DISCLAIMER: I do not claim to be an authority, expert, or anything more than just a gearhead with a little formal training in mechanical engineering. I am fascinated with cars, science, and all manner of machinery, I get excited about it and occasionally will become long-winded and share some of what I have observed during my life of obsessing over cars
I have been wanting to write a tech article about how to calculate the center of gravity of your car and how changing the placement of items (such as the battery or a ballast) changes things. It is a very lengthy topic of discussion and requires quite a lot of visuals that I would need to sketch up so for now I want to blabber about a somewhat related topic: Wheels, Flywheels, and Polar Moment of Inertia.
Physics term review time: Inertiais the resistance of any physical object to a change in its state of motion or rest, or the tendency of an object to resist any change in its motion.
or in other words, a linebacker is harder to stop than a quarterback or kicker, but the linebacker is also a bit slower to get going off the line.
So naturally you may think about "inertia" as being purely a function of mass. This is normally true; however, when an object rotates about an axis (such as a wheel, or a flywheel) the distribution of mass throughout that shape can make all the difference.
Imagine now that you are swinging a sledgehammer
Now imagine swinging that same sledgehammer while holding it the wrong way, with the head of the hammer in your hand, striking objects with the handle of it instead. It certainly does less damage, but it is far easier to swing.
This is the concept of polar (rotating about an axis) moment of inertia. If your wrist joint is the axis of rotation, the hammer has a much greater moment of inertia when you swing it correctly, than when you hold it and swing it incorrectly.
What does this have to do with wheels? (I was about to bust out some calculations but will refrain...)
Behold the beauty of the mesh wheel design. Superior balance of elegance, light weight, and strength. Another cool design feature of a mesh wheel is a low moment of inertia since the spokes tend to be a thin cross-section and the majority of the wheel mass is centered close to the hub/axis of rotation (think of swinging a hammer the wrong way).
In contrast, the VW Huff wheel design, while kinda cool looking, focuses more mass towards the outer radius of the wheel (think of swinging a hammer the correct way)
So when considering 2 wheels with identical weight, lets say 16 lbs. The wheel with the lower polar moment of inertia (more of that 16lbs at the center of the wheel) will provide better acceleration and braking performance than the 16 lb wheel with a greater polar moment of inertia ( more of the 16lbs is located at the outer rim).
"BUT WAIT, THERES MORE!"
Designing a rotating component with polar moment of inertia in mind can be a very powerful thing
I was once faced with the challenge of reducing the overall weight of a small flywheel-driven device in order to get it below a maximum weight specification that it had exceeded. The problem was, 90% of the object's weight in the flywheel and since the device was driven by the flywheel, giving it a lighter flywheel would hinder its performance. I did the best that I could removing weight from everything BUT the flywheel but it was no-where near enough. If I wanted the device to meet the weight restriction, the flywheel HAD to be lightened :\
I used the polar moment of inertia to my advantage though:
The flywheel was originally composed of 4 steel plates of the same diameter (lets say 8"). In order to reduce the total weight of the system without compromising the performance of the flywheel (its ability to store kinetic energy as inertia) I increased the diameter of the flywheel to the maximum permissable dimensions (lets say 13" diameter) made its center section out of a lightweight composite, and bonded a high density (lets just say lead) ring around the outer diameter of it. So in total, the flywheel weighed about half of what it did before, but by increasing its diameter and concentrating the mass to the outer radius of it, I was able to make something which was lighter weight and had identical performance....
Removing weight from the outer radius of a flywheel is typical for cars since the priority is decreasing the inertia/letting the car rev more quickly (the opposite of my design problem where I wanted as much energy stored in the flywheel as possible)
Also it is a main reason why a 3000 lb BMW handles (changes direction) so much better than a 3000 lb Audi. The engine being the massive end of the hammer and it being further from the axis of rotation in the audi than it is in a BMW.
That is all, hopefully something was learned and perhaps some inspiration for future performance mods has been obtained.
So remember kids, its not necessarily the weight of something that determines its performance, its where the weight is distributed!
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DISCLAIMER: I do not claim to be an authority, expert, or anything more than just a gearhead with a little formal training in mechanical engineering. I am fascinated with cars, science, and all manner of machinery, I get excited about it and occasionally will become long-winded and share some of what I have observed during my life of obsessing over cars
I have been wanting to write a tech article about how to calculate the center of gravity of your car and how changing the placement of items (such as the battery or a ballast) changes things. It is a very lengthy topic of discussion and requires quite a lot of visuals that I would need to sketch up so for now I want to blabber about a somewhat related topic: Wheels, Flywheels, and Polar Moment of Inertia.
Physics term review time: Inertia is the resistance of any physical object to a change in its state of motion or rest, or the tendency of an object to resist any change in its motion.
or in other words, a linebacker is harder to stop than a quarterback or kicker, but the linebacker is also a bit slower to get going off the line.
So naturally you may think about "inertia" as being purely a function of mass. This is normally true; however, when an object rotates about an axis (such as a wheel, or a flywheel) the distribution of mass throughout that shape can make all the difference.
Imagine now that you are swinging a sledgehammer
Now imagine swinging that same sledgehammer while holding it the wrong way, with the head of the hammer in your hand, striking objects with the handle of it instead. It certainly does less damage, but it is far easier to swing.
This is the concept of polar (rotating about an axis) moment of inertia. If your wrist joint is the axis of rotation, the hammer has a much greater moment of inertia when you swing it correctly, than when you hold it and swing it incorrectly.
What does this have to do with wheels? (I was about to bust out some calculations but will refrain...)
Behold the beauty of the mesh wheel design. Superior balance of elegance, light weight, and strength. Another cool design feature of a mesh wheel is a low moment of inertia since the spokes tend to be a thin cross-section and the majority of the wheel mass is centered close to the hub/axis of rotation (think of swinging a hammer the wrong way).
In contrast, the VW Huff wheel design, while kinda cool looking, focuses more mass towards the outer radius of the wheel (think of swinging a hammer the correct way)
So when considering 2 wheels with identical weight, lets say 16 lbs. The wheel with the lower polar moment of inertia (more of that 16lbs at the center of the wheel) will provide better acceleration and braking performance than the 16 lb wheel with a greater polar moment of inertia ( more of the 16lbs is located at the outer rim).
"BUT WAIT, THERES MORE!"
Designing a rotating component with polar moment of inertia in mind can be a very powerful thing
I was once faced with the challenge of reducing the overall weight of a small flywheel-driven device in order to get it below a maximum weight specification that it had exceeded. The problem was, 90% of the object's weight in the flywheel and since the device was driven by the flywheel, giving it a lighter flywheel would hinder its performance. I did the best that I could removing weight from everything BUT the flywheel but it was no-where near enough. If I wanted the device to meet the weight restriction, the flywheel HAD to be lightened :\
I used the polar moment of inertia to my advantage though:
The flywheel was originally composed of 4 steel plates of the same diameter (lets say 8"). In order to reduce the total weight of the system without compromising the performance of the flywheel (its ability to store kinetic energy as inertia) I increased the diameter of the flywheel to the maximum permissable dimensions (lets say 13" diameter) made its center section out of a lightweight composite, and bonded a high density (lets just say lead) ring around the outer diameter of it. So in total, the flywheel weighed about half of what it did before, but by increasing its diameter and concentrating the mass to the outer radius of it, I was able to make something which was lighter weight and had identical performance....
Removing weight from the outer radius of a flywheel is typical for cars since the priority is decreasing the inertia/letting the car rev more quickly (the opposite of my design problem where I wanted as much energy stored in the flywheel as possible)
Also it is a main reason why a 3000 lb BMW handles (changes direction) so much better than a 3000 lb Audi. The engine being the massive end of the hammer and it being further from the axis of rotation in the audi than it is in a BMW.
That is all, hopefully something was learned and perhaps some inspiration for future performance mods has been obtained.
So remember kids, its not necessarily the weight of something that determines its performance, its where the weight is distributed!
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