Have you ever seen a spoke pull through an aluminum rim? When a spoke pulls through, it leaves a massive hole in the rim. Rim material is not as strong as spoke material. How can one stretch a harder spoke with a softer rim?
Spokes do get looser after a couple of week of riding, We have all seen new wheels come back for a free bike tune up and the tension is 40 to 60 Kgf. I have been told that spokes are made of magical metal, some mechanics will tell you spoke metal cures after the original stretching and become a harder metal and stops stretching. WOW
If mechanics read the book The Wheel by Jobst Brandt they would find out that spokes get EFFECTIVELY longer, losing tension but they do not stretch. The elbow will crush a little metal in the hub flange, making the spoke effectively longer, under tension the elbow will change angle making the spoke effectively longer, and the spoke will become straighter becoming effectively longer. and the nipple can crush any burr in the rim and seat into the hole. Once the elbow is seated in the hub, and the angle corrected for pull direction, the spoke straightened and the nipple seated, If the spoke threads have been chemically prepped or has thread locking nipples to keep the nipples from untwisting, The spokes should keep their final tension after being prestressed.
Pre stressing can be done a number of ways, most are ineffective, not verifiable, not consistent, or destructive. Some major wheel builders use thick gloves to squeeze pairs of spokes around the wheel until all spokes are grasped as hard as the mechanic can squeeze. Problem: Thor and Wimpy squeeze spokes with different forces. Thor has good days and is tired at the end of the day and when he is sick he is not consistent. A nine year old girl and a 250 football player will have very different results with this method. Yes I do teach both little girls and huge men how to build wheels with equal quality.
A method taught by a school in Colorado puts the handle of a wrench between crossed spokes and bends the spokes over each other. No tension value is measured, and spokes get slightly sprung (bent by the wrench)
Mechanics at the Schwinn School were taught to lay a wheel on the floor and stand on the spokes all over the wheel until all the spokes were prestressed. This did bend the rim, is not a measurable nor repeatable by all people to the same effect. It is fun to try once on a damaged rim.
One wheel builder tensioned all spokes to 150 Kgf and the rider would detune the wheel to 100 Kgf by just riding it. Not a bad idea, If the nipple is lubed at the rim to reduce galling and the spoke wrench does not deform the nipple (4 sided wrenches are superior to 3 sided ones to achieve higher tension)
There is also a machine that pre stresses all spokes to 150 Kgf with a long torque wrench that a 9 year old girl and a big burly football player can turn with ease and accuracy. It was invented in the 1880's and brought back to existence by Dietrich Rolf. Anyone that can read a spoke tension meter can use this machine with accurate repeatable results every time.
How do you prestress the wheels of your customers to increase quality and life span of the spokes?
If you have an idea to share, send me a note.
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