Ride quality of 2 wheelers an elucidation

Since a scooter has a low center of gravity, is it easier to handle than other bikes like FZ

One of the reasons why motorbikes are better than scooters when it comes to handling is because they have higher Centre of Gravity.

You might believe lower cg always gives better handling. Because all sports car, super cars are designed to have lower cg. Even Formula 1 cars which are fastest and “best handling cars in the world” have low cg (one of the lowest cg when it comes to cars).

In case of Four Wheelers it is essential to keep the cg as low as possible so as to prevent “Rolling”. Higher the cg, higher will be the rolling.

But in case of motorbikes you need this rolling to lean the bikes more effortlessly and effectively.

Because higher cg enables the rider to shift weight easily while cornering. Bikes like KTM RC/Kawasaki Ninja have higher cg compared to Royal Enfield Bikes. Any rider who has experience with both these bikes will tell you that shifting weight on a RC/Ninja is better than a Bullet. Now you may say Bullets are heavy but so is Benelli TNT300 but still it's easier to lean on a Benelli. One of the reasons? Higher cg.

Also with higher cg, the lean angle required to make the corner will be less. With that you can lean even more aggressively and corner even faster.

If cg is kept as low as possible in bikes it will stay straight, even if you try to lean it the bike will try to resist the lean. It will act somewhat like that Gyro Bike that keeps the two-wheeler balanced without any mechanical support using Gyroscope technology. The gyro bike has the gyro tech to move the cg off the balance during cornering but in the above case the cg will be fixed or with negligible amount of variations available.

Oh! By the way, the bikes with “lower” cg will have good high speed stability (limited to straight line motion) and low speed balancing.

Also, coming to the part where you compare the maneuverability of an average scooter to the FZ, the answer is that the scooter will be easier to handle at low speed (below 40kmph). Now this is not because of lower centre of gravity but due to the fact that it will be lighter.

Nimbleness is a cambination of wheelbase, rake, weight and Front Tyre size and profile.

A bike with shorter wheelbase turns quickly
Less rake angle gives more agility
Lower vehicle weight helps in quick steering
Smaller radius and thin Tyre will both help in agility.
Wide handle bars will help in easy turning

How better is scooter as compared to fz? ride and feel!

Ride both! What is better handling? If it’s just nimbleness, the Scooty wins by a mile. If it’s straight line stability, the FZ wins for higher speeds.

A low centre of gravity means more stability, however, but weight (or a higher centre of gravity ) can also help the bike lean into turns, and conversely, resist standing up returning to upright , so you need more power/acceleration to get a high CG bike to straighten up.

Does a motorcycle’s lower center of gravity make it more stable than a bicycle?

No. Having a low center of gravity makes it easier to hold up a motorcycle when at rest, but it doesn’t make it more stable. Having a high center of gravity actually makes a motorcycle easier to balance at low speeds. You can see why if you go out and balance a broom with the handle end in your palm and with the straws up. It has a high center of gravity so if it starts to fall over it has a low moment of inertia and moves slowly and you can easily move your hand under it. Now try the same thing with a whisk broom. It’s short and has a low center of gravity; so it can fall over quickly. Similarly a scooter is hard to balance at walking speed.

At medium to high speed the height of the center of gravity has no significant effect on stability. What does have an effect is the roll moment of inertia of the bike about it’s center of gravity. Road racing bikes try to minimize this because it makes it easier to flick the bike from right to left in an S-bend. Because of the gyroscopic forces of the wheels, motorcycles tend to be to stable at high speed…to hard make them change directly quickly. So the tendency in racing has been to smaller diameters. Over the years road racing motorcycles have gone from 19″ wheels in the 50s to 18″ wheels in the 70s to 17″ wheels in the 90s to 16″ wheels in the 2000s. Now they seem to have settled on 16.5″.

Why is it difficult to control a motorcycle if it goes slow and you have a heavy person sitting behind you?

Let say that your motor cycle tyre width is 6 inches, and the heavy weight person whose seat width is 30 inches, now the weight of motor cycle + your weight +weight of heavy person is supported by 6 inch tyre and on move slowly the gravitational force acting on the is considerably high so it will try to shift, where as vehicle at high speeds the G force reduced and holds the vehicle steady. This is experience too and analysis.

How do I find the center of gravity of a car or a motorcycle?

Pretty simple.

Assume you are measuring the center of gravity for the motorcycle. Refer to the image. Assume L is the wheelbase of the bike. CG is the center of gravity.

Take 'b' as the distance of CG from the rear wheel. So its distance from the front wheel will be 'L-b'.

Now, if you keep two weighing machines, one under each wheel, then you'll be able to get the normal forces 'N1' and 'N2'.

Now, since the motorcycle is in stable equilibrium, the sum of moments about the CG should be zero. This gives -

N1(b) = N2(L-b)

You'll get 'b' from this equation.

Mass, Weight and Center of Gravity

To better understand the motorcycle’s suspension and how to interpret suspension data to improve setup, it’s helpful to look at weight and the center of gravity, and how much load is on the suspension as the bike turns, brakes and accelerates. With GPS data, it’s possible to put values to the acceleration parameters, and from there calculate how much weight is transferred and to which wheel, and the actual load on the suspension.

The center of gravity of an object is denoted by the circle/cross symbol. A typical motorcycle’s CG is located midway between the front and rear axles, and each wheel supports half the motorcycle’s weight (top). If more of the motorcycle’s mass is concentrated toward the front wheel (bottom), the center of gravity would correspondingly move forward. The front wheel would support more weight, the rear wheel less.

First, it’s important to understand the difference between mass and weight when considering weight transfer and center of gravity. Mass is a measure of the amount of “stuff” in an object, and is measured in kilograms or pounds. For example, the typical combined mass of a motorcycle and rider is approximately 250 kilograms. Weight, on the other hand, is a measure of the force exerted on an object, generally due to gravity. On earth, gravity is a constant and weight and mass can be used somewhat interchangeably. However, because we are applying acceleration to the motorcycle’s mass, and this acceleration is added to the acceleration due to gravity,this has a major effect on the suspension. As such, we are more concerned with weight rather than mass. Static weight (that is, weight of the motorcycle solely due to gravity) is defined as:

w=mg

w=weight in Newtons or kilograms-force

m=mass in kilograms

g=9.81 $\frac{m}{s^2}$ (on earth)

The 250kg motorcycle and rider have a weight of 2452 Newtons or 250 kilograms-force. Typically, the “force” is dropped from the units and you can see where confusion begins to arise. To simplify matters, we will drop the “force” from the kilogram-force units and stick to kilograms when dealing with weight and weight transfer. While not technically correct, this makes it much easier when looking at data and trying to evaluate various scenarios as the motorcycle’s weight shifts around.

Another important term to understand is center of gravity (CG), or center of mass. The CG is best visualized as the point at which an object can be supported such that the object is perfectly balanced. For example, you can support a heavy book on the tip of your finger if your finger is at the exact center of the book – its CG. While a book is a simple object and its CG is easily calculated, finding the CG of a more complicated shape such as a motorcycle is a difficult matter. The usefulness of the CG is that forces acting on a body can be considered to be acting on its CG, drastically simplifying calculations.

Horizontal Center of Gravity Position

The horizontal position of a motorcycle’s CG can be easily found by using two scales and finding how much of the motorcycle’s weight is on each wheel.

$x=\frac{W_r * WB}{(W_f+W_r)}$

x=horizontal position of cg from front axle

Wf=front weight

Wr=rear weight

WB=wheelbase

This value is sometimes expressed as a front/rear weight bias, in percentage. For example, our 250-kilogram motorcycle with an equal weight of 125 kilograms on the front and rear wheels has a 50/50 weight bias. Knowing the center of gravity’s horizontal position or the front and rear weight measurements, weight bias can be calculated as:

$\text{rear weight bias} = \frac{x}{WB} \text{*100 or} = \frac{W_r}{(W_f+W_r)} \text{*100}$

$\text{front weight bias} = \frac{(WB-x)}{WB} \text{*100 or} = \frac{W_f}{(W_f+W_r)} \text{*100}$

Typically, the horizontal position of the center of gravity is midway between the wheels and Wf=Wr, giving a 50/50 weight bias. This is a very usable approximation for many calculations.

Vertical Center of Gravity Position

Finding the vertical position of the motorcycle’s center of gravity is more difficult, but in his book “The Racing Motorcycle: A Technical Guide for Constructors,” John Bradley outlines a method using two scales and again measuring weight on the front and rear wheels, but this time with one end of the motorcycle in an elevated position.

$h=cot(arcsin(\frac{y}{WB}))*(\frac{W_r*WB}{(W_r+W_f)}-x)+\frac{(R_f+R_r)}{2}$

h=height of CG

y=height front wheel is raised above rear

Rf=radius of front tire

Rr=radius of rear tire

By raising one end of the motorcycle, the vertical position of the CG can be determined from the weight on each wheel.

A typical value for the vertical position of the CG and a usable approximation for many calculations is half of the wheelbase. For example, if your motorcycle’s wheelbase is 1400mm, a good approximation of the vertical CG position is 700mm.

Note that we are interested in knowing the position of the center of gravity with the rider onboard, and these measurements must be taken with the rider in position, making it somewhat difficult. Also note that when the motorcycle is on-track, the rider moves around a considerable amount and the bulk of the motorcycle’s mass pitches about on the suspension. This means the CG’s vertical and horizontal positions are constantly changing.

To take at least the suspension movement into account and its effect on CG position, it is sometimes useful to consider the CG as a point relative to the motorcycle’s swingarm pivot rather than the front or rear axle. This helps determine a more accurate position of the CG based on suspension travel, in turn giving more accurate data in some calculations.

Now that we know the position of the motorcycle’s center of gravity, we can use this information to calculate the total weight or total load of the motorcycle (which takes into account the bike’s mass as well as load from lateral acceleration) and then split that into values for front and rear weight that take longitudinal acceleration into account.