August 11, 2013 1 Comment
A rider's stubborn insistence on moving forward creates tension in the chain that pulls the gear on the back wheel. The twisting (torque) force goes from hub to rim to ground and away he goes.
Since the wheel is such a sparse structure, it must manage this torque efficiently without flex, fatigue, or excess weight. Through the cleverness of tangential spoking, a rear wheel can transmit torque with ease.
Wire spoke wheels were in use for decades before tangential spoking was devised. James Starley is given credit for the first tangential spoke product, the Ariel bicycle of his Coventry Sewing Machine Company.
The Ariel front wheel has two (and the rear, one) radial levers among the spokes. From the end of each is a wire connection to the rim. Tightened, and the semi-loose spokes of each wheel become snug as they all acquire a slightly tangential angle from hub to rim. This angle makes torque transfer more efficient.
This is exactly what happens when any fully radial wheel needs to transmit torque. A very slight windup occurs so that each spoke has a small lever with which to handle torque. Until that windup, a radial wheel is theoretically unable to transmit torque. Here's a simplified wheel showing only 4 pulling and 4 pushing spokes. Their tangential angle to the hub means they all do work when transmitting torque.
The great news is how well the wheel handles torque. The strength required to support vertical loads (by far its biggest job) incidentally makes over-design for torque. Wheels carry torque like chains. No human is strong enough to break a bicycle chain with sheer leg power. Chains break from rust, defects, and bad connecting links. Wheels can carry all the torque of super strong riders with tiny spoke gauges and counts.
When a wheel fails during strong pedaling, a sprint for example, it's not the torque that does it. Other chaotic things happen during heavy pedaling. Fishtailing losses of control, the wheels may actually be hopping from side to side with pedals strokes like prize fighter punches. These create secondary forces, lateral and vertical, that can fail a wheel. It wasn't the rider's strength no matter how much he wants to believe it.
Let's be clear about the role of spoke pattern. Here is a simplified example of a perfectly tangential spoke attachment.
Such tangential geometry in spoking comes from higher cross numbers. Remember that the angle is affected by spoke count. Hence, cross 2 is perfectly tangential for 24 hole (image above), cross 3 for 28, cross 4 for 36, and cross 5 for 48.
Lower cross number also work. Here's a much lower angle of spoke attachment.
Lower cross pattern, creating a shorter lever arm is exactly like pedaling on a smaller front chainring: chain tension is higher for a given pedal force. That's why you feel stronger for hills (not faster!). For a wheel, the higher spoke tension is not a big problem but greater cycles of tension change contribute to metal fatigue and more stress for the hub and rim where the spoke attaches.
Understand spoke pattern but don't obsess because wheels certainly don't. If you want a good example how easy wheels do torque work, think for a moment about modern road wheels. There must be a million road bikes with 24 hole rear wheels. Most of them have radial patterns on the non-drive side for style and aerodynamic bling. Radial can transmit no torque until a tiny windup. Since the drive side, crossed pattern is rotationally stiffer it carries the torque load before the poor non-drive side gets a chance. It's not unwilling, just too slow. The rear wheel's left side is completely unable to assist for torque loads. All it can do is carry vertical weight, for which the whole unit is grateful.
So, we have millions (maybe) of rear wheels with only 12 spokes managing pedal power. That's 6 pulling and 6 pushing spokes, each weighing as little as 5 grams. So 2 ounces of wire carries the muscular effort of the strongest riders whose thunder thighs produce thousands of watts and hundreds of Nm for years of cycling. The wheel's effectiveness no longer surprises me, but it still boggles the mind.
Makes us proud to be makers, users, and fans of the bicycle wheel! Next up, suspension. Yup, the wheel does its part, on which the bike more than the rider seriously depends.
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