Failed Disc

[Marchrider]

Evidently the irony of that statement is as lost on you as the bulk of guidance and suggestions contributed by other members..

I really don't know why you are saying this - Its a Public Enquiry into a bicycle part failure (what could be more important), any evidence needs to be peer reviewed and tested. There was a couple of different thoughts on the first page and they were adequately explored and debated as they should be.

I think it became clear early on that it was the correct size of rotor and the most likely cause of the failure was excessive wear. Rotor should have been replaced at 1.5mm but was still in use with parts of the rotor down to 1.1mm (thats 25% below its design capacity) and that combined with my brothers fast/aggressive riding style would be likely to cause failure.

And the size of the disc was absolutely confirmed with the picture of the 160mm rotor fitted into its caliper - it looked very much like 10mm short

The one thing I am still not happy about is the rate it has worn, apparently it was fitted last May, and I remember this as he had to have his wheel repaired just before his ride to here last summer (across Germany, France, thru Wales, across to Dublin, then Belfast then across Scotland, he knocks out the miles) and now I remember him saying the guy in the bike shop replaced the disc/rotor because he thought it was dangerous. He certainly does a very big milage (10-15k miles at a guess) but that shouldn't wear a rotor out

And I do know he changes pads as he raised an interesting question, why do Shimano always include 3 split pins in the pack? Now this makes me think, is the third pin just a spare, Or is there a third pin used on assembly that he does not know about.? (a split pin short of a full braking system!)

 

[JonBuoy]

As a point of reference regarding disc wear: The pair of discs in the photo that I posted earlier in the thread had done about 12000 km and were down to about 1.4mm(R) and 1.2mm(F). However this was on an MTB that gets used in a lot of slop. I have been through a fair number of sets of brake pads from various suppliers but haven't been counting them.

 

[silva]

In the context of a disc with a minimum recommended thickness of 1.5mm then 0.1mm is 6.67%, which is a pretty significant amount.

"Wildly off" is wildly off "pretty significant".

 

[rualexander]

"Wildly off" is wildly off "pretty significant".

Depends on your standards and the context.
If the Olympic 100m course turned out to be only 93m you could describe that as "wildly inaccurate".

 

[Ming the Merciless]

why do Shimano always include 3 split pins in the pack?

They don’t, at least for the ones I get for my Shimano XT hydraulics. Always one pin per pair of pads.

 

[silva]

Depends on your standards and the context.
If the Olympic 100m course turned out to be only 93m you could describe that as "wildly inaccurate".

Replacing a brake disc at a measured 1.6 mm that is actually 1.5 mm won't have dire consequences.

 

[rualexander]

Replacing a brake disc at a measured 1.6 mm that is actually 1.5 mm won't have dire consequences.

Never said it would, doesn't alter the fact that 0.1mm is a significant proportion of 1.5mm.

 

[silva]

Never said it would, doesn't alter the fact that 0.1mm is a significant proportion of 1.5mm.

A 0.1 mm tolerance on a range of 2.8 > 1.5 is more "wildly accurate" than "wildly off".
You declared it "significant" based on a comparison with a 100 meter sprint track length, which IS a "wildly off" comparison.

 

[figbat]

A 0.1 mm tolerance on a range of 2.8 > 1.5 is more "wildly accurate" than "wildly off".
You declared it "significant" based on a comparison with a 100 meter sprint track length, which IS a "wildly off" comparison.

This is just a matter of scale, to demonstrate the point. I would describe 7% as a pretty significant variance on any measure.

 

[Marchrider]

you also have to consider what you are measuring, in the case of a well worn disc/rotor it will not have a uniform thickness. micro warping (not enough to feel when braking) will have taken place and this will have led to some areas wearing more than others. My brother seems to have got various measurements between 1.1 & 1.3 (or something like that) So when these variations are occurring, is there any point in getting down to accuracies of 10's of microns

I wonder what the safety factor is in considering the limit of wear to be 1.5mm ?
if tests showed failure occurred around 1.0 then they would add a safety factor, maybe 50 % and get to the 1.5mm ? it would be interesting to know how this is calculated. some exponential factors probably ?

@Reynard - is this vaguely your area ? design limitations of metals ? calculating safety factors

 

[silva]

This is just a matter of scale, to demonstrate the point. I would describe 7% as a pretty significant variance on any measure.

It's not "just a matter of scale".
It's a matter of accuracy required to make a decision.
7% wear tolerance is not significant in a decision to replace or not replace a brake disc.
2.8 downto 1.5, or, downto 1.6, is not "wildly off", the accuracy suffices, by far, to decide.

 

[silva]

I wonder what the safety factor is in considering the limit of wear to be 1.5mm ?

Since a ring is as strong as its weakest spot, the logic is nowhere thinner than that.

 

[rualexander]

It's not "just a matter of scale".
It's a matter of accuracy required to make a decision.
7% wear tolerance is not significant in a decision to replace or not replace a brake disc.
2.8 downto 1.5, or, downto 1.6, is not "wildly off", the accuracy suffices, by far, to decide.

You're not making any sense unfortunately.

 

[wafter]

You're not making any sense unfortunately.

After wading through all nine pages of this thread, it seems nothing in the world makes sense any more..

 

[Marchrider]

Since a ring is as strong as its weakest spot, the logic is nowhere thinner than that.

I think this is very true, so if you measure a brake disk in 6 places then the minimum should be the considered measurement. and bear in mind that any inaccuracies in measuring (esp if using flat faced calipers) will err on the greater side - so if you are getting figures between 1.1 & 1.4 then assume the disc is below 1.1