Life Span of a Chainset Spider & Cranks?

[Claud]

My Stronglight Impact chainset is ~15 years old with 35-40k miles on the clock, just replacing the rings as needed. I know alloy fatigues and can occasionally catastrophically fail. My question: Would any such failure usually be preceded with visible hairline cracks? So practically, is regular close inspection good enough, or should I just decide at some point that the spider and cranks are too old and replace the lot?

[Pebble]

mine is 32 year old with 80k miles, don't think I have ever inspected it - you have me worried now.

[PT1029]

Clean your bike (or cranks) on a reasonably regular basis. Once clean give the cranks a look over for cracks.
On vary rare occasions cranks just break, complete brittle fracture, with no warning.
However, "no warning" doesn't (usually) mean there were no signs, it is just the owner did not see the warning. Most items that break have been cracking for some time, once say 1/3 has cracked through, the remaining 2/3 can't take the strain. When it does break, 2/3 of the fracture surface will be clean and bright. The other 1/3 will be dull, as it has been cracked for some time, and the clean cracked surface has had time to oxidise to a dull grey.
While riding cracked cranks is a bad idea (they will break under maximum stress, ie, when you are pushing hard, and so will deposit you in the road, usualy with a big gouge in your leg). Having said that, I have seen some vey badly cracked cranks being ridden, the owner being unaware they were cracked. The message here being if you inspect regularly, you will detect a crack well before its crack by date.

Crack or scratch? A crack will be VERY narrow (to start with at least), and invariably will be a finely wiggly/jagged line. Scratches tend to be wider and straighter.

crack.jpg (13.04 KiB) Viewed 456 times

In the photo, the crack started from the bottom, then it failed suddenly.

If worried, there is a dye penetration method for crack detecting.

[fastpedaller]

I had a pair of Stronglight 49D cranks on my fixed-wheel bike - they had been in service at least 10 years. I cleaned the bike and polished the cranks - they did look lovely! The next day I rode a few hundred yards to post a letter, and whilst doing so, looked down and thought "Ha Ha a hair on my crank" and attempting to remove it, I realised it was a crack! Frightening that one can appear so quickly..Whilst stationary I gave it a good push with my foot (in hindsight maybe not sensible?) It didn't break, but I didn't go riding that day, and changed the cranks before using the bike again.

[andrew_s]

Just carry on riding it.
The Stronglight Impact (a rebadged Sugino XD2) is at the tough end of things, and 35-40k miles isn't all that high a mileage.

Even if it does crack, it's not necessarily catastrophic - none of my 4 failed cranks were.

XT & Middleburn - cracked through to the taper and started making creaking noises (don't ignore them).
TA Zephyr RH - a bolt tab came off its spider arm.
TA Zephyr LH - the pedal hole cracked through to the outside, and the crack opened enough to allow the pedal to hop a couple of threads so it was at ~80° to the crank arm

[Claud]

Thanks all - reassured to continue with the 'regular checks' method

[Claud]

35-40k miles isn't all that high a mileage.

Yeh, fair enough, some folk do that every year