The drivetrain of a motor vehicle is the group of components that deliver power to the driving wheels. This excludes the engine or motor that generates the power. In contrast, the powertrain is considered as including both the engine or motor, and the drivetrain.
For a motorcycle the rear
wheel is also part of the power train as it delivers the power to the
Wikipedia Gear Ratio is:
But in a motorcycle the are more then one gear at work so there also are more gear ratio. All these are working together and form the overall ratio. Most gearing consists off:
The overall gearing of a motorcycle consists of 3 major parts:
All these ratio combined results in the Overall Gearing Ratio of the power train:
Changing any of these parts or changing the rear wheel tire's dimensions will effect speed and / or torque results. All of these changes can be simulated by using the GC by changing involved gearing fields. To be able to do this correct, all ratio, sizes and RPM's are necessary input.
primary drive is the ratio between the engine's crankshaft and the incoming shaft
of the gear box or clutch. Usually this ratio can not be easily altered unless you
perform major surgery on your bike.... But if you do, don't forget to change the
Primary Drive Ratio field in the GC accordingly.
box is the only gearing part that can change its ratio very easily and
that is by you changing the gear while you drive. Normally a gear box is
dedicated for an engine or bike so you do not have to change gear ratio
in the gearbox itself. But sometimes the stock gears are not suited for
all circumstances and manufacturers offer a way to change the internal
gear box ratio by either offering different cogwheels or by offering a
so called cassette gearbox that can be swapped easily. That is why in
the GC you can change the gear ratio of each gear by changing the white
gear box ratio fields for each gear.
The Final Drive is sometimes also referred to as 'the product of all ratios involved' but I prefer to call that the 'Total' or 'Overall' ratio. In our example the Final Drive ratio is the ratio of the final part of the gearing, the ratio between Rear and Front sprocket. This is also referred to as 'Secondary Ratio'.
Below a table containing the possible final drive gearing changes and their results on Speed and Torque.
As a rule of thumb, changing the front sprocket with 1 tooth, amounts to the same effect as changing the rear sprocket with 3 teeth. This does not exactly apply to every bike but as a rule of thumb it will do and explains the results in the table below.
can see, the effect of adding a front tooth and keeping the rear the
same has about 3 times more influence on the speed then removing 1
tooth in the rear and keeping front the same. Changing only the rear
with 1 tooth does not have a lot of effect, you can use it to 'fine
tune' the final drive. In combination with also
changing the the front it either amplifies or weakens the total effect.
The exact increase or decrease
in both % and actual speed for your bike can be calculated using the Gearing Commander tables.
all these individual ratios results in the
Overall power train ratio or the Overall ratio.
As the rear tire is a major part of the total bike gearing, changing its size does effect this gearing and thus effect speed and torque at certain RPM's. This is easy to explain: the chain (or drive shaft) is rotating the rear wheel at a certain RPM. Now when it is a small wheel in diameter it will have a small circumference and when rotated it will cover only a small distance. A bigger wheel will cover a bigger distance when rotated equally fast.
changing for instance your rim size from 16" to 17" would mean a higher
(top-)speed at the same RPM.
A modern tire has size marks on it like this '190 / 50 / 17' which means: the width is 190 mm, the rim size is 17" and the height of the tire is 50% of the width ( 50% of 190 = 95 mm).
Let's asume you have an older bike (1982 Suzuki GS750T) which was originally fitted with a 4.5H17 4PR rear tire which is not available anymore and you want to replace it with a similar modern tire. The original tire has a width of 4.5" which is 114.3 mm. It's circumference was 2074.7 and theroretical top speed 208.1 Km/h. A similar wide modern tire could either be a 120/90/17 or a 130/80/17.
Though all 3 mentioned tires have the same rim size, as their width and heights are different, so will their circumference be different:
4PR - width = 114.3 mm, circumference = 2074.7 mm, theoretical top
speed: 208.1 Km/h
shown above, the tire size does affect the speed of the bike but it is not
part of the commonly used 'Overall Gearing Ratio'. Say we want to
compare 2 identical bikes, so having the same primary drive ratio, the
same gearbox ratio and also the same final drive ratio but having rear
tires with different sizes. This will resullt in an Overall Gearing
Ratio which is the same for both bikes but they will perfom differently
due to the different rear tire circumferences. So at a fixed RPM they
will both reach different speeds.
Now say Custom bike 'C' is like bike 'B' (so with the wider rear tire) and also has a smaller front sprocket, 30 instead of 32.
This means the Overall Ratio will be different as the final drive changed and the Total Ratio will change because of the changed Overall Ratio and a changed rear tire.