Expensive wheel is barely faster

| 6-01-2022, 16:42 | English

There are so many cyclists racing on the beautiful roads around the big city these days that the outsider can’t help but notice that they dress more and more insanely and that they ride on increasingly strange bicycles. Weird frames, weird handlebars. A new trend is the use of wheels with ‘high rims’ and a greatly reduced number of spokes. Not 36 but 20 or less.

 

 

 

It is better not to give them any attention, but in the long run you still want to know why they use those circus wheels. Because they are not cheap. A shopkeeper who explains it on YouTube does not turn around that: the wheels are one thousand euros per set, but can also cost ‘just two thousand euros’. You don’t put it on your bike for nothing.

For what then? “Some think it just looks cool,” says the shopkeeper. And he knows even more: the wheels have an aerodynamic advantage over more conventional wheels that can be as much as 10 percent. With these types of wheels you can reach a speed of 33 kilometers per hour (km/h) where you could previously have reached 30 km/h with the same effort, he translates it to the potential buyer. He thinks it is ‘large’ and mentions another advantage: wheels with high rims are stiffer. There are also disadvantages: wheels with high rims are heavier and more sensitive to crosswinds than regular wheels.

Rotational air resistance

From 30 to 33 km/h with the same power input is not wrong. Would it also be true? The literature that Google Scholar has dug up gives little hope. At the speeds that amateur cyclists reach, 90 percent of the drag to be overcome comes from air resistance, and this is mainly provided by the body and frame. Until recently, the common wheels together only accounted for about 10 to 15 percent of the air resistance. Most of it is of a kind that also arises on the body and frame and which is more or less proportional to the frontal area. At most a quarter of the air resistance of conventional wheels comes from the creamy effect of the spokes. It is this ‘rotational air resistance’ that decreases if you equip wheels with high rims (deep rims) and decrease the number of spokes. The effect is maximum with completely closed wheels (disc wheels) but they are too sensitive to crosswinds.

There are indications that high rims and a reduced number of spokes reduce rotational resistance by a quarter, but even if the rotational resistance were to disappear completely, the air resistance of bicycle plus rider would only decrease by about 3 percent. That would not bring the mentioned 30 km/h to 33 km/h, but to a maximum of 31 km/h, as can be deduced from graphs showing the relationship between speed and required power (for example, those in Bicycling Science by DG Wilson).

Cycling on a roller bench

One problem is that the effect of wheel adjustment is difficult to measure. Field trials, wind tunnel tests and calculation models (computational fluid dynamics, CFD) gave different results. Intriguing are recent tests in which a cyclist sat on a roller bench with his bicycle in a laboratory and maintained a constant, high speed for a minute as best he could. In this setup, the air resistance comes solely from the rotational resistance (and a little bit from the moving legs). The favorable effect of the high rims was surprisingly clear from a registration of the power used. But the tests were not very reproducible.

There is room for amateur research. For example, in windless weather, the enthusiast can drive down a long slope (such as the exit of a bridge) without pedaling and in this way determine the influence of rims and spokes. The weight difference between the wheels must be compensated for, because a heavy cyclist rides down a slope faster than a light one. Parents who drive down a bridge with children (without pedaling) observe this time and again. The mutual weights differ much more than the ‘frontal surfaces’ that determine the air resistance.

fallacy

That’s what this wheel piece is good for: to point out that most people assume that parents and children would go equally fast. They think this is what Galileo Galilei came up with in his refutation of Aristotle’s views that heavy objects fall faster than lighter ones. Galileo made it plausible that all objects fall at the same speed, but he also showed that this only applies if there is no air resistance. American psychologists included this ‘Galileo bias’ in their description of ‘naive physics’ in 2005.

Many people also think that the effort it takes them to cycle up a high bridge is compensated by the ease with which they roll off the other side. Or, better example, that the same wind as tailwind makes up for exactly what it ruined as headwind. The poet Jan Hanlo called this ‘intellectual optical illusion’ and showed in a calculation example where the fallacy lies. Hence it took him little effort to show that there is no reason to doubt the existence of God, Creation and Providence.

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