Do underweight rims warp?

Phil Lee said:

I built up a stupid-light 1420g MTB disc/v-brake wheelset using Sun
UFO rims. They're a 395g claimed weight, but they ended up being
360g each, about a 10% underweight.

Quoted message said:

After building it up with ~100kgf tension, I test-rode it using
v-brakes with abysmal results. The bad results were from the wheel
width not being constant or something, so as I brake, the whole bike
shudders when it encounters what seems to be every other spoke.
This is more pronounced on the front, I would guess as a result of
the pretty significant dish on the front wheel. The wheel is very
true on the stand and I can barely see any sort of warping of the
rim as it spins.

Can you see discoloration on the brake track that looks like the
ripples you feel? How many spokes does this wheel have? Are the brake
tracks anodized?

Quoted message said:

My question: are the rims warping from them being too light? Would
it be alleviated by reducing tension? I would venture to guess that
it would eventually stop once I rode it through enough mud to grind
off enough material on the sides to make it constant-width.

If there is something more than anodizing or the like affecting
friction on the side walls I'm at a loss to guess what it is.

Quoted message said:

In case you're wondering, the hubs I used were WTB Laserdisc Lite
disc hubs, which were the lightest and cheapest of any disc or
non-disc mountain hubs. I'm using this wheelset for light weight,
so v-brakes only.

If there is some distortion cause by spoking, it will not come out by
loosening spokes, because to affect braking it would be a permanent
(plastic) deformation. Spoke pattern shadows in anodizing on brake
tracks is common but it never has caused brake shudder as one might
imagine.

Jobst Brandt

Phil said:

I built up a stupid-light 1420g MTB disc/v-brake wheelset using Sun UFO
rims. They're a 395g claimed weight, but they ended up being 360g each,
about a 10% underweight.

After building it up with ~100kgf tension, I test-rode it using v-brakes
with abysmal results. The bad results were from the wheel width not being
constant or something, so as I brake, the whole bike shudders when it
encounters what seems to be every other spoke. This is more pronounced on
the front, I would guess as a result of the pretty significant dish on the
front wheel. The wheel is very true on the stand and I can barely see any
sort of warping of the rim as it spins.

My question: are the rims warping from them being too light? Would it be
alleviated by reducing tension? I would venture to guess that it would
eventually stop once I rode it through enough mud to grind off enough
material on the sides to make it constant-width.

In case you're wondering, the hubs I used were WTB Laserdisc Lite disc hubs,
which were the lightest and cheapest of any disc or non-disc mountain hubs.
I'm using this wheelset for light weight, so v-brakes only.
--
Phil, Squid-in-Training

Sun's web site shows the UFO rim:
http://www.sun-ringle.com/prods/mtbrims.html
Looks like a pretty normal rim.

I've had bad brake shudder upon braking with a flexible fork on my
cyclocross bike. What kind of fork are you using?

Quoted message said:

Phil Lee said:

I built up a stupid-light 1420g MTB disc/v-brake wheelset using Sun
UFO rims. They're a 395g claimed weight, but they ended up being
360g each, about a 10% underweight.

Quoted message said:

After building it up with ~100kgf tension, I test-rode it using
v-brakes with abysmal results. The bad results were from the wheel
width not being constant or something, so as I brake, the whole bike
shudders when it encounters what seems to be every other spoke.
This is more pronounced on the front, I would guess as a result of
the pretty significant dish on the front wheel. The wheel is very
true on the stand and I can barely see any sort of warping of the
rim as it spins.

Can you see discoloration on the brake track that looks like the
ripples you feel? How many spokes does this wheel have?

and how relevant is spoke count to principle? it affects period &
amplitude, but not the fact that local elastic distortion occurs.

Quoted message said:

Are the brake
tracks anodized?

why don't you bother to look them up like dianne did? no, the brake
track is not anodized.

Quoted message said:

Quoted message said:

My question: are the rims warping from them being too light? Would
it be alleviated by reducing tension? I would venture to guess that
it would eventually stop once I rode it through enough mud to grind
off enough material on the sides to make it constant-width.

If there is something more than anodizing or the like affecting
friction on the side walls I'm at a loss to guess what it is.

....

Quoted message said:

Quoted message said:

In case you're wondering, the hubs I used were WTB Laserdisc Lite
disc hubs, which were the lightest and cheapest of any disc or
non-disc mountain hubs. I'm using this wheelset for light weight,
so v-brakes only.

If there is some distortion cause by spoking, it will not come out by
loosening spokes, because to affect braking it would be a permanent
(plastic) deformation.

absolute rubbish!!! rims elastically distort as a direct result of
spoke tension!!! that is /not/ plastic deformation!!! how you gave the
gall to make this stuff up amazes me.

Quoted message said:

Spoke pattern shadows in anodizing on brake
tracks is common

true.

Quoted message said:

but it never has caused brake shudder as one might
imagine.

more jobstian rubbish. you just can't resist trying to hang a bunch of
garbage on one small fact can you.

Quoted message said:

Jobst Brandt

Phil said:

I built up a stupid-light 1420g MTB disc/v-brake wheelset using Sun UFO
rims. They're a 395g claimed weight, but they ended up being 360g each,
about a 10% underweight.

After building it up with ~100kgf tension, I test-rode it using v-brakes
with abysmal results. The bad results were from the wheel width not being
constant or something, so as I brake, the whole bike shudders when it
encounters what seems to be every other spoke. This is more pronounced on
the front, I would guess as a result of the pretty significant dish on the
front wheel. The wheel is very true on the stand and I can barely see any
sort of warping of the rim as it spins.

My question: are the rims warping from them being too light? Would it be
alleviated by reducing tension? I would venture to guess that it would
eventually stop once I rode it through enough mud to grind off enough
material on the sides to make it constant-width.

In case you're wondering, the hubs I used were WTB Laserdisc Lite disc hubs,
which were the lightest and cheapest of any disc or non-disc mountain hubs.
I'm using this wheelset for light weight, so v-brakes only.

i'm with dianne on this one. the frequency of the whole bike juddering
is orders of magnitude below the frequency of spoke encounter. i'd look
for other issues.

in principle, yes, you do get some small elastic distortion of a rim due
to spoke tension, but it should be small in relation to the flexibility
of the brake pad rubber and the length of the brake pad block. this
distortion /will/ be slightly exaggerated if the rim is lighter than
normal, but i doubt it's likely to be an issue. it's also possible for
the rims to be subject to some kind of manufacuring defect where the
brake tracks are not parallel but again, that's not likely.

bottom line, check the braking system. if you can see the frame flex
when you squeeze the rear brake hard, consider one of those horseshoe
reinforcer braces. one of those cured a rear brake judder on a light
steel mtb frame i have.

Quoted message said:

Phil Lee said:

I built up a stupid-light 1420g MTB disc/v-brake wheelset using Sun
UFO rims. They're a 395g claimed weight, but they ended up being
360g each, about a 10% underweight.

Quoted message said:

After building it up with ~100kgf tension, I test-rode it using
v-brakes with abysmal results. The bad results were from the wheel
width not being constant or something, so as I brake, the whole bike
shudders when it encounters what seems to be every other spoke.
This is more pronounced on the front, I would guess as a result of
the pretty significant dish on the front wheel. The wheel is very
true on the stand and I can barely see any sort of warping of the
rim as it spins.

Can you see discoloration on the brake track that looks like the
ripples you feel? How many spokes does this wheel have? Are the brake
tracks anodized?

I haven't ridden these any more than 50 feet. I'm actually still waiting on
more singlespeed conversion parts. The brake track is still painted (not
anodized) black like the rest of it. It has 32 spokes, of course.

Quoted message said:

Quoted message said:

My question: are the rims warping from them being too light? Would
it be alleviated by reducing tension? I would venture to guess that
it would eventually stop once I rode it through enough mud to grind
off enough material on the sides to make it constant-width.

If there is something more than anodizing or the like affecting
friction on the side walls I'm at a loss to guess what it is.

Quoted message said:

In case you're wondering, the hubs I used were WTB Laserdisc Lite
disc hubs, which were the lightest and cheapest of any disc or
non-disc mountain hubs. I'm using this wheelset for light weight,
so v-brakes only.

If there is some distortion cause by spoking, it will not come out by
loosening spokes, because to affect braking it would be a permanent
(plastic) deformation. Spoke pattern shadows in anodizing on brake
tracks is common but it never has caused brake shudder as one might
imagine.

That's what I was thinking, too. What's weird is that even if the rim does
get pulled, the width of the rim should remain relatively constant, even if
the rim, overall, twists a small bit.

I did forget to mention that the left-right alternating spoke offset on
these rims is virtually nothing. Maybe this would increase the magnitude of
rim twist?

I'll probably ride these in mud to grind the rims away to the proper width,
once I get the whole bike working, of course.

--
Phil, Squid-in-Training

Quoted message said:

Phil said:

I built up a stupid-light 1420g MTB disc/v-brake wheelset using Sun
UFO rims. They're a 395g claimed weight, but they ended up being
360g each, about a 10% underweight.

After building it up with ~100kgf tension, I test-rode it using
v-brakes with abysmal results. The bad results were from the wheel
width not being constant or something, so as I brake, the whole bike
shudders when it encounters what seems to be every other spoke.
This is more pronounced on the front, I would guess as a result of
the pretty significant dish on the front wheel. The wheel is very
true on the stand and I can barely see any sort of warping of the
rim as it spins.

My question: are the rims warping from them being too light? Would
it be alleviated by reducing tension? I would venture to guess that
it would eventually stop once I rode it through enough mud to grind
off enough material on the sides to make it constant-width.

In case you're wondering, the hubs I used were WTB Laserdisc Lite
disc hubs, which were the lightest and cheapest of any disc or
non-disc mountain hubs. I'm using this wheelset for light weight, so
v-brakes only. --
Phil, Squid-in-Training

Sun's web site shows the UFO rim:
http://www.sun-ringle.com/prods/mtbrims.html
Looks like a pretty normal rim.

Quoted message said:

I've had bad brake shudder upon braking with a flexible fork on my
cyclocross bike. What kind of fork are you using?

Right now, a RockShox Duke, but I'll be putting a rigid on it shortly.
Neither of them are very flexy. What would be the cause of shudder with a
flexy fork?

--
Phil, Squid-in-Training

Phil said:

I built up a stupid-light 1420g MTB disc/v-brake wheelset using Sun UFO
rims. They're a 395g claimed weight, but they ended up being 360g each,
about a 10% underweight.

After building it up with ~100kgf tension, I test-rode it using v-brakes
with abysmal results. The bad results were from the wheel width not being
constant or something, so as I brake, the whole bike shudders when it
encounters what seems to be every other spoke. This is more pronounced on
the front, I would guess as a result of the pretty significant dish on the
front wheel. The wheel is very true on the stand and I can barely see any
sort of warping of the rim as it spins.

Is it true vertically, or does it have a hump in it? If it's out of
true in the vertical direction, it will shudder when you brake,
regardless of how true it is in the side-to-side direction.

Quoted message said:

My question: are the rims warping from them being too light? Would it be
alleviated by reducing tension? I would venture to guess that it would
eventually stop once I rode it through enough mud to grind off enough
material on the sides to make it constant-width.

In case you're wondering, the hubs I used were WTB Laserdisc Lite disc hubs,
which were the lightest and cheapest of any disc or non-disc mountain hubs.
I'm using this wheelset for light weight, so v-brakes only.

Southern Comfort said:

Quoted message said:

If there is some distortion cause by spoking, it will not come out
by loosening spokes, because to affect braking it would be a
permanent (plastic) deformation.

Quoted message said:

absolute rubbish!!! rims elastically distort as a direct result of
spoke tension!!! that is /not/ plastic deformation!!! how you gave
the gall to make this stuff up amazes me.

This would have to be a change in the cross section of the rim, not a
wave of out-of-trueness. Therefore, it would be a permanent
distortion because for it to affect braking its amplitude would need
to be larger than a few thousandths of an inch, the limit for elastic
deformation transverse to the length of the rim.

Quoted message said:

Quoted message said:

Spoke pattern shadows in anodizing on brake tracks is common

Quoted message said:

true.

Quoted message said:

Quoted message said:

but it never has caused brake shudder as one might imagine.

Quoted message said:

more jobstian rubbish. you just can't resist trying to hang a bunch
of garbage on one small fact can you.

Instead of that sort of reply, would you please explain why you
believe this to be so. Before machined sidewall, this was the main
cosmetic complaint I heard for anodized rims but I never heard anyone
say it made the braking shudder.

Jobst Brandt

Quoted message said:

Southern Comfort writes:

Quoted message said:

Quoted message said:

more jobstian rubbish. you just can't resist trying to hang a bunch
of garbage on one small fact can you.

Instead of that sort of reply, would you please explain why you
believe this to be so. Before machined sidewall, this was the main
cosmetic complaint I heard for anodized rims but I never heard anyone
say it made the braking shudder.

Even thick anodising is on the order of tens of microns, is it not? Then
the deformation (not making comments about plastically or elastically)
must be on that order of magnitude to get the neat wavy effect for any
length of time.

I would imagine that paired-spoke wheels have much less of that kind of
deformation than regular wheels with the same low number of spokes,
incidentally.

Jasper

Quoted message said:

Southern Comfort said:

Quoted message said:

If there is some distortion cause by spoking, it will not come out
by loosening spokes, because to affect braking it would be a
permanent (plastic) deformation.

Quoted message said:

absolute rubbish!!! rims elastically distort as a direct result of
spoke tension!!! that is /not/ plastic deformation!!! how you gave
the gall to make this stuff up amazes me.

This would have to be a change in the cross section of the rim, not a
wave of out-of-trueness. Therefore, it would be a permanent
distortion because for it to affect braking its amplitude would need
to be larger than a few thousandths of an inch, the limit for elastic
deformation transverse to the length of the rim.

eh? why are you confusing true with changes in section??? jobst, look
at one of these rims. there's a big patch of wear on the anodizing with
spokes orinted to the near side and a small patch with the spoke doing
to the far side. that's all spoke-induced elastic distortion. want me
to post a pic?

Quoted message said:

Quoted message said:

Quoted message said:

Spoke pattern shadows in anodizing on brake tracks is common

Quoted message said:

true.

Quoted message said:

Quoted message said:

but it never has caused brake shudder as one might imagine.

Quoted message said:

more jobstian rubbish. you just can't resist trying to hang a bunch
of garbage on one small fact can you.

Instead of that sort of reply, would you please explain why you
believe this to be so. Before machined sidewall, this was the main
cosmetic complaint I heard for anodized rims but I never heard anyone
say it made the braking shudder.

er, if you have a partially worn rim with pathces of what amounts to two
different materials being present at different locations around a rim's
circumference, and each material has a different coefficient of
friction, are you telling me you seriously don't see any periodic change
in the force exerted on the caliper as the rim passes through???

Quoted message said:

Jobst Brandt

Threeducks said:

Phil said:

I built up a stupid-light 1420g MTB disc/v-brake wheelset using Sun
UFO rims. They're a 395g claimed weight, but they ended up being
360g each, about a 10% underweight.

After building it up with ~100kgf tension, I test-rode it using
v-brakes with abysmal results. The bad results were from the wheel
width not being constant or something, so as I brake, the whole bike
shudders when it encounters what seems to be every other spoke. This is
more pronounced on the front, I would guess as a result of
the pretty significant dish on the front wheel. The wheel is very
true on the stand and I can barely see any sort of warping of the
rim as it spins.

Is it true vertically, or does it have a hump in it? If it's out of
true in the vertical direction, it will shudder when you brake,
regardless of how true it is in the side-to-side direction.

Nope, this is a new wheelset I built. This isn't a single hump. We're
talking 16 (or so) tire-skid-inducing vibrations on each revolution. Maybe
not 16, but it's quite obvious that it occurs at spoke holes.

--
Phil, Squid-in-Training

Squid-in-Training said:

Threeducks said:

Phil said:

I built up a stupid-light 1420g MTB disc/v-brake wheelset using Sun
UFO rims. They're a 395g claimed weight, but they ended up being
360g each, about a 10% underweight.

After building it up with ~100kgf tension, I test-rode it using
v-brakes with abysmal results. The bad results were from the wheel
width not being constant or something, so as I brake, the whole bike
shudders when it encounters what seems to be every other spoke. This is
more pronounced on the front, I would guess as a result of
the pretty significant dish on the front wheel. The wheel is very
true on the stand and I can barely see any sort of warping of the
rim as it spins.

Is it true vertically, or does it have a hump in it? If it's out of
true in the vertical direction, it will shudder when you brake,
regardless of how true it is in the side-to-side direction.

Nope, this is a new wheelset I built. This isn't a single hump. We're
talking 16 (or so) tire-skid-inducing vibrations on each revolution. Maybe
not 16, but it's quite obvious that it occurs at spoke holes.

Dear Phil,

Dumb question, but if the bare wheel is true on the stand,
have you checked it on the stand with a fully inflated tire
mounted?

Carl Fogel

jim beam said:

Phil said:

I built up a stupid-light 1420g MTB disc/v-brake wheelset using Sun
UFO rims. They're a 395g claimed weight, but they ended up being
360g each, about a 10% underweight.

After building it up with ~100kgf tension, I test-rode it using
v-brakes with abysmal results. The bad results were from the wheel
width not being constant or something, so as I brake, the whole bike
shudders when it encounters what seems to be every other spoke. This is
more pronounced on the front, I would guess as a result of
the pretty significant dish on the front wheel. The wheel is very
true on the stand and I can barely see any sort of warping of the
rim as it spins. My question: are the rims warping from them being too
light? Would
it be alleviated by reducing tension? I would venture to guess that
it would eventually stop once I rode it through enough mud to grind
off enough material on the sides to make it constant-width.

In case you're wondering, the hubs I used were WTB Laserdisc Lite
disc hubs, which were the lightest and cheapest of any disc or
non-disc mountain hubs. I'm using this wheelset for light weight, so
v-brakes only.

i'm with dianne on this one. the frequency of the whole bike
juddering is orders of magnitude below the frequency of spoke
encounter.

But that's what I'm talking about. I'm talking super-jerky braking, so much
that it would lock the rear wheel 16 times each revolution. I'm going to go
back and double-check that it correlates with the spoke holes, but I'm
pretty sure this is the case.

Quoted message said:

in principle, yes, you do get some small elastic distortion of a rim
due to spoke tension, but it should be small in relation to the
flexibility of the brake pad rubber and the length of the brake pad
block. this distortion /will/ be slightly exaggerated if the rim is
lighter than normal, but i doubt it's likely to be an issue. it's
also possible for the rims to be subject to some kind of manufacuring
defect where the brake tracks are not parallel but again, that's not
likely.

This is a machined, then painted, rim. The paint very clearly and uniformly
shows the machining marks underneath.

Quoted message said:

bottom line, check the braking system. if you can see the frame flex
when you squeeze the rear brake hard, consider one of those horseshoe
reinforcer braces. one of those cured a rear brake judder on a light
steel mtb frame i have.

I've had Rhyno Lites (550g rim) and Mavic x221 (probably 450g or so) rims,
both handbuilt by me, on the same frame, with never a problem. I'm
currently having the issue with both front and rear wheels. The fork is a
RockShox Duke, but it also does it on the rigid fork I have.

--
Phil, Squid-in-Training

Phil said:

jim beam said:

Phil said:

I built up a stupid-light 1420g MTB disc/v-brake wheelset using Sun
UFO rims. They're a 395g claimed weight, but they ended up being
360g each, about a 10% underweight.

After building it up with ~100kgf tension, I test-rode it using
v-brakes with abysmal results. The bad results were from the wheel
width not being constant or something, so as I brake, the whole bike
shudders when it encounters what seems to be every other spoke. This is
more pronounced on the front, I would guess as a result of
the pretty significant dish on the front wheel. The wheel is very
true on the stand and I can barely see any sort of warping of the
rim as it spins. My question: are the rims warping from them being too
light? Would
it be alleviated by reducing tension? I would venture to guess that
it would eventually stop once I rode it through enough mud to grind
off enough material on the sides to make it constant-width.

In case you're wondering, the hubs I used were WTB Laserdisc Lite
disc hubs, which were the lightest and cheapest of any disc or
non-disc mountain hubs. I'm using this wheelset for light weight, so
v-brakes only.

i'm with dianne on this one. the frequency of the whole bike
juddering is orders of magnitude below the frequency of spoke
encounter.

But that's what I'm talking about. I'm talking super-jerky braking, so much
that it would lock the rear wheel 16 times each revolution. I'm going to go
back and double-check that it correlates with the spoke holes, but I'm
pretty sure this is the case.

Quoted message said:

in principle, yes, you do get some small elastic distortion of a rim
due to spoke tension, but it should be small in relation to the
flexibility of the brake pad rubber and the length of the brake pad
block. this distortion /will/ be slightly exaggerated if the rim is
lighter than normal, but i doubt it's likely to be an issue. it's
also possible for the rims to be subject to some kind of manufacuring
defect where the brake tracks are not parallel but again, that's not
likely.

This is a machined, then painted, rim. The paint very clearly and uniformly
shows the machining marks underneath.

ok, in that case, and based on what you say above, see if you can remove
the paint. or go find a nice muddy hill.

Quoted message said:

Quoted message said:

bottom line, check the braking system. if you can see the frame flex
when you squeeze the rear brake hard, consider one of those horseshoe
reinforcer braces. one of those cured a rear brake judder on a light
steel mtb frame i have.

I've had Rhyno Lites (550g rim) and Mavic x221 (probably 450g or so) rims,
both handbuilt by me, on the same frame, with never a problem. I'm
currently having the issue with both front and rear wheels. The fork is a
RockShox Duke, but it also does it on the rigid fork I have.

Quoted message said:

Squid-in-Training said:

Threeducks said:

Phil, Squid-in-Training wrote:
> I built up a stupid-light 1420g MTB disc/v-brake wheelset using Sun
> UFO rims. They're a 395g claimed weight, but they ended up being
> 360g each, about a 10% underweight.
>
> After building it up with ~100kgf tension, I test-rode it using
> v-brakes with abysmal results. The bad results were from the wheel
> width not being constant or something, so as I brake, the whole
> bike shudders when it encounters what seems to be every other
> spoke. This is more pronounced on the front, I would guess as a
> result of
> the pretty significant dish on the front wheel. The wheel is very
> true on the stand and I can barely see any sort of warping of the
> rim as it spins.

Is it true vertically, or does it have a hump in it? If it's out of
true in the vertical direction, it will shudder when you brake,
regardless of how true it is in the side-to-side direction.

Nope, this is a new wheelset I built. This isn't a single hump.
We're talking 16 (or so) tire-skid-inducing vibrations on each
revolution. Maybe not 16, but it's quite obvious that it occurs at
spoke holes.

Dear Phil,

Dumb question, but if the bare wheel is true on the stand,
have you checked it on the stand with a fully inflated tire
mounted?

Yep. Were you thinking reduced spoke tension or the bowing out due to air
pressure would cause a problem?

--
Phil, Squid-in-Training

Squid-in-Training said:

Quoted message said:

Squid-in-Training said:

Threeducks wrote:
> Phil, Squid-in-Training wrote:
>> I built up a stupid-light 1420g MTB disc/v-brake wheelset using Sun
>> UFO rims. They're a 395g claimed weight, but they ended up being
>> 360g each, about a 10% underweight.
>>
>> After building it up with ~100kgf tension, I test-rode it using
>> v-brakes with abysmal results. The bad results were from the wheel
>> width not being constant or something, so as I brake, the whole
>> bike shudders when it encounters what seems to be every other
>> spoke. This is more pronounced on the front, I would guess as a
>> result of
>> the pretty significant dish on the front wheel. The wheel is very
>> true on the stand and I can barely see any sort of warping of the
>> rim as it spins.
>
> Is it true vertically, or does it have a hump in it? If it's out of
> true in the vertical direction, it will shudder when you brake,
> regardless of how true it is in the side-to-side direction.

Nope, this is a new wheelset I built. This isn't a single hump.
We're talking 16 (or so) tire-skid-inducing vibrations on each
revolution. Maybe not 16, but it's quite obvious that it occurs at
spoke holes.

Dear Phil,

Dumb question, but if the bare wheel is true on the stand,
have you checked it on the stand with a fully inflated tire
mounted?

Yep. Were you thinking reduced spoke tension or the bowing out due to air
pressure would cause a problem?

Dear Phil,

Really just wondering if that had been eliminated. Sounds as
if it has.

If I understand things, the rim seems true enough on the
stand even with an inflated tire, but the brake seems to
grab the sidewalls as if they aren't true.

The brakes would (I think) take a little lateral wobble in
stride, so . . .

Either the rim's sidewalls are symmetrically bulging in (or
out), presumably at the spokes, enough to grab the brakes,
but not enough for you to see on the truing stand.

Or the sidewall surface has been scraped or somehow changed
between the spokes so that the grab is due to a new surface,
not bulging in or out.

Or both.

Carl Fogel

Phil said:

I built up a stupid-light 1420g MTB disc/v-brake wheelset using Sun UFO
rims. They're a 395g claimed weight, but they ended up being 360g each,
about a 10% underweight.

After building it up with ~100kgf tensio n, I test-rode it using v-brakes
with abysmal results. The bad results were from the wheel width not being
constant or something, so as I brake, the whole bike shudders when it
encounters what seems to be every other spoke. This is more pronounced on
the front, I would guess as a result of the pretty significant dish on the
front wheel. The wheel is very true on the stand and I can barely see any
sort of warping of the rim as it spins.

1. Why is the front wheel dished?
2. Try taking a caliper to the rim to check for constant cross-section
width. If you don't have a dial caliper or micrometer, you can use the
brake pads, get them to almost touching and rotate the wheel, see if
the gap opens or closes.
3. If not, i.e. if the section is in fact constant, run your fingers
along the track, or again with the brake pads, this time just touching,
and see if the coefficient of friction changes- if so, check for some
defect in the paint surface.
4. If the section is not constant: these are very light rims with thin
sidewalls and V-brakes have a lot leverage. Is it enough to deform the
cross-section under load?

jim beam said:

eh? why are you confusing true with changes in section??? jobst, look
at one of these rims. there's a big patch of wear on the anodizing with
spokes orinted to the near side and a small patch with the spoke doing
to the far side. that's all spoke-induced elastic distortion. want me
to post a pic?

Please post the pic so that we lesser mortals know what you are talking
about.

Jasper

Phil said:

I built up a stupid-light 1420g MTB disc/v-brake wheelset using Sun UFO
rims. They're a 395g claimed weight, but they ended up being 360g each,
about a 10% underweight.

After building it up with ~100kgf tension, I test-rode it using v-brakes
with abysmal results. The bad results were from the wheel width not being
constant or something, so as I brake, the whole bike shudders when it
encounters what seems to be every other spoke. This is more pronounced on
the front, I would guess as a result of the pretty significant dish on the
front wheel. The wheel is very true on the stand and I can barely see any
sort of warping of the rim as it spins.

My question: are the rims warping from them being too light? Would it be
alleviated by reducing tension? I would venture to guess that it would
eventually stop once I rode it through enough mud to grind off enough
material on the sides to make it constant-width.

In case you're wondering, the hubs I used were WTB Laserdisc Lite disc hubs,
which were the lightest and cheapest of any disc or non-disc mountain hubs.
I'm using this wheelset for light weight, so v-brakes only.

Perhaps the problem is in the brake, grabing, flexing, winding up and
springing back, kinda chattering back and forth at frequency of 16 times
a revolution.

Are the brake mounting studs set very wide for this narrow rim?

Check the bushings, watch the brake arms.

Try a different brake.

-Marcus