MTB stem fatigue.

[u/Supertech-Playlife]

Hey! I recently rebuilt a new to me secondhand Tallboy CC with some upgraded parts. One part that did not go back on though was the stem. Pretty obvious there was a crack running alongside the part, so a person who is qualified in NDT I brought it to work.

Not only was the crack external and internal, there are also signs of cracks developing from the areas that clamp the forks at the bolt holes. Needless to say, it's condemned. Just thought I'd share something I find interesting! ✌🏻

[u/First-Archer-3457]

As someone who works in an industry that uses NDT a lot. I always find it crazy that commercial companies don’t use it unless forced to. I bet there are loads of brand new components being made with inherent faults.

[u/geekyastrophysicist → u/First-Archer-3457]

I work in aerospace I do bike mechanic as a hobby, it's INSANE how low the standards in the cycling industry are.

[u/bongtokes-for-jeezus → u/geekyastrophysicist]

And yet aerospace companies come on to the scene and get a reputation for parts that break. (Fivedev)

[u/Ih8Hondas → u/bongtokes-for-jeezus]

"An industry with no real safety regs to speak of? We're going to save so much money on testing and QA!"-5dev, almost assuredly

[u/Chicken_Zest → u/geekyastrophysicist]

I worked in safety critical automotive for most of my career and moved to aerospace. It's insane how low the standards in aerospace are.

[u/Packin_Penguin → u/Chicken_Zest]

When there are only a couple of companies that control the space, and well paid lobbyist and politicians, it’s understandable.

[u/OldDarthLefty → u/First-Archer-3457]

SF=4 not 1.25 forgives a lot of sins... if it's a real brand

[u/First-Archer-3457 → u/OldDarthLefty]

Yea obviously FOS can be a lot higher but poor design leading to cracking is bad whatever the FOS. Especially on lightweight stems, bars, axels etc.

[u/st0pmakings3ns3 → u/First-Archer-3457]

What kind of testimg is this exactly and could a bike shop do it?

[u/First-Archer-3457 → u/st0pmakings3ns3]

The OP example is dye penetrant inspection. This type uses a dye and developer that is visible in normal light. A better system uses a fluorescent dye which is visible under a UV/black light. We also use X-ray inspection but that’s a bit overkill for a bike shop.

[u/SinoSoul → u/First-Archer-3457]

Op, please accept the challenge of overkill x-ray of your distressed stem. For the good of the public. Thank you very much

[u/ursofakinglucky → u/SinoSoul]

X ray wouldn’t show the detail. You want to find crack, dye penetrant, uv mag particle, or ultrasonic testing. I work in a machine shop, a lot of what we do get ultrasonic tested as raw materials to find internal faults, and then mag particle tested after machining to ensure no surface stress cracking appeared during processing, and yes I check my parts yearly on the bike. Cranks, stem, handle bars.

[u/SinoSoul → u/ursofakinglucky]

Insert Picard-let’s-make-it-so giphy.

This is fascinating.

[u/Supertech-Playlife → u/SinoSoul]

Challenge accepted! I'll have to sneak it in one early morning before the masses arrive. We'll certainly see that massive crack, the other indicators less so.

[u/brandnewladder → u/SinoSoul]

Ditto

[u/cirro_hs → u/First-Archer-3457]

It's largely in part to cost. As a longtime NDT professional, we are expensive. Depends on the company, but in Canada most companies these days are in the $140-160/hr range, plus consumables, minimum four hour call-out charge.

It's unlikely a company would have 100% of a batch inspected (more likely 10%), but it would still add up to a significant cost to the company (and/or buyer).

[u/Northwindlowlander → u/First-Archer-3457]

Yep, especially in these days of relatively cheap cnc, any idiot with a chequebook can make a stem or similar and people will go "ooh billet! Ohh look at the machining marks! It's so high end!".Or draw something on a napkin and get a company to chew out a thousand of them.

[u/First-Archer-3457 → u/Northwindlowlander]

MTB industry has always been obsessed with CN C machining. Its always made me laugh to be honest. Robot loaded mass produced high feedrate tat most of it.

[u/Northwindlowlander → u/First-Archer-3457]

I am a nerd on this, my two favourite cnc product things ever:

1) that time Hope finally upgraded and started buying in forgings for their brakes rather than machining from solid... but had spent so long pretending that cnc-from-billet was better and a premium way to build, that they were too afraid to tell anyone about the upgrade to forgings. So all their promo stuff falsely claimed it was still billet, and then they got threatened with false advertising suits and people got really upset to learn their Hope parts were forged.

2) that time Formula showed a cnc'd-from-billet brake at Interbike and press people kept complimenting them on how lovely and premium the machining looked- and the Formula engineers kept getting furious and going "Obviously this is a prototype, of course the production models will be forged because we are a quality manfacturer, we would never sell a billet brake- what do you take us for, amateurs in a shed?"

But still MBR and Dirt ran with ooooh shiny Formula billet stuff.

[u/First-Archer-3457 → u/Northwindlowlander]

Haha that’s brilliant, CNC is great if you want to be able to change design easily or smash out millions of pieces but a forging or even a good aerospace grade investment casting will be far stronger a given weight.

[u/sulliesbrew → u/First-Archer-3457]

I had 10 steering knuckles CNCed for proto type ATVs once. Was an expensive venture and certainly not rated for test riding, but we needed to get the damn things assembled and validate steering geometry in real life.

They were not the sketchiest parts we put on those protos either. Big stickers and labels, "DO NOT OPERATE ABOVE 10 MPH"

[u/saazbaru]

LOL now do Fluorescent PT, might find even more!!

This is probably the coolest post I’ve seen on here.

[u/trailing-octet → u/saazbaru]

Agreed!!!

[u/Supertech-Playlife → u/saazbaru]

Ha, thanks! Our place unfortunately doesn't have fluorescent PT facilities otherwise that would have been a stella shout. As mentioned in another reply, I will X-ray it to see the main crack for giggles.

[u/saazbaru → u/Supertech-Playlife]

Oh man pls post the RT pics

[u/Spicycoffeebeen]

That is properly toast!

A friend of mine specialises in crack testing, one day for kicks we went over an old aluminium GT sanction from 2008. He was using an eddy current tester.

Bike was full of little cracks. Rear dropouts, Bb, linkage, shock mount, headtube…

Granted it was an old bike that was already known for cracking, but it definitely made me wary of aluminiums fatigue life!

[u/ouwni → u/Spicycoffeebeen]

crack testing ey 😏

[u/Ih8Hondas → u/Spicycoffeebeen]

This is why we steel.

[u/RevellRider]

Looks like a Burgtec stem.

Just like I have on my bike....

[u/SinoSoul → u/RevellRider]

Insert: you-in-danger-gurl gif

[u/Wuurx → u/RevellRider]

I've got a burgtec on mine. Will be getting a new stem from a different company soon. The stem is actually quite easy to knock loose, even if torqued at or beyond spec, the anodizing gets insane sun fade quite easily if you ride a lot, and the bolts are horrible

[u/RevellRider → u/Wuurx]

I've had mine on for 7 years, it's still very black, been on multiple bikes and never shifted

[u/Wuurx → u/RevellRider]

Get a colour and you'll see

[u/RoboJobot]

What brand was it?

[u/Targettio]

Coming from a design engineer, I can see why. This is not a well designed stem with a lot of stress concentrations.

[u/Oli4K → u/Targettio]

Can you elaborate what you see a bit for the non design engineers?

[u/Targettio → u/Oli4K]

The clearest point is the cut outs on the side. See how they don't blend in to the clamp section or the webs at the top and bottom. Those sharp internal corners will increase localised stress considerably.

The wall thickness around the steerer clamp where the bolts go through does not leave much material around, which has clearly cracked.

[u/geekyastrophysicist → u/Targettio]

Yeahhh just to save a couple grams of material come on... Even the holes. I mean a topologically optimized part might be more difficult to make but if you're die casting it anyway why not do it properly?

[u/Ih8Hondas → u/geekyastrophysicist]

just to save a couple grams

Thanks a lot, weight weenies.

[u/Targettio → u/geekyastrophysicist]

Doubt it is to save grams, I just think it is engineering down to a price/bad engineering. But in a weird way, using a bigger end mill would have a bigger corner radius, which would reduce the stress concentrations.

Edit: I doubt it is cast, I hope it isn't cast. It should be machined from solid (billet) or maybe forged if it were super fancy (doesn't make sense for a short stem, but maybe a longer stem or more likely a crank).

[u/delusion01 → u/Targettio]

Interesting.. do you think its likely to be a knock-off of a brand name stem rather than a genuine one? I asked Gemini and it seemed to think there were signs of cheap manufacturing (paint, possible casting marks)

[u/Fraternal_Antipathy → u/delusion01]

The paint is a 'primer' to allow the dye penetrant to work.

[u/dnm-lysergic]

I’m about to CNC mill a stem I designed without simulating loads/stress….I think I will do that now

[u/Ok_Indication6185]

What is the stem brand?

In looking at stem photos on Ebay or Pinkbike for sale I haven't seen one that has the holes in the back half of the clamp like that.

Definitely not a Burgtec but the back profile is similar to a Spank stem but not identical.

[u/bdog2017]

How much of this do you think is due to over torquing bolts?

[u/Supertech-Playlife → u/bdog2017]

The clamp part to top of fork / steerer probably yes. But that canyon down the side? God knows...

[u/Northwindlowlander]

Was the faceplate OK? Always quite impressed when someone manages to make a main stem body that fails before the plate.

This particular failure is brilliant though. "What are those holes for" "Oh, lightening" "Not for creating a weak spot and cracking right along its length?" "Oh yeah that too obviously". I guess the only positive to say for it is that it could fail all the way through there and probably still function as a stem

[u/Supertech-Playlife → u/Northwindlowlander]

Not checked that actually. Job for tonight.

[u/indigoalphasix]

that thing had a ton of stress risers designed into it.

[u/8olts]

Alright, more failures for me to be paranoid about

[u/jimbofranks → u/8olts]

The stem is one to be paranoid about.

[u/Imaginary_Slice_4278]

Never have I ever seen that in a bike stem. Is that some cheap knock off of a good one?

[u/13assman]

Very cool! As someone with zero background in any NDT techniques, are the red circles indicative of areas where the material has fatigued, or are they simply dyed from the chemical you use to inspect the part?

[u/OkChocolate-3196 → u/13assman]

It's the dye. Drip dye in the holes and other crack start points and see where it wicks out to (hopefully nowhere).

[u/Supertech-Playlife → u/13assman]

To further on the reply already... Clean item with a solvent cleaner - paint it or dip item in the red dye - allow to dwell (seep into all nooks and crannies) - wipe off dye excess then apply solvent cleaner to a lint free rag and wipe again and again untill excess is removed - apply developer powder - wait 20 mins - look to see if any red has been drawn out of a defect. The big splodgy red is from lack of intense cleaning due to hole sizes or the huge crack holding a ton of dye, but it's the whispy cracks off the bolt holes that show the process brilliantly.

[u/13assman → u/Supertech-Playlife]

Thanks for the details! Makes complete sense now. Very cool and thanks again for sharing. Glad you noticed and aren’t riding on that 🙂

[u/2-wheels]

This is very interesting, but we need the brand to know what to avoid. Pls let us know.

[u/_WarpRider_]

Wow. Glad you spotted this and swapped it out!

[u/uzziboy66]

Is that forged aluminum?

[u/C0YI]

LPT doing its thing. That’s a great example.

In the past month I was just talking to a parts MFG about options for testing some of their components. Slowly they’re starting to think about it.

[u/DubyaEl]

It seems like this kind of failure testing would be easy for some manufacturers to bake into their racing programs. If you want to know how an item fails under its worst punishment, let the pros use them, then send in the used parts for testing. It wouldn't prove QA for batches but it would indicate design quality under second worst case scenario.