Gear Ratios

The other day when I wrote about ratios in game design, Adam Liss reminded me of gear ratios.  Thank you for this entry.

To begin with gears can transfer motion, slow things down or speed them up, create changes in torque, and control  motion.

Gears and gear ratio play a huge part in most bicycles with more than one speed.  I have one with like 15 gears but I seldom use more than 3 or  4 depending on the hills I have to go up.

The first thing to consider is the gear ratio which is the ratio of teeth on each gear.  Which means if you have a gear with 8 teeth and another with 40 teeth, the gear ratio is 40/8 or 5/1.  This tells us the smaller gear will rotate 5 times to one time for larger 40 tooth gear.  It also means the smaller one will rotate 5 times faster than the other.  This type of set up is designed to increase torque.

On a bicycle, its easier to see gears because of its design.  On a bike, the gear ratio is also referred to as the velocity ratio.  Usually the pedals are attached to the largest gear while the back week has several gears of differing sizes attached to it.  When you pedal, it moves the wheel which moves the gears and the chain.  The more teeth the back gear has, the easier it is to pedal up hill.

A racing bike is normally has 52 teeth on the front with 13 teeth on the back so its a 4 to 1 ratio while a regular bike might have the best gear ratio of 44 teeth to 16 teeth or a 2.75 ratio.  It all depends on the type of bike and the type of gears installed.  Many bicyclists will customize their gears based upon their needs.  I've got a hybrid which will go off road or on road if needed but I don't know the gear ratio only because I never bothered asking for that information.  I gave the bike person my request and she put it together.

If you look at your car manual you might find something about the pinion gear found on the motor of an electric car, and the spur gear found on the drive axle of a car.  So if your pinion/spur gears is 18:90, although they usually use the spur/pinion gear ratio of 90:18 or 5 to 1.  This is similar to the example. There are also transmission ratios, final drive ratios and other ratios associated with cars.

If you do a bit of looking, you find out that first has a ratio of about 3.166 to 1, second is about 1.882 to 1 while third is about 1.296 to 1 and fourth is around 0.972 to 1.  It would be easy to have students calculate the number of teeth by calculating the actual ratio with whole numbers.  This would be doing a reverse calculation.

To figure out the actual ratio, it is recommended you take apart the car enough to find the actual gears, otherwise car manuals tend to only list the reduced ratios.

Thank you to Adam again for this idea.  Have a great day everyone.