Failed Disc

[Yellow Saddle]

Some people are their own worst enemies. Alloing a disc to get that thin is pure...let's say, something that sounds like liquidity.

That disc is probably 0.3mm thick, way, way way beyond the acceptable level. I'm pretty certain that bike has three or four other death traps too.

 

[wafter]

Looks very thin; and possibly also of sandwich format with an ally centre between two steel wear faces.

It does look like the swept area of the pads impinges on the arms at their ends, but I'm not sure if this was the primary failure mode. Thinking about how the forces act on the disc under braking (torque subjecting the disc to compression as it rotates into the restraint provided by the caliper and tension after it exits) I reckon the whole thing probably just got too thin, lost stiffness and buckled before it entered the caliper.

As for the swept areas of the arms; this would have to be a mis-alignment or manufacturing issue rather than sizing since they step up in 20mm increments on the OD, so the incorrect size would be very obvious.

 

[Drago]

A pre ride check would have saved him an emergency change of underwear. Old pilots, bold pilots.

 

[Marchrider]

It looks like the caliper was set incorrectly so the pads wore the bit where it snapped,

They do. Most commonly 140 or 160mm for road bikes, often larger for mountain bikes.

If you fit the wrong size, it either won't go into the caliper at all, or you may need to change the caliper bolts. It may be possible to fit a larger one without adjusting the calipers and have them catching the wrong place.

But the noise & vibration if the pads aren't pulling against the correct surface would be intolerable, would have thought, so that sounds unlikely.

OK, I put it to him the disc could be the wrong size, and here is the return email

When I was measuring the thickness of the old disc, I noticed it said “180 mm” on it … but the new one is 160 mm, and indeed is the one on my rear wheel (as per my spreadsheet). I now think that when I bought a new front wheel from a shop here in Munich (may 24) which came with a disc brake attached that the guy in the shop had sorted out for me, that I hadn’t realised the disc was a different diameter and I’ve been, indeed, using a 180 mm when I should have been using 160 mm. I never noticed.

I'm struggling to believe this was not obvious with some sort of knocking noise, when braking - he will of undoubtedly done a huge mileage since last may, it is his main transport and does massive tours - visits me from Munich 1000+miles away on his bike, knocks out 120-150mile day in day out sort of nonsense. - it couldn't have been that obvious riding, he would have known, but it should have been visually, but then he's not into mechanics and maintenance in the same way i am.

oh, and he had measure the thickness of the failed one at between 1.1 and 1.3 , the new one is 1.6 with a min thickness of 1.5 shown.

I think he has dodged a bullet wit this

 

[si_c]

do these discs come in different diameters ? could the wrong on have been fitted ?
(I know nothing about these discs, my bikes are on rims)



I have one of these for checking my rims

The disc is the right diameter, it's just way way way past the point it should have been replaced - the fact that the metal of the disc has folded like a bit of tape is a clear indicator.

I check mine periodically for thickness, 1.2mm is the thinnest I've ever been, I'd be surprised if that was even 0.5mm.

 

[Marchrider]

The disc is the right diameter, it's just way way way past the point it should have been replaced - the fact that the metal of the disc has folded like a bit of tape is a clear indicator.

I check mine periodically for thickness, 1.2mm is the thinnest I've ever been, I'd be surprised if that was even 0.5mm.

did you read the post above ? #19

 

[si_c]

did you read the post above ? #19

Nope. I don't think I saw the second page

 

[Marchrider]

I'm still not clear who fitted the wheel - my brother or the bike shop?

a bit worrying if it was the bike shop

 

[silva]

Lol it's like wearing shoe soles until walking bare feet like in the Flintstones.

 

[Yellow Saddle]

OK, I put it to him the disc could be the wrong size, and here is the return email


I'm struggling to believe this was not obvious with some sort of knocking noise, when braking - he will of undoubtedly done a huge mileage since last may, it is his main transport and does massive tours - visits me from Munich 1000+miles away on his bike, knocks out 120-150mile day in day out sort of nonsense. - it couldn't have been that obvious riding, he would have known, but it should have been visually, but then he's not into mechanics and maintenance in the same way i am.

oh, and he had measure the thickness of the failed one at between 1.1 and 1.3 , the new one is 1.6 with a min thickness of 1.5 shown.

I think he has dodged a bullet wit this

I smell a rat. The disc in question was ground down in a calliper set at the correct distance. That's what I see in anyway. A 160mm disc in a 180mm calliper will not grind down with a outer perimeter ridge like the one in the picture. Further, for him to have measured the disc he would have had to use a micrometer, since a vernier would only have measured the thickness of the ridge, not the friction surface. Someone with that little mechanical knowlege won't know how to use a micrometer, nevermind own one.

 

[Marchrider]

No - it was a 180 mm disc in a caliper fitted for a 160mm disc

the image seems to fit very well with what he is saying - it looks like the pad has missed the last 10mm of the disc and caught the last 10mm of the legs that hold the disc - seems very plausible with what he is saying
and to back that up the spec for the bike says it should be 160mm disc, and the one the bike shop fitted is 180 mm - no rat to smell

as for measuring - of course you can measure the disc with digital (or or vernier) calipers, just stick a washer or similar to each side of the shiny bit and bridge the edge. And with all those snapped off bits you probably don't even have to do that

and he may of even used a micrometer (and you get vernier gauges on micrometers), all that stuff will be in his workplace - He just has little to no interest in the mechanical workings of a push bike and gets the bike shop to most of it, And I would have full confidence in measuring something like that to 0.1 of mm

 

[Regular.Cyclist]

I fail to see the a 180mm disc would fit into a bike set up for a 160mm disc as it would foul on the rear of the caliper long before it would sit into the dropouts.

 

[Marchrider]

I fail to see the a 180mm disc would fit into a bike set up for a 160mm disc as it would foul on the rear of the caliper long before it would sit into the dropouts.

I will get him to send a picture of the new disc inside the caliper and we should be able to see if there was 10mm of space to accommodate a bigger disc
and if there was not enough space how has the pad missed the out perimeter and caught 10mm of the supporting legs?

 

[Drago]

I smell a rat. The disc in question was ground down in a calliper set at the correct distance. That's what I see in anyway. A 160mm disc in a 180mm calliper will not grind down with a outer perimeter ridge like the one in the picture. Further, for him to have measured the disc he would have had to use a micrometer, since a vernier would only have measured the thickness of the ridge, not the friction surface. Someone with that little mechanical knowlege won't know how to use a micrometer, nevermind own one.

Indeed. It smells fishier than Accy's home made apple crumble.

 

[wafter]

Indeed. It smells fishier than Accy's home made apple crumble.

F*ck's sake