crash after stem broke at 50 km/h

[DawieO]

My mate had a similar scare in Dec, sans the violent end.  We took the bikes off the carrier to go for a little Harkerville ride and the handlebar came right off in his hands!

 

The face plate on his KCNC scandium stem had cracked right through at the bolt holes.

[bustthesickness]

 

kcnc part are machined in china' date=' now my limited knowlegde of material quality control leads me to suspect that there could have been a cold joint or a flaw in the original billit the stem was machined from... hard to notice unless ultra sonic or radiographic testing is done before the material is machined...

 

use it don't use it...

[/quote']

I come from a rock climbing background where 90% of the hardwear is forged from aluminium. It makes VERY strong equipment this process. Each caribiner [clips used to join ropes to gear to rocks etc] and item gets x-rayed for flaws and microscopic cracks that may occur during manufacture. The rule of thumb too is that if one ever drops an item of equipment it gets destroyed to avoid further use. These components are critical. Certain pieces like screwgates are so critical that if they were to fail it could kill you and your climbing partner.

 

Basically, long story short, it's common practice to x-ray inspect these items, as they are crucial. The same should apply to a stem. It's failure can lead to a very bad accident, and possibly death too. People in climbing gear shops understand this and thus gear is never dropped or rough handled. A very likely possibility in the case in question here is that the KCNC stem might very well have passed all QC inspections, only to be dropped, overtightened etc afterwards.

 

While machining parts has some advantages, in the strength through process department, forging is a really strong solution. Tricky to tool up for certain stem designs [and often dual processes happen where the bulk gets forged and the rest is machined].

 

As was said now, check your gear all the time. Inspect critical components before each ride. If you hear any strange noises, find the problem.

 

[Serendipity]

I had a good hard look at my stem after seeing that. I think the really scary bit is that you can probably feel something like that breaking for a few seconds, knowing that you can do absolutely nothing to safely stop.

[Troi Sport]

 

I think the really scary bit is that you can probably feel something like that breaking for a few seconds' date=' knowing that you can do absolutely nothing to safely stop.[/quote']

have to agree with Serendipity on this , the feeling of being totally out of control when the stem breaks is really scary.

I have the scars and the therapy bills to prove it

[DeltaOscar]

Amazing how it stayed attached to the steerer tube........

[Bos]

I think like "bustthesickness" said, be aware of any new noises on the bike... thats one thing I have experianced..a crack in a frame, bar etc.. It makes noises.. stop, find it! or the next noise wil be the "snaP!" followed by Oo F@K!!

[Johan Bornman]

I'll answer a bunch of questions at once.

Forging:

Forging is the hammering of a cold or hot metal into the desired shape. It aligns the metal crystals in such a way as to improve strength. Also, it gets rids of internal voids in the metal by literally hammering them closed.

In manufacutiring, forging is reserved for he most critical components due to the  high cost of the process. Aircraft landing gear is forged, jet engine surrounds are forced and, good bicycle parts are forged - brakes, stems, cranks etc.

To set up a factory to forge something cost about 100 times more than to set it up to make it from CNC'd billets.

Components could also be cast, but that's the weakest and cheapest. Cheap brake calipers are cast, then mid-range ones are CNC'd and the best is forged. Shimano and Campag forges, the rest CNC's. This is a broad statement and there will be exceptions. I like sweeping statements.

 

A good, forged component will be ductile and strong and since it is strong and definitely has fewer internal imperfections than a billet, can be made thinner and thus lighter.

 

A good forged component will not break/crack like glass when dropped, as was suggested. They are robust and reliable. Mountaineering equipment can survive knocks and drops. The sport would be different otherwise.

 

Someone suggests keeping your bike clean as a safety precaution. He's absolutely right. If you lovingly clean your bike you'll find lots of faults long before they become critical.

 

I see this often. Customers bring me wheels which have cracked rims. They always say that after an accident they inspected the bike and found the cracks, which they deduce were caused by the crash. It isn't like that. They only saw the cracks 'cause they crashed and took the trouble to inspect the bike afterwards.

 

As for you falling first and the component then breaking. This doesn't happen. A fall per se doesn't do much damage to a bike. It certainly doesn't crack a frame or a rim. Exceptions to this rule does apply. (I hate having to safeguard my sweeping statements).

 

We usually fall because something has broken. It's the same with old ladies. They fall because their hips have broken. Not fall and broken their hip. Osteoporosis weakens the bone to a point where it simply breaks under load.

 

Cracks are seldom the result of a single event but usually from fatigue over time.

 

Greatwhite (A Hubber) once said that most of these stupid lightweight components are "sacrificial parts". I'm beginning to think he's right - we sacrifice ourselves at 50kph to have light parts.

 

My stem is forged and I check it regularly. The bolt is not forged and I check that one even more regularly.

 

 

 

[DeltaOscar]

What's your opinion on carbon stems JB?

(steps back & waits....)

[Johan Bornman]

That is why I like the BBB stems I use on my Tandems. The torque specification is clearly printed on them.

 

JB' date=' does that mean i'm skating on thin ice here with the 2 bolt Easton EA70 stem I have put on my BMC?

 

I don't think EA70 qualifies as weight weenie stuff but is it a proper, strong stem? I looked at the KCNC Fly Ride which was cheaper but somehow I didn't trust it and decided to go with easton...

 

Regards

 

[/quote']

 

Sorry, only saw this now.

 

I just can't say how safe it is, other than to say it is infinitely less safe than a 4-bolt design. You have no redundancy there. If the bolt goes you go.

 

If you don't overtighten the bolts, if you don't abuse your bars (i.e. pump hard when riding out of saddle), and if if if, it'll be OK. Also, you probably have two 6mm bolts in there wheareas the others are 4 4mm bolts.

 

However bolts do crack and lightweight aluminium parts even more so.

 

I poep myself each time I charge (anything over 40kph at my age is charge) down Beyers Naude and I hit those rough spots. I always think of my stem and my face.

 

Next time, get a 4-bolt.

 

 

 

 

 

 

 

[Johan Bornman]

What's your opinion on carbon stems JB?

(steps back & waits....)

 

You get good carbon stems and bad carbon stems. Just like you get good aluminium/steel and bad aluminium/steel stems.

 

It is not the material, as we've seen in the sister thread to this one (KCNC), but the method of manufacture. You can put Scandium in a stem and it is still rubbish.

 

Carbon comes with its own problems. Fellow hubber Christie is somewhat of an expert at the challenges carbon brings to making components where the forces T-bone each other as in a stem.

 

Christieeeeeee! Jou telefoon lui.

 

 

[bustthesickness]

 

A good forged component will not break/crack like glass when dropped' date=' as was suggested. They are robust and reliable. Mountaineering equipment can survive knocks and drops. The sport would be different otherwise.

 [/quote']

contrary to your convoluted opinion, fractures can develop if forged components are dropped. In terms of climbing pieces, I'm not talking about a little drop from your bag onto the ground. I'm talking dropping gear from height. And trust me, when your life is entrusted to a single interlink, and your are dangling 200 meters up in the air, you make bloody sure you don't take chances.

I only brought up this point because of the similarities. Both critical components; both forged; both made from the same/similar materials; both have high impact use.

 

Use it dont use it.

[JPW#]

Phew, I have one of those stem's on my bike, they seem solid, guess I have to sell the stem now, any willing buyers? I'm going back to the heavy Ritchey stems!(only heavy in price tho!) a titanium stem or a steel stem should be the strongest, I haven't seen any Ti stems on the market lately tho.

[JPW#]

 According to all carbon fork manufacturers you have to have at least 10mm of spacers on top of your stem and that fork had none I could see in the picture, I wonder if that had anything to do with the incident.....just thinking out loud.

[Johan Bornman]

A good forged component will not break/crack like glass when dropped' date=' as was suggested. They are robust and reliable. Mountaineering equipment can survive knocks and drops. The sport would be different otherwise.

 [/quote']
contrary to your convoluted opinion, fractures can develop if forged components are dropped. In terms of climbing pieces, I'm not talking about a little drop from your bag onto the ground. I'm talking dropping gear from height. And trust me, when your life is entrusted to a single interlink, and your are dangling 200 meters up in the air, you make bloody sure you don't take chances.
I only brought up this point because of the similarities. Both critical components; both forged; both made from the same/similar materials; both have high impact use.

Use it dont use it.

 

Wont use it.

 

 

Here's a nice selection of climbing equipment. I've excluded wedges, nuts and hexes. They're not even forged, just little lumps of aluminium with steel cables or webbing loops.

 

You'll notice that most of the stuff is anodised, a clear sign that this stuff is not prone to fatigue. Most of it is lightweight and attached to webbing or rope, which when falling, brings it to terminal velocity very quickly.

 

You'll notice that none of it is made of glass. Nicks and dents don't affect their strength since they're not subject to fatigue. A single event (fall) does not develop a stress crack. Either it breaks during that fall or it does not.

 

But keep on shifting the goalposts. The only option still open for you is to increase the height so that it drops from the moon.

 

Neither bicycle nor mountaineering uses equipment that is subject to high impact. Dream on and keep on buying new stuff each time you drop it from "height."

 

 

[Barend de Arend]

 

 According to all carbon fork manufacturers you have to have at least 10mm of spacers on top of your stem and that fork had none I could see in the picture' date=' I wonder if that had anything to do with the incident.....just thinking out loud.

[/quote']

 

Easton does that (require 10mm on top) on their lightest forks. but not the midrange.  Isaac just recalled bikes because of a similar problem.

 

However, it seems to be fork related, and not stem related.

 

Same as overtightening the stem bolts: it's likely to crack the steerer, but the stem will survive.

 

[#Pete#]

"We usually fall because something has broken. It's the same with old

ladies. They fall because their hips have broken. Not fall and broken

their hip. Osteoporosis weakens the bone to a point where it simply

breaks under load."

 

For once I can correct you as we are now in my terrain ;-)

 

Osteoporotic hip fractures (and radial fractures) are almost always resultant from a fall. Spinal fractures however are not.