J
Jobst Brandt
Guest
Wayne T Dunlap writes:
>>>> Weight savings is an extra bonus. The real reason is that a wheel with DB spokes will be more
>>>> durable, not necessarily stronger.
>>> If the straight gauge is stronger wouldn't it be more durable than double butted under heavy
>>> loads?
>> If... but they are not, because their ends, where they fail, are identical for both types of
>> spoke, having the same cross section. The swaged spoke having been made from a straight gauge
>> spoke blank.
>> Fatigue is a process by which a metal is torn apart by repeated loading much lower than that
>> needed to forcefully break the part. Therefore, it is not rupture strength but rather stress
>> levels under cyclic loading that cause spokes to fail. That is why stress relieving is highly
>> important in building durable wheels.
> So, essentially what you are saying is that the fact that the middle of a double butted spoke is
> thinner, it allows it to flex and take the stress off the ends of the spoke where most spoke tend
> to break. But wouldn't the fact that the middle of a double butted spoke flexes more than a
> straight gauge, cause the middle section to fatigue more and, therefore, cause more mid section
> breakage?
No. That spokes do not break in their slender mid sections is well established. Failure occurs at
stress concentrating features where the spoke is not uniformly loaded and where it has corners in
which stresses are naturally higher. An analogy would be a uniformly wide hallway that makes a right
angle turn with a stream of people walking in the same direction. Congestion at the corner is
inevitable as is stress concentration in a spoke's threads, elbow, and head.
To visualize an extreme example, cut 10cm from the middle of a spoke and replace it with a coil
spring with a stiffness of 200lbs/inch of stretch. Instead of tightening the spoke by turning the
spoke nipple, stretch the spring an inch and attach it to hooks on the cut ends of the spoke. This
tensions the spoke to 200lbs. Deflecting that spoke under the usual loads (causing a length change
of 0.003-0.005") would cause less than 1lb change in spoke load instead of 50lbs or more.
By adding elasticity, swaging spokes emulates that function.
Jobst Brandt [email protected] Palo Alto CA
>>>> Weight savings is an extra bonus. The real reason is that a wheel with DB spokes will be more
>>>> durable, not necessarily stronger.
>>> If the straight gauge is stronger wouldn't it be more durable than double butted under heavy
>>> loads?
>> If... but they are not, because their ends, where they fail, are identical for both types of
>> spoke, having the same cross section. The swaged spoke having been made from a straight gauge
>> spoke blank.
>> Fatigue is a process by which a metal is torn apart by repeated loading much lower than that
>> needed to forcefully break the part. Therefore, it is not rupture strength but rather stress
>> levels under cyclic loading that cause spokes to fail. That is why stress relieving is highly
>> important in building durable wheels.
> So, essentially what you are saying is that the fact that the middle of a double butted spoke is
> thinner, it allows it to flex and take the stress off the ends of the spoke where most spoke tend
> to break. But wouldn't the fact that the middle of a double butted spoke flexes more than a
> straight gauge, cause the middle section to fatigue more and, therefore, cause more mid section
> breakage?
No. That spokes do not break in their slender mid sections is well established. Failure occurs at
stress concentrating features where the spoke is not uniformly loaded and where it has corners in
which stresses are naturally higher. An analogy would be a uniformly wide hallway that makes a right
angle turn with a stream of people walking in the same direction. Congestion at the corner is
inevitable as is stress concentration in a spoke's threads, elbow, and head.
To visualize an extreme example, cut 10cm from the middle of a spoke and replace it with a coil
spring with a stiffness of 200lbs/inch of stretch. Instead of tightening the spoke by turning the
spoke nipple, stretch the spring an inch and attach it to hooks on the cut ends of the spoke. This
tensions the spoke to 200lbs. Deflecting that spoke under the usual loads (causing a length change
of 0.003-0.005") would cause less than 1lb change in spoke load instead of 50lbs or more.
By adding elasticity, swaging spokes emulates that function.
Jobst Brandt [email protected] Palo Alto CA