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In general, ball bearings are applicable in higher speeds because of the smaller contact area and smaller friction force, but roller bearings can carry greater loads because the load applied to the contact area produces smaller contact stresses.
-- Principles of Mechanical Engineering Design, Liubomir Dimitrov

In bicycle applications there are neither high speeds nor high loads (compared to cars or industrial machines). However, I would assume that the "smaller contact stresses" would result in significantly longer service life of the bearing, even if water and contaminants penetrate the bearing. And still the only time I have heard of bicycle roller(pin) bearings is in the hyped Lefty.

Why are ball bearings used exlusively in bikes e.g. in the headset where some minor friction would not be a problem?

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    Ball bearings are much simpler to construct and maintain. They're also lighter and more compact. – Daniel R Hicks Sep 29 '15 at 12:21
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    "It's all ball bearings nowadays" - Fletch – user25511 Apr 15 '16 at 17:54
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I think several of your assumptions are incorrect:

  1. no high speeds: rim dynamos spin quite fast, up to 10,000rpm, and that does require effort on the part of the designers. Typically the drive wheel on those will have a radius of about 1cm, or a circumference of 3 1/3rd cm and 30 revolutions per metre. At 10m/s (30kph) that means 300rps or 18000 rpm. This is why they make such a high pitched whining noise when in use.
  2. no high torque: a 100kg cyclist standing on a 165mm crank is exerting about 1kN at 0.165m, or 165Nm. That's comparable torque to a mid-range car. I repeatedly snapped a 8mm high tensile bolt in the drive system of my four wheel load bike, and replacing that with industrial freewheels meant buying two of the highest rated units I could find (German conveyer belt parts rated to 250Nm)
  3. No high loads: a touring cyclist with 50kg on the rear rack, plus half their 100kg body weight is putting a load of 100kg or 1kN on a 10mm diameter hollow rear axle. Those axles and the dropouts they bear on must be made of hard steel to stand the load, where the load bearing parts of a car or truck are typically made of much softer steel.
  4. Only ball bearings are used: modern sealed bottom brackets commonly use roller or needle bearings, as do many headsets. Internally geared hubs have used needle bearings for many years. Sheldon Brown describes roller bearings in his glossary, for example.

The answer is that various types of bearings have been used in bicycles and will likely continue to be. Again, Sheldon discusses needle bearings in Shimano hub gears from some years ago and they're still mentioned in the marketing.

One problem not unique to bicycles but especially relevant to them, is that more precise bearings are more vulnerable to grit and also to damage. So especially needle bearings need to be carefully protected from dust, sand, and forces outside their design range. Unfortunately doing that inside a suspension unit is hard, and last time I looked Cannondale had not solved the problem (my HeadShock is unmaintainable after only about 10Mm, I've seen multiple lefty shocks fail during a race because of light rain and fine grit causing them to abrade to uselessness very quickly)

But inside a hub or headset, for example, they can be amazing. I've talked to people who've ridden 20Mm on an Alfine 8, for example, with no problems.

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Firstly I disagree with the comment about high speed. The highest velocity bearings are plain: crankshaft bearings in a full size car engine are plain, whereas in a radio controlled model, ball bearings are used. Where velocity (rpm x diameter) is high the oil wedge effect tends to centre the shaft (hydrodynamic lubrication). This can be further enhanced if the oil is pumped.

Straight roller bearings are great at supporting radial loads, but useless at supporting axial loads. Where there is no axial load (for example a generator driven by an engine, either directly through a sliding spline or via belt drive) they are a good solution. In most bicycle applications, axial forces are present, therefore a total of 3 bearings must be considered: roller, plus axial thrust in each of two directions. (in some applications you might find a plain bearing for small axial thrust, but I can't think of one on a bike.)

Ball bearings are much simpler to make, and two ball bearings forms a great all-purpose joint. On a bike wheel, the two bearings are at the ends of the axle, as far apart as possible, which is exactly where we need them to resist non-axial torque applied to the wheel. In this respect, the central part of a roller bearing would be doing nothing, so you might want two roller bearings, one at each end, plus some additional bearings for the axial thrust (getting complicated isn't it?) The better solution is to have a single ball bearing at each end of the axle, with a conical form, which enables it to resist both radial and axial forces. Tapered roller bearings do exist, but they need a cage to help guide the rollers, which avoids the need for four bearings on a wheel, but adds other complexity.

Finally, headsets definitely do need some kind of antifriction bearing. There is an enormous nonaxial torque: your weight is trying to push the forks up and forward, except when you apply the brakes, when they will be pushed back. To be able to steer properly, you need a smooth movement there. Anyone who has tried to ride a bike with a damaged headset will know that it is possible, but uncomfortable and dangerous.

  • WRT "you might find a plain bearing for small axial thrust" I have seen one made by drilling a hole in the end of the shaft and inserting a ball bearing used in sidewall dynamos. I suspect they're not much needed, but since the end cap/thrust bearing was also the seal, removing it would have likely destroyed the dynamo. – Móż Nov 15 '15 at 3:08
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The current answers give lots of technical reasons for the use of ball bearings while noting that other types of bearings are sometimes use on bike.

However they miss the most important factor. Ball bearings are cheap!

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You will see sealed cartridge bearings in many bottom brackets, which use roller bearings. Also common in head sets. Ball bearings were more widley used up to around 10 years ago. But still common in pedals and hubs.

  • That's interesting. I never opened a sealed BB. Why BBs but not the much slower rotated headsets? Why only in the past? – Vorac Sep 29 '15 at 10:12
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    Some headsets do use roller bearings, as said in the answer. As to the reasons why: probably becasue of tradition, possibly economical reasons, ... – stijn Sep 29 '15 at 10:17
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    @Vorac - Conventional roller bearings do not take thrust very well. – Daniel R Hicks Sep 29 '15 at 22:41
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Roller bearings and ball bearings, are efficient. If you take plain bearings for the wheels, e.g. PTFE-bushings, the bicycle will work, but becomes a bit heavy to pedal, not so fun... Plastic bearings also makes the bicycle lesser stiff, which can feel uncomfortable, it can easily get into self-resonance and start wobbling if too low egen-frequency, but that part depends a lot of the detail design. You can use metal bronze bearings, but they need some lubrication.

For the headset ball bearings are also normally used. I just replaced the trust ball bearings for the headset on my old bicycle with home-made delrin (Acetal plastic) bearings I made myself on the lathe, because the old one were rusty and I found new one a bit expensive... Result: Yes, delrin plain bearings works fine for the headset, it will be heavier to turn it if you wish same rigidly as with ball bearings, but not a problem... It can be a bit of stick-slip with plain bearings in the head-set, this is probably the main reason why ball bearings are usually used - it is stiff and it turns smooth - and you will feel control.

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