Larger lighter tire vs smaller heavier tire - Page 2 - Toyota 4Runner Forum

02SE · 02-29-2024, 09:55 PM

Quote:

Originally Posted by Bmnorm2

Rotational Inertia of a tire = m*r^2

Falken: I = (28.123kg * 0.4165m^2) = 4.88 kgm^2

Firestone : I = (24.358kg * 0.4305m^2) = 4.51 kgm^2

So, the Firestone will actually take less energy to turn. The Firestone results in ~ 7.5% inertia reduction. Due to the squared radius, often the larger tire will have a higher moment of inertia however in this case the mass is reduced enough for the taller tire to be a lower moment of inertia.

If both tires have the same rolling resistance, you may be slightly better fuel economy with the firestone. I have no idea what their rolling resistances are.

In general, I wouldn’t expect to see any notable difference.

The tread pattern definitely effects rolling resistance. As does casing construction, and rubber compound.

IslandJim · 03-01-2024, 11:06 AM

Can tell you that my MT tires are the first set of tires I've had in which I've never had any punctures. The tread blocks are thick and never had anything ever puncture them..has been great. Every tire set have had before always got a puncture(usually debris from the shoulder is most likely culprit).

That being said, will most likely get some AT's for the next set....

El Dusty · 03-01-2024, 11:47 AM

Quote:

Originally Posted by Bmnorm2

Rotational Inertia of a tire = m*r^2

Falken: I = (28.123kg * 0.4165m^2) = 4.88 kgm^2

Firestone : I = (24.358kg * 0.4305m^2) = 4.51 kgm^2

So, the Firestone will actually take less energy to turn. The Firestone results in ~ 7.5% inertia reduction. Due to the squared radius, often the larger tire will have a higher moment of inertia however in this case the mass is reduced enough for the taller tire to be a lower moment of inertia.

If both tires have the same rolling resistance, you may be slightly better fuel economy with the firestone. I have no idea what their rolling resistances are.

In general, I wouldn’t expect to see any notable difference.

You have a close approximation but I = MR^2 only works for thin hoops which a tire is not really. The mass is distributed throughout the radius (serious calculus required). This approximation becomes worse when comparing MTs with heavier sidewalls to ATs and also different load / construction tires like we are doing here.

I think percentage difference between the two measurement is probably very close (7.5%) but the inertia you are calculating are higher than reality.

Bmnorm2 · 03-01-2024, 03:59 PM

Quote:

Originally Posted by El Dusty

You have a close approximation but I = MR^2 only works for thin hoops which a tire is not really. The mass is distributed throughout the radius (serious calculus required). This approximation becomes worse when comparing MTs with heavier sidewalls to ATs and also different load / construction tires like we are doing here.

I think percentage difference between the two measurement is probably very close (7.5%) but the inertia you are calculating are higher than reality.

I’m aware. I intentionally made the broad assumptions required to seriously reduce the complexity of the problem. I agree very different tread patterns/depths can result in massive error from the assumptions made.

Also agree on your last paragraph. Was looking for a relative difference with same assumptions, not necessarily accuracy of the calculations.

Bmnorm2 · 03-01-2024, 04:00 PM

Quote:

Originally Posted by 02se

the tread pattern definitely effects rolling resistance. As does casing construction, and rubber compound.

100%

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