Why spokes of bikes are kept under tension

Dozens of spokes must be perfectly ordered and balanced otherwise the wheel will suffer a horrible bias. Building a wheel is an act of repetition, concentrating on one spoke at a time. Assembly: spokes are inserted into the hub flanges according to the desired lacing pattern radial, two-cross or three-cross patterns are the most common for road wheels and attached to the rim with the nipples.

Tensioning: The nipples are gradually tightened in a step-wise manner to slowly and evenly increase spoke tension in the wheel. Wheelbuilders typically count the number of turns of the spoke key as they tension a wheel to ensure every spoke is tensioned to the same degree.

Rounding and truing: As each spoke is tensioned, the rim is pulled towards the centre of the hub, as well as in the direction of where the spoke originates. Dishing: The dish of a wheel is a measure of how far the rim is displaced from the end of the hub axle and should be identical for both sides of the wheel. Adjusting the balance of spoke tension between the left and right sides of the wheel adjusts the dish of the wheel.

If the rim is round—perfectly round—and the hub has a perfectly centred axle with truly round flanges and precisely positioned spoke holes, then the wheel will be round and true once assembled with evenly tensioned spokes. As a wheel turns, the spokes are loaded and unloaded and it is the repetition of this strain that takes its toll, especially at the head of the spoke. Under-tensioned spokes are especially susceptible to this strain. While there is nothing that can be done to avoid spoke fatigue per se, even spoke tension ensures that it develops at the same rate for every spoke.

Where once wheelbuilders had to rely on their senses to judge spoke tension, now there are sophisticated tension meters that provide an accurate measure. And according to Zak Smiley, the owner and operator of Skunkworks Bikes in Sydney, they have become indispensable for wheelbuilding. Zak uses a digital tension meter made by Wheel Fanatyk. The resolution of this meter affords him enormous precision that he uses to diligently tension the wheel.

A good spoke tension meter not only removes a lot of guesswork from wheelbuilding, Zak has also found it invaluable for diagnosing and remedying problem wheels. An example of one such wheel is shown in Figure 2 below. Not only was it over-tensioned, there was also too much variation in the spoke tension, so Zak set to work righting both.

Over the course of an hour, he loosened then gradually re-tensioned the spokes while keeping an eye on the roundness and lateral true of the wheel. But low spoke tensions are also a problem because the nipple is more likely to come loose [unwind] when they are de-stressed through impacts or road vibrations, causing the wheel to come out of true. Sapim, one of the leading spoke manufacturers, produces million spokes a year, and shops around to maintain quality and stay competitive across its product range.

Once accepted, the wire off the spools is straightened by machine and cut. Plain gauge wire can also be made into butted spokes where the central portion is made narrower by drawing the wire through a die. Once butted, the head of the spoke and the J-bend are forged and the thread at the other end is rolled not cut. Finished spokes are inspected both by machine vision systems and by human eye and hand.

One machine is able to make 20, butted spokes a day, which explains why different labour costs have little impact on the price of a finished spoke and why manufacturers worldwide can sell at similar prices.

But why butt a spoke anyway? They are wider in the plane of the wheel, which is the direction of the torsional force, so there is more material to resist it.

Also, they flex a little more in the perpendicular plane, so they are better at distributing the compression load across the wheel. The traditional spoking pattern of a bicycle wheel comprised 32 or sometimes 36 spokes, crossed three times.

The interwoven pattern of the spokes in a traditionally laced wheel, far from being just a pretty kaleidoscopic arrangement, is actually a functional part of the wheel design. In terms of lateral rigidity the points where the spokes intersect allow each one to brace against another as it is placed under tension, as well as support it as it is compressed.

The most vital role of the three-cross lacing pattern is in a rear wheel, where the spokes must transmit pedalling power from the hub. In this case the spokes are loaded with much greater torsional loads thanks to the twisting force from the drivetrain. Spokes on the cassette side, leaving the hub tangentially, transfer a rotating force torque from the hub to the rim.

Radial spokes which follow a path from the centre of the hub directly to the rim, without crossing another are much less able to cope with this type of loading and would be more likely to fail. This saves weight, as the spokes can be shorter and fewer are needed to create a laterally stiff wheel. Do it on the right side of the wrench and then rotate the wrench degrees and do it again on the other side.

Take a look at my wrenches. If you mark your wrenches, all you have to remember to do is to push on the dot when tightening a spoke! And turn the wrench the other way to loosen a spoke. I advised him to replace the spoke, as it was twisted a full turn. I also provided some additional instruction off-line. A couple of others were twisted, too. Just not as badly. I also bought the spoke wrench you recommended. Read his full bio. He is a life-long cyclist from the UK who has spent most of his adult life in Canada.

Mike has been a passionate home wheel builder for the past 52 years and specializes in taking the mystery out of wheels and wheel building for Newbies. Hundreds of cyclists have built their first wheels with online help from his wheel building website, MikeTechInfo.

Send your questions about wheel building and wheel maintenance to Mike at [email protected].