Why wheels go out of true

Whether you ride a top of the range lean and mean carbon machine, a heavy tourer or a tricycle, at some point you have probably experienced a wheel out of alignment. Typically panicking and seeking advice from your local workshop or the internet wisdom, the answer you got was most likely along the line of: “you hit a pothole” or “you’re too lardy for your wheels” or “your wheels are lightweight, you need stronger ones”.

roubaix

Paris-Roubaix Cyclo 2014: Mons en Pevele and 80 Km to go!

What I want to do here is convince you that none of this is true, or to put it better none of this is the REAL reason why your wheel is not true. A bicycle wheel is designed to be light, therefore large strong pillars that connect the hub to the rim are replaced by metal wires, that we call spokes. Spokes don’t have any real strength, a toddler can bend a spoke easily. However, when they are in tension, they become extremely strong. You can hang several hundred Kg to a spoke before it snaps. So the key to make them strong is tension: the more, the better. The job of a wheel is to turn or rotate and as it does so the spokes experience cyclic load and off load. When they are at the bottom they are in compression and when they are at the top they are under tension. If the tension of the spokes is low, every time they find themselves at the bottom, they are slack and become weak: they are free to unscrew from the nipple and lose even more tension and they can move in the flange which causes metal fatigue. Moreover, if a portion of the spokes are not in tension, the wheel as a whole has less strength. If they work themselves loose of course your wheel goes out of alignment. You take it to the shop, they tweak the nipples and bring it back into alignment, but crucially they don’t check the tension. Next time you’re out on the bike the same happens… again and again. The bike shop tells you they don’t stay true because they are cheap/old/worn out and you need new ones like the ones they stock, when in fact all they need is some damn tension, which nobody has yet checked!

The load a wheel can carry depends on the number of spokes and the tension. If you have more spokes at the bottom part of the wheel, they can carry more load in compression without bending, while if you have fewer, they need to be loaded with higher tension to do the same job… in simple words you need 8 men to carry a piano, or 4 strong ones or 2 scrum props… spokes are only strong if they are in tension. A tiny 1.5 mm gauge spoke in tension is way stronger than a plain gauge 2 mm one which is almost slack. 90% of the wheels out of true are not ready for the skip, they just need to be brought to tension.

The remaining 10% of the wheels that have lost alignment, have been damaged. If you crash and experience a strong side load or if you hit a deep pothole with inadequate tyres, the spokes will temporarily experience compression, bend and allow the rim to deform permanently. If the rim goes out of shape by only 1 mm, the wheel will appear buckled and out of true. Sometimes it is possible to compensate for a bent rim by altering the tensions of the spokes in that spot, other times the bend is too severe and tension alterations will result in a wheel with no strength, which will show all the symptoms of low tension, as above. Basically it will go out of true again within a few miles. In this case you are typically looking at a rim replacement. In my experience a rim which is bent by as little as 3 mm is beyond repair.

I used to be of the old school: if you ride Paris-Roubaix you need wheels with 32 plain gauge spokes… over time I have changed my mind and realised it’s perfectly fine to ride lower spoke count wheels, as long as the tension is kept high enough to compensate. This is not always possible as many rims are too light and fragile to tolerate a spoke tension in excess of 1100-1200 N for example, so high spoke counts are still the way to go if you want to ride lighter and shallower rims.

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