What contributes most to Rotational inertia?

[Kyknet]

Hi. How would my rotational inertia be affected if I changed from 225/55/19 to 255/55/18? The new wheels would be 7 pounds lighter per wheel but the heavier tire would cancel some of that out. At best the 18s would be 3 - 4 pounds lighter per wheel. Would I see any performance benefits or will the slightly larger diameter and width affect it negatively?

 

[leeharvey418]

It really depends on the specific tire. You can make generalizations about the mass of the wheel rim moving toward the center of rotation, but if the tread on the 255/55R18 is thicker, it will likely offset that gain.

At the end of the day, though, you're not going to see a drastic change as long as the estimated three to four pound total weight savings is true.

 

[B-ham Gary]

Inertia is represented by the distribution of mass by the square of the distance from the rotational axis. So it's not just the total mass of a wheel and tire, but how far the mass is from the rotation (and in this case, CG) axis that matters. Your example, assuming identical but smaller tire and wheel construction, would certainly have a lower inertia, but whether it's enough to matter is unknown (and your odometer/speedometer would be off 3.5%).

 

[MikeM.]

Hi. How would my rotational inertia be affected if I changed from 225/55/19 to 255/55/18? The new wheels would be 7 pounds lighter per wheel but the heavier tire would cancel some of that out. At best the 18s would be 3 - 4 pounds lighter per wheel. Would I see any performance benefits or will the slightly larger diameter and width affect it negatively?

At high speeds the extra aerodynamic drag of the wider 18" tires would make them slower. At low speeds the setup with the lowest rotational inertia would be quickest but my estimation is that rotational inertia would be roughly the same due to the extra 1.2 inches of width on the wider tire (and the extra rim width also). IOW, even if the 18" setup weighs around 3 lbs. less, they would likely have about equal rotational inertia.

Perhaps the biggest effect of this change would be that the 18" combo will raise the effective gearing making it slower off the line.

 

[B-ham Gary]

At high speeds the extra aerodynamic drag of the wider 18" tires would make them slower. At low speeds the setup with the lowest rotational inertia would be quickest but my estimation is that rotational inertia would be roughly the same due to the extra 1.2 inches of width on the wider tire (and the extra rim width also). IOW, even if the 18" setup weighs around 3 lbs. less, they would likely have about equal rotational inertia.

Perhaps the biggest effect of this change would be that the 18" combo will raise the effective gearing making it slower off the line.

Given the way Mazda tucks their tires inboard of the body, aero effects would likely be small. Rolling resistance differences would like be a more significant effect, and that could vary widely depending on the tires being compared.

 

[MikeM.]

Given the way Mazda tucks their tires inboard of the body, aero effects would likely be small. Rolling resistance differences would like be a more significant effect, and that could vary widely depending on the tires being compared.

Rolling resistance remains constant relative to speed so at some point the aerodynamic drag of the wider wheels/tires will exceed the rolling resistance. I would estimate that speed to be around 70-80 mph. The aerodynamic drag of wheels/tires is more than their frontal area might suggest because of the way they interfere with the laminar flow between the road surface and the under-fairings. And on a high ground clearance vehicle like the CX-5, the effect will be more pronounced than the typical sedan or hatchback because more tire is exposed to the air flow. The effect is significant enough that Mazda engineers bothered to design two small fairings for the front tires that direct the air around the tires in a more laminar manner. These increase the part count by two but increase performance and MPG on the highway slightly.

 

[B-ham Gary]

Tire rolling resistance varies widely based on carcass construction and tread compound, amongst other factors. One can't assume two tires with different dimensions will have comparable RR. (Ten years as an engineer at Michelin R&D responsible for drum-based RR testing, but I didn't sleep in a Holiday Inn last night). Aero resistance differences of a 30-mm wider tire at normal operating velocities, is still "small". Either way, we're just blathering on about engineering nit-picks. For city driving, tire/wheel rotational inertia trumps all this other silliness.

But it is fun to ponder...

 

[MikeM.]

Tire rolling resistance varies widely based on carcass construction and tread compound, amongst other factors.

True, and rotational inertia varies significantly with tire construction specifics also. Because the question did not contain specific tire models I was answering the question while assuming undefined variables remained the same.

One can't assume two tires with different dimensions will have comparable RR. (Ten years as an engineer at Michelin R&D responsible for drum-based RR testing, but I didn't sleep in a Holiday Inn last night).

I agree, with similar rubber compounds, similar carcass stiffness and similar tread designs, the wider tire will have more rolling resistance. And as a former engineer focused on tire rolling resistance for Michelin, I can see how easy it would be to focus on rolling resistance while minimizing the relative importance of aerodynamics at high speed.

Either way, we're just blathering on about engineering nit-picks. For city driving, tire/wheel rotational inertia trumps all this other silliness.

But it is fun to ponder...

Yes, aerodynamics do not come into play (significantly) at city speeds. That's why I limited my comment to "high speed" driving. And the only reason I brought up aerodynamics which is an (admittedly) small effect is because the two tire/wheel sizes in the initial question appear to have very similar rotational inertia.

As an engineer experienced in measuring rolling resistance, I have to ask, how large of a test drum was considered large enough to reduce the inaccuracies introduced by the drum surface being round, not flat like a road, to an acceptable level? Because I can see how two tires with the same RR on a flat road could have different measured RR if the drum diameter was too small.

Also, did any of your RR test drums have actual sharp, coarse gravel glued to them to simulate the kind of chip seal roads so popular on many secondary roads in the American West? Or is the RR testing primarily limited to carcass construction and not so concerned with rubber compounds and tread design differences?

One last thought to ponder. If I put 2" wide knobbies on my mountain bike, use a stand to elevate the wheels off the ground, put the bike in high gear and use my hands to pedal the crank at a normal 60 rpm riding cadence, the rear tire creates quite a breeze and I must maintain significantly heavy pressure on the crank to maintain 60 rpm's. Conversely, if I do the same thing with 2" wide city slicks mounted, very little breeze is created and very light crank pressure will maintain 60 rpm's. This difference simulates 50% of the aerodynamic efficiency difference between slicks and knobbies (and none of the RR differences) because in actual use there is also a front wheel spinning (and there is friction with the ground).

As a RR test engineer, how were aerodynamic efficiency differences handled. I can see they might be ignored as being insignificant or any small differences could be considered to be "rolled into" RR measurements or a fan and a circular fairing could be closed around the test tire to minimize aerodynamic effects. Or, were you testing different carcass constructions only and the tires all had either the same tread or were slicks?

I would happily sleep in Holiday Inns for a week if I could learn more about tire testing.

 

[piotrek91]

Hi. How would my rotational inertia be affected if I changed from 225/55/19 to 255/55/18? The new wheels would be 7 pounds lighter per wheel but the heavier tire would cancel some of that out. At best the 18s would be 3 - 4 pounds lighter per wheel. Would I see any performance benefits or will the slightly larger diameter and width affect it negatively?

This site has some discussion and fun calculators to play with.
http://hpwizard.com/rotational-inertia.html

 

[B-ham Gary]

@MikeM -- yikes that's a lot of text. I'll just answer a bit as this has deraied the OP's thread enough.

Passenger car tires were tested on a 1.7 meter dia. drum, smooth steel. Sure it's more curved than a flat surface, but much better than the old tiny twin drum test machines. There were some correction factors that attempted to account for the curve. The available flat track machines used a textured Safety Walk surface on the belt, but the force transducers weren't precise enough for RR work.

No way would there be texture or cleats added; the hub-mounted load cells used to measure RR were hand-made and incredibly sensitive. The goal was to measure energy loss for a given tire design as accurately as possible. Additional factors due to road surfaces would be done via track testing (coast down and other fun things).

My apologies to the OP...