In this thread we've found about 4-5 cases while on the other hand there are over 1.000.000 Bromptons out there. I would not call this a spectacular failure rate. Even less so when having in mind that the cranks should be changed after 5.000 miles and most (if not all) of those that snapped were way over that limit.
How many bikes do you know that have over 1.000.000 samples in the wild?
Would you complain if you bought a Diesel car, ignore the change interval for the belt, and complain when it collapses (and trashes your motor) that they were using "cheap parts"? Possibly not...
I'm not claiming it to be a spectacular failure rate; I am however questioning whether it's not still significant since no other brand appears to have a reputation for crank breakage in this way. Granted Shimano have an issue with some of their higher-end Hollowtech cranks failing; but this is through a bonding issue rather than outright material failure.
I suspect the number of cranksets in use from Shimano is orders of magnitude higher than the million Bromptons you reference.
My issue is with Brompton essentially specifying the crankset as a consumable, when no other manufacturer seems to consider them in this regard. On the one hand I respect their candour, on the other it could be argued that they're only doing so to cover their arses for supplying parts that fail before many would expect them to.
I imagine they've consciously kept the arms slim to keep mass down, which in turn has reduced their stiffness and predisposition to failure. One might postulate that they've pushed this relationship unacceptably far, considering that stiffness rises (and stress falls) to the fourth power of the beam thickness and reducing stress massively increases fatigue life.. so for a modest increase in size, mass and cost they could have made the cranks last basically forever.
I don't consider your Diesel engine example legitimate since a cam belt is clearly a higher-stressed, cheaper, obviously consumable part which you'd struggle to increase the service life of through design changes (other than swapping it for a chain!). I'd suggest a more appropriate example would be having to replace the pistons in an engine after 50k miles on the manufacturer's advice; when everyone else's pistons last for the lifetime of the vehicle.
They should be cheaper than they are but at the same time you complain that the parts used are not expensive enough - while at the same time there is the t-line available with way more expensive parts but you did not go for it but bought a second hand C-line. Isn't that a bit of a weird combination?
Again, a straw-man argument that simplistically, naively assumes the cost of an item is directly representative of its quality.
I'm suggesting that the bikes offer poor value when compared to others - being over-priced for what they are and suggesting that either the price should be lower, or the components better quality than they are.
I bought the bike I did as it was as much I could afford, and I expect it to be fit for purpose. I didn't buy a T-Line because unfortunately, somewhat obviously I don't have
over four thousand pounds to spend on a bicycle 🙄
EDIT: Further from some brief fag-packet calculations, increasing the thickness of the crank arms by 20% in both planes would have more than doubled their stiffness; which for a given crank load would have more than halved stress. From the graph of no. cycles versus load for a decent alloy posted in Kell's other thread on the subject, halving the stress would potentially increase the service life by a factor of hundreds or even thousands - making the cranks essentially "fit and forget" like everyone elses; rather than something that requires regular replacement at a cost not insignificant to the value of the bike itself.
