Centre of gravity, Roll center, Roll axis and some important factors(VD-Part 1) -By Shrey Mishra

Before making a racing car,the team first needs to decide the aim i.e what parameters needs to be fulfilled.

The parameters includes:

a. Maximum speed.

b. Vehicle’s Endurance on the track.

c. Safety.

Maximum speed: Speed can be increased by equip ting the vehicle with upgraded EV power train system consisting battery and motor that can provide high power outputs.If that was the only case than vehicles today would have been much bulkier than what they are now.

     There is one more aspect which plays very important role in decreasing the speed of the vehicle i.e mass.

We all know,

a = F / m

Where a is the acceleration of vehicle,F is force that acts between tire and road that moves vehicle forward and m is the mass of vehicle.

Since the mass and acceleration have inverse relationship therefore on increasing mass the acceleration decreases.

Endurance: Endurance is basically is measure of how long, a vehicle can can run on track maintaining its performance in lap time scenario.

In the above event the vehicle’s speed,mileage and durability is put to test

There are two major factors that is needed to be looked after i.e weight of car and air drag.

Air drag is basically resistance that air provides to the moving vehicle.

Every vehicle has a limited power source,the vehicle needs to overcome its weight and air drag in that limit only, for a long period of time.

Drag force:

FD = Cd * A *(ρ *v*v )/2

Where CD is the drag force,A is the reference area,ρ is density of fluid,V is flow velocity relative to the object.

Here density of air is constant,to reduce drag we can’t really compensate velocity as it is an important aspect that is needed in a race car.

Some Measures to reduce drag and weight of the vehicle:

a. CD depends on the surface roughness of the exposed part to air therefore by polishing the surface of vehicle,the co-efficient of drag can be minimised.

b. A is the area of exposed part,usually front nose section of race car.By decreasing the surface area in such a way so that minimum surface is exposed to incoming air,the drag force can be minimised. Therefore these days maximum vehicle have spherical air foil shape for which surface area is minimum.

c. Weight of vehicle can reduced by using lighter,composite material for building vehicle’s frame,which are strong, durable and have higher strength to weight ratio.

d. Components with less weight and high efficiency can used in place of bulkier ones.

Centre of gravity and its importance in vehicle dynamics :

What is mint by the centre of gravity? Centre of gravity is that point at which a system or  vehicles body behaves as if all its mass were centred at that point. Acceleration, braking and cornering forces act through this point.

In the above figure the CG for the car is shown and its behaviour in rolling condition.

What is the importance of the CG in vehicle dynamics?

The vehicle is in motion is subjected with different number of forces like centrifugal force on cornering,load transfer in braking and acceleration.All these forces are calculated with respect to position of centre of gravity.

The dynamics of vehicle can be studied by observing the behaviour of CG in dynamic condition.One can really decide its position,as CG is the just average value of concentrated mass.

All the above mentioned forces tend to disturb the stability of vehicle in motion therefore one can optimise CG position so that magnitude of above forces can be minimised.

Roll center and roll axis:

Roll center for a vehicle is a point at about which the cornering forces in the suspension system acts or is point about which the body tend to roll on cornering situation.

The figure shows roll center of rear side of vehicle

In the above figure there are two wheels,four suspension links,two for left and two for the right.The line extended from upper and lower wishbone links of left meets at point A.In the same way we get point B.Intersection between line extended from B and A connecting tire contact patch is the roll center.

This is for a rear side of vehicle and similar procedure is followed for a front side.In this way we get two roll

 Centre, one for rear and one for front.

The line joining both the roll center is called roll axis .

Above figure shows roll axis- line joining both front and rear roll center.

Conclusion: This blog discusses some important factors and its role in vehicle’s design and dynamics.


Mechatronics Lab, SME, KIIT DU, Patia, Bhubaneswar, Odisha 751024, India

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