An unexpected end to my ride today

[Eldron]

Aluminiun's major failure mode is a fatigue fracture as JB correctly states and in most of the failrues there is indeed an initiation point (stress raiser) where the fracture initiates.

 

Typically the stress raiser is a scratch/change in wall thickness/weld/impurity in the alloy etc.

 

The "evidence" is known as beach marks - typically these radiate out from the initiation point (think a stone thrown in water and the ripples radiating from it).

 

Have a look at the fracture face and see if you can spot the initiatiion point and beach marks - that'll give you and idea of why it broke. In bars I would sat the most common initiation point is a scratch caused by the stem rubbing the bars, a scratch made when cutting the bar tape (as someone else stated) or an impurity in the alloy (worsenede bt drawing the bars).

[Eldron]

Another potentially nasty injury awaits those who engrave a nice mark around their seatpost .........

 

Quite right!

 

Never, ever make marks on your bars/seat pillar - use a bit of tape. Match the colour if you think it looks ugly. Also check the inside of the seat tube before inserting the seat pillar - lightly sand away any burrs you find - this wll also stop your seat pillar getting scratched.

 

I change my bars and seat pillar every year - mainly due to the 20 stitch scar in my left calf made caused by the broken seat pillar...

[Napalm]

My frame also snapped yesterday, and if it wasn't for my lighting quick reflexes I would have a aluminum stuck in places where aluminum really shouldn't go.

 

[firewolf]

Oh mu f***! A snapped bar!  That could have been deadly serious. Will you contact the maunufacture/dealer/LBS?

[Dirt*girl™]

Sheesh, that is really scary stuff! I'm gonna get all paranoid when I'm riding now!

[Cyclewizz]

My frame also snapped yesterday' date=' and if it wasn't for my lighting quick reflexes I would have a aluminum stuck in places where aluminum really shouldn't go.
[/quote']

 

What frame ?

Where did it snap ?

[Capricorn]

no need to go uberemo dirtgirl just check tubes in areas around the clamps for scratches etc Difficult to say how deep is bad, but i reckon scratches in areas where the pipe is under tension is not a good thing. Try to sand it away if it's not very deep, or if you can, rotate the tube such that the scratch is at the bottom, or at a position where it experiences compression more than tension. For example, top of bars are in tension when you resting your weight on the bars, the bottom will be under compression. Deep scratches are a near definite: no-no.

 

I wonder if there are fatigue studies that correlates time to failure with number of load cycles given a certain defect depth, defect being a scratch, for various materials used on bicycles.

[zeepee]

http://i157.photobucket.com/albums/t57/grahamr_photo/F430/IMG_1069v2.jpg

 

 a roadie that doesnt shave his legs????

[Dirt*girl™]

ahhh, thanks for that Capri...I'll check my tubes everyday!

[AndreZA]

a roadie that doesnt shave his legs????

 

 

 

you will see he rides a metal bike aswell. So he is no ponce and I will ride with him.

[Slowbee]

oh my google bannanna flavoured cakesisters.

 

How serious is this "nicking" of the seat post???

 

 

I marked my seat post for the DD with a nick as I did not trust the tape since I knew the tape might move. How serious is this ? Do I need to replace the seat post ?

 

[Capricorn]

a nick is fine i reckon. heaven's knows i literally stand by that comment cos my DH frame is full of nicks Thing is, it's the thin walled components that are more easily prone to fracture has happened with the OP. Eldron seems pretty clued up as well on this phenomena. I wonder if he has some guidance as to what constitutes a deep enough scratch which would warrant component replacement. It's not a black and white question and answer scenario. There are quite a few variables that come into play: the load, the load frequency, defect size, temperature, material composition, time, etc...

 

The best general advice is to ensure the deflect is blended into the rest of hte material: no sharp internal edges which are called stress raisers. It's from these sharp internal edges where crack propogation normally starts.

Soz for the jargon, but i'm y'all will appreciate why certain things are done a certain way> makes doing it yourself a lot easier cos you know why and ?how.

[GoLefty!!]

Marking the frame or bars with a light scratch in the anodising does not a stress riser make.

If you go sawing into the metal, yes then you have a problem.

 

Have you ever crashed or fallen with those bars in the past?

How old are they?

 

Whetehr your bars are Carbon or Aluminium, they should be replaced at least as often as the manufacturer recommends.

 

[whistler]

This is bad, imagine this happens in a berm at a relative high speed. ouch.

 

luckily you're not heavily injured!

[Napalm]

 

My frame also snapped yesterday' date=' and if it wasn't for my lighting quick reflexes I would have a aluminum stuck in places where aluminum really shouldn't go.

[/quote']

 

What frame ?

Where did it snap ?

 

GT Avalanche. Going down a steep hill at a somewhat stupid speed it snapped where the seatpost goes into the frame. Luckily I could stop without killing myself. I did have to wash my cycling shorts when I got home though

 

[Capricorn]

 

GoLefty: when it comes to thin walled items, especially those items ear marked for the weightweenies, I would be a bit hesitant saying a light scratch is not a stress raiser. Defect depth relative to overall wall thickness is the proper indicator IMO, not what we think is deep or not. Granted, minor surface scratches that are more like rub marks you get on CDs when u dont look after them, are never going to be a concern.

If you look at the scratch mr Bornman pointed out, that's not very deep, and neither did the OP go postal with a hack saw to make it.

But again, load, load cycles and time come into play to determine when or even if failure will occur.

 

?

 

But prevention in this case is a lot easier than understanding the failure mechanism:
Just ensure none of the clamps and area underneath the clamps have ridges that can scratch thin tubes such as bars and seatpost. If there are, just lightly sand it down till the surface is flat and level with surrounding surfaces.

When removing seat posts and bars, undo the clamps properly. Some have a tendency to barely loosen it, and then wriggle the bar or seat back and forth to free it up. Granted, we might not have a choice but to do it. So check afterward if the tube has any deep scratches in it. If there are scratches which concern you, and can be sanded out by hand, give it a go.

I'm sure JB et al will have some wisdoms to share.

Capricorn2010-01-12 04:24:01