the posts half an inch up in the air.



Gussets are always a good idea when there are odd concentrated loads. They are not always needed but it is very hard to work out those occasions when they are not.



PRESUMING that ali welding is similar to steel welding, there is no reason why you cannot weld onto weld, provided that you do it properly, cleaning off any slag between passes and deal with differential temperature stresses. If we need to do a 25 mm weld then we will typically use a series of 10 6mm passes building up the weld. The problems here would be that

a) the strengthening piece could have to be cut to the bizarre 3D shape to fit the curved tube (predictable to part of a mm) AND to fit to the initial weld (not so predictable), and

b) there might be fatigue problems with any stress raiser that happens due to an odd shaped or incomplete weld.



So all sorts of welds are possible but you need expert knowledge (far greater than mine since I tend to work with old structures) to decide what is a good idea.

Cheers

A

April 10, 2014 - 11:58am
Rager

Country: France


Just to let people know that I have now had a couple of emails from Amanda, in response to my query a few weeks ago. Nothing really specific, so like everybody, awaiting with baited breath to hear the official position on the frames.

Regards.

April 10, 2014 - 12:53pm
bugaboosun

Country: USA


Regarding holes and water collecting in the tubes and gusset, there are two weep holes that prevent water from collecting, one in the top tube and one in the gusset plate.

April 10, 2014 - 1:07pm
bugaboosun

Country: USA


My pet hypothesis:

Welders joined the gusset to the top tub first, aligning then edges in a single plane. Then the the tube and gusset assembly was butted up against the hinge plate and welded. But the assembly never butted up flush like on a Verge tube, because of the protruding bulge around the center hole in the hinge plate. Gusset was welded to the hinge plate with a large gap between. You can see the welding material squirted through the gap in some photos.

In this scary scenerio, theboly place the tube and gusset assembly would touch the hinge before welding would be at the very top of the tube and the bottom of the tube. No gusset contact and only two points of contact for the tube.

April 10, 2014 - 3:24pm
Nerv

Joined: 2013-10-09
Country: Belgium


I' have been following this thread for a while...

Amazed at how people react from both sides (Tern & clients).

But that aside:



In the picture below I think I see that a large part of the gusset is broken and that it's not the weld that has broken on that place: bottom right corner of the hingeplate.

So in the place where there are most forces (on the bottom of the hinge-tube weld - pulling the pieces apart) it is the gusset that has sheared in two FIRST and then the crack propagated further along the weld. The joint was then already comprimised so it's logical that the crack propagated along the welds...

But if the crack hasn't started in the welds but rather just the gusset shearing in two and then cracking along the weld, maybe the fact Tern saying it's due to a bad weld is just a guess and the real cause is something different?

I'm propably wrong, I hope i'm wrong.






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April 10, 2014 - 5:22pm
bugaboosun

Country: USA


Weld broke, not gusset. Gusset looks broken because of impact with pavement and remaining weld material attached on on side.

April 10, 2014 - 10:54pm
edwinic



Country: Philippines


I share the obsevation by Nerv.

If the main tube has been welded along its full circumference and the gusset later added on, the these will compensate for the greater tension the bottom connection is undergoing.

That probably was the intention of the Tern designer. However, bypassing the full welding of the main tube has relegated the assembly into an SGM-like collapse as shown by bikerrman picture some post earlier.
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Tern P9

April 11, 2014 - 8:30am
bugaboosun

Country: USA

edwinic wrote:

I share the obsevation by Nerv.

If the main tube has been welded along its full circumference and the gusset later added on, the these will compensate for the greater tension the bottom connection is undergoing.

That probably was the intention of the Tern designer. However, bypassing the full welding of the main tube has relegated the assembly into an SGM-like collapse as shown by bikerrman picture some post earlier.

A short cut made by a rogue welder on a few bikes, or consistently by all welders accros the whole Link d8 line?

April 11, 2014 - 10:32am
edwinic



Country: Philippines

bugaboosun wrote:
A short cut made by a rogue welder on a few bikes, or consistently by all welders accros the whole Link d8 line?

I pray to Thor, its not the later.
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Tern P9

April 11, 2014 - 4:17pm
orbit

Country: UK


How may frames could a rogue welder make in a shift? For how many shifts? Could be a lot of bikes. With out an official explanation from Tern we are still only guessing the scale of the problem.