Cable discs

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

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

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.

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

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.

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.

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

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.

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.

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.