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Learn About Your Bike 101: Rims

beginner bike tech Feb 06, 2021

There's a lot of lingo to learn when you get into cycling, not least of which is the naming of the bike parts themselves. This blog is focused on rims, a bike part that can often be overlooked when understanding more about bike mechanics and maintenance.

What does it do?

If you've watched CWRB's 5 Basic Bike Concepts webinar, you'll know that there are two things important to understanding a bike part: What does it do (its purpose), and What properties does it need to do its job.

No matter what kind of bike you ride, your rim has two main jobs (purpose):

  1. Holding the tyre in place
  2. Providing the surface onto which the spokes attach.

Given its main jobs, what properties does it need? Well, to hold the tyre in place and provide a surface for the spokes to attach, it needs to be shaped a certain way.

If you cut through the rim of 5 different bicycles and compared their cross sections, you will see a big difference in what they look like. Some may be narrower (racing road bikes) compared with much wider (downhill MTB). Some may have a lot of spoke holes (up to 48 for DH bikes) and some may have only 20 (road racing bike).

As with any component, there is a reason for the way a rim is designed - and it's likely to achieve a very specific purpose.

Rim braking

One example is that some rims are designed for rim brakes (where the brake pads squeeze onto the rim of the bicycle to slow you down) and so need to provide a specific braking surface for the brake pads to grip.

Bicycles with disc brakes or hub brakes will not require this surface so can be a different shape or even made of different material because they don’t have to withstand the same braking force.

What are rims made out of?

Bike rims can be made of many materials, each offering different benefits and drawbacks. Some of the more expensive rims might be lighter (ie made of aluminium alloy or a carbon fibre composite) while some rims on commuters or dutch style bikes may be made of steel and so are a bit heavier (because saving seconds over every kilometre doesn't matter as much when you're going to the shops as when you're doing a time trial).

 

Looking to upgrade?

Although rims all perform a similar job, there are a wide variety of rims for different uses.

For road and trail riding, stiff and lightweight rims reduce rotational weight, helping you go faster for less effort. ... However for disciplines such as BMX and downhill, rims must be strong and wide enough to take considerable punishment without literally buckling like a taco (I've seen it firsthand - not good). 

The important thing to remember is the type of rim you choose must be matched to the type of riding you do.

MTB

A lightweight set of XC rims simply can’t withstand the beating dished out by a super rocky descent with lots of drops. Conversely, a super-strong pair of DH rims will be hard work to pedal for any length of time.

If you're a mountain biker, you also need to remember that there are three different common wheel sizes: 26", 27.5" and 29". If you're looking to upgrade, whichever rim diameter you opt for will be determined by your bike type (eg you can’t put 27.5” rims on a bike made for 26” wheels).

Width
The general rule is that narrower, lighter-weight rims are used for XC, marathon and common off-road riding, whereas tough, wide rims are used for more gravity-orientated adventure.

Narrower rims (around 23mm) accommodate narrower tyres (around 2.1") and thicker rims (36mm) can accommodate thicker tyres (around 2.7").

Spoke number

Rule of thumb with spokes is that less spokes means a lighter wheel. The problem with this as a 'rule' is that the more spokes a wheel has, the more the load is spread and the stronger the wheel is.

So wheel building and spoke choice and pattern are really about balancing the weight of the wheel with its strength - which again depends on what kind of riding you're doing.

For general trail riding, a 32-spoke rim is standard. If you are after more lightweight race wheels, your rim may feature 28- or 24-hole drilling. Of course, if you're into more extreme riding styles that will call for more strength - so you may find that 36 spokes are more common in Enduro and DH wheelsets.

Street riders may opt for anything up to 48 spokes in order to handle the force of impacts dished out by big landings.

Road

If you're a roadie, rims may be designed to take the more common clincher tyres or the tubular type (‘tubs’). We talk more about this in the Bicycle 101: Tyres blog.

Though they may look more similar to one another than XC vs DH rims, road bike rim widths also vary. Generally speaking, narrow rims accommodate narrow tyres that can be pumped to a high pressure for more speed - the standard for high-performance bikes. Commuters/tourers may have wider rims for the oppsite reasons.

Another aspect to consider regarding road bike rims is their depth. Wait, what? So you probably will have seen newer generation aero rims and wheelsets which feature deep-section rims? These were invented for improved speed (and stiffness), but as with any modification, there is a tradeoff in that the deeper the rim, the more bike handilng is affected. 

A ‘traditional’ road rim may therefore have between 28 and 36 spoke holes, while lightweight aero wheels may have as few as 20.

Quick Facts: Bike Rims

  • Before the 1980s, most bicycle rims were made of steel. Historically, they were even made of wood!
  • Aerodynamics, mass and inertia, stiffness, durability, tubeless tyre compatibility, brake compatibility and cost are all considerations in how a rim is made
  • Rims and spokes work together to distribute the force of moving the bicycle!