rim seam stiffness/tolerance stuff

Yesterday I built up a Rhynolite / XT 756 / DT 2.0/1.8 wheelset, 32h
for both. These are the cheap Rhynolites - all silver, I think brushed
and not anodized, not machined, and not welded. These are maybe the
14th and 15th wheels I've built, and I've built 4 other Sun rims
previously.

Since these are for me and are probably never going to be used with rim
brakes, I basically paid little attention to radial trueness and
tension balanced them fairly precisely using a tensiometer, and then
made the necessary small adjustments for lateral trueness. When you do
this, the radial trueness at all areas except the seam basically comes
out as a reflection of the rim's precision. At the seam, there are some
additonal factors having to do with the construction of the rim there,
as well as some other stuff that I'm kinda in the dark about.

The idea of building wheels this way is that in order to correct
signifcant radial errors that are still there after the tension balance
is good, you'd have to throw tension balance out the window in those
spots, since that's how much change in tension is needed to produce
much change in radial trueness.

What I'm wondering about is this: on at least one one of these rims
(can't remember now), the seam had a significant radial dip (error
inwards toward the hub, not outward). With tension balance looking
good, the rim was maybe 1.5 - 2mm in there. So you'd need to have some
spokes stupidly loose there to make things look good. This is pretty
normal, especially for Sun. However, some rims behave in the opposite
way at the seam. They want to have outbound errors and need
tighter-than-average spokes to be radially true at the seam. And I
built an Aerohead once (sleeved seam, I think) where I wasn't willing
to sacrifice much radial trueness for tension balance, and it wanted to
have one spoke at the seam be really, really tight and an adjacent one,
coming from the other flange and on the other side of the seam, be
really loose.

Can someone who knows what they're talking about explain how exactly
this works? I assume that it all has to do with different seam joining
and rim forming processes producing different effects, but what's the
explanation for why the rim I just built wanted to have a dip at the
seam where others go outward? I realize that the rim probably had a
flat spot there to begin with, but can some joining processes make the
rim more elastic there, causing it to respond differently to the same
amount of tension as the rest of the rim? If it's just a flat spot (and
I'm guessing that's all it ever is), then what causes that and what
joining processes is it an inherent problem with? If the type of
irregularity I'm describing usually or always are just simple flat
spots, then is the same true of high spots at the seam? Or in those
cases, is something going on that makes the rim stiffer there, like
sleeves providing unwanted reinforcement? What types of joining are
actually the best and worst at minimizing irregularities at the seam?
Does anyone who's built a lot with many of the current brands and
models have an opinion about which are CONSISTENTLY good about seam
stuff? (I've noticed that it can vary a lot from rim to rim and perhaps
model to model).

Thanks in advance.
Nate Knutson

Nate Knutson wrote:

What I'm wondering about is this: on at least one one of these rims
(can't remember now), the seam had a significant radial dip (error
inwards toward the hub, not outward).
....
This is pretty normal, especially for Sun. However, some rims
behave in the opposite way at the seam.

Can someone who knows what they're talking about explain how exactly
this works?

The rim joint in a tensioned wheel is under a great deal of
compression. Any gap in the joint will be closed by the pressure. If
the faces of the cuts don't lie fairly precisely along a plane that
passes through the wheel's axis, a bump in some direction will result.
If there is a gap along the rim's hub-facing surface, the rim will
bulge outward at the seam when tensioned. If there is a gap along the
spoke bed, the rim will "flat spot" when tensioned.

Welded rims can be cut sloppily because the weld fills any gap that
they would have had. This is one of the reasons that welded rims are
cheaper to make, and it partly accounts for their widespread adoption
by manufacturers.

Chalo Colina

Chalo wrote:

If there is a gap along the spoke bed, the rim will "flat spot" when tensioned.

By "spoke bed" I really meant to say the rim's tube-facing wall-- the
"fond de jante".

Chalo

Chalo wrote:
Nate Knutson wrote:

What I'm wondering about is this: on at least one one of these rims
(can't remember now), the seam had a significant radial dip (error
inwards toward the hub, not outward).
...
This is pretty normal, especially for Sun. However, some rims
behave in the opposite way at the seam.

Can someone who knows what they're talking about explain how exactly
this works?

The rim joint in a tensioned wheel is under a great deal of
compression. Any gap in the joint will be closed by the pressure. If
the faces of the cuts don't lie fairly precisely along a plane that
passes through the wheel's axis, a bump in some direction will result.
If there is a gap along the rim's hub-facing surface, the rim will
bulge outward at the seam when tensioned. If there is a gap along the
spoke bed, the rim will "flat spot" when tensioned.

Chalo,
Thanks for the reply. I can see how the precision of the rim's cuts
would do this, but unlaced rims of any quality don't come to you with
gaps at the seam - or if they do, it's not easily visible and so I'd
assume wouldn't be enough to cause a 1.5mm error. Is it possible that
the gaps you're talking about have already been closed up by the
joining process, and cause a built-in low or high spot in that way?

Do you believe that imprecisely cut joint faces account for all/most
irregularities at the seam?

Welded rims can be cut sloppily because the weld fills any gap that
they would have had. This is one of the reasons that welded rims are
cheaper to make, and it partly accounts for their widespread adoption
by manufacturers.

Chalo Colina

Nate Knutson wrote:

Do you believe that imprecisely cut joint faces account for all/most
irregularities at the seam?

That's my impression for pinned rims. I have come across a small few
that had a step at the joint on both sides, with one cut end slightly
wider than the other.

Chalo

Hi Nate

You can actually significantly reduce this problem by "cold setting"
(for lack of a better term) the rim joint. It never ceases to amaze me
how well this works. Strive to bring the joint out (away from the hub,
in the case of this thread), but not all the way to radial equality.
This will cause the adjacent sections on either side of the joint to
extend beyond the radial average, but that's ok IMO. The average
roundness will be improved. (See "The Bicycle Wheel" P114 -5 for more
detail)
I don't know how close you keep the tension balance, but it doesn't
have to be perfect. You can play w/ adjacent same side tension, in
particular be aware that a crossed spoke has a strong influence on the
tension of it's mate, & a lesser affect on near by spokes through the
rim, which can sometimes give a beneficial effect. The idea is to
spread the differing tension requirements over a range of spokes. I
also believe that the tension of the spokes ~180 deg away can
influence radial tension @ the point of interest, if the opposite side
spokes tension can be changed w/o being too detrimental to their
balance.
I really love taking an old beat up wheel & manipulating it back into a
state of reasonable trueness w/ hi & even tension. It gives me a sense
of satisfaction that is hard to describe. "Life is good" as I think
Jeff Starr says. Even though it takes longer that building a new wheel,
& is economically unviable.
Also Bicycle Research makes a tool that facilitates aligning the sides
of rim joints.
One more thing, 1.5mm to 2mm is a huge radial joint misalignment.
That's 0.079", man that's a lot! Looks like a huge gap on the stand.
Did you say that is typ. for Sun rims? I hope not. I've only seen one
rim that was that bad, it was also new.

Good luck, John

Nate Knutson wrote:
Yesterday I built up a Rhynolite / XT 756 / DT 2.0/1.8 wheelset, 32h
for both. These are the cheap Rhynolites - all silver, I think brushed
and not anodized, not machined, and not welded. These are maybe the
14th and 15th wheels I've built, and I've built 4 other Sun rims
previously.

Since these are for me and are probably never going to be used with rim
brakes, I basically paid little attention to radial trueness and
tension balanced them fairly precisely using a tensiometer, and then
made the necessary small adjustments for lateral trueness. When you do
this, the radial trueness at all areas except the seam basically comes
out as a reflection of the rim's precision. At the seam, there are some
additonal factors having to do with the construction of the rim there,
as well as some other stuff that I'm kinda in the dark about.

The idea of building wheels this way is that in order to correct
signifcant radial errors that are still there after the tension balance
is good, you'd have to throw tension balance out the window in those
spots, since that's how much change in tension is needed to produce
much change in radial trueness.

What I'm wondering about is this: on at least one one of these rims
(can't remember now), the seam had a significant radial dip (error
inwards toward the hub, not outward). With tension balance looking
good, the rim was maybe 1.5 - 2mm in there. So you'd need to have some
spokes stupidly loose there to make things look good. This is pretty
normal, especially for Sun. However, some rims behave in the opposite
way at the seam. They want to have outbound errors and need
tighter-than-average spokes to be radially true at the seam. And I
built an Aerohead once (sleeved seam, I think) where I wasn't willing
to sacrifice much radial trueness for tension balance, and it wanted to
have one spoke at the seam be really, really tight and an adjacent one,
coming from the other flange and on the other side of the seam, be
really loose.

Can someone who knows what they're talking about explain how exactly
this works? I assume that it all has to do with different seam joining
and rim forming processes producing different effects, but what's the
explanation for why the rim I just built wanted to have a dip at the
seam where others go outward? I realize that the rim probably had a
flat spot there to begin with, but can some joining processes make the
rim more elastic there, causing it to respond differently to the same
amount of tension as the rest of the rim? If it's just a flat spot (and
I'm guessing that's all it ever is), then what causes that and what
joining processes is it an inherent problem with? If the type of
irregularity I'm describing usually or always are just simple flat
spots, then is the same true of high spots at the seam? Or in those
cases, is something going on that makes the rim stiffer there, like
sleeves providing unwanted reinforcement? What types of joining are
actually the best and worst at minimizing irregularities at the seam?
Does anyone who's built a lot with many of the current brands and
models have an opinion about which are CONSISTENTLY good about seam
stuff? (I've noticed that it can vary a lot from rim to rim and perhaps
model to model).

Thanks in advance.
Nate Knutson

depends on the sleeve and the facing of the joint. the manufacturer
should cut/join in anticipation of build tension and subsequent rim
compression. if that's not done, the problems are as you describe.
welded joints are generally better.

Chalo wrote:
Nate Knutson wrote:

What I'm wondering about is this: on at least one one of these rims
(can't remember now), the seam had a significant radial dip (error
inwards toward the hub, not outward).

...

This is pretty normal, especially for Sun. However, some rims
behave in the opposite way at the seam.

Can someone who knows what they're talking about explain how exactly
this works?


The rim joint in a tensioned wheel is under a great deal of
compression. Any gap in the joint will be closed by the pressure. If
the faces of the cuts don't lie fairly precisely along a plane that
passes through the wheel's axis, a bump in some direction will result.
If there is a gap along the rim's hub-facing surface, the rim will
bulge outward at the seam when tensioned. If there is a gap along the
spoke bed, the rim will "flat spot" when tensioned.

Welded rims can be cut sloppily because the weld fills any gap that
they would have had. This is one of the reasons that welded rims are
cheaper to make, and it partly accounts for their widespread adoption
by manufacturers.

Chalo Colina

welded rims are more expensive to manufacture. mavic are
electro-pressure welded. the weld itself may be cheap, but the
subsequent refinishing, including the cut under the rim bead hook, is
/much/ more expensive to execute than a simple sleeve joint where no
subsequent machining operations are required.

jim beam wrote:

Chalo wrote:

Welded rims can be cut sloppily because the weld fills any gap that
they would have had. This is one of the reasons that welded rims are
cheaper to make, and it partly accounts for their widespread adoption
by manufacturers.

welded rims are more expensive to manufacture. mavic are
electro-pressure welded. the weld itself may be cheap, but the
subsequent refinishing, including the cut under the rim bead hook, is
/much/ more expensive to execute than a simple sleeve joint where no
subsequent machining operations are required.

My expertise in a lot of this stuff is that of an enthusiast, but I am
a research and development machinist by trade. Running a face plate
setup on a lathe to hack off the braking surfaces of a rim and leave a
nice pretty tool mark is _much_ less labor intensive than finishing the
same weld by hand, as was done previously by Rigida, Araya, and others.
I expect that it's also significantly cheaper than making a pinned
joint that won't cause a major blip on the brakes (though it's plain to
see that not all pinned rims provide that).

Now if a welded rim were heat treated after welding, then that would
almost certainly make it more expensive to produce than an equivalent
pinned rim. Mavic clearly don't do a post-weld heat treat. That's why
their rim joints are the the weakest part of the rim, and why their
rims can't support the same spoke tension as comparable pinned rims of
equal weight.

The surfaces of a Mavic rim's welded joint, outside of the lathe-cut
braking surfaces, are not as carefully finished as you suggest. The
sticker on recent Mavic rims hides both a nasty scar from the electrode
clamps and a hastily trimmed weld. I have used welded rims that had an
even sloppier treatment to the inside of the bead hooks than Mavics,
but those rims cost less than a third of what Mavics do.

Chalo Colina

Chalo wrote:
jim beam wrote:

Chalo wrote:

Welded rims can be cut sloppily because the weld fills any gap that
they would have had. This is one of the reasons that welded rims are
cheaper to make, and it partly accounts for their widespread adoption
by manufacturers.

welded rims are more expensive to manufacture. mavic are
electro-pressure welded. the weld itself may be cheap, but the
subsequent refinishing, including the cut under the rim bead hook, is
/much/ more expensive to execute than a simple sleeve joint where no
subsequent machining operations are required.


My expertise in a lot of this stuff is that of an enthusiast, but I am
a research and development machinist by trade. Running a face plate
setup on a lathe to hack off the braking surfaces of a rim and leave a
nice pretty tool mark is _much_ less labor intensive than finishing the
same weld by hand, as was done previously by Rigida, Araya, and others.
I expect that it's also significantly cheaper than making a pinned
joint that won't cause a major blip on the brakes (though it's plain to
see that not all pinned rims provide that).

any hand finishing operation is very expensive, but i'm talking
automatic machining operations. with a pinned joint, the pin is fitted
and the two ends are pressed together. end of story. with welding,
once the weld is done, it requires machining of all surfaces, including
under the hooks. even when automated, there's no way that's going to be
cheaper than just fitting a pin.


Now if a welded rim were heat treated after welding, then that would
almost certainly make it more expensive to produce than an equivalent
pinned rim. Mavic clearly don't do a post-weld heat treat.

sorry, you can't say that based on visual inspection. i don't know
whether mavic heat treat or not, [although i know some rims are] but
there's no "clearly" about it.

That's why
their rim joints are the the weakest part of the rim, and why their
rims can't support the same spoke tension as comparable pinned rims of
equal weight.

the material coherence of a welded joint is much better than that of a
pinned joint. i would therefore expect the fatigue strength to be much
higher.


The surfaces of a Mavic rim's welded joint, outside of the lathe-cut
braking surfaces, are not as carefully finished as you suggest. The
sticker on recent Mavic rims hides both a nasty scar from the electrode
clamps and a hastily trimmed weld. I have used welded rims that had an
even sloppier treatment to the inside of the bead hooks than Mavics,
but those rims cost less than a third of what Mavics do.

Chalo Colina

agreed, they're not mirror finish, but they're functional in that
there's minimal material loss. and the cosmetics are addressed by using
labels in the offending spots. my old cxp12's were more extensively
finished on both sides of the weld, and it led to a definite "blip" in
the brake track - presumably what led mavic to go for machining in the
first place with their cxp30 rims, etc.