[Note: this is #1 in a series of 4 about bearings]
Hidden from sight in greasy, gritty recesses, bearings have the power to transform a bicycle’s performance. No wonder engineers pay attention. Nothing affects wheels and our quest to make great ones like bearings. Wheel building is faster and more informed the more you know about bearings. As we seek highest trueness and lowest rolling resistance, bearing design, function, and fitting becomes a major factor.
Overlooked for decades, bearings are one of the simplest yet most important parts of the bicycle. They transfer power, absorb nasty side loads, reduce friction, and help an otherwise unstable machine balance; all this with little weight, cost, or recognition. These dynamic elements have been around as long as the wheel and have played a central role in some of man's greatest achievements.
STONE AGE ROOTS
When Neanderthals spun sticks above a central contact point, generating enough heat to start a fire, they had essentially harnessed the concept of rotation about a fixed center. That combination of load and motion would lead to the development of the wheel. It is also why the wheel center experiences concentrated wear. To reduce that wear, man used hard woods and animal fat. Those bearings were crude, but still, the burden was off our backs and into the wagon.
By the time the Stone Age finished, materials with lower friction and greater wear resistance were available. Metallurgy offered options with lower friction than wood and longer life, but it was only the perfection of rolling elements that reduced bearing friction to negligible levels. That creation had to wait until the 19th century and with it came the bicycle’s meteoric development.
Rolling elements (initially balls) must be extremely round, smooth, and uniform in size. Making these is a mechanical marvel and required most of the advances of the industrial revolution. The biggest lynch pin was hard materials. Alloying iron with carbon created steel hard enough to survive the concentration of load in a small bearing. This discovery and further refinements make ball bearings possible. For a glimpse at these dazzling processes, check here. Note particularly after 2min, the making of the balls.
BYE, BYE FRICTION
Reduction in friction is miraculous. Archibald Sharp in his 1896 masterpiece, Bicycles and Tricycles, asserted that the rolling friction of ball bearings is 1/1,000 the weight of the rider. In other words, thanks to ball bearings, it takes a thousand times more force to lift your body than to push it forward (the same distance). By that measure, less than 1 per cent of riding energy is absorbed by bearing friction.
One-percent is a pretty sweet number considering the size of human motors. Great competitors are reputed to sustain 500 watts of power for 40 minutes. Impressive but, nothing personal, that’s less than 3/4 horsepower. A pretty feeble lawnmower. Me, I probably generate less power than a fruit juicer. We need ball bearings.
So why are they often overlooked? Because they are tiny and their friction has been considered negligible for more than a century. More ball-bearing-related patents were filed for bicycle use than for any other purpose. Today, the proliferation of electric motors fuels an industry making low friction ball bearings that fit nicely into bikes. The bike industry concentrates on comfort, cost, weight, and efficiency. Bearing are available off the shelf and friction remains negligible.
How are bearings so efficient? Simple: they concentrate loads onto tiny contact points with very hard materials. The entire weight of bicycle and rider is transferred to the ground through the bearing mechanism. Within the bearing, each ball rests on a steel raceway at a spot that is microscopic. With super hard (undeformable) metals for both surfaces, friction is microscopic too.
ALL SHAPES AND SIZES
There are several types of bearings used in bicycles. First are bearings with rolling elements, what we've discussed so far. Second are plain bearings, which consist of close fitting, smooth surfaces. Bushings are plain bearings. Hubs use rolling bearings with tiny balls. Derailleur pulleys, brake pivots and rear-suspension links often use bushings.
Rolling bearings can be categorized in at least two primary ways: the shape of the rolling element and the angle of contact. The rolling element can be a sphere (ball), a cylinder, or a shape that resembles a cone or barrel (toroidal). Bearings with balls offer the lowest friction because their contact point is so small, but those with other shapes can support greater loads because they have larger contact area. Since bicycles see relatively low loads compared to many other machines, there is little need for non-ball rolling elements.
Thanks to its single-track nature, bicycles and their bearings see few side forces. By single track, I mean one wheel following the other. When cornering, bike and rider lean into a turn. G-forces generated by the turn are in the plane of the frame, right down to the tire-ground contact. From a wheel's point of view, these are radial loads. So, radial-contact (annular) ball bearings suit the job.
One area where non-radial loads exist is steering. In a headset, loads are huge and sideways to the bearing. But at the same time, a single-track vehicle requires low steering friction. Why? How?
That’s where we’ll explore in the second of four Dirty Secrets installments about bearings. Our point: for wheel builders to know about bearings to build better, faster wheels, by understanding the role they play as we center the hub in the rim with high, uniform tension. Otherwise, you might think the wheel was actually interfering with your build!
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Tom Wray
November 02, 2021
Forces are not absorbed but resisted. Also, friction only exists when there is relative motion between two surfaces. Although ball bearings do slide a bit when in contact with the races and create friction, mostly there is rolling contact between the two surfaces with no relative motion and therefore no friction. It’s the deformations of the materials in contact that create opposing torques that oppose motion – the same idea as a wheel rolling on a surface, there’s no friction unless the tire slips on the road.