The LBS said this was safe to ride...

[sl3pr]

2012-05-01 19.42.59.jpg
Here's the story: I brought my bike in for a tune-up and I asked the mechanic to check and make sure it was safe to ride. After servicing the bike, he said it was old, but in decent shape and didn't have any issues.

As I was walking out the door I noticed the crack in picture. I told the mechanic it wasn't there before, and asked him if I should be concerned, (especially since I'm roughly 250lbs). He said "you probably didn't notice it" and that I shouldn't be concerned. I'm no expert on bikes, and took his word for it. Stupid me.

I took it to another LBS the next day, and they said I should replace the crank arm immediately, and it is pretty dangerous.
The LBS who serviced the bike said they weren't at fault, and that there was nothing they could have done in their shop that would cause this type of damage.

Is this true??

[HillRider]

1. Yes, replace that crank arm IMMEDIATELY. I don't care if you weigh 250 or 110 pounds, it's not safe to ride.

2. The first LBS might have caused that crack by overtorquing the crank bolt if they checked the tightness as part of their inspection but it would require a lot of force unless the crank was already weakened.

[Sixty Fiver]

Find a new shop.

[bsektzer]

^^ This

[DannoXYZ]

Yes, you should replace the crankarm immediately. It's a very, very rare occurence that I've seen only 2x before during my 10-years working at a shop, then another 10-years racing.

There's anecdotal sayings going around that greasing the bottom-bracket spindle and repeatedly tightening the crankarm-bolt will push the crankarm up the taper and crack it. However, this was dispelled through empirical analysis and modeling by several mechanical-engineers who've published a lot of papers on bicycle applications. I've broken numerous crankarm-bolts well before the crankarms were pushed on anywhere nearly enough to split it. Also the crack is not at one of the corners of the square-hole in the crankarm; where it would most likely split first.

Replace it immediately. Then never go back to that 1st shop ever again. Perhaps even take photos and show everyone you know as there may be others who have gotten dangerous advice from that shop.

[Burton]

So what exactly are we looking at?

If thats just a crack in the plastic wrapping of a steel pedal arm the the first shop is correct - its perfectly safe. If its an alloy arm ( doesn't look like one) then its a different story.

[Bianchigirll]

So what exactly are we looking at?

If thats just a crack in the plastic wrapping of a steel pedal arm the the first shop is correct - its perfectly safe. If its an alloy arm ( doesn't look like one) then its a different story.

I thought of that for a seccond then saw your post. I kind of agree it looks like that cheap silver plastic covering.

[strock]

So what exactly are we looking at?

If thats just a crack in the plastic wrapping of a steel pedal arm the the first shop is correct - its perfectly safe. If its an alloy arm ( doesn't look like one) then its a different story.

The crack appears to run through a dimple near the BB shell. Certainly that end of the crank arm is out of round. This might support your interpretation and perhaps suggests that the damage is due to some sort of impact.

[xenologer]

In my own experience, overtorquing a crank bolt does not crack the crank; the bolt breaks off first.
With repeated removal and retightenings however, the taper hole does continue to widen and deform, eventually leading to failure of the crank arm; this is not from just a single install though.
Similar wearing can be seen on the pedal eye threads of the crank arm... With that in mind I consider cranks to be disposable wear parts (like brake pads and rims).

So, I would say you can't fault the 1st shop for causing it to break, it was either old and it was finally its time to go, or you really never noticed a crack forming over time.
Their advice that it is safe to ride though is definetly incorrect and irresponsible. So find a different lbs who's more safety informed.

[DannoXYZ]

It's a painted alloy arm. Something like 105 series. The rear-surface of the arm doesn't appear to be parallel with the BB-cup lockring. It may have been damaged previously, cracked, and re-installed.

The crack may have started at a weak spot in the material, porosity is common with poorly-cast parts. Could also have been some sort of impact that notched the surface, leading to a stress-riser.