BICYCLE CRANKSET – BUYERS GUIDE

Shaun George BOTS Guides, Mechanics

Looking at a crankset upgrade? Maybe your chainrings have worn out. Whatever it may be, we’ve got you covered. Cranksets are often thought of as overly complicated components that are of various types, which further adds to the complication. Things can seem complicated at first which is why we’re here to simplify things.

WHAT IS A CRANKSET?

A crankset often referred to as a chainset, is a group of components that lie at the very heart of your drivetrain. The crankset essentially connects the chain to the drive wheel (rear wheel) of the bike.

The chainset rotates using the bottom bracket as a support and housing. To start the wheels turning, you need to turn the cranks. When the chainset travels in a clockwise direction, it provides momentum to move the bike forwards.

FRONTAL DRIVETRAIN ANATOMY

Frontal drivetrain anatomy

1. CRANK ARMS

Both crank arms collectively are called cranks. The two crank arms are mounted on either side of each other at 180 degrees to each other, connected by an axle. The axle sits within the bottom bracket.

Crank arms are available in a variety of materials, including steel, aluminum, and carbon fiber. Steel cranks are common on older and more basic bicycles. Aluminum on the other hand is found on most low to mid-range road and MTB cranksets. Lightweight carbon fiber cranks are generally reserved for high-end cranksets intended for racing.

Campagnolo’s Super Record crankset – full carbon cranks

For road and MTB bikes, aluminum cranks are considered to be tough, stiff and light for the price. Most manufacturers will make some efforts to shed weight while maintaining stiffness, with some favoring hollow crank arms and others machining away excess metal.

FUN FACT – Japanese manufacturer Shimano still swears by aluminum cranks on even their highest-end offerings such as on the Dura-Ace crankset. They believe that they can match weight and stiffness requirements even with aluminum.

2. CHAINRINGS

Charings are essentially the large sprockets you see in front. They enable the chain to drive the rear wheel, with the spaces in the chain fitting in between the teeth around the edge of each chainring.

These rings are generally made from lightweight aluminum or, as is often the case with the small and middle rings on mid-range cranksets, less expensive steel. These rings are bolted onto a four or five-arm piece called the spider.

Most road bikes have two chainrings at the front, known as a ‘double’. Touring bikes and most basic bicycles sometimes have three chainrings up front, known as a ‘triple’. Track and single speed bikes have just one chainring.

The size and number of teeth on the rings will depend on the bike type and riding discipline, with road bikes often having more teeth on the biggest ring in front to be used with the smallest at the back for maximum speed.

A type of chainring called an oval chainring also exists. As the name suggests, the chainring is oval in shape rather than round. But why? Watch the following video to understand this fully.

Credits – World Wide Cyclery

These chainrings are designed to equalize the inconsistency of power through the pedal stroke. Road/MTB riders who are very powerful but have inconsistent pedal motion benefit the most, especially during phases of consistent high power delivery, such as during sprints and breakaways.

HOW TO CHOOSE CRANKSETS

Here are a few factors you need to take into consideration when choosing cranksets in order to make the right buying decision.

1. COMPACT VS STANDARD CHAINSETS

A chainset is described as a “compact” one when it has 34/36teeth inner chainring (smaller) and a 50teeth outer chainring (bigger) fitted. Compact chainsets are a great alternative to a triple chainset as they offer a good wide ratio of gearing and provide a good chain line, thus giving increased chain strength and life.

A Shimano Ultegra compact chainset

Standard chainsets have gear ratios that are higher and closer than those found on a compact chainset. They are fitted with 39/42teeth inner and 52/53teeth outer chainrings. These chainsets are for racing or aggressive climbing.

Fewer teeth = lower gearing = easier to climb.

2. CRANK LENGTH

Crank lengths are variable and are adaptable to your height and bike frame size. Most road cranks are 170-175mm long, but riders with legs shorter or longer than the average may feel more comfortable on cranks that better match their measurements. Options from 165mm to 180mm cover different limb lengths.

Crank length is usually indicated on the inside of each crank arm, close to the threaded hole for the pedal.

This crank arm is 172.5mm in length

Choosing your crank length is a wide open debate on what is best for you and your leg length. Road chainsets differ progressively in 2.5mm increments. If you are in any doubt you should head over to your local bike shop for help.

3. BOLT CIRCLE DIAMETER

When looking out for different chainrings you will probably hear of the term “BCD” or Bolt Circle Diameter. The BCD of a crankarm is the diameter of the circle that goes through the center of all of the bolts on your chainring.

This dimension is usually measured in millimeters. It is critical to know the BCD of your crankset when you are selecting a new chainring for your bike. In many cases, the BCD is printed on the inside (rear) of the chainring.

Cranks are also available in 4 and 5 arm versions so if replacing rings ensure you have the correct bolt pattern on your rings to match the cranks.

4. CHAINRINGS

Depending on which bike you have, it will either have single, double or triple rings.

Credits – Bike Exchange
  1. Single ring (1X) – More commonly spotted on mountain bikes, single chainrings were originally used by gravity riders who spent most, of their time descending, and so didn’t need a wide gear range. More recently though, SRAM has introduced 11 and 12-speed cassettes that have widened the gear ratios achievable with a single ring up front.
  2. Double ring – Double rings are a standard on almost all road bikes these days – they offer good range without the weight penalty of a triple. Some MTB riders still use two rings but their gearing ranges have been equaled by the huge ratios achieved by 11- and 12-speed cassettes used with single-ring cranksets.
  3. Triple ring – Something that’s seen only on basic mountain bikes and hybrids – triple-ring cranksets are now all but an obsolete technology. They offer wide gear ratios but with a significant weight penalty and added drivetrain complexity.

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