Motorcycle Safety Site

Delta V
Affects all motorcycles, not just 'crotch rockets'

By: James R. Davis

Following are two facts:

Assuming that your speed remains constant while in a turn, you are accelerating constantly.

Assuming you maintain the same speed in a turn as you had while riding in a straight line, your gasoline mileage goes down.

Though these facts do not appear to be true, they are. And if you will bear with me through a brief bit of physics you will learn why it is important to understand them.

Speed does not equal velocity. Speed is merely a measure of how fast you are moving. Velocity, however, is speed in a particular direction. Changing either speed or direction, therefore, changes velocity. Finally, the definition of 'acceleration' is changing velocity.

[Changing velocity, as any watcher of NASA activities knows, is called 'Delta V'.]

So? This is not some subtle play on words. There really is a significant difference between changing speed and changing velocity.

We all know that there is a limit to how much traction you can consume before you break loose a tire. We know that acceleration and braking (deceleration) eat up traction. It is important to understand that you do not have to roll-on your throttle to be accelerating! By changing direction you are definitionally changing velocity (Delta V) even if you maintain speed.

That means you are consuming traction simply by being in a curve. To make this crystal clear, when you apply any force to any mass you change its velocity (i.e., you accelerate it.) When you are in a curve you know that there is a force involved that is not there when you travel in a straight line - centrifugal. [Actually, the new force is centripetal. Centrifugal is only an apparent force.] Anyway, you can feel that force, you know it doesn't come from out of nowhere, and you know it is not 'free.' When you roll-on your throttle when traveling in a straight line, your head is forced backwards and you experience acceleration. In a car, if you maintain speed but travel in a circle, your head is pushed towards the outside. (i.e., you experience centrifugal force.) On a motorcycle, because you lean in a curve, even if you maintain a steady speed, you feel heavier. That is, you experience acceleration (called 'centripetal acceleration'.) If you have Excel on your system you may wish to click this link (http://www.msgroup.org/images/Bike.xls) in order to access a model that shows this pretty clearly.

So, now you know that centrifugal force demonstrates acceleration.

Going back to 'Delta V' - if you acknowledge that acceleration IS changing velocity, then it follows that more energy is consumed (gasoline burned) in a curve than when riding in a straight line, even if both rides are at the same speed. [Note, however, that more energy is not actually being 'consumed'. Rather, more of it is being converted to heat, and your tires are getting warmer as a result.]

Again, so?

It is not an academic insight. If you generate more energy with your engine to maintain your speed while in a curve, then that energy MUST be consuming traction. [This, thanks to the law of 'conservation of energy'.] In other words, confirmation that you should think twice before aggressively rolling your throttle ON or OFF while in a curve.

[Just for a complete understanding, and so you do not think I pulled a fast one here, there is yet another reason why your gasoline mileage goes down if you maintain the same speed in a curve as when you drive in a straight line: You are riding on a part of the tire that has a smaller diameter while in a curve. Thus, the wheel has to make more revolutions in order to travel the same distance. That means, of course, that the engine must turn faster in order to maintain the same real speed. The speedometer will read high while riding in a curve.]

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