Originally Posted by
nickw
Alan S / Cyccommute - you guys should take the time to read the articles cited, mathematical calcs showing the stresses. One of the articles even points to the semantics regarding compression vs tension. You are getting into a dew loop over compressive load vs stress.
Perhaps you should go back and read my post 77. I
have read the articles cited and looked at the calculations for the stresses, along with seeing...and even quoting...the semantics. All I can say at this point is that you need to look at a wheel and see where you can "compress" a spoke. There simply is no "compressive" load on a spoke.
By the way, I think the term you are looking for is a "DO loop". And you seem to have fallen into your own DO loop.
Originally Posted by
nickw
You are really underestimating us if you think we need to push on a string....or spoke....to understand what would happen.
Let's try it this way. Say you start with each spoke at a tensive force of 500 N (newtons) pulling from the rim towards the hub. As the spoke passes through the bottom of the stroke, according to "Ian", the force on the bottom spokes reduces until the one at the bottom is reduced by 325 N (or there about). It is still under 175 N tensive force. The
tensive force has been reduced but that doesn't the spoke itself has been compressed. The rim has been compressed and the tension on the spoke has been reduced but the
spoke hasn't been compressed. You can even apply enough force to permanently deform the rim (hit a curb sometime) and the tensive force can completely to zero but the spoke hasn't been compressed. If you attempt to compress the spoke, it will buckle like those shown by BobG...or like a string that you try to compress.
Let's further reduce the problem to a simple one spoke/one rim/one hub problem. Connect a spoke to a rim. Hold the rim up so that the hub is hanging from the rim by the spoke. Now grab the hub and let the rim hang from the hub. If there is any compressional load on the spoke, you should be able to release the hub and the hub will "stand" on that spoke. I've built far too many wheels to know that the hub will stand on the rim. On the other hand, the hub will hang from the rim for (nearly) an eternity as long as you hold up the rim.
The problem
is, indeed, semantics. If you assume that compression is the opposite of tension and proceed accordingly, you reach the wrong conclusion...no matter who you are. If you understand that compression and tension are, indeed, forces in opposite direction but that the lack of one does not necessarily mean the presence of the other, you reach a very different conclusion. It also helps to look at real world examples rather than mathematical models.