Why Bullets have Lesser Top Speed

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I take great pride in riding my bull and am very vociferous about it. This startling point came up in the midst of a typical outing with my younger cousin. Here is how the conversation went:

Cousin: Do you ride the Royal Enfield?

Me (Proudly): Yes

Cousin: I have heard it doesn’t get punctured?

Me: It’s a myth, it does!

Cousin: I have also heard the engine runs fine even after 600 Kms and it doesn’t get heated up.

Me: Myth again! Though it runs fine, the engine does gets heated up and needs to be cooled down.

Cousin: Hmm… Yours is a 350 cc right?

Me: Yes.

Cousin: What’s the top speed?

Me: Max. 100 Kmph

Cousin: That’s it!!!  My 150 Pulsar tops at 110! And much quicker too! 350 should be topping 200 no?

Me (confused): Eat your ice cream!

That got me thinking. The boy is right! Why is that so? Then another thought stuck me that this question may be haunting many of my biker friends! So I got down to the bottom of it. Now let me set the expectation straight here, this article will not have numbers from physics. This will just have the understanding from a layman’s perspective. Let’s have some basic terminologies listed out here.

Cubic Capacity: Volume of air + fuel mixture the engine takes. In Cm3

Bore: The diameter of a cylinder

Stroke: The to-and-fro motion of the piston during the combustion cycle.

Cylinder: Where the entire combustion cycle happens.

BHP:  Torque X RPM

Torque: angular force that affects the rotational motion.

RPM: Rotations per minute.  The number of time the crank shaft rotates in a minute.

The assumption here is that every reader is aware of the cycle of the Internal Combustion Engine. The following diagram illustrates the parts named above:

BHP or Brake Horse Power is again in layman’s terms the power generated in a given period of time. I am deliberately avoiding any mathematical or physics terms in this article as it is aimed at a layperson.

Any motorbike manufacturer’s biggest challenge is to get maximum BHP out of the bike. How they do it actually defines how a bike would turn out to be. There are two ways maximum BHP is attained:

  • High torque at low RPM
  • High RPM at lower Torque.

Torque is basically force in a rotational form. To give you an example, when you are shutting a door, some amount of force is applied to pull the door. The door is connected to its hinges that results in the rotational movement which is directly proportional to the applied force (torque). The higher torque (force with which the door moves), the greater slamming.

To apply torque in automobiles, the force applied on the crankshaft by the piston that will result in rotational movement of the engine.

Coming back to maximum BHP, the high RPM at lower torque is suitable to the sports bike or commuter’s bike. The high RPM is achieved by making the bore diameter longer than stroke. That means the height of the piston movement is less than the width of cylinder. At higher RPM, the life of the engine is lesser than its other counterparts.

Now the point of interest for us is the category of “high torque at low RPM”. Most of the Cruiser bikes like Harleys and Royal Enfields of the world use this to attain max BHP. In this, the bore diameter is lesser than the stroke length. That means the stroke has longer way to go to-and-fro, hence attaining lower RPM at high torque at one stroke.

To understand it better, let’s consider a very simple experiment. This may not be the best way to explain rotational motion but it is apt in the concept of Bore X stroke. Take a basketball. Try dribbling it with your hand from your knee length. The ball would bounce back faster and more number of times and the force with which it hits the ground is lower. Now try dribbling the basketball from the height of your hips. The number of dribbles is less and the force with which the ball hits the ground is more and hence bounces back more. In short, the basketball is your piston stroke, the height from where the dribbling (knee or hips) is your bore diameter and the force with which it hits the ground is the torque generated.

To conclude, both the category of bikes, the BHP may be the same, the cubic capacity may be the same, but the way a bike would perform would depend on its Bore X Stroke  and Torque to  RPM proportion.

Below are the representations of the cylinder dimensions in both the cases. This is the simplest way I could explain the concept.

Hence, my much-loved bullet will not win me any races for sure! However, the experience on the highway for a long ride is unparalleled! Moral of the story- the next time anyone asks about top speed, dribble the basketball!

Written by Sam 

 

 

 

 

 

 

 

 

 

 

saurabh says:

damn good article man!
Keep writing…

Truly a great article its good to have things like this explained in simple terms without the technicalities especially for a novice like me

A wonderful article indeed. It’s good to have something explained without the confusing technicalities especially for novices like me.

Punit Dubey says:

This is the one of the very best wriiten, easy to understand article on Bore vs Stroke thing. I especially like the basketball example, and that the life of engine depends on RPM is new to me.

Indraneel says:

Also if we tinker with the gear ratio or in layman terms the no of teeth, can we not play with the topspeed or pickup of the bike?

Gen says:

Well Explained – Sam

You can increase the RPM in Bullets and also increase BHP – few decades ago someone increase the BHP of a 350 CC to some 48 BHP. Guess one can ask Google Singh 🙂

Gods of bullets have built Bulls which clock 100 Miles (160+ Kms) no need to tell dribble Basketball – Sam