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user079622

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I understand what is moment of inertia is, flywheel with more mass at the edge has more inertia than flywheel that has mass closer to center.

quote from link:https://decarreteres.wordpress.com/2019/04/24/chassis-engineering-polar-moment-of-inertia/

Why they assume that car rotate around c.g. just like seasaw rotate around fixed pivot point?

I think their pivot point is not ok..

So how to find moment of inertia for car in turn?

quote from link:https://decarreteres.wordpress.com/2019/04/24/chassis-engineering-polar-moment-of-inertia/

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We will only consider the engine and gearbox as poles of inertia, since they are the elements that have the greatest influence on a vehicle. In a real calculation all the elements of the car should be taken into account.

The lengths are calculated from the vehicle’s center of gravity, to the center of gravity of each of the elements.

Let’s assume that the Porsche 904 engine in the example has a total mass of 100kg and its gearbox weighs 50kg. Also assuming that the center of gravity of the engine is 0.4m away from the center of gravity of the car, and that the gearbox is 0.8m.

The calculation result for the Porsche 904 will be:

ΣM = m1*d1² + … + mn*dn² = 100kg *(0’4m)²+50kg*(0’8m)²= 48kgm²Let’s see now what the same calculation shows in the case of a car with it’s weights far away from the center of gravity, such as an Audi, with it’s engine ahead of the front axle.

Assuming the approximate values are 100kg for the engine mass (the real one would be closer or more than 200kg), 50kg for the gearbox; The engine’s cog is at a distance of 1.5m from the center of gravity and the cog of the gearbox at 0.8m.

ΣM = m1*d1² + … + mn*dn² = 200kg *(1’5m)²+50kg*(0’8m)²= 482kgm²

After this calculation we can see that the polar moment of inertia is 10 times higher in this case, due solely to the position of the engine since the rest of the values are identical in both examples.

This means that we will need to apply 10 times more power to move the engine and gearbox in the Audi than in the Porsche. No other than the tires have the ability to do that, therefore they are suffering 10 times more load.

For this reason the vast majority of racing cars have mid engines and try to ensure that all the masses are as close as possible to the center of gravity, having as a result a very agile vehicle, even though it also means abrupt reactions.

The location of the poles of inertia influences not only the car’s nimbleness, but also the type of reactions it will have. A car will understeer if its masses are mostly in the front and will oversteer otherwise."We will only consider the engine and gearbox as poles of inertia, since they are the elements that have the greatest influence on a vehicle. In a real calculation all the elements of the car should be taken into account.

The lengths are calculated from the vehicle’s center of gravity, to the center of gravity of each of the elements.

Let’s assume that the Porsche 904 engine in the example has a total mass of 100kg and its gearbox weighs 50kg. Also assuming that the center of gravity of the engine is 0.4m away from the center of gravity of the car, and that the gearbox is 0.8m.

The calculation result for the Porsche 904 will be:

ΣM = m1*d1² + … + mn*dn² = 100kg *(0’4m)²+50kg*(0’8m)²= 48kgm²Let’s see now what the same calculation shows in the case of a car with it’s weights far away from the center of gravity, such as an Audi, with it’s engine ahead of the front axle.

Assuming the approximate values are 100kg for the engine mass (the real one would be closer or more than 200kg), 50kg for the gearbox; The engine’s cog is at a distance of 1.5m from the center of gravity and the cog of the gearbox at 0.8m.

ΣM = m1*d1² + … + mn*dn² = 200kg *(1’5m)²+50kg*(0’8m)²= 482kgm²

After this calculation we can see that the polar moment of inertia is 10 times higher in this case, due solely to the position of the engine since the rest of the values are identical in both examples.

This means that we will need to apply 10 times more power to move the engine and gearbox in the Audi than in the Porsche. No other than the tires have the ability to do that, therefore they are suffering 10 times more load.

For this reason the vast majority of racing cars have mid engines and try to ensure that all the masses are as close as possible to the center of gravity, having as a result a very agile vehicle, even though it also means abrupt reactions.

The location of the poles of inertia influences not only the car’s nimbleness, but also the type of reactions it will have. A car will understeer if its masses are mostly in the front and will oversteer otherwise."

Why they assume that car rotate around c.g. just like seasaw rotate around fixed pivot point?

I think their pivot point is not ok..

So how to find moment of inertia for car in turn?

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