Archive for March, 2015

Gear Ratios

March 14, 2015 5:11 pm

Gearing on a bike might be a bit confusing to some since there are several things that affect how low the gearing is.  There are effectively four circles or gears involved:

  • The Crank Arm.  This is measured in mm, with a valued like 170 or 172.5
  • The Front Cassette, which is usually either a Triple which as 3 rings that have 53/39/30 teeth each, a Double which two rings with 53/39 teeth each or a Compact with 50/34 teeth each.
  • The Rear Cassette.  This also defines the number of speeds, usually either 10 or 11 speed and the smallest cassette is typically 11 or 12 teeth and the largest anywhere from 23 to 36 teeth.
  • The Tire Size.  For mountain bikes theses are either 26 in, 27.5 in or 29 in.  For road bikes these are typically 700c.  It is the outside circumference of the tire that is the factor so a fatter tire will have a larger circumference.

To get the lowest gear you would go with the largest crank, smallest front chaining, largest rear cassette cog and smallest tire.  A gear ratio is defined by the the ratio of the number of teeth (or circumference) between two gears but we effectively have 4 gears at work so to get a lower gear ration you go from a large gear driving a small gear, driving a large gear, driving a small gear.

On a bicycle it is a bit more complicated than if all these were simple gears, in which case the middle gears would be idler gears and not affect anything.  But the connection between the crank arm and the chain ring is direct coupled.  The crank arm acts as a lever and like any lever, the longer the lever the easier it is because the longer the crank arm, the more distance your foot moves on each rotation and more distance for the same rotation means less force, or effectively a lower gear.  Likewise the cassette is directly coupled to the rear tire.  As the tire size decreases, each rotation of the tire means moving less of a distance, which means less force is needed for each rotation of the tire.

When you move to a larger tire size (such as the 29ner mountain bike) the same crankset and rear cassette will have higher gear ratios (harder to pedal) than on a 26 in bike.

The crank arm length is picked based on the rider and the bike tires are what works on the bike.  So all that most riders can change is the chain rings size and the rear cassette range.

There are some online calculators you can use where you put in the crank size, tire size, front chain rings and rear cassettes and it will give you gear ratios.  A lower number means a lower gear (easier to pedal).  These will help you decide if you can switch to a Compact Crank instead of using a Triple Crank.

I was considering to buy a new bike for my wife but her current bike has a Triple Crankset with a11/34 cassette on the rear, which I am able to have her use by installing a mountain bike rear derailleur.  I was hoping to get a bike for her with electronic shifting but the lowest option would be a Compact crank set and a Ultegra long cage derailleur that will work with a 11/32 cassette.  With her current bike she rarely goes into the very lowest gear, but on some of the hills she goes in one gear down (30 tooth cog), so she is using a 30 tooth chain ring in the front and a 30 tooth cog  in the rear.  Plugging in the crank arm length (170 mm), tire size (700×25) that is a gear ratio of 2.1.  Now switching to a Compact crank and a 11/32 cassette, the lowest gear ratio is also 2.1.  So she is only giving up the last gear only although that last gear is relatively big jump.