What does a report card of a bike look like? There are only two subjects, basically torque and power. On the basis of these two subjects, a bike is categorized under good or poor performer. Closed serious tracks and circuits becomes the classroom. Repeated practice runs on a race track make it perform better and better. The ultimate test/examination is when a bike undergoes dyno test. So you think that a bike with more power or torque tops the exam? Well, that is what you are going to find out. This article will help you to understand torque and power with their respective curves and is a part of a series of 3 articles. After reading all the 3 articles, you will have complete knowledge from torque to stage 3 tuning as all of these are interrelated.

Let me start off with what actually torque is. I know that many of you guys hate math just like me but sit back and just relax because I will keep it simple and make sure math does not interfere.

Torque

Basically, a torque is a twisting force applied on an object to rotate it on its axis.

Mathematically, torque is perpendicular distance multiplied to applied force.

The most basic example of torque is tightening a bolt using torque wrench. One applies a force(f) on the wrench. This applied force is at a distance(d) from the pivot point(O) where the bolt is located.

                            

so torque becomes,

                                 T= f*d

where, ‘f’ is applied force in Newton

          ‘d’ is the distance of applied force from pivot point.

           ‘T’ is torque in N-m, kgf.m, pound-feet

Let me take you back to the time when it all started...........THE CYCLE. Do you know that the simplest transmission system on this planet is the one which is found on a geared cycle?!!

 This is my Scrapper SHX bicycle with a 6-speed transmission system. For an instant, imagine you are riding this cycle on a flat road on 3rd gear. So a certain amount of force is transferred by you to this gear while pedaling. Suddenly you approach a slope and start losing speed. Immediately, you switch to lower gear say 1st gear which in turn helps you to climb the slope. You choose to apply force from a greater distance by switching to lower gear with larger diameter from the previous one. This is where torque comes into picture. So now you are spinning the pedal more with less induced energy and effortlessly climb the slope. Remember that 1st gear has the least power which made climbing a highly gut-wrenching job. 

A bike on the other hand has no pedals and require an engine to move forward from its static state.    

Crankshaft is where this twisting force or torque is applied. Force(f) is exerted on the piston when mixture ignites in the cylinder. This force acts tangentially on the crankshaft which initially pushes the piston down resulting in rotation of the crankshaft. This force is transmitted from piston head to the crankshaft by a connecting rod which is circumferentially assembled about pivot point(O). This force acts as a twisting force due to the arrangement. The force in turn rotates the crankshaft and ultimately converting linear force into twisting one which is actually torque and moves the bike.

In a manual transmission bike, there are basically 5 gears of different sizes starting from the largest gear and in deceasing order ends with the smallest gear. To be precised, 1^st gear is the largest gear whereas 5^th is the smallest.

Now you won’t face any problem in answering the question that states which gear will have more torque. Of course, 1^st gear has the maximum torque as the distance of applied force from center is more due to larger diameter of the gear. Similarly, smallest gear i.e. 5^th would produce the least torque.

How to get more torque?

                                  T= f*d

By the linear equation, it becomes easy to understand that either the force or the distance should be manipulated in order to increase the torque. It is not always easy to increase the force and totally depends on the object or process producing that particular force so the simplest way to increase torque is to increase the distance. According to this theory, if you cannot open a bolt with a short arm wrench then just replace it with a long arm wrench which means that you are applying same force but now from a greater distance. Installations of paddock stand in pits, long armed jacks are some applications in automobile world.

  

Power

Power is simply how fast torque is applied and is expressed in horsepower(hp), bhp(brake horse power), kilowatt(kW) etc. Again, take your position on the cycle where you are pedaling on 3rd gear. By directly switching to 6th gear, you find that pedaling gets reduced even when you were putting the same amount of energy in the process. In turn to achieve the same pedaling speed as was there in 3rd gear, you will have to induce more energy in terms of force. This will increase the gear rpm as well as the overall speed of the cycle. This is what power is in the simplest way. Hence, 6th gear is the most powerful gear but least torquey. 

Low rpm

High rpm

How fast means the instantaneous speed of engine which is displayed by the rpm tachometer on the cluster. Power produced is calculated when rpm is introduced to the torque.

                       Power= Torque*rpm

 Even if torque is left undisturbed, a bike could travel faster just by increasing the rpm. Greater rpm means greater power production. So now you might be clear about what you need to make your Ducatis, BMWs, Pulsars faster. Applying the same logic, 1^st gear would produce least power and 5^th gear would produce the most which is again due to the different size of the gears. Greater diameter 1^st gear means it would rotate less under a fixed torque. Small diameter 5^th gear means it would rotate more under the same torque. This is all because of the introduction of rpm but understand that power is nothing without torque. Power will require torque every time a bike wants to go faster.

Torque vs. Power Curve

The overall performance of a bike as mentioned earlier is observed by torque vs. power curve. It is simply a graphical representation between torque and power with respect to rpm.

This is how a typical curve looks like. It shows how an engine behaves when revved to redline. In the graph, both torque and power increase initially with engine speed and soon after reaching their top values, the curves start to fall. The fall is due to increase in friction between gears under high rpm. Optimum values reached across power and torque band are called peak values. The graph shows that the famous Kawasaki H2 SX(No, this is a tourer version of record holder H2 and not H2 itself) cranks out a peak torque of 89.08 N-m at 8500 rpm and peak power of 166.1 hp at 10,200 rpm. You might have come across max torque and power in any automobile brochure which is nothing but peak torque and power. It should be noted that overall performance of a bike say Kawasaki H2 in this case would not be determined by those peak values but the area under the curves. This type of torque vs. power curve is plotted by running dyno test on a bike using a special m/c called dynamometer. Do not miss out the next article of this series where I have covered how dyno test is run on a bike.

  

by AutoVogue