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8c.1 Stress Relieving Spokes


This article is from the Bicycles FAQ, by Mike Iglesias with numerous contributions by others.

8c.1 Stress Relieving Spokes

From: Jobst Brandt <jobst.brandt@stanfordalumni.org>
Date: Mon, 29 Nov 1999 17:13:28 PST

> I wonder if "stress-relieving" is entirely correct? I see it as a
> yielding/hardening process, in which the yield load is increased by
> embedding the spoke elbow in the hub, bending the elbow to a
> different angle, etc. When unloaded from a high load, this area of
> the spoke should be more or less elastic.

> So I think the term should be "overloading" or "hardening" -- any
> thoughts?

Yes. It appears that the process of stress relieving is obscure to
many if not most people, because after seeming to have made it clear,
comments like the above surface. Spokes are cold formed from wire
that is (at least DT) as hard and work hardened as it can become.
Tensioning does not further harden spokes, there being no plastic
deformation. Besides, wire ductility is important in both forming
spokes and in use.

The coiled wire from which spokes are made is straightened by running
it first between rollers staggered in X and then in Y, the wire moving
in the Z direction. Reverse bending acts as a degausser, having ever
diminishing excursions that affect ever shallower depths of the wire.
This stress relieves the wire while removing the curl of being shipped
in a coil. If it had no curl, releasing its free end on the spool
would allow it to uncoil explosively into a huge birds nest.

Wire is cut into suitable lengths, the first operation being to cold
form a spoke head onto one end with one axial blow of a die, after
which the spoke is cut to a specific length before rolling the thread
and bending a 100 degree elbow.

Threads, head, and elbow, contain metal that was plastically deformed
(beyond yield) as well as metal that was elastically deformed, each
having elastic memory. In these transitions, parts that yielded and
ones that did not conflict, each wanting to return to or stay in a
different shape. This is why a spoke bent by hand springs back only
partially when released.

On lacing spokes into a wheel, elbows are often additionally bent
(brought to yield), thus remaining at or exceeding yield stress during
tensioning. Threads also have internal tensile stress besides local
compressive stress at the threads. The thread core is already in
tension from the lengthening effect of thread rolling and its stress
only increases with tensioning.

Therefore, spokes in a newly built wheel have locations where stress
is near yield, some more so than others. Because fatigue endurance of
a metal at or near the yield stress is short, cyclic loads in such
spokes will cause failures at high stress points. In normal use, a
wheel only unloads spokes, but with spokes near yield, even these
stress cycles readily cause fatigue failures. Only the lightest
riders on smooth roads might be spared failures with a wheel whose
spokes have not been stress relieved.

Stress relieving to relax these high stress points is accomplished by
over-stressing them in order to erase their memory. It is not done to
bed the spokes into the hub, as is often stated. Bedding-in occurs
sufficiently from tension. However, stretching spoke pairs with a
strong grasp at midspan, can momentarily increased tension by 50% to
100%. Because spokes are usually tensioned no higher than 1/3 their
yield stress, this operation has no effect on the spoke as a whole,
affecting only the small high stress zones where spokes are near
yield. By stretching them, these zones relax below yield by as much
as the overload.

Stress relieving with a light grasp of spoke pairs is worthless, as is
bouncing the wheel or bending it in a partially opened drawer.
Pressing axially on the hub, while supporting the rim, requires a
force larger than is manually possible but is effective for spoking
machines (except the left side rear spokes that would collapse the
rim). Another not recommend method, is laying the wheel on the floor
and walking on it with tennis shoes, carefully stepping on each pair
of crossed spokes. The method works but bends the rim and is
difficult to control.

It is STRESS RELIEVING! Even though people insist on calling it
pre-stressing or seating-in. The wheel is already prestressed when

Jobst Brandt <jbrandt@hplabs.hp.com>


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