Understanding Wheel Building

I recently came across some Shimano instructions on spoke lacing for
disc brake hubs. This lead me to a web search and a tech discussion on
Velonews (by Lennard Zinn) on the subject of building disc brake
wheels.

For both sides of the the front wheel and the disc-side of the rear
wheel, all parties seem to agree on the following: the spokes that are
loaded in greater tension on braking, i.e., the "leading" spokes,
should be installed such that they run on the outside of the hub. The
reason given is that braking generates higher loads on spokes (as much
10 times greater than pedaling) and spokes on the outside of the hub
are less likely to fail in the head or elbow because they fit tight to
the hub flange and therefore either transfer some of the load to the
flange (by friction?) or, due to the tight fit, they flex less and
therefore are more resistant to fatigue failure.

I could accept [some of ] that; however, this seems to be in conflict
with another design consideration that is commonly advised: In order
to maintain clearance between the spokes and the brake calipers, the
spokes should be laced such that the braking torque causes the
[crossed] spokes to move inward rather than outward during braking. A
similar design guideline has been applied to the drive-side of rear
hubs, i.e., the spokes should be laced such that the pedaling torque
causes the [crossed] spokes to move inward rather than outward during
pedaling to maintain clearance from the derailleur.

Unless I am missing something, here is the apparent conflict: If the
leading spokes [which experience increased tension during disc
braking] are run along the outside of the hub flange, they normally
will cross under a trailing spoke. At the point of crossing contact,
both spokes are slightly bent. On braking, tension changes in both
spokes (leading = increased tension, trailing = decreased tension),
moves the crossing point outward. If there is extremely low initial
tension in the spokes, it may be possible for the unconstrained
trailing spoke to buckle and perhaps contact the brake caliper.

Is it possible to lace a wheel and satisfy both of these design
objectives, is one more important than the other, or does the lacing
pattern make insignificant difference on wheel performance?

There is a third alternative, which is to not interlace the spokes. If
they clear the derailleur and/or brake calipers when static, they will
clear it regardless of the spoke load.

You could also do what Mavic does on their MTB wheels. They use a hub
designed for straight pull spokes with the paired flanges offset, so
that crossing spokes go directly to the rim without touching each other.

I gave that some thought, not interlacing the spokes. That would seem
to achieve all of the design objectives. I just have not seen it in
practice nor have I seen it stated in any of the wheel building
guides. Why isn't it commonly done?

On Jan 1, 4:25*pm, Brian Nystrom wrote:
There is a third alternative, which is to not interlace the spokes. If
they clear the derailleur and/or brake calipers when static, they will
clear it regardless of the spoke load.

You could also do what Mavic does on their MTB wheels. They use a hub
designed for straight pull spokes with the paired flanges offset, so
that crossing spokes go directly to the rim without touching each other.

HKEK wrote:
Is it possible to lace a wheel and satisfy both of these design
objectives, is one more important than the other, or does the lacing
pattern make insignificant difference on wheel performance?

Your last answer is the right one. I don't buy the argument that the
"elbows out" spokes have some special strength advantage. I also don't
believe that leaving the spokes uncrossed will help at all... if there
is a clearance issue that would be the worst case... the elbows-out
spokes would stick out farther than they ever would if crossed. So I
generally lace wheels so the pulling spokes are elbows-in and cross
over the outside of the other spokes.

Another possible consideration, especially on a front wheel and when
using a light shallow rim, is the pulling spokes distorting the rim.
If you lace each side the same way, then you will end up with pulling
spokes (and pushing spokes) in pairs at the rim. This will tend to
make the rim unround when braking hard... moreso than if the pushing
and pulling spokes alternated at the rim. I haven't heard any reports
of this causing problems, but it is easy enough to lace the non-disc
side with the pulling spokes elbows-out.

There is a myth that having ebows-out on one side and elbows-in on the
other will tend to pull the rim over when subjected to torque... but
in reality the pulling spokes on each side of the rim will increase in
tension and the pushing spokes will decrease the same amount... so
you'll have the same average tension on each side that you started
with... and the rim will still be centered.

HKEK wrote:
I gave that some thought, not interlacing the spokes. That would seem
to achieve all of the design objectives. I just have not seen it in
practice nor have I seen it stated in any of the wheel building
guides. Why isn't it commonly done?

On Jan 1, 4:25 pm, Brian Nystrom wrote:
There is a third alternative, which is to not interlace the spokes. If
they clear the derailleur and/or brake calipers when static, they will
clear it regardless of the spoke load.

You could also do what Mavic does on their MTB wheels. They use a hub
designed for straight pull spokes with the paired flanges offset, so
that crossing spokes go directly to the rim without touching each other.

Interlacing helps spread to maintain tension in the spokes that are
detensioned by the torque load, by putting a side load on them as the
spokes bearing the try to straighten. If the wheel is built with high
enough spoke tension relative to the torque load, it shouldn't matter
whether the spokes are interlaced or not. The Mavic wheels I spoke of
have low spoke counts and relatively high spoke tension, which works
well. I don't know if this would be a reasonable idea for a typical
32/36 spoke wheel or not. Although I've seen wheels built that way, I
don't recall if I've ever built one myself. Perhaps Jobst could speak to
this.

Brian Nystrom wrote:
Interlacing helps spread to maintain tension in the spokes that are
detensioned by the torque load, by putting a side load on them as the
spokes bearing the try to straighten.

True... but a very small effect.

The Mavic wheels I spoke of
have low spoke counts and relatively high spoke tension, which works
well.

It works anyway. The reason Mavic does it is because it doesn't make
sense to lace those fat aluminum spokes, and... they can. A lot of
this biz is just doing things different so you can claim that you are
using some new "technology".

HKEK wrote:
reason given is that braking generates higher loads on spokes (as much
10 times greater than pedaling)

Wanted to comment on this also... since it isn't true. The limits of
torque will be either loss of traction or an endo. If you do the basic
force balance you will see that it is possible to generate as much
rear wheel torque via stomping in a low gear, as what you could
achieve via braking on the front.

"Ron Ruff" wrote in message
...
HKEK wrote:
reason given is that braking generates higher loads on spokes (as much
10 times greater than pedaling)

Wanted to comment on this also... since it isn't true. The limits of
torque will be either loss of traction or an endo. If you do the basic
force balance you will see that it is possible to generate as much
rear wheel torque via stomping in a low gear, as what you could
achieve via braking on the front.

FWIW, the torque follows a sine wave over time during pedaling, whereas it
exists through the full 360 of rotation during braking. It seems that
fatigue is more of an issue at that point and would be more of a concern for
the rear than the front wheel, especially if the rear is disc also.

Phil

On Jan 2, 2:36*pm, Ron Ruff wrote:
HKEK wrote:
reason given is that braking generates higher loads on spokes (as much
10 times greater than pedaling)

Wanted to comment on this also... since it isn't true. The limits of
torque will be either loss of traction or an endo. If you do the basic
force balance you will see that it is possible to generate as much
rear wheel torque via stomping in a low gear, as what you could
achieve via braking on the front.

If this is true, and I have no reason to disbelieve it, then none of
this matters. The weight of the rider plus bike creates more tension
in the wheel than pedaling torque. Therefore, there is no reason to
consider braking torque, because it's less than the tensions of
coasting, and wheels are already adequate to sustain coasting.

Tom Reingold
Noo Joizy

On Fri, 2 Jan 2009 18:54:01 -0800 (PST), Tom Reingold
wrote:

On Jan 2, 2:36*pm, Ron Ruff wrote:
HKEK wrote:
reason given is that braking generates higher loads on spokes (as much
10 times greater than pedaling)

Wanted to comment on this also... since it isn't true. The limits of
torque will be either loss of traction or an endo. If you do the basic
force balance you will see that it is possible to generate as much
rear wheel torque via stomping in a low gear, as what you could
achieve via braking on the front.

If this is true, and I have no reason to disbelieve it, then none of
this matters. The weight of the rider plus bike creates more tension
in the wheel than pedaling torque. Therefore, there is no reason to
consider braking torque, because it's less than the tensions of
coasting, and wheels are already adequate to sustain coasting.

Tom Reingold
Noo Joizy

Dear Tom,

"Braking with a caliper brake causes a small but significant radial
load that affects spoke tension. Under hard braking, the brake shoes
retard the rim with a force of up to 500 N by pushing rearward with
250 N force and pulling on the front half of the rim equally. This
increases compression in the rear half of the rim and decreases
compression in the front half about the same as the increase from tire
pressure."

"Spokes in the forward half of the wheel become about 5% looser and
ones in the rear, 5% tighter. At the caliper and the ground contact
point, where forces act on the wheel, there is little effect so
tension remains unchanged. The bending stiffness of the rim and the
direction of the braking force cause a smooth transition in spoke
tension as the rim passes through the brake caliper. Of all the loads
on a wheel, braking is the only one that causes an significant
increase in rim compression, and severe braking can cause an
overtensioned wheel to collapse into a saddle shape (pretzel)."

--The Bicycle Wheel, 3rd edition

Earlier editions used 50/25 kg for 500/250 N and 5 kg for 5% (assuming
a 100 kgf spoke tension).

Cheers,

Carl Fogel