Bicycle Mechanics - Powertap SL+ drive side spoke tension
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04-15-11, 08:12 PM
Hi everyone, I have a question that probably has an obvious answer but wanted to be sure I was on the right path. I have a pt sl+ that has worked great until I recently had it put into a new wheel (dt swiss 415). As soon as I got this wheel something didn't feel right - primarily that under torque on a hills it felt soft. I started getting some strange power numbers as well. Anyways, I checked the spoke tension on the non cassette side and they are very loose relative to the side on the cassette. My understanding is that Saris considers the non cassette side of the hub the drive side, and those spokes are supposed to be tight relative to the non drive side, ie the side with the cassette. Anyways, my question is how loose should spokes be, and is my understanding of drive vs non drive right? Thank you!
On a dished rear wheel, the right and left side tensions will vary because if they were equal the rim would move over to the center between the flanges. The relative tension is determined by the amount on asymmetry, not by the builder. Normal practice is to make the closer flange as tight as possible, and letting the chips fall where they may on the far flange.
Within reason, it's possible to bring the near flange tension slighter higher than you would otherwise so the left flange can be tighter, but that's limited.
BTW- I don't believe that you can feel the difference between wheels by riding unless one is ridiculously loose.
04-15-11, 08:39 PM
Hi FB, thanks for the reply! It sounds like by the PT design that the drive side spoke tension should be twice that of the non drive side, at least this is what I have seen as being recommended by Saris. It is hard to describe what I feel when I ride, but it feels like the rim is moving side to side. In terms of loose, I can take the non cassette spokes where they cross and essentially move them upwards of a centimeter in either direction... in other words, where they cross I can squeeze and they move along each other for a substantial ways. Not sure this is normal but it seems a little off especially from the cassette side.
As I said, left side spokes are usually much looser than right. If you're concerned, have a shop with a tension gauge check the right side, making sure that it's properly tight If it is then the left is OK despite feeling loose.
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