How do I check for cracks?

For discussions about bikes and equipment.
johnmac
Posts: 510
Joined: 19 Jan 2007, 9:45pm

How do I check for cracks?

Postby johnmac » 7 Jul 2013, 9:46am

Recently, we've seen a photo of a broken crank arm. A sudden failure is not a nice prospect, so I think I need to check. Am I right in thinking that in industry, dye is used to reveal cracks? And what about painted parts, like forks?

Any advice on how to check for cracks would be appreciated.

Brucey
Posts: 39495
Joined: 4 Jan 2012, 6:25pm

Re: How do I check for cracks?

Postby Brucey » 7 Jul 2013, 9:59am

see

http://www.rotax-owner.com/pdf/spotcheck2.pdf

also a wikipedia article on dye penetrant testing.

BTW most cracks that might be dangerous in cranks are visible without dye-pen, but you need to look carefully.

cheers
-------------------------------------------------------------------------------
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

johnmac
Posts: 510
Joined: 19 Jan 2007, 9:45pm

Re: How do I check for cracks?

Postby johnmac » 7 Jul 2013, 11:57pm

Thanks Brucey. Do you use dye, or do you just have a close look?

Brucey
Posts: 39495
Joined: 4 Jan 2012, 6:25pm

Re: How do I check for cracks?

Postby Brucey » 8 Jul 2013, 2:35am

I usually just look carefully, but sometimes scratches look like cracks and vice-versa. I will use a magnifying glass or binocular microscope to have a closer look, and if I'm still not sure, I'll use dye-pen.

Steel frames -and in fact many other painted parts- have the distinct advantage that if there is a crack in the part, the paint will usually show a clear crack also. This does require that you actually bother to look.... but such cracks are far easier seen in light coloured parts than dark colours.

Some folk prefer white framesets because of this. Jobst Brandt has only ridden yellow bikes for years now; same reason.

Having recently had a dark coloured frameset break on me, I am increasingly in favour of this approach myself. Disturbingly, it seems that the crack in my failed frame may have started in behind where some cables run, so I'd not have seen it unless I'd have moved the cables to one side. The frame in question was if anything, a 'mileage minnow' and shouldn't have failed. It just goes to show you can't take anything for granted.

Stuff like pedal spindles, BB spindles, some rear axle designs, steerer tubes etc need special attention; these parts can all crack in unseen places, and if they fail in use, the consequences might be severe. On well-used bikes I'd suggest annual stripdown and inspection of these hidden parts where possible.

cheers
-------------------------------------------------------------------------------
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

User avatar
NATURAL ANKLING
Posts: 11803
Joined: 24 Oct 2012, 10:43pm
Location: English Riviera

Re: How do I check for cracks?

Postby NATURAL ANKLING » 8 Jul 2013, 9:47am

Hi,
One way of doing this I have been told is to emerse the arm in parrafin over night.
Then degrease a dry rag will suffice, with a quick wash in soapy water, dry again with rag.
Then sprinkle with talculm powder and wait.
Any cracks will show as a dampining of talc, I think bread flower would work too :?:
NA Thinks Just End 2 End Return + Bivvy
You'll Still Find Me At The Top Of A Hill
Please forgive the poor Grammar I blame it on my mobile and phat thinkers.

Brucey
Posts: 39495
Joined: 4 Jan 2012, 6:25pm

Re: How do I check for cracks?

Postby Brucey » 8 Jul 2013, 10:56am

the correct dye-pen fluid is essentially a light paraffin, but with a strong dyestuff in it to make it easier to see.

cheers
-------------------------------------------------------------------------------
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

drossall
Posts: 4847
Joined: 5 Jan 2007, 10:01pm
Location: North Hertfordshire

Re: How do I check for cracks?

Postby drossall » 8 Jul 2013, 10:19pm

I've told this story before, but, years ago, as a physics student, I broke a crank in a team time trial. Luckily I was on the back at the time. Also luckily, I was at Leeds, only a short ride from Ron Kitching's at Harrogate, the Zeus importer.

My final-year project supervisor enjoyed his mostly-utility riding. I must have shown off my wounds or something, and he was fascinated by the concept of a human being able to break a crank. (I knew it was not uncommon.) He sent me to a friend who lectured in mechanical engineering.

The friend examined the broken crank, and did a dye test on the other one, which he proclaimed sound. I broke it some months later, riding home from visiting my future wife. This time, I knew the signs and did not fall off.

I'm still riding the two replacements, thirty years later, though that bike doesn't get used so much now.

User avatar
NATURAL ANKLING
Posts: 11803
Joined: 24 Oct 2012, 10:43pm
Location: English Riviera

Re: How do I check for cracks?

Postby NATURAL ANKLING » 8 Jul 2013, 11:00pm

Hi,
I dont think breaking a crank is a virtue ( no disrespect ) more fatique of poor design / material / manufacture.
I dont agree that a peculiar power stroke is responsible either.
NA Thinks Just End 2 End Return + Bivvy
You'll Still Find Me At The Top Of A Hill
Please forgive the poor Grammar I blame it on my mobile and phat thinkers.

Brucey
Posts: 39495
Joined: 4 Jan 2012, 6:25pm

Re: How do I check for cracks?

Postby Brucey » 8 Jul 2013, 11:05pm

NATURAL ANKLING wrote:Hi,
I dont think breaking a crank is a virtue ( no disrespect ) more fatique of poor design / material / manufacture.
I dont agree that a peculiar power stroke is responsible either.


I agree that not all cranks are created equal breakage -wise, but re the pedal stroke; the peak load has the biggest effect on fatigue damage. Plot force vs distance and the area under the curve is the work done. A smooth pedal stroke can have a low peak load vs a jerky pedal stroke, but be more powerful. The jerky pedal stroke can break the crank more quickly though, if the peak force is higher.

cheers
-------------------------------------------------------------------------------
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

johnmac
Posts: 510
Joined: 19 Jan 2007, 9:45pm

Re: How do I check for cracks?

Postby johnmac » 9 Jul 2013, 7:58am

If you were to replace components on a regular basis to guard against fatigue induced failure, maybe it would make things worse, as eventually you may end up buying a faulty component?

Brucey
Posts: 39495
Joined: 4 Jan 2012, 6:25pm

Re: How do I check for cracks?

Postby Brucey » 9 Jul 2013, 9:05am

I would say that in the case of cranks, there are many, many more that start to crack because they have been damaged (scratches, corrosion etc) than because there is a tiny flaw in the bulk of material itself.

There are (or more accurately, also, were...) lots of crank designs out there which had bad stress raisers in them by design; used Campagnolo Nuovo Record cranks (and other similar designs) can pretty much be relied upon to be cracked in at least one place unless modifed. [ Back in the day many hard-riding racers would change their cranks every year regardless, just because they knew that they were liable to break otherwise. If they did another year on the training bike it was a bonus, but I saw plenty of two or three year old cranks break on training bikes. Cranks are generally better designed these days, fatigue-wise...]

My preference is for cranks that have natural polished aluminium finish, or failing that are plain anodised. In either case cracks will be seen fairly easily when cleaning the bike, well before they are likely to cause a breakage.

I have one set of well-used cranks where the finish is slightly matte; I plan to polish these cranks before putting very many more miles on them, since otherwise I won't easily see any cracks that might develop.

I don't think I've ever seen a broken (polished or anodised) crank where the crack couldn't have been seen well beforehand, if only someone had looked.

cheers
-------------------------------------------------------------------------------
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

mig
Posts: 2228
Joined: 19 Oct 2011, 9:39pm

Re: How do I check for cracks?

Postby mig » 9 Jul 2013, 10:29am

drossall wrote:I've told this story before, but, years ago, as a physics student, I broke a crank in a team time trial. Luckily I was on the back at the time. Also luckily, I was at Leeds, only a short ride from Ron Kitching's at Harrogate, the Zeus importer.

My final-year project supervisor enjoyed his mostly-utility riding. I must have shown off my wounds or something, and he was fascinated by the concept of a human being able to break a crank. (I knew it was not uncommon.) He sent me to a friend who lectured in mechanical engineering.

The friend examined the broken crank, and did a dye test on the other one, which he proclaimed sound. I broke it some months later, riding home from visiting my future wife. This time, I knew the signs and did not fall off.

I'm still riding the two replacements, thirty years later, though that bike doesn't get used so much now.


i was thinking that the story would end.. "and ron kitching welded it and i got back on to finish in a course record time..." :shock:
the only crank i've ever broken was, i think, an SR which sheared of flat near the pedal end approximately 3mm before the hole for the pedal. cruising downhill at the time so just got a bit of a wobble and a "what on earth!" type feeling.

drossall
Posts: 4847
Joined: 5 Jan 2007, 10:01pm
Location: North Hertfordshire

Re: How do I check for cracks?

Postby drossall » 9 Jul 2013, 1:16pm

Ah sorry, no, the time trial was in Staffordshire. We were left stranded nowhere in particular, so rode one-legged (in my case) to the next marshal. He offered his bike, but we were ten or fifteen minutes down by then. I went to Ron Kitching's later.

I've known various brands of crank to break, usually (as with both Zeus ones) at the pedal eye.

rjb
Posts: 3844
Joined: 11 Jan 2007, 10:25am
Location: Somerset (originally 60/70's Plymouth)

Re: How do I check for cracks?

Postby rjb » 9 Jul 2013, 10:30pm

Have a look at these. Shows some of the things you can check.
http://www.m-gineering.nl/techdexg.htm
Click on the "oops" link in the technical section on the left pane.
:shock:
At the last count:- Peugeot 531 pro, Dawes Discovery Tandem, Dawes Kingpin, Raleigh 20, Falcon K2 MTB dropped bar tourer, Longstaff trike conversion on a Falcon corsa. :D

Binkyboy
Posts: 83
Joined: 29 Oct 2010, 5:05pm

Re: How do I check for cracks?

Postby Binkyboy » 10 Jul 2013, 12:59pm

I have successfully used the soak in paraffin and then wipe it off then dust with talcum powder version before.

The paraffin seeps into even the tiniest crack and the talcum powder will draw it out and stick to the crack and show as a dark/damp line.