Dye Penetrant Testing


I’ve recently acquired some Suntour Superbe cranks (Record copies), and also run some Sugino Super Mighty’s (also Record copies).

I was having a close look at the Superbes to check for fatigue cracking at the arm-spider interface, as it’s a known problem on this style of crank, and saw what looks like a couple of crack initiations.

Given that many of us use vintage parts with unknown histories under heavy loads, in the interests of not ‘becoming unseated’ (also known as binning it), I plan to grab some ink & lacquer from work and test all my cranks/stems/bars. Depending on how much I can get, I’ll offer to test anyone else’s parts, as long as you can get them to me (cleaned!).

If you catch the crack early, you can either destroy the part, or remove it with a burr grinder to prevent further growth and subsequent failure. I’d bin handlebars, but probably try and save cranks with small crack-like indications.

Lots of bike failures, many crank failures:

Another reason not to ride large OD, thin walled frames, buckling instability:

Doubly unfortunate, crank fails, rider maintains poise, then seatpost fails (I’ve had the seatpost bolt fatigue before, saddlebag kept it in place):


Sounds good Blakey, I’ll put my hand up for that!

(and I’m hoping that you don’t find anything wrong…)

From little cracks, big cracks grow…

Visual only for now. Dye pen to come.

Arrrgh, you are bring back memories of materials lectures, scary stuff for a chem eng.

Damn stress risers, personally never broken a crank and in no rush to find out. Pedal eyes are another well reported failure point, not good when foot and pedal drop off the end of your crank!

If you’re worried about this concept, stay away from conversions and track bkes. Buy something new??? If you don’t have the money, welll…

Well not sure who you are having a go at, but Blakey is offering to help people and I’m just ssaying that not every thing old was well designed, take the attitude that they have with planes and inspect stuff that is known to break.

New stuff is not infalible, there is a reason that there was two redisgns of the Isis crank followed by a new style of crank coinnections, as engineer I can say that engineers stuff shit up (and lots of it is bigger than a crank) google: tacoma narrows bridge!

I completely agree, Engineers have no fucking idea (never trust a professional with a Uni degree, useless/unless), My experience has been, either use "used’ parts and expect failure or use new parts and have a piece of mind that ‘stuff could happen’.

Glass half empty much? :roll:

My point was:
Ride it until it breaks, then replace! (I am so, Zen)

Well, you could also use used parts that are tested and known to be good, or known to have flaws but are still fit for service (AS3788/API579 for the enginerds) with an acceptable safety margin at original stresses or at reduced stresses.

Which is more reliable, a 20 year old part that is put into service and then inspected every 12 months with a method that has a 95% probability of detection of >0.5mm cracks [1], or a new part put into service and never inspected?

There are a lot of old parts with unknown history (For Sale: Record Pista cranks, only ridden to church on Sundays) out there, and I’d wager that the majority of us here are using old bits too. It doesn’t hurt to have a look occasionally and lower your probability of failure.

Edit: ‘Ride it until it breaks’ is all well and good, but sudden crank/bars/stem failure is probably going to experience final fracture under a heavy load, which generally means high speed/acceleration, so the consequences are likely to be nasty.

PS: Engineers are useless, they don’t understand how things are really done, I know. But we’re the ones that have to put our signature on paper and take responsibility if it doesn’t work. :roll:

Is it time for engineers to bitch about managers now? You know, not giving us enough funding or expecting unrealistic outcomes? :smiley:

All in fun!

[1] Numbers vary. Go ask an NDT tech

Engineers are only worried about their BAC! Comming from a ‘fucking’ manager!!! :wink: :smiley:

Blood alcohol content?

I think the engineers in Sakhalin have more of a problem with the BAC of their workers!

Crikey! I feel uneasy when the Westgate moves slightly underneath me… and people drove on that bridge in the 40’s with it moving like that!

Sorry Blakey, back to the cracks and dye… :slight_smile:

Not for long :stuck_out_tongue:

Yeah, even thought it must have looked stupidly dangerous, I suppose they were TOLD it was safe…

So are you an engineer Blakey? I’m studying engineering (just more people for CraigC to hate :-D)

This all get s pretty interesting, I think its a great idea to be able to inspect your own bits and pieces that are known to fail/crack from time to time, I guess the problem I have is that many bike shops already wont sell 2nd hand parts /bikes becasue they “dont know the history of the bike/ part” and therefore dont want to be responsible for any future failures.
So i guess my point is that this could be another measure that people (say on ebay/ other online forums) may use to prove that a product is structurally sound and give more reason for bike shops to just refuse to trade in any 2nd stuff as they wont be bothered to check each item, meaning that a lot of good sound stuff along gets chucked with the occasional suspect item.
Which can lead to people being channeled into buying a new bike (like the $59 dual suspension MTB landfill i saw at KMART last week seriously!) … getting of the topic I know… apologies for being long winded

Is it that obvious? (Materials/metallurgy, currently masquerading as structural)

There are only two kinds of people.

  1. People Craig hates
  2. People Craig has yet to hate


“There are only two kinds of people”.

  1. Engineers
  2. Non-engineers


Beat me to it.