Tacoed rear wheel

[[email protected]]

It's not quite what you think...I trued the rear wheel on my older
bike - the rim is Araya pinned, 36h laced to Ultegra (6sp) hubs. I
trued it and then brought up the overall tension as the spokes felt
undertensioned. I then proceeded to stress relieve by pulling up at 12
o'clock while pushing with elbows at 9&3. The wheel just caved and now
I have a potato chip/taco. It isn't so bad, and I can try loosening
and truing but the problem is the rim joint is misaligned.

So the question is, can this be fixed or undone, or did I just ruin
the rim? The more basic question is why did it taco (so severly)? I've
built several wheels with modern rims to very good success. Based on
Brandt and Schraner, it would seem that the tension was too high.
Perhaps for the older rim that it is, I put too much tension?

Josh

 

[sergio]

On Apr 3, 8:28 am, [email protected] wrote:

> Perhaps for the older rim that it is, I put too much tension?

Yes, indeed.
I made the same mistake many years ago and trashed my new rim. Not by
doing the stress relief, which I was not aware of at that time, but
upon the slightest shock the wheel received while riding along.

Sergio
Pisa

P.s. As said many times over. Too bad manufacturers don't give
specifications as to the highest tension a rim (and its holes) can
sustain.

 

[Phil, Non-Squid]

[email protected] wrote:
> It's not quite what you think...I trued the rear wheel on my older
> bike - the rim is Araya pinned, 36h laced to Ultegra (6sp) hubs. I
> trued it and then brought up the overall tension as the spokes felt
> undertensioned. I then proceeded to stress relieve by pulling up at 12
> o'clock while pushing with elbows at 9&3. The wheel just caved and now
> I have a potato chip/taco. It isn't so bad, and I can try loosening
> and truing but the problem is the rim joint is misaligned.
>
> So the question is, can this be fixed or undone, or did I just ruin
> the rim? The more basic question is why did it taco (so severly)? I've
> built several wheels with modern rims to very good success. Based on
> Brandt and Schraner, it would seem that the tension was too high.
> Perhaps for the older rim that it is, I put too much tension?

Spoke tension is uneven. Release tension on all spokes, then tighten by
counting the same number of turns each time, not by how quickly they
tension. Otherwise, one side will be tensioned before the other, and you
will end up with uneven tension, allowing the rim to settle in a
lower-energy state. If tension on all spokes was truly even, the wheel
would automatically be in its lowest energy state and could not taco
further.

I've done this back before I knew what I was doing and the rim was
salvageable. I suggest you try the same.

--
Phil

 

[David L. Johnson]

[email protected] wrote:

> trued it and then brought up the overall tension as the spokes felt
> undertensioned. I then proceeded to stress relieve by pulling up at 12
> o'clock while pushing with elbows at 9&3. The wheel just caved and now
> I have a potato chip/taco.

You went a step too far. Over-tensioned.

> It isn't so bad, and I can try loosening
> and truing but the problem is the rim joint is misaligned.

Try straightening it in a jig. I think Brandt's book has detailed
instructions.
>
> So the question is, can this be fixed or undone, or did I just ruin
> the rim? The more basic question is why did it taco (so severly)? I've
> built several wheels with modern rims to very good success. Based on
> Brandt and Schraner, it would seem that the tension was too high.
> Perhaps for the older rim that it is, I put too much tension?

Certainly the tension was too high. This is not usually fatal; you
should be able to pop it back mostly true, then start over truing it
with not quite so high a tension. But the joint failure may doom the rim.

--

David L. Johnson

Arguing with an engineer is like mud wrestling with a pig...
You soon find out the pig likes it!

 

[Michael Press]

In article <[email protected]>,
"Phil, Non-Squid" <[email protected]> wrote:

> [email protected] wrote:
> > It's not quite what you think...I trued the rear wheel on my older
> > bike - the rim is Araya pinned, 36h laced to Ultegra (6sp) hubs. I
> > trued it and then brought up the overall tension as the spokes felt
> > undertensioned. I then proceeded to stress relieve by pulling up at 12
> > o'clock while pushing with elbows at 9&3. The wheel just caved and now
> > I have a potato chip/taco. It isn't so bad, and I can try loosening
> > and truing but the problem is the rim joint is misaligned.
> >
> > So the question is, can this be fixed or undone, or did I just ruin
> > the rim? The more basic question is why did it taco (so severly)? I've
> > built several wheels with modern rims to very good success. Based on
> > Brandt and Schraner, it would seem that the tension was too high.
> > Perhaps for the older rim that it is, I put too much tension?
>
> Spoke tension is uneven. Release tension on all spokes, then tighten by
> counting the same number of turns each time, not by how quickly they
> tension. Otherwise, one side will be tensioned before the other, and you
> will end up with uneven tension, allowing the rim to settle in a
> lower-energy state. If tension on all spokes was truly even, the wheel
> would automatically be in its lowest energy state and could not taco
> further.
>
> I've done this back before I knew what I was doing and the rim was
> salvageable. I suggest you try the same.

A potato-chip wheel is in a lower energy state, even
starting with equal tension on all spokes. A potato
chip is a surface with negative Gaussian curvature. A
sphere is a surface with positive Gaussian curvature.
Cut a disk out of a sphere such as a basketball then
try to flatten it. The outside portion is not big
enough around and wants to tear as you flatten the
disk. A surface of negative curvature is different. It
wants to fold over at the outside as you try to flatten
it. A Lettuce leave is another example of a surface
with negative curvature.

On a positive curvature surface a circle circumference
is greater than Pi times the diameter. On a negative
curvature surface a circle circumference is less than
Pi times the diameter.

A potato-chipped wheel has the same circumference as
when it started, but the spokes are shorter. Hence, the
energy is much less.

The experiment is to measure the spoke tension on a
wheel as you over-tension it until it potato-chips,
then measure the spoke tension.
--
Michael Press

 

[Ben C]

On 2007-04-04, Michael Press <[email protected]> wrote:
> In article <[email protected]>,
> "Phil, Non-Squid" <[email protected]> wrote:
[...]
>> Spoke tension is uneven. Release tension on all spokes, then tighten by
>> counting the same number of turns each time, not by how quickly they
>> tension. Otherwise, one side will be tensioned before the other, and you
>> will end up with uneven tension, allowing the rim to settle in a
>> lower-energy state. If tension on all spokes was truly even, the wheel
>> would automatically be in its lowest energy state and could not taco
>> further.
>>
>> I've done this back before I knew what I was doing and the rim was
>> salvageable. I suggest you try the same.
>
> A potato-chip wheel is in a lower energy state, even
> starting with equal tension on all spokes. A potato
> chip is a surface with negative Gaussian curvature. A
> sphere is a surface with positive Gaussian curvature.
> Cut a disk out of a sphere such as a basketball then
> try to flatten it. The outside portion is not big
> enough around and wants to tear as you flatten the
> disk. A surface of negative curvature is different. It
> wants to fold over at the outside as you try to flatten
> it. A Lettuce leave is another example of a surface
> with negative curvature.
>
> On a positive curvature surface a circle circumference
> is greater than Pi times the diameter. On a negative
> curvature surface a circle circumference is less than
> Pi times the diameter.
>
> A potato-chipped wheel has the same circumference as
> when it started, but the spokes are shorter.

Are they all shorter? If I have a perfect front wheel with a round rim,
spokes all the same length and tension, won't _any_ deformation have to
result in some spokes getting a bit longer?

It may be that the total tension will be lower in the tacoed wheel
though, I don't know.

> The experiment is to measure the spoke tension on a
> wheel as you over-tension it until it potato-chips,
> then measure the spoke tension.

 

[sergio]

On Apr 4, 9:49 am, Ben C <[email protected]> wrote:
> It may be that the total tension will be lower in the tacoed wheel
> though, I don't know.

As an extreme, suppose you fold a circumpherence into a perfectly
symmetric 8-shape on a plane.
Where does the center go? How far from it will the 'eyelet holes'
be?

Sergio
Pisa

 

[Michael Press]

In article <[email protected]>,
Ben C <[email protected]> wrote:

> On 2007-04-04, Michael Press <[email protected]> wrote:
> > In article <[email protected]>,
> > "Phil, Non-Squid" <[email protected]> wrote:
> [...]
> >> Spoke tension is uneven. Release tension on all spokes, then tighten by
> >> counting the same number of turns each time, not by how quickly they
> >> tension. Otherwise, one side will be tensioned before the other, and you
> >> will end up with uneven tension, allowing the rim to settle in a
> >> lower-energy state. If tension on all spokes was truly even, the wheel
> >> would automatically be in its lowest energy state and could not taco
> >> further.
> >>
> >> I've done this back before I knew what I was doing and the rim was
> >> salvageable. I suggest you try the same.
> >
> > A potato-chip wheel is in a lower energy state, even
> > starting with equal tension on all spokes. A potato
> > chip is a surface with negative Gaussian curvature. A
> > sphere is a surface with positive Gaussian curvature.
> > Cut a disk out of a sphere such as a basketball then
> > try to flatten it. The outside portion is not big
> > enough around and wants to tear as you flatten the
> > disk. A surface of negative curvature is different. It
> > wants to fold over at the outside as you try to flatten
> > it. A Lettuce leave is another example of a surface
> > with negative curvature.
> >
> > On a positive curvature surface a circle circumference
> > is greater than Pi times the diameter. On a negative
> > curvature surface a circle circumference is less than
> > Pi times the diameter.
> >
> > A potato-chipped wheel has the same circumference as
> > when it started, but the spokes are shorter.
>
> Are they all shorter? If I have a perfect front wheel with a round rim,
> spokes all the same length and tension, won't _any_ deformation have to
> result in some spokes getting a bit longer?

Yes some of the configurations between a true rim and a
potato-chipped rim have longer spokes, more highly
tensioned spokes, and higher energy than the starting
and ending configuration. The true wheel and the
potato-chipped wheel are both local energy minima. The
latter has lower energy than the former because the
spokes are shorter, hence have given up mechanical
energy that was stored in their elastic deformation in
the true wheel configuration.


> It may be that the total tension will be lower in the tacoed wheel
> though, I don't know.
>
> > The experiment is to measure the spoke tension on a
> > wheel as you over-tension it until it potato-chips,
> > then measure the spoke tension.
--
Michael Press

 

[sergio]

On 4 Apr, 19:02, Michael Press <[email protected]> wrote:
The true wheel and the
> potato-chipped wheel are both local energy minima. The
> latter has lower energy than the former because the
> spokes are shorter,

That's what is called having two equilibrium configurations between
which the thing can flip.
I have always understood that the spokes under tension cause
instability from the former to the latter, more so the higher the
tension.
If you tension spokes too high you make it very unstable (something
that stess relieving already signals to you) and likely to flip into a
permanent deformation.

Sergio
Pisa

 

[Ben C]

On 2007-04-04, sergio <[email protected]> wrote:
> On Apr 4, 9:49 am, Ben C <[email protected]> wrote:
>> It may be that the total tension will be lower in the tacoed wheel
>> though, I don't know.
>
> As an extreme, suppose you fold a circumpherence into a perfectly
> symmetric 8-shape on a plane.
> Where does the center go? How far from it will the 'eyelet holes'
> be?

Well, what were two diametrically opposite points on the rim actually
end up touching each other at the centre of the 8. The points on the
top and bottom of the 8 I'm less sure about.

I do know that a circle has the highest area/perimeter ratio of any 2D
shape. So distorting the shape into something non-circular means
perhaps a smaller area, and therefore less total spoke tension.

But it's rather complicated because when the rim bends it becomes a 3D
shape and we're talking about the area of a non-flat surface, in fact
one with quite an interesting curvature.

 

[Ben C]

On 2007-04-04, sergio <[email protected]> wrote:
> On 4 Apr, 19:02, Michael Press <[email protected]> wrote:
> The true wheel and the
>> potato-chipped wheel are both local energy minima. The
>> latter has lower energy than the former because the
>> spokes are shorter,
>
> That's what is called having two equilibrium configurations between
> which the thing can flip.
> I have always understood that the spokes under tension cause
> instability from the former to the latter, more so the higher the
> tension.
> If you tension spokes too high you make it very unstable (something
> that stess relieving already signals to you) and likely to flip into a
> permanent deformation.

The overload of stress relief presumably is what actually adds the
temporary energy to get it over the hill and to flip into the
taco-configuration on the other side.

This would be why tacoing would be detected after stress relief.

But it's evidently not so easy to "flip" it back into being a true
wheel. You just have to detension and true it back up again.

Any attempt to manipulate it probably just results in it flipping
between a series of similar but differently oriented taco shapes.

 

[[email protected]]

Someone writes::

> trued it and then brought up the overall tension as the spokes felt
> under tensioned. I then proceeded to stress relieve by pulling up
> at 12 o'clock while pushing with elbows at 9&3. The wheel just
> caved and now I have a potato chip/taco.

Don't do that!!! There is a reason for the stress relief method
outlined in "the Bicycle Wheel" and this is one of them.

> It isn't so bad, and I can try loosening and truing but the problem
> is the rim joint is misaligned.

Don't do that unless the warp is on the order of +-mm or so. Follow
the wheel straightening description.

> So the question is, can this be fixed or undone, or did I just ruin
> the rim? The more basic question is why did it taco (so severely)?
> I've built several wheels with modern rims to very good success.
> Based on Brandt and Schraner, it would seem that the tension was too
> high. Perhaps for the older rim that it is, I put too much tension?

I don't know how strongly the wheel is warped but straightening a
warped wheel is best done with all spokes in place as described in
"the Bicycle Wheel". Loosen the spokes a turn or so (this depends on
spoke thickness) that the wheel has light tension (not slack). Then
carefully bend the wheel back to straight. When it is close, do the
rest with the spoke wrench.

Jobst Brandt

 

[[email protected]]

here's a method to consider. the procedure wroks. i had 2 rims
vandalized and no excess cash flow. two problems surface right off the
bat.1)where to hold the rim to straighten it (see JB on keeping the
spokes in) and 2) how to evaluate the bend sightwise. 3) how to exert
enough force just so to straigthen the bend.

tho the method wroks, our amateur opinion: one should consider BENT
rim straightening as an entertaining pastime not a maintenance
procedure. the maintenance procedure for a BENT rim is to buy a new
unbent rim, how does that song go...?

i use the two spoke squeeze method also known as the group two sissy,
method of releiving stress (mine) followed with a road test of one to
two weeks then a retuning and more sissy relief.

the method:
i have not built the tool decribed in "rim repair tool" in tech
archives but trued a bent CR-18 27" a few days ago using the 1" plus
particle board covered with formica like material-counter tops stuff.
The pieces i have were desktop end supports.

i was run over by a killer from Miamuh. The car hit the rear wheel
giving a good bend of 1' plus over 16"


I placed the rim between two countertops and jumped rope on the slabs
for 20-30 minutes truing the rim over several times.


This produced knee pain and a rim (i took the spokes out) rim that
was
bent in two parts that is the rim rocked at two opposite points when
laid on the slab.


I eyeballed sighted the rim to produce a large trued effect from
hmmm
70% of the rim surface to my right holding the rim down and before
me-like sighting the chain line from gear cluster cassette to CR-one
sights until the surfaces disappear and you're directly behind the
gears. Try this with the bent rim held bottom at waist and the near
side up toward your eyes then turn the rim until you get the greatest
in-line or true area to the right and the bent area to the left. This
can be a fooley-the rim looks true but really its not. If you turn
the
rim some more and sight down the bent part up close to your eyes then
it'll get ugly again.


Mark the rim's bent areas beginnings with fingers then place these
pints at the lower slabs upper left edge. Place the upper slab over
the
rim flush with the lower edge. The bent area may have a one up one
down
bend-start with the larger length of the two.


Place a tuba4 or tuba4's forbuh side down and steo on the upper slab.
Helps to have a fat friend here to stand behind you as you step down
on
the edges.


Continue these steps until you work your rim's way to a lesser chord
where stepping on the upper slab doesn't seem to produce any more
cure.


Then hit the sucker with an 11 pound engineers hammer (on another
tuba
placed atop the rim stupid not directly onto the rim) one whap! maybe
two then resight and recheck.


More correction. Then try the smaller bend.
Take a break, and let the almost true rim set forawhile before you
overbend it in your enthusiasm for this unreal save.

 

[[email protected]]

my usual crystal clear tech writing failed here in the above rim tool
description.

two slabs of particle board (covered with formica) hold the rim's
straight section or straighter section(s)

the tubah4 goes under the rim's bent radius, bend down, outside the
particle board, with weight given the more bent section, we assume
where the bend started. kinda angle the tubah iover toward the big
bend.

the downward bend thus gets bent upward when you jump up and down on
the particle board.

the method works for the cr-18, i assume for 'similar' mavic rims.

i haven't reread brandt's method. holding an unspoked rim and
rebending to straight bends the sections held. if you figure a way
around that let us know.

preloading spokes needed to pull the bent section back in line.mark
the bent area then seat all nipples, turn bent side nipples 1/2 turn
more then start trueing or if able mark bend's center, 1/4 out each
side then grade the turns from center outward say 1/2 then 3/8 then...

 

[David L. Johnson]

[email protected] wrote:
> my usual crystal clear tech writing failed here in the above rim tool
> description.

?????

--

David L. Johnson

If all economists were laid end to end, they would not reach
a conclusion. -- George Bernard Shaw