Spoke tension question

Marcin J? writes:

Do I need to worry about the variability in spoke tension?

First of all, it's important to know that the left and right side
spokes of rear wheels for derailer bikes, and on several other
types of rear wheel, use different tensions. There's more tension
on the drive side spokes, which isn't a good thing but is inherit
to any wheel where the spoke-end centerline is going to end up
anywhere but exactly between the two flanges. It's unclear whether
you know this or not.

I'm not sure if I understood all of the wisdom (I'm pretty sure I
didn't) and I'm not sure if you understand my English but I have to
ask a question:

What about SRAM hubs (and probably some others too) which have
flanges located symmetrically from the hub center (rim is exactly in
the middle between the flanges)? They should allow to build wheels
with equally tensioned spokes at both sides. However the distance
between flanges is reduced. Are wheels build on these hubs stronger
than "classic" ones?

That depends on ho closely the flanges are spaced. I'm sure that the
reason for flange spacing is obvious because otherwise bicycle wheels
would not need two flanges, the lateral strength of the wheel coming
from the lateral slope of the spokes, the larger the wheel, the
greater the flange spacing needs to be.

Even with unequal tension in spokes on left and right, the more widely
spaced asymmetrical rear wheel is stronger than the narrow spaced
symmetrical one for lateral stability but if spoke tension of the
left side is too low, these spokes may become loose under radial
loading and allow spoke nippled to unscrew.

With machine built wheels these days, sufficiently tensioned reliable
wheels are harder to find because machines, up to now anyway, cannot
master spoke twist that occurs as spoke become tighter, therefore,
they are set to cease tensioning at a lower than adequate level. For
this reason thread lock lubricants have been introduced to arrest spoke
nipple rotation after a wheel is built. This is a band-aid, so to
speak, that covers a wheel deficiency without correcting it.

Jobst Brandt

Gary Robinson wrote:

Do I need to worry about the variability in spoke tension?

Yup. The shop probably just did the minimum to make it true... and that
is not good. On a wheel with lots of spokes it isn't difficult to make
a wheel run true even if the tension is uneven... especially if the
tension is lower than optimum.

Do these have Shimano 2200 hubs and generic 14g spokes? If so, then I
have the same wheels. Since machine-built wheels come undertensioned in
the rear (as Jobst mentioned), I increased the tension soon after I got
mine. I don't have a tensiometer, so I just guessed on tension. I also
didn't worry about getting the tension perfectly even, which would have
taken a lot more work. They went for 5,000 miles, perfectly true
without being touched... but then I started popping spokes. So far 1 on
the front, and 2 on each side on the rear in the last 2,000 miles. I
don't know if the cause was a poor build or bad spokes, or both... but
if I could do it over again, I'd loosen all the spokes until they were
slack, then use Jobst's procedure for building a wheel. Sheldon Brown
also has a basic procedure for building a wheel on his website. The
only tool you need is a spoke wrench... and some lube.

If any more spokes break, I'm going to buy a whole new set of DB spokes
and start over. I'm quite happy with the rims... they seem to be
durable (no spoke hole cracking!) and have a thick braking surface.
Hubs are fine too... cheap, but they work.

writes:

Even with unequal tension in spokes on left and right, the more widely
spaced asymmetrical rear wheel is stronger than the narrow spaced
symmetrical one for lateral stability but if spoke tension of the
left side is too low, these spokes may become loose under radial
loading and allow spoke nippled to unscrew.

I don't believe that this is entirely true. An exception is a wheel
with paired spokes; a pure radial load changes the tension in both
spokes proportionally, so both reach zero at the same applied force.
Asymmetric loosening of spokes in a paired spoke rim is due to lateral
loading. For a normal rim, things are more complicated.

Joe Riel

Joe Riel wrote:
writes:

Even with unequal tension in spokes on left and right, the more widely
spaced asymmetrical rear wheel is stronger than the narrow spaced
symmetrical one for lateral stability but if spoke tension of the
left side is too low, these spokes may become loose under radial
loading and allow spoke nippled to unscrew.

I don't believe that this is entirely true. An exception is a wheel
with paired spokes; a pure radial load changes the tension in both
spokes proportionally, so both reach zero at the same applied force.
Asymmetric loosening of spokes in a paired spoke rim is due to lateral
loading. For a normal rim, things are more complicated.

I'm not following your logic. If say, the left side spokes are half the
tension of the right side in order to center the rim, then a radial
load that completely detensions the left side will only reduce the
right side tension by a little over half. Doesn't really matter if the
spokes are paired or not.

Joe Riel wrote:
writes:

Even with unequal tension in spokes on left and right, the more widely
spaced asymmetrical rear wheel is stronger than the narrow spaced
symmetrical one for lateral stability but if spoke tension of the
left side is too low, these spokes may become loose under radial
loading and allow spoke nippled to unscrew.

I don't believe that this is entirely true. An exception is a wheel
with paired spokes; a pure radial load changes the tension in both
spokes proportionally, so both reach zero at the same applied force.
Asymmetric loosening of spokes in a paired spoke rim is due to lateral
loading. For a normal rim, things are more complicated.

I guess my other post disappeared, so I'll try again...

If you displace the rim radially, the spokes on both sides will see
nearly the same tension change... in other words a radial load will
completely detension the left side spokes long before the right,
because the left side has much lower initial tension. Doesn't matter if
it is paired-spoke or not.

That's why it makes sense (to me) to use light DB spokes (1.6mm or
less) on the left side, since it takes a greater displacement for them
to lose tension.

Joe Riel wrote:
writes:

Even with unequal tension in spokes on left and right, the more widely
spaced asymmetrical rear wheel is stronger than the narrow spaced
symmetrical one for lateral stability but if spoke tension of the
left side is too low, these spokes may become loose under radial
loading and allow spoke nippled to unscrew.

I don't believe that this is entirely true. An exception is a wheel
with paired spokes; a pure radial load changes the tension in both
spokes proportionally, so both reach zero at the same applied force.

I don't understand how this could possibly be true as you put it. This
doesn't apply to conventional wheels and I fail to see how it would be
different for paired-spoke ones. I could see how there might be factors
that make it so that adjacent spokes coming from different sides of the
hub lose tension somewhat disproportionately when there's a load
originating directly between them (I don't have Jobst's book handy, but
it's data would say whether this was true or not), but I'd like to know
where to go for an explanation of how it could happen that both spokes
of different tensions would reach zero tension at the same time from
such a load. And I'm not saying your wrong, because I don't deal with
paired spoke wheels and don't have experience with them and sure
haven't ever seen a technical analysis of them that I trust.

Asymmetric loosening of spokes in a paired spoke rim is due to lateral
loading. For a normal rim, things are more complicated.

Joe Riel

"Nate Knutson" writes:

Joe Riel wrote:
writes:

Even with unequal tension in spokes on left and right, the more widely
spaced asymmetrical rear wheel is stronger than the narrow spaced
symmetrical one for lateral stability but if spoke tension of the
left side is too low, these spokes may become loose under radial
loading and allow spoke nippled to unscrew.

I don't believe that this is entirely true. An exception is a wheel
with paired spokes; a pure radial load changes the tension in both
spokes proportionally, so both reach zero at the same applied force.

I don't understand how this could possibly be true as you put it. This
doesn't apply to conventional wheels and I fail to see how it would be
different for paired-spoke ones.

Fortunately, you don't have to do any math to demonstrate this. Start
with a spoked wheel with zero tension in the spokes, but all just
snugged up (we're going to ignore the effect of "bedding in"). Now,
rather than tightening the wheel we're going to put it in tension by
expanding the rim (don't worry that this isn't practical, this is a
thought experiment). Assume that the change in length of the spokes
during this process is negligible compared to the geometry of the
wheel. Because the spokes don't change length, the starting angles
are identical to the final angles after the rim is expanded. That is,
the rim doesn't move laterally as it is expanded.

Expanding the rim is a pure radial load. If we reverse it, that is,
apply a pure radial compressive load, the tensions in the spoke will
decrease and, because they started with identical (zero) tension
will each reach zero tension concurrently. Note this is independent
of the spoke angle.

Now, it isn't true that the spokes don't change length. I'll have
to think about this a bit more, but I don't believe it affects the
argument. That is, even though the rim may shift laterally as the
spokes compress/elongate, reversing the procedure (as in the thought
experiment) just causes them to move oppositely, so both sides
reach zero tension at the same time (some hand-waving involved 8-).

Joe Riel

Jobst Brandt wrote:

I think the term would better be "right side" spokes, some hubs today
having the drive (crossed spokes) on the left and radial on the right.
The asymmetry arising from the gear cluster being on the right side
and displacing the hub to the left. Besides, "drive side" sounds
pretentious and has a ring of insider jargon that thrives in
bicycling....

However, people like Sheldon Brown confuse the issue by building
left-hand-drive bicycles: http://sheldonbrown.org/gunnar/index.htm.


--
Tom Sherman - Fox River Valley

Joe Riel wrote:
writes:


Even with unequal tension in spokes on left and right, the more widely
spaced asymmetrical rear wheel is stronger than the narrow spaced
symmetrical one for lateral stability but if spoke tension of the
left side is too low, these spokes may become loose under radial
loading and allow spoke nippled to unscrew.


I don't believe that this is entirely true. An exception is a wheel
with paired spokes; a pure radial load changes the tension in both
spokes proportionally, so both reach zero at the same applied force.
Asymmetric loosening of spokes in a paired spoke rim is due to lateral
loading. For a normal rim, things are more complicated.

Joe Riel

you are correct about pure radial loads. but being as we don't see pure
radial loads in the real world, and spoke angle affects tension delta on
lateral loading, left side spokes on highly dished wheels do "appear" to
slacken first because they do loosen.

Gary Robinson wrote:
Hello,

I've been using a set of wheels with CXP22 rims and Shimano hubs.

When checking the bike over I noticed that a number of spokes on the rear
wheel were very loose. I started tweaking them up, but the wheel went out of
true. Not being experienced with wheels, I took it down to the LBS,
explained the problem and asked them to true and tension it.

Checking the wheel, I've found that while it's nice and true, there's a lot
of variability in spoke tension on the non-drive side and some of the spokes
have a lot less tension in them than others.

My favourite ride is a 10 mile climb, followed by a 10 mile descent. The
road surface for the first couple of miles of the descent is pretty bad.
Lots of potholes and cracks, resulting in a rough and bumpy ride. I'm not a
lightweight either, so I'm sure the wheels aren't having an easy time.

Do I need to worry about the variability in spoke tension?

Thanks
Gary


if the rims are still true, and the spokes have been bedded in
sufficiently, the spoke tension should remain relatively even [on the
same side, not side to side] in use. find a decent wheel builder
[unfortunately not so easy] or learn how to do this job yourself.
there's plenty of online sources of info on this subject. check out
sheldon's web site in particular.