Hairline crack in crank...

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Rabbit
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Hairline crack in crank...

Post by Rabbit »

I've just bought a Campagnolo Super Record chainset for my Mercian rebuild and on cleaning it noticed a tiny hairline crack on the V of the spider arm - Arggh! I have read that these can be filed out to prevent them spreading but not sure about how or whether to do that. Any advice - or anyone down Hampshire/Dorset way who might be willing to have a quick look and see if it can be saved or is one for the bin? Thanks -
Brucey
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Joined: 4 Jan 2012, 6:25pm

Re: Hairline crack in crank...

Post by Brucey »

many of them are like that (*) which is why buying in person is the best way with these things.

If the crack is small it can be filed out, but if it is larger it is (barring adventurous weld repairs) a scrapper.

photos?

(*) back in the day, several of my racing chums wouldn't ride more than one season on campag NR/SR cranks because they knew they were likely to break.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~
pliptrot
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Re: Hairline crack in crank...

Post by pliptrot »

Brucey wrote:back in the day, several of my racing chums wouldn't ride more than one season on campag NR/SR cranks because they knew they were likely to break.
My goodness. That is quite a shocker. Don't let onto the retrobike forum lot.
Rabbit
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Joined: 19 Aug 2012, 10:43pm
Location: The New Forest

Re: Hairline crack in crank...

Post by Rabbit »

Thanks - I'm embarrassed to say I did look at the chainset before I bought it AND I did look for cracks because I heard they could happen... must have just missed it! Thought it was a grease smudge but turns out it's not - inexperienced eyes, I suppose. It's only about 3mms long right on the inside of the V where the right hand crank connects. Will try to take a picture so you can see.
Brucey
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Re: Hairline crack in crank...

Post by Brucey »

In all fairness, the 'design spec' for these cranks -of there was one- would have been 'last a season's racing, then another year on the hack bike'. Any more weight/durability than was required for this would arguably have been too much. [I'm sure the typical british winter conditions don't help either; the first thing to crack is the anodised layer, and corrosion is therefore concentrated at the point of highest stress, which helps the crack along nicely.]

However the design is also rather flawed, both in conception and execution. In conception, the web where the crank joins the spider arms; the sharp edge at this point is guaranteed to crack (I've yet to see one that was well used which wasn't cracked here, and I've had to file every set I've owned). In execution because the cranks can also break via cracks initiating elsewhere. Typical locations for crack initiation are;

- the spider web (as above)
- the stamp marks on the back of the crank
- the machining marks in the flute near the pedal eye
- the stamp marks round the pedal eye
- (very rarely) the square taper points on the LH crank
- (some models) the stamp marks/flute ends near the middle of the crank.

Campagnolo copies obviously suffer in the same way, at least as badly. I've also seen other cranks crack in the same places, including shimano/Dura-Ace ones.

It is worth mentioning that the efforts of racing cyclists can be very much more destructive than those of typical touring cyclists (unless the latter are strong and ride hammer and tongs everywhere, which I'm sure some do). 10000 miles at (say) 150W average, 500W peak is probably not even half as damaging as the same distance at 250W average, 1000W peak.

If the crack is in the web and 3mm long it should be possible to file it out BTW.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~
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Si
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Re: Hairline crack in crank...

Post by Si »

Brucey wrote:
(*) back in the day, several of my racing chums wouldn't ride more than one season on campag NR/SR cranks because they knew they were likely to break.

cheers


It was plain Campag Strada that did for me....despite checking for cracks... :x
Rabbit
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Joined: 19 Aug 2012, 10:43pm
Location: The New Forest

Re: Hairline crack in crank...

Post by Rabbit »

I'm thinking perhaps I should go for a different chainset - but they look so damned nice!!
daddig
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Re: Hairline crack in crank...

Post by daddig »

Try polishing using either a Dremmel or similar , or wrap file in smooth emery and use that. Keep the area around the crack as smooth as possible.
Mike G
mig
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Joined: 19 Oct 2011, 9:39pm

Re: Hairline crack in crank...

Post by mig »

re. the design spec of lightweight chainsets. in the bigger scheme of things how much advantage does losing a few (hundred) grammes worth of weight from a chainset really offer? (excluding the tour riders)
thirdcrank
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Re: Hairline crack in crank...

Post by thirdcrank »

pliptrot wrote: ... My goodness. That is quite a shocker. ...


Don't forget that in those days, it's widely reported that the Campag van was there at all the big events freely swapping equipment for the big teams. There was one incident of the "Rolls Royce axles never break" type - and if I could remember the name of the rider involved I'd look for a link - when there was a dramatic failure of a Campag crank which was speedily replaced and then disappeared.
LWaB
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Joined: 26 Nov 2010, 5:33am

Re: Hairline crack in crank...

Post by LWaB »

Laurent Fignon (pre-Tour de France days) lost Paris-Tours due to a broken Campag Ti BB axle. The next year, the Renault-Elf team wasn't using Campag.

Back in the day, Campag had a generous warrantee for all their customers. Not any more.
Brucey
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Joined: 4 Jan 2012, 6:25pm

Re: Hairline crack in crank...

Post by Brucey »

This is Fignon sat on the roadside instead of winning Paris-Tours

Image

you can see the remains of a Campag SR left crank and broken Ti BB spindle lying right there. Apparently there was an incident in the Giro that year, too.

If you make every part on a bike 10% heavier you might go 1% slower which is no big deal until you lose a race by a fraction. Go too far the other way and you don't get to the end of the race...

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~
Colin Stanley
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Re: Hairline crack in crank...

Post by Colin Stanley »

Any photos guys that would indicate the crack initiation areas please?
Brucey
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Re: Hairline crack in crank...

Post by Brucey »

the 'spider web' crack goes like this;
Image
http://materials.open.ac.uk/mem/mem_ccf4.htm

if left long enough. The initiation point is at the bottom right of the fracture in this image. Such cracks are easily seen on clean crank if you can be bothered to look.

Lots more busted campag cranks here;

http://pardo.net/bike/pic/fail-001/FAIL-008.html

and here

http://pardo.net/bike/pic/fail-001/FAIL-035.html

etc

cheers
-------------------------------------------------------------------------------
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~
Colin Stanley
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Joined: 12 May 2007, 7:05pm
Location: Somewhere in Kent

Re: Hairline crack in crank...

Post by Colin Stanley »

Thanks for the links to the photos.
Well I never did, and I hope I never have these problems as I just tend to sit and pedal at my age. However being an old engineer who was used to introducing a fillet radius on his component designs, I might get a dye penetrant kit - just in case. Interesting that some cranks have failed along their length with no obviously apparent stress raiser. On highly stress parts (steel), we used to shot peen them to put the outer fibres into a compressive state to reduce chance of failure. Never attempted this with aluminium alloy forgings.
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