Torque, Horsepower, Bore and Stroke
While these topics can get very scientific here is a overview that you will find useful to understand the characteristics of a particular engine. This can assist you to make an informed choice based on scientific grounds when comparing two motorcycles.
Practically what does this mean for me the rider? Let us take the BMW F650 as an example. The bike's manual gives the following technical specs
From this we can see that the difference between max torque and max power is 1300rpm - quite a big gap that we must utilise for different conditions.
You are traveling along on your motorcycle on a country road behind a long truck doing about 85km. The oncoming traffic is heavy. Things are going to be tight. You will need maximum torque for fast acceleration and maximum horsepower for the necessary speed to take you past and beyond.
The gap you are waiting for arrives! You check your rev counter and it is sitting around 3000rpm. The specs tell us that 5200rpm delivers the maximum torque. The situation demands the maximum. You change down a gear and the revs shoot up drawing on the torque available. You open the throttle up and gun the bike into the gap.
The bikes speed and revs climb quickly. There is not much point taking the revs past 6500 as the horsepower begins to level off after that (the maximum rated capacity). You change to a higher gear and the revs drop back towards the 5200 mark allowing you to again draw on the torque to give added impetus to the bike's acceleration.
Again the bike's speed and revs climb quickly. As you do not have another gear to change to you keep the throttle wide open gunning for 7000 rpm. Here the bike is using its horsepower to cover greater and greater distance in a shorter and shorter time period (distance over time i.e. speed)
By now the truck is a speck in your mirror and it's time to relax and check to see that your passenger was not left somewhere behind on the tarmac!
You are travelling up a steep gravel-road mountain pass on a 4 x 4 route. You do not want the motorbike to falter and equally you do not want to go too fast. Thus you change to whatever gear that maintains the revs around 4500rpm and maintains the desired speed. This still gives you some extra torque to use if you need it (from 4500 to 5200 rpm)
If both the speed and revs begin to increase you can change to a higher gear and bring them back down. If the speed and revs begin to drop below 4000rpm it is time to change to a lower gear and bring them back up. At all times the idea is to keep the bike in striking distance of the maximum torque level (sometimes called the powerband - yes, confusing as it has nothing to do with horsepower)
The great thing about this topic is that we naturally know all of this just by doing. Listening to the engine and feeling the amount of power still left untapped is the real teacher in this instance.
Bore and Stoke
The combined attributes of bore and stroke makes motorcycle engines very different. Bore is the diameter of the piston (or the diameter of the inside of the cylinder). Stroke is the maximum distance that the piston travels up and down inside the cylinder
Example: The Ducati 999R has a bore of 104mm and a stroke of 58.8mm while a Harley Davidson Road King Classic has a bore of 95.3mm and a stroke of 111.3mm. These big differences in bore and stroke makes the first a superbike with enormous horsepower and the latter a cruiser with a lot of torque.
To make comparisons between bikes we divide the bore by the stroke. If the resultant number is more than 1, then the bike is horsepower orientated and must be operated at high revs. If the number is less than 1 then the bike is torque orientated and is operated at low revs. In the case of the HD Road King above the number is 0.86, therefore a cruiser. The Ducati by comparison yields a number of 1.76, a number comparable to a formula one racing car. The average result for motorcycles is generally around 1.2
NOTE: A motorcycle is not only a product of bore and stroke as many other factors come into play e.g. number of cylinders, gearbox, suspension, rake angle etc. Nevertheless the trend is clearly visible in the table below.
* Other factors is design in fact make this BMW motorcycle a capable off road touring bike as well.
NOTE. Older cruiser engine designs have the longest stroke measurement. . Engines with long strokes have a lower compression ratio, poorer fuel economy, poor exhaust emissions, greater friction and have crankshafts that are subject to greater destructive forces. For this reason modern performance cruisers like the HD V-Rod have moved quite far up the table as a result of their shorter strokes.
A good website to find out technical information about your motorcycle is www.bikez.com .