carlfogel
Computer
- Feb 29, 2004
- 4
Hello,
Here's a stainless-steel stress-relief question.
Briefly, can stainless-steel bicycle spokes be usefully stress-relieved by grabbing a pair of spokes already tensioned to 200-400 pounds in a wheel and squeezing them together with your hands?
This procedure is one of the sacred beliefs of bicycle wheel-building, but I can't find any actual testing or data connected with it.
Now for the details.
Typical bicycle spokes are 314 (8/18) stainless steel, about 290mm to 300mm long with an short elbow and flattened button at the hub end and a threaded end for the nipple.
Spokes vary from 1.6mm to 2.0 mm in thickness and are often double-butted. The theory is that thicker ends resist fatigue, while a thinner mid-section is more elastic. (Pre-tensioned spoked wheels are a system that supports loads by elastically losing tension in the lower spokes and effectively standing on them.)
Except for rare crash damage, spokes fail by fatigue at either the nipple/thread junction or at their hub elbow. In well-built wheels, spokes often outlast several rims over 50,000 miles and ten years. (The rims slowly wear out under caliper-pad braking.)
Two major spoke manufacturers are Sapim and DTSwiss:
During wheel-building, the spoke elbow may be bent slightly (less than five degrees) to make it lie flat along the flange of the hub. Here's the prevailing theory underlying stress relief for stainless steel spokes:
"Metals such as steel and aluminum are elastic materials that spring back if they are deformed or bent. If they are bent far enough they take a set, and do not return entirely to their original shape. The stress level at which a metal takes a set is called its elastic limit or yield point. Below the elastic limit the metal works in its elastic stress zone. Above the elastic limit it is forced into its plastic stress zone. Beyond the plastic zone is the
failure stress at which the metal breaks. Brittle metals have little or no plastic region and break shortly beyond the yield point. Bending causes tension and compression on opposite sides of a piece of metal. Both forces cause the
same kind of failure."
--"The Bicycle Wheel," Jobst Brandt, 1981, p. 31-32
[an idealized stress vs. strain graph follows:]
S o o o breaking point
t o
r o
e o
s elastic o
s limit o
o o
o o o
o o o
o
o
o
Strain (deflection)
[figure 15; stress here is tension and strain is elongation]
It's been pointed out that stainless steel does not show this drop-off after any elastic limit, but instead shows a steadily climbing stress-strain curve more like aluminum. Indeed, figure 69 in the same book shows the results of
actual testing of butted and unbutted spokes in 2.0mm and 1.8mm diameters from DT and WheelSmith, which all show this kind of stress-strain curve:
o o o o o
o
o
o
o
o
o
o
o
o
o
o
[figure 69's actual spoke tests show no dip after elastic limit]
The long passage following my signature details the prevailing theory and procedure for relieving residual stresses in spokes. It can be found in the FAQ
section of rec.bicycles.tech at:
Unfortunately, I can't find any author addressing this spoke-squeezing stress-relief procedure other than Jobst Brandt. I'm pig-ignorant and just trying to find out whether non-bicycle metals engineers find it plausible.
Naturally, I'll do my best to answer any questions and scan and provide links to any graphs if it will help. Sorry if this is covered in Engineering for Dummies or is considered inappropriate by any members. (I ended up here because I found a 304 wire stress-relief thread.)
Thanks,
Carl Fogel
carlfogel@comcast.net
Subject: 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 elastically 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 tensioned.
Jobst Brandt <jbrandt@hplabs.hp.com>
[end of FAQ section]
Here's a stainless-steel stress-relief question.
Briefly, can stainless-steel bicycle spokes be usefully stress-relieved by grabbing a pair of spokes already tensioned to 200-400 pounds in a wheel and squeezing them together with your hands?
This procedure is one of the sacred beliefs of bicycle wheel-building, but I can't find any actual testing or data connected with it.
Now for the details.
Typical bicycle spokes are 314 (8/18) stainless steel, about 290mm to 300mm long with an short elbow and flattened button at the hub end and a threaded end for the nipple.
Spokes vary from 1.6mm to 2.0 mm in thickness and are often double-butted. The theory is that thicker ends resist fatigue, while a thinner mid-section is more elastic. (Pre-tensioned spoked wheels are a system that supports loads by elastically losing tension in the lower spokes and effectively standing on them.)
Except for rare crash damage, spokes fail by fatigue at either the nipple/thread junction or at their hub elbow. In well-built wheels, spokes often outlast several rims over 50,000 miles and ten years. (The rims slowly wear out under caliper-pad braking.)
Two major spoke manufacturers are Sapim and DTSwiss:
During wheel-building, the spoke elbow may be bent slightly (less than five degrees) to make it lie flat along the flange of the hub. Here's the prevailing theory underlying stress relief for stainless steel spokes:
"Metals such as steel and aluminum are elastic materials that spring back if they are deformed or bent. If they are bent far enough they take a set, and do not return entirely to their original shape. The stress level at which a metal takes a set is called its elastic limit or yield point. Below the elastic limit the metal works in its elastic stress zone. Above the elastic limit it is forced into its plastic stress zone. Beyond the plastic zone is the
failure stress at which the metal breaks. Brittle metals have little or no plastic region and break shortly beyond the yield point. Bending causes tension and compression on opposite sides of a piece of metal. Both forces cause the
same kind of failure."
--"The Bicycle Wheel," Jobst Brandt, 1981, p. 31-32
[an idealized stress vs. strain graph follows:]
S o o o breaking point
t o
r o
e o
s elastic o
s limit o
o o
o o o
o o o
o
o
o
Strain (deflection)
[figure 15; stress here is tension and strain is elongation]
It's been pointed out that stainless steel does not show this drop-off after any elastic limit, but instead shows a steadily climbing stress-strain curve more like aluminum. Indeed, figure 69 in the same book shows the results of
actual testing of butted and unbutted spokes in 2.0mm and 1.8mm diameters from DT and WheelSmith, which all show this kind of stress-strain curve:
o o o o o
o
o
o
o
o
o
o
o
o
o
o
[figure 69's actual spoke tests show no dip after elastic limit]
The long passage following my signature details the prevailing theory and procedure for relieving residual stresses in spokes. It can be found in the FAQ
section of rec.bicycles.tech at:
Unfortunately, I can't find any author addressing this spoke-squeezing stress-relief procedure other than Jobst Brandt. I'm pig-ignorant and just trying to find out whether non-bicycle metals engineers find it plausible.
Naturally, I'll do my best to answer any questions and scan and provide links to any graphs if it will help. Sorry if this is covered in Engineering for Dummies or is considered inappropriate by any members. (I ended up here because I found a 304 wire stress-relief thread.)
Thanks,
Carl Fogel
carlfogel@comcast.net
Subject: 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 elastically 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 tensioned.
Jobst Brandt <jbrandt@hplabs.hp.com>
[end of FAQ section]