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Out at the weeekend with a bunch of Scouts, we had oppurtunity to use
our first aid skills, after one of them grabbed a big handful of front brake and went over the bars. He had been using his rear cable operated disc to slow down on a steep downhill. He said there was a bang and the brake stopped working, so he grabbd the front. Later examination showed that the moving pad was, say, 1mm from the disc. The Book of Words says it should be 0.3mm. (I Phoned a Friend so he could look it up online). This meant the actuating arm hit the stop before full force could be put on the disc. Later that evening by a bit of front rear pad swapping and adjusting I got both brakes working again. However the next day,I noticed the wheel was rubbing in the frame as he pedalled. This in a bike with vertical dropouts. Further examination showed the rear axle wwas broken. It was held together by the QR spindle. The wheel was not rubbing the previous day as he cycled up Box Hill. 1) Presumably the bang the lad heard could have been the axle breaking. 2) I'm trying to work out the mechanism which leads to a broken axle stopping the brake working properly. The disc only need to be a gnat's crotchet off line, but I can't see how the two are linked. Ideas? -- Tim |
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#2
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Tim Hall wrote:
2) I'm trying to work out the mechanism which leads to a broken axle stopping the brake working properly. The disc only need to be a gnat's crotchet off line, but I can't see how the two are linked. Ideas? How about the axle shears in torsion. The brake _works as intended_, (or snags - but I don't know if that can happen) retarding force is transmitted via the disc to the axle. The bike and so the wheel wants to keep on going providing the equal and opposite. BANG! Once broken there is no way for the retarding force to get transmitted through the axle to the wheel. Hence the "it stopped working". I think the pad gap was a red herring. As I have never had disc brakes this is pure theoretical speculation. -- Peter 'Prof' Fox Multitude of things for beer, cycling, Morris and curiosities at http://vulpeculox.net |
#3
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On Tue, 16 Jun 2009 08:43:53 +0100, Peter Fox
wrote: Tim Hall wrote: 2) I'm trying to work out the mechanism which leads to a broken axle stopping the brake working properly. The disc only need to be a gnat's crotchet off line, but I can't see how the two are linked. Ideas? How about the axle shears in torsion. The brake _works as intended_, (or snags - but I don't know if that can happen) retarding force is transmitted via the disc to the axle. The bike and so the wheel wants to keep on going providing the equal and opposite. BANG! ITYF that the broken part, as described, is more of a spindle than an axle, in that the rear hub, onto which the disk is mounted, is rotating, via bearings, on the 'axle', which is stationary. So it's unlikely to have happened as you suggest :-} The only thing I can think of is that the dropouts themselves are so weak that they were twisting under braking, causing a sheer stress on the wheel spindle. Sounds unlikely, but if it's a cheap bike, as suggested by the cable-operated disk brakes, then perhaps the materials of the frame and the spindle were themselves of inferior quality. Once broken there is no way for the retarding force to get transmitted through the axle to the wheel. Hence the "it stopped working". I think the pad gap was a red herring. More likely the broken spindle caused the entire wheel to skew, which then created the gap and stopped the brake working further. |
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On Tue, 16 Jun 2009 10:46:48 +0200, Ace wrote:
snip ITYF that the broken part, as described, is more of a spindle than an axle, in that the rear hub, onto which the disk is mounted, is rotating, via bearings, on the 'axle', which is stationary. So it's unlikely to have happened as you suggest :-} Exactly. The only thing I can think of is that the dropouts themselves are so weak that they were twisting under braking, causing a sheer stress on the wheel spindle. Sounds unlikely, but if it's a cheap bike, as suggested by the cable-operated disk brakes, then perhaps the materials of the frame and the spindle were themselves of inferior quality. It wan't that cheap a bike, although your point about cable discs rather than, say, hydraulic discs is well made. Specialized something or other. Probably the best bike amongst those the Scouts were riding (although another lad did have something fitted with a Brooks saddle) Once broken there is no way for the retarding force to get transmitted through the axle to the wheel. Hence the "it stopped working". I think the pad gap was a red herring. More likely the broken spindle caused the entire wheel to skew, which then created the gap and stopped the brake working further. I think so, but with the spindle (not axle, as you point out) in compression so I was half thinking that would be sufficient to keep everything in the right place. Obviously not. -- Tim |
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On Tue, 16 Jun 2009 08:43:53 +0100, Peter Fox wrote:
Tim Hall wrote: 2) I'm trying to work out the mechanism which leads to a broken axle stopping the brake working properly. The disc only need to be a gnat's crotchet off line, but I can't see how the two are linked. Ideas? How about the axle shears in torsion. The brake _works as intended_, (or snags - but I don't know if that can happen) retarding force is transmitted via the disc to the axle. The bike and so the wheel wants to keep on going providing the equal and opposite. The axle does not rotate. The disc and the wheel rotate in unison around it. Axles fail from bending loads. |
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On Tue, 16 Jun 2009 10:17:13 +0100, Tim Hall
wrote: On Tue, 16 Jun 2009 10:46:48 +0200, Ace wrote: The only thing I can think of is that the dropouts themselves are so weak that they were twisting under braking, causing a sheer stress on the wheel spindle. Sounds unlikely, but if it's a cheap bike, as suggested by the cable-operated disk brakes, then perhaps the materials of the frame and the spindle were themselves of inferior quality. It wan't that cheap a bike, although your point about cable discs rather than, say, hydraulic discs is well made. Specialized something or other. Probably the best bike amongst those the Scouts were riding (although another lad did have something fitted with a Brooks saddle) I guess the key question is what type of hub it was. If it's anything half-decent he might be well advised to send it back to the manufacturer, via a sympathetic LBS if available, as it sounds like it must have been a manufacturing fault. More likely the broken spindle caused the entire wheel to skew, which then created the gap and stopped the brake working further. I think so, but with the spindle (not axle, as you point out) in compression so I was half thinking that would be sufficient to keep everything in the right place. Obviously not. Yes, I'd have thought so too, but if the broken spindle had moved internally, the cones on which the hub is mounted would also shift, so there would be a twisting motion which the Q/R is probably not designed to deal with. |
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The axle does not rotate.
The disc and the wheel rotate in unison around it. Axles fail from bending loads. My learned friend is correct. I was trying to use some lateral thinking where the OP had all the bits in plain view and still couldn't connect suggesting to me a non-mechanic with a genuine puzzle. At 2nd hand I wondered if "axle" was layman's usage for the 'bit in the centre'. It is pleasing to know that despite the 'troubles' in this ng that within minutes more probable solutions had been provided. "Engineers ... are not superhuman. They make mistakes in their assumptions, in their calculations, in their conclusions. That they make mistakes is forgivable; that they catch them is imperative. Thus it is the essence of modern engineering not only to be able to check one's own work but also to have one's work checked and to be able to check the work of others." To Engineer Is Human. Engineers Creed(Henry Petroski) -- Peter 'Prof' Fox Multitude of things for beer, cycling, Morris and curiosities at http://vulpeculox.net |
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On Jun 16, 12:18*pm, _
wrote: The axle does not rotate. The disc and the wheel rotate in unison around it. Axles fail from bending loads. I'm watching you. |
#9
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On 16 June, 08:07, Tim Hall wrote:
.. 2) I'm trying to work out the mechanism which leads to a broken axle stopping the brake working properly. The disc only need to be a gnat's crotchet off line, but I can't see how the two are linked. Ideas? -- Tim My son complained of a wobbly rear wheel. Sure enough, the tyre was rubbing against one of the arms. I straightened it out, got him to take it for his ride (about 8 miles). He reported that he'd had to adjust the wheel alignment again. I couldn't work out what was causing the wheel to loosen, as I'd tightened up the (quick-release) very tightly. In the early hours sub-conscious did some work on the problem, and I thought of a broken axle. Left note for son NOT to ride that day. Sure enough, on stripping the wheel down in the evening, axle was in two pieces. He has V-brakes. No idea how it happened. Lucky he didn't come a cropper on it. |
#10
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In ,
al Mossah tweaked the Babbage-Engine to tell us: On 16 June, 08:07, Tim Hall wrote: . 2) I'm trying to work out the mechanism which leads to a broken axle stopping the brake working properly. The disc only need to be a gnat's crotchet off line, but I can't see how the two are linked. Ideas? -- Tim My son complained of a wobbly rear wheel. Sure enough, the tyre was rubbing against one of the arms. I straightened it out, got him to take it for his ride (about 8 miles). He reported that he'd had to adjust the wheel alignment again. I couldn't work out what was causing the wheel to loosen, as I'd tightened up the (quick-release) very tightly. In the early hours sub-conscious did some work on the problem, and I thought of a broken axle. Left note for son NOT to ride that day. Sure enough, on stripping the wheel down in the evening, axle was in two pieces. He has V-brakes. No idea how it happened. Lucky he didn't come a cropper on it. Curious. I b0rked the rear axle on the Big Red Roockhoooper many years ago and rode it for weeks without any wheel alignment issues (waiting for new axle), though it did emit an horrible grinding noise from time to time. The only difference I can think of is that said bike had a solid rear axle, as back in those days it was Well Known that q/r axles weren't strong enough for off-road use. -- Dave Larrington http://www.legslarry.beerdrinkers.co.uk Cheery urchin reporting for duty. |
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