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Stress-relieved? How can you tell....?(5 posts)

Stress-relieved? How can you tell....?opencl
Oct 9, 2002 6:51 PM
Hi, how can you tell if a wheel is stress relieved or not?
re: Stress-relieved? How can you tell....?divve
Oct 9, 2002 10:48 PM
Ever hear stories about wheels that go out of true as soon as you look at them the wrong way? That's basically what happens when you don't stress relieve it.
There are two types of "stress relief"...TFerguson
Oct 10, 2002 4:03 AM
Proponents of one type believe that the main purpose is to unwind the spokes. When you tighten the nipples while building the wheel, some of the spokes get twisted and will stay that way due to the resistance of the nipple to turn against the rim. These will unwind as the wheel is used which changes their length causing the wheel to go out of true. The usual way to "relieve" this is to lay the wheel on its side on the axle and push down with one hand 180 degrees from the other. This loosens the spokes on the other side and allows them to unwind. You go around the wheel and do this in 8 - 10 places. You will hear a little pinging noise when they unwind. You then do this to the other side of the wheel. If you do this to your wheel and hear pinging - bad build.

Proponents of the other type believe that the builder should be able to tighten the nipples without leaving them twisted by turning the nipple too far and then backing it off and feeling the torque require to turn the nipple in each direction. The purpose of "relieving" in this case is to "stretch" the spoke more than it will be stretched in use. The belief is that when a spoke is stretched the first time it does not return to its full length by a minute but significant amount. This type of relief is usually done by grabbing a pair of spokes in your hand and squeezing them together. I don't know how you would tell if this has been done to your wheel.

Both of the types are also used to tell when the spokes are just a little too tight because the rim will start to taco. At this point you would back off each nipple and finish the wheel at this tension.
Neither, quiteJofa
Oct 10, 2002 1:00 PM
Stress-relieving is one thing only. It serves a subtle function that is not easily intuitively understood. When spokes are (cold) formed, residual uneven stresses are retained in their threads and elbows. These disparate and "peaked" zones are inclined to concentrate stress, rather like a sharp corner or scratch in a larger component. Spokes which have not been stress-relieved are consequently liable to failure.

Stress-relieving is the process of temporarily increasing the tension of a spoke, such that those areas which are near yield are stressed beyond yield, causing the spoke to finally take a permanent set. This equalizes the internal stresses in the spoke. The spoke is not necessarily lengthened, and if it is then that is an inconvenience.

The phrase is sometimes wrongly applied, as you say, to the process of relieving spoke twist, which as you also mention, should be dealt with during the build. This confusion is unhelpful and is the source of all sorts of odd practices in workshops. Stress-relieving can be done by hand by grapping opposite pairs of spokes and squeezing them together tightly. No other method provides enough of a tension increase, excepting those used by dedicated machines.

There is no way of detecting whether a wheel has seen this process, but it can be applied retrospectively. The wheel will need retrueing afterwards.

Jofa
DIYoff roadie
Oct 10, 2002 8:20 AM
As far as I know, there's no harm in "re-relieving" a wheel, as long as it is propperly done. So the best way I know to be sure it is done is to do it yourself. I'm a proponant of the second theory / method above, BTW. You can damge a wheel by pressing on the rim while it rests on the hub, but its very hard to over-stress the spokes by squeezing them. I actually use the "stick between the spokes" method Sheldon Brown mentions in his wheelbuilding guide.

One obvious sign that spokes are probably NOT stress releived is if they do not lie flat and flush on the oustide of the hub, or have wide curves near where they intersect, instead of fairly sharp bend. Stress relieving won't itself correct a bad spoke line, but no spoke that isn't propperly seated and set to its position can ever be propperly stress relieved.