Wednesday, October 10, 2007

Q&A: wheelbuilding spoke tensioning

On tensioning spokes, Jim, I saw your page on rebuilding a wheel and I have looked at several other sites and it's kinda like, "well it takes lots of experience to know how much to tighten a spoke." I have an old Schwinn tandem I am fixing up for my wife and I to ride. Since it will be double the weight on the same amount of spokes, I know that spoke tension is critical. Isn't there a way to say, "tune each spoke to middle C on the piano." Or whatever pitch you need. Even a tuning fork would help to know how tight to tighten it. When you tighten, is the musical pitch of the spoke the same from wheel to wheel? If so, what is that pitch? It seems to me that would solve a lot of guesswork as to how much to tighten, even if it were a span of 2-3 notes on the piano.

Thanks for the email, Jim. Actually going by the sound the spokes make is a great shortcut when truing wheels and fixing broken spokes and one of my favorite tips. It's in all of my wheelbuilding and truing articles on my site, in fact. However, there's a big difference between truing a built wheel and building a wheel from scratch. With the built wheel you have spokes that are at the right tension so you can pluck them and then match the pitch with the new spoke you're installing or the spoke that was loose and needed tightening. Using that technique you can usually get a wheel almost perfectly true without ever using a truing jig. It's usually that accurate.

So, since you're probably working on a built wheel on that Schwinn tandem it should be relatively easy to compare the pitch and make sure that the same-side spokes are at the same pitch. If some sound dead they are likely loose and need some snugging up. Most likely that wheel had heavy gauge spokes, which is what Schwinn typically used on its "family style" tandems (if you're working on one of the 10-speed types, that would have lightweight wheels). Tensioning a wheel like this should be pretty easy because those spokes don't stretch much and don't need to be overly tight to make for a strong wheel. You'll see what I mean when you turn a few nipples to tighten the wheel. It should go pretty easily if it's got heavy gauge spokes.

If you're planning on rebuilding a tandem wheel off a lightweight bicycle it's tougher to find the right tension because you're dealing with a unique mix of components and materials and there's nothing to compare it to unless you can find the exact same combination. And, it has to be the exact same in every respect. If you had a built wheel truly identical to the one you plan to build then you could pluck that finished-wheel's spokes and get a pitch to shoot for in tensioning your newly built wheel.

I like music but the talent in my family went to my brother and sister so I don't know much about musical pitch or notes. So I called a bicycle mechanic friend who knows a lot more about music and even tunes pianos on occasion and has studied it. I asked him if it would be possible to come up with a range of notes to tune spokes to on different types of common wheels. His reply was that it would be impossible to do this with any amount of accuracy due to there being way too many variables. Even small things like how well you seated the nipples in the rim or dirt or oil on the spokes would change the pitch, so he felt there would be no way to know what's right for a given combination of components besides checking a built wheel and even then, one small change could make it irrelevant for the new wheel.

What I recommend on my wheelbuilding articles is to compare your wheel to a similar wheel at a bike shop or on a friend's bike. Squeeze or pluck the spokes and compare. By doing both you should get a feeling for what's tight enough. That's a start. You can also buy spoke tensionometers these days for around $100, not that huge an investment if you plan to build a number of wheels. And, you'll also learn what's right after you've built a few. This usually happens by building them to what you think is tight enough and then finding out when you ride that the spokes aren't tight enough because the wheel goes out of true. You then true the wheel and add another round of tension and try again. If the wheel stays true that's a good sign that the spokes are now tight enough. If not, you'll need to add another layer of tension, and so on. That's exactly how I learned how to properly tension a wheel because back when I was learning there was no such thing as a tensionometer. It was all done by feel.

Sorry I can't give you an easy solution but experience is often the best teacher and in this case it's actually kind of fun learning what tension is correct so it's not all that bad a thing. If you're a musician and want to use a tuning fork or some such device to put a pitch on spoke tension on a variety of modern wheels perhaps you can come up with something that will help future wheelsmiths. It would be an interesting experiment and I'd be happy to share your results to my readers.

Happy wheelbuilding,

No comments: