What’s all the hubbub about hubs?
by Brent Robinson Illustration by Chris Simonen
There really is nothing that compares to riding your bike on a lovely spring day. The sun beaming on your face and the wind rippling through your hair (or lack thereof.) Balanced on your two wheels, you glide through the park like a butterfly on a spring breeze. What really allows you to have that amazing sensation of unstoppable forward force? Sure tires play a part of it, as do your cranks, and your frame makes a difference, but really what is it that allows your wheels to keep on spinning? It’s your hubs. This article will overview the basics of hub internals to help you understand exactly how they work and why they're so important.
Hubs are arguably the most important component on a bicycle. Every bicycle has them, no matter what size, type or purpose. The hub, together with the spokes and rim, form the complete wheel. Onto the wheel the tire and tube are mounted, along with gears on the rear wheel. Hubs provide the the support base for the spokes, which are adjustable for tension at the rim. While the rim and the spokes provide support for the tire and tube, the hub is what the entire bike frame rests on and how the weight of the bike and rider are transferred to the spinning interface of the wheel.
Hubs exist at the centre of every front and rear wheel. Front and rear hubs are very similar in purpose, although rear hubs are more complex as they also have to deal with the drive (pedalling) of a bike. We'll start with a basic overview of the front hub as it's a little bit simpler, and then get on to what differs with a rear hub.
All hubs have an axle, a bearing interface, and a hub shell. The front fork or rear dropouts rest on the axle, so the entire weight of the bicycle and rider is transferred through the two axles. Hollow axles are usually attached to the fork or dropouts using a quick release skewer which holds the hub on but is not load bearing. On older bikes, single speed bikes, and internally geared bikes, a solid axle extends past the fork and dropout so that a nut may be threaded on. The axle stays stationary while the bike is in motion, while the hub shell (which the spokes are laced to) rotates around it by using a bearing interface.
While there are many different sizes and types of bearings, there are two basic bearing interfaces. Cup-and-cone type bearings have a cone which threads onto the axle, and a cup or “race” which is integrated into the hub shell. Both of these surfaces are machined to be curved so that ball bearings may roll freely along their surface. The cones in a hub can adjust to apply proper tension on the bearings. If these cones are too tight the wheel won't spin freely, but too loose and the bearings will begin to move around and break down.
Cartridge bearings integrate the cup and race into one sealed, cylindrical unit. Generally these are pressed in to the hub shell and then the axle inserted. Cartridge bearings are more resilient to weather and can roll more smoothly due to tighter bearing tolerances, but are non-adjustable and must be replaced with ones of exactly the same size otherwise the wheel could be damaged.
The hub shell is the outer part of the hub that you actually see. The hub shell integrates either the race (for cup-and-cone bearings) or the machined press-fit surface (for cartridge bearings) with a steel or aluminum shell. The shell also contains the flange and spoke holes, which the spokes are inserted through and brace against to hold tension with the rim. The number of spoke holes in the hub shell dictates how many spoke holes the rim must have. Hub shells generally do not wear out or break, although loose or badly-fitted bearings could cause the bearing surface to become damaged, rendering the hub useless.
Rear hubs have the same components as front hubs, but add gearing to the mix. A rear hub may be either set up for a freewheel or a cassette ("freehub"). A freewheel is a gearing system that threads on to the hub, and integrates the ratcheting system that allows the rider to coast within it. A cassette slides onto a carrier which has the ratcheting system integrated into the hub. Generally speaking, a cassette type hub is more durable than a freewheel type hub. In either situation if the rider stops pedalling while the bike is in motion, the hub will continue to rotate, but the freewheel or cassette will be stationary.
Hubs are what allows a bicycle's wheels to rotate, keep moving forward and inevitably stay upright and continue rolling. It's essential to keep them well maintained because they truly are at the heart of your bicycle.
Hubs are what allows a bicycle's wheels to rotate and keep rolling. Hopefully this article has helped to give some understanding as to what a hub does and what it's components are. Hubs can easily feel fine while you're riding your bike but if you're concerned about how they're feeling (or sounding) or what the overall well-being of your hub might be, it's best to leave the technical adjustments of a hub to your local bike shop mechanic. Most bike shops will check them out for a minimal fee and overhaul them for a bit more if it's needed.
Don't forget: always keep the rubber side down.
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