When things go wrong

Dave Vandervies said:

... it turns out that one
of my front brake pads somehow got wedged under the rim of the wheel,

Quoted message said:

What would have caused the brake
to get wedged in there in the first place?

I'm not picturing "under the rim of the wheel." Where is the brake pad
wedged? Did the brake pad(s) come out and and get wedged between the
wheel and the fork? Or did the brakes bend so that the pads are rubbing
on the tire? If so, what portion of the tire?

{r.b.tech might be the more appropriate group to post this to.}

RFM

In article <[email hidden]>,
Fritz M <[email hidden]> wrote:

[moved to top]

Quoted message said:

{r.b.tech might be the more appropriate group to post this to.}

Crosspost added.
For the new readers: Went over the handlebars today after hitting a bump
or hole in the ground (not sure which), noticed that a front brake got
wedged under the rim of the wheel (which I suspect is what put me over
the handlebars), now I'm trying to figure out how it got there before
the bike went down.
The long version (including the story with details of exactly what
happened and commentary not directly related to that question) should
be findable at
http:[email hidden]

Quoted message said:

Dave Vandervies said:

... it turns out that one
of my front brake pads somehow got wedged under the rim of the wheel,

Quoted message said:

What would have caused the brake
to get wedged in there in the first place?

I'm not picturing "under the rim of the wheel." Where is the brake pad
wedged?

With the friction surface facing the spokes and the top against the part
opposite the tire's road contact surface (at the outer end of the spokes).
It was still the rubber part of the brake pad in contact with the wheel
at that point, just not the part that's normally used for braking,
and it was still attached to the rest of the brake assembly as well.

These are fork-mounted cantilever brakes, so it was just the one "arm"
assembly getting pushed in farther than it should have and stuck there
rather than putting the friction surface against the edge of the rim
(where the cable pull would normally put it).

Quoted message said:

Did the brake pad(s) come out and and get wedged between the
wheel and the fork?

No, still attached to the brake (which was still attached to the fork
and in the right position, just turned too far in on the axis it pivots
around in normal operation) and not moved along the circle of the rim.
(I think that was me not being clear - the pad was the part that was
actually in contact with the wheel, but the entire brake assembly on
that side was stuck in that position.)

Quoted message said:

Or did the brakes bend so that the pads are rubbing
on the tire? If so, what portion of the tire?

Nope, as far as I could tell it wasn't bent, and it wasn't actually
in contact with the tire, it was the inner rim (underneath the tire).
As noted above, these are cantilever brakes, so each pad and brake
arm assembly can be moved independently, and it was just one side that
was stuck.

dave

--
Dave Vandervies [email hidden]

If in doubt, go round in circles.
--Richard Heathfield in comp.lang.c

Dave Vandervies said:

For the new readers: Went over the handlebars today after hitting a
bump or hole in the ground (not sure which), noticed that a front
brake got wedged under the rim of the wheel (which I suspect is what
put me over the handlebars), now I'm trying to figure out how it got
there before the bike went down.

Quoted message said:

The long version (including the story with details of exactly what
happened and commentary not directly related to that question)
should be findable at

Quoted message said:

http:[email hidden]

Commenting on the basis of the short synopsis..., I am aware of brake
pop-under ability of cantilever brakes, something to which V-brakes
are more immune because their levers would hit the tire first.

The cantilever brake boss/pivot is far enough to the side to allow a
wheel with a large MTB tire to be easily removed. A straight line
drawn from the pivot to the braking surface of the rim lies at about
45 degrees to the wheel central plane, so the motion of the pad is
downward at that angle. With pad wear, and no vertical adjustment,
brake shoes can dive past the braking surface. Because this occurs
during hard braking, the brake shoe will not return on its own return
spring, and wikk remain locked under the bed of the rim by the metal
brake pad holder.

In the event of an over-the-bars crash, the brake cable can snag on a
foreign object or the riders arm as he dived forward. The force of
that brake application can be enough to cause brake pad pop-under.

Jobst Brandt

J. Brandt said:

...
The cantilever brake boss/pivot is far enough to the side to allow a
wheel with a large MTB tire to be easily removed. A straight line
drawn from the pivot to the braking surface of the rim lies at about
45 degrees to the wheel central plane, so the motion of the pad is
downward at that angle. With pad wear, and no vertical adjustment,
brake shoes can dive past the braking surface. Because this occurs
during hard braking, the brake shoe will not return on its own return
spring, and wikk remain locked under the bed of the rim by the metal
brake pad holder....

S. C. Brown has an instructive drawing of a cantilever brake:
<http://www.sheldonbrown.com/cantilever-geometry.html>.

--
Tom Sherman - Fox River Valley

J. Brandt said:

...
The cantilever brake boss/pivot is far enough to the side to allow a
wheel with a large MTB tire to be easily removed. A straight line
drawn from the pivot to the braking surface of the rim lies at about
45 degrees to the wheel central plane, so the motion of the pad is
downward at that angle. With pad wear, and no vertical adjustment,
brake shoes can dive past the braking surface. Because this occurs
during hard braking, the brake shoe will not return on its own return
spring, and wikk remain locked under the bed of the rim by the metal
brake pad holder....

S. C. Brown has an instructive drawing of a cantilever brake:
<http://www.sheldonbrown.com/cantilever-geometry.html>.

--
Tom Sherman - Fox River Valley

Quoted message said:

Dave Vandervies said:

For the new readers: Went over the handlebars today after hitting a
bump or hole in the ground (not sure which), noticed that a front
brake got wedged under the rim of the wheel (which I suspect is what
put me over the handlebars), now I'm trying to figure out how it got
there before the bike went down.

Quoted message said:

The long version (including the story with details of exactly what
happened and commentary not directly related to that question)
should be findable at

Quoted message said:

http:[email hidden]

Commenting on the basis of the short synopsis..., I am aware of brake
pop-under ability of cantilever brakes, something to which V-brakes
are more immune because their levers would hit the tire first.

The cantilever brake boss/pivot is far enough to the side to allow a
wheel with a large MTB tire to be easily removed. A straight line
drawn from the pivot to the braking surface of the rim lies at about
45 degrees to the wheel central plane, so the motion of the pad is
downward at that angle. With pad wear, and no vertical adjustment,
brake shoes can dive past the braking surface. Because this occurs
during hard braking, the brake shoe will not return on its own return
spring, and wikk remain locked under the bed of the rim by the metal
brake pad holder.

In the event of an over-the-bars crash, the brake cable can snag on a
foreign object or the riders arm as he dived forward. The force of
that brake application can be enough to cause brake pad pop-under.

Or it's possible that the bike simply landed on one of the front
cantis, forcing the other end under the rim. Look for abrasions on
the "outside end" of the canti arm - I'll be there are some.

Unless the OP grabbed all the brake available in the middle of the
corner for some unknown reason, I can't imagine that it's a "brake
failure" of any type.

Mark Hickey
Habanero Cycles
http://www.habcycles.com
Home of the $795 ti frame

In article <[email hidden]>,

Mark Hickey said:

[email hidden] wrote:

[snip]

Quoted message said:

Quoted message said:

I am aware of brake
pop-under ability of cantilever brakes,

[snip]

Quoted message said:

Quoted message said:

In the event of an over-the-bars crash, the brake cable can snag on a
foreign object or the riders arm as he dived forward. The force of
that brake application can be enough to cause brake pad pop-under.

Quoted message said:

Or it's possible that the bike simply landed on one of the front
cantis, forcing the other end under the rim. Look for abrasions on
the "outside end" of the canti arm - I'll be there are some.

I'm beginning to think it's this. I walked through the same spot I went
over the bars later this afternoon and checked the ground, and there's
a fairly deep hole (deeper than I'd noticed because the grass growing in
it is cut almost level), so I wonder if that might've been what actually
put me over the bars and landing on the brake is what made it pop under.

(Is an over-the-bars crash consistent with what's expected to happen
when the front wheel hits too steep an upslope? I know it's a Bad Thing
to have happen, but this would (if it was that) be the first time I've
done it, so I don't have much to go by.)

I've already taken the bike in to the shop to get the wheel replaced,
but when I get it back I'll have to remember to check the brake arm -
I suspect I'll probably find some evidence of a hard landing there
(though it might just be a bit of grass stuck in it, since that's what
it landed on).

Quoted message said:

Unless the OP grabbed all the brake available in the middle of the
corner for some unknown reason, I can't imagine that it's a "brake
failure" of any type.

Nope, quite sure I didn't touch the brake at all. I'd assumed that
something with the bump it took caused the brake to go wrong (which then
caused the flip over the bars), but something else causing the flip and
that pushing the brake under seems more likely, especially if there's
a reasonable way that would happen and not a reasonable way to have
hitting a bump cause the pop-under.

Even if I'd hit the brake as hard as possible, that alone wouldn't've
been enough to put me over the bars, since the old (steel) rims weren't
playing all that nicely with the brake pads (hence replacing the wheel
instead of trying to repair it).

dave

--
Dave Vandervies [email hidden]
If you find misleading, off-topic and partly incorrect information given by an
individual having horrible misconceptions about how C works helpful: your
decision. Just be prepared for others to disagree. --Irrwahn Grausewitz, CLC

"Dave Vandervies" wrote: (clip) (Is an over-the-bars crash consistent with
what's expected to happen when the front wheel hits too steep an upslope?
(clip)
^^^^^^^^^^^^^^^^
When I first put Rock Shocks on my mountain bike, I expected too much of it.
I tried to ride over a square wooden beam without slowing down, and I endoed
over the bars. The front wheel stops, and everything else keeps moving,
causing your center of gravity to rise and rotate around the front axle. I
had to go to my optometrist and have my glasses straightened.

(Dave Vandervies) said:

In article <[email hidden]>,
Mark Hickey <[email hidden]> wrote:

Quoted message said:

Quoted message said:

Unless the OP grabbed all the brake available in the middle of the
corner for some unknown reason, I can't imagine that it's a "brake
failure" of any type.

Nope, quite sure I didn't touch the brake at all. I'd assumed that
something with the bump it took caused the brake to go wrong (which then
caused the flip over the bars), but something else causing the flip and
that pushing the brake under seems more likely, especially if there's
a reasonable way that would happen and not a reasonable way to have
hitting a bump cause the pop-under.

I suspect it was just "geometry" that put you over the bars.

Your bike has "automatic stability" because the tire is touching the
ground BEHIND the steering axis (look down along the center of your
bike's head tube and you'll see that the wheel is "trailing behind"
the steering axis, much like a grocery cart wheel).

When you hit that bump, chances are the contact was AHEAD of the
steering axis, which would try to force the wheel to turn to the side
(which would cause an immediate endo).

Mark Hickey
Habanero Cycles
http://www.habcycles.com
Home of the $795 ti frame

Dave Vandervies said:

In article <[email hidden]>,
Fritz M <[email hidden]> wrote:

[moved to top]

Quoted message said:

{r.b.tech might be the more appropriate group to post this to.}

Crosspost added.

stuff snipped..

poorly set up cantis are more prone to this than correctly set up ones
... this becomes even more true as they wear. If they aren't set up like
the pic on sheldons site (in another post) or in the Shimano
instructions then they are wrong.

Quoted message said:

From my experience with London Ontario Bike stores I'd bet they were

wrong.

In article <[email hidden]>,

Mark Hickey said:

I suspect it was just "geometry" that put you over the bars.

Your bike has "automatic stability" because the tire is touching the
ground BEHIND the steering axis (look down along the center of your
bike's head tube and you'll see that the wheel is "trailing behind"
the steering axis, much like a grocery cart wheel).

When you hit that bump, chances are the contact was AHEAD of the
steering axis, which would try to force the wheel to turn to the side
(which would cause an immediate endo).

I'm pretty sure that's what happened. Thanks for the explanation -
I'd worked out the self-stabilizing under normal conditions but wasn't
sure what the failure mode was when the contact point gets ahead of the
steering axis. Now I know both on the theory side and from personal
experience.

dave

--
Dave Vandervies [email hidden]
Of all possible chronic diseases, it was probably inevitable that
I'd get the one with Shiny! toys.
--Colin Andrew Percival in the scary devil monastery

In article <[email hidden]>,

Quoted message said:

poorly set up cantis are more prone to this than correctly set up ones
.. this becomes even more true as they wear. If they aren't set up like
the pic on sheldons site (in another post) or in the Shimano
instructions then they are wrong.

Since I'd had it adjusted recently, and it had to put a pretty serious
bend in the wheel to get where it ended up, I'm pretty sure it wasn't
because it was set up wrong.

Quoted message said:

From my experience with London Ontario Bike stores I'd bet they were
wrong.

I'm not in London, either.

dave

--
Dave Vandervies [email hidden]
Of all possible chronic diseases, it was probably inevitable that
I'd get the one with Shiny! toys.
--Colin Andrew Percival in the scary devil monastery

When things go wrong, as they sometimes will,
When the road you're trudging seems all up hill,
When the funds are low and the debts are high
And you want to smile, but you have to sigh,
When care is pressing you down a bit,
Don't come whining to me; I don't give a [censored].

In article <[email hidden]>,

Bob the Cow said:

When things go wrong, as they sometimes will,
When the road you're trudging seems all up hill,
When the funds are low and the debts are high
And you want to smile, but you have to sigh,
When care is pressing you down a bit,
Don't come whining to me; I don't give a [censored].

Actually, it was more of a "When things go wrong, after I've dealt
with any immediate problems and figured out What Not To Do Again, the
engineer in me still isn't happy until I've figured out all the details
of everything that happened".

dave

--
Dave Vandervies [email hidden]
People who are *really good* can do awesome things with C++. People who are
not so good with C++ produce awe of a ... different ... kind.
--Michael Hudson in the scary devil monastery

"Dave Vandervies" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

Actually, it was more of a "When things go wrong, after I've dealt
with any immediate problems and figured out What Not To Do Again, the
engineer in me still isn't happy until I've figured out all the details
of everything that happened".

Well, yeah, but the first thing that popped into my "mind" when I saw the
subject line was that poem. Sorry...