My son came home injured yesterday after flying off his Brompton when the crankarm snapped at the pedal eye.
Is this a known issue, or a fluke?

Not seen that before.
I seen cotters shred,square tapers come off, shafts snap,cranks bent inwards (after fall).

You would imagine the crank would bend on its long lever as opposed to fracturing at the end.

Manufacturing fault??

I would say fluke. I have had one fail the same way on a 1970 Raleigh Sports, 25 years ago. Take a close look at the fracture area, better yet post a couple of pictures. It may have had a defect in the forging or a crack may have developed. FWIW in 35+ years of wrenching I have not seen more than a couple fail that way.

Aaron

Aluminum cranks, some times do that regardless of brand..

scratches become 'stressrisers' and the cracks develop as the metal is flexed.

Right or Left arm?

I find close inspection helps, to catch cracks before they become fractures.

I found a stress crack developing in my folding pedal
I got a friend with a Macro focus camera to highlight the crack , and the company
supplied a replacement pedal, through the Post, Direct.

Like it was mentioned -- left or right? The left pedal is the weakest due to it being fold-able. Was you son standing on the pedals while riding at the time? Sometimes I wonder if folding pedals can handle a lot of the stress put on them, especially if you are standing on them. Much more pressure is applied in the power or downward stroke, so to speak, when standing.

I hope you son is alright.

Cheers
Wayne

In the recent thread

https://www.bikeforums.net/showthread...on-pedal-broke

it was actually the RH pedal that broke. The design flaw was introduced, since corrected, by trying to make that pedal lighter. Ourselves we actually changed the RH pedal in our Brompton to a folding one that lacks the particular flaw.

I've not heard of this type of failure on the Brompton crank arms, and looking closely at the threaded holes where the pedal spindles screw in, there is no apparent difference in the amount of material or thickness between the left and right crank arms. Whether the pedal is folding or not shouldn't affect the load or stress on the threaded hole and surrounding material.

I'm guessing the cause is a manufacturing defect rather than a design flaw. Can you post close-up photos of the failure?

EDIT - Reading the OP, I thought it was the arm that broke at the threaded hole, not the pedal. If it was the folding pedal that failed, I can see it being a design flaw.

The boy was not doing urban skate bowl X games tricks on it like a BMX bike was he?

we have seen the videos of some treated like that. has to have long term repercussions ..

Another possibility: if the pedal spindle wasn't torqued properly into the crank arm and the threads backed out two or three turns, I could see the crankarm snapping at the pedal eye.

I've see a lot of scary gouges in crank arms caused by the typical sharp edges on the "wrench flats" of the pedal spindles, and have wondered why we don't see more reports like this. I always use pedal washers (contoured side toward the crank arm) and either Loctite or copper anti-sieze depending on mood (I've been trying to figure out which is best for 20+ years) when installing pedals.

John

The crank snapped on the drive side crank arm pedal eye while he was standing on the pedals. He's got a short ride and likes to go fast.
From the dark spot on the cracked surface in the attached photo, its obvious that the crack started much earlier than the break. Close inspection would have prevented the accident. But my son is not a bike rider, he's a commuter ... he bought the bike new hoping that it would be trouble-free for years to come. (I should mention that he bought it new after many years of riding old fixer-uppers that I gave him). This problem may be a fluke but it should never happen on a new (1 year old) bike !

Ok, it's the right side.. Contact the dealer, there, you bought through, + UK manufacturer,

with your picture, it does not require the resolution I had to find to show cracks 10mm long..


complaining here is not going to bear fruit. ( other than, I could sell you the one I took off)
not much possible resolution from others half way around the globe, .. will agree it's rare.

but selling millions of any thing. 1 in 10000 is not 0 .



you don't have to use a Brompton, crank I took mine off,
and fitted a Swiss made 2 speed, internal gear crankset.

Possible non direct replacements ..

you might get a track crank. or BMX, another single speed,
If the 3 speed then the 1/8" chainring.. if the 6 speed, then a 3/32" thick chainring..

consult, in person, with a bike shop, there, about the compatibility..

I would contact Brompton and let them know about it. They should know about defects in their products.

Looks like a defective part. Contact the retailer you bought the bike from and see if there's any warranty coverage. For parts, usually 1 year from purchase, so depending on if you are over or under a years, you maybe out of or in luck.

Where that dark area is showing should nudge them in the direction of replacing under warranty.

Unlike some warranty situations where replacement is due to operational or even cosmetic issues, this is clearly a safety issue and someone along the line should go out of their way to make this right for you.

What brand of crank?

AFAIK Brompton has Stronglight in France Make their Cranksets,
at least the ones with the swaged chainring/arm connection.

1 Year Old that's probably what was fitted..

(who makes their new Spider crank , could be the same supplier)