# B The Physics of Tires

1. Mar 18, 2016

### katteelynnn

This is not a homework problem.

I have been thinking of this for a while now. I am only in physics 2 my professor isn't smart as he thinks he is. I want to figure out how to calculate the gas milage difference from tire size. I would like help to point me in the right direction in order to solve this.
What i think i need to use for this.. I know i am wrong but hopefully someone can point me in the right direction.
I was thinking we would need to know the rotational kinetic energy, while having friction of being on a road. But how do I find the power base off of kinetic energy?

I think this would be absolutely amazing if I could figure this out.

2. Mar 18, 2016

### jerromyjon

I think the final drive ratio between engine and wheels is the biggest factor. The rotational inertia of the wheels is very minor when the weight of the entire vehicle is much greater.

3. Mar 18, 2016

### Staff: Mentor

Welcome to the PF.

@jerromyjon is probably correct about the moment of inertia differences being a very small component of the gas mileage for cars (at least for small changes in tire diameter and type). There are a couple other things that you should be thinking about:

• The change in aerodynamic drag with larger diameter tires -- this can have a pretty large effect on gas mileage
• The change in rolling resistance, mainly through any difference in recommended tire pressures
I know from personal experience that a major disadvantage of going to bigger tires on SUVs and trucks is that your air resistance goes up a fair amount (because of the bigger opening under your vehicle for air to pass through. (Well, that and the part where your speedometer is now off -- you have to keep correcting mentally for the difference).

4. Mar 18, 2016

### sophiecentaur

This are good points but the mpg over rough ground could be helped somewhat with big wheels. I have recently started using a Landrover Freelander and a Toyota Hilux, both with large wheels. They glide up and over curbs and bumps that a 14" wheel would baulk at.
I would not be surprised if the Prof was just referring to the effect on the odometer reading.

5. Mar 18, 2016

### Staff: Mentor

You drive funny...
That's an excellent point!

6. Mar 18, 2016

### sophiecentaur

I now drive over curbs because I can! It opens up many more possibilities for parking.

7. Mar 18, 2016

### drvrm

to increase fuel efficiency;
a few things are important
1.wider tire is less aerodynamic but is more than made up with the lesser rolling resistance.
2.The larger of the two wheels of the same weights is less Fuel Efficient because the weight is farther from the center thus using more energy to get going.When it comes to wheels, lighter weight is better.
3.The ultimate win is a lighter, smaller rim and a larger diameter tire.
4. Air drag minimization is more important for fuel efficiency.

8. Mar 18, 2016

### sophiecentaur

I always thought that was to get a low unsprung weight, to make the suspension better and keep the wheels in contact with the road after hitting a bump on a corner.
I really can't imagine the rotational KE is much of a factor in fuel efficiency.

9. Mar 18, 2016

### jerromyjon

Don't tell anyone but I have driven on the road with cement filled tires. (Demolition Derby trick) The acceleration that is possible is reduced somewhat by the added weight but the reduced rolling resistance improved fuel economy noticeably. You just can't go very fast because the handling suffers considerably.

10. Mar 18, 2016

### Staff: Mentor

LMAO! Fuel economy is a pretty major consideration at a Destruction Derby...

11. Mar 18, 2016

### jerromyjon

It was an observation as I was modifying the vehicle for entrance, the first things I had to do was relocate the battery and replace the fuel tank with a smaller interior tank. (for safety)

12. Mar 18, 2016

### rcgldr

The key factor is rolling resistance. Since the tires are partially enclosed by a car's body, the difference in aerodynamic drag doesn't have as much effect as rolling resistance.

Rolling resistance is related to how much the tire deforms at the contact patch, some of it is radial deformation, some along the tread surface of the tire in the direction of rotation, and some compression / expansion perpendicular to the direction of rotation, sometimes called squirm. A wider or larger diameter tier has less rolling resistance due to radial deformation if the sidewall stiffness is about the same as a narrower tire, but the tread surface deformation factors would remain about the same.

13. Mar 18, 2016

### jbriggs444

Larger radius wheels rotate more slowly for a given vehicle speed. This exactly offsets the increase in moment of inertia due to the increased radius. There is neither advantage nor disadvantage from this factor.

14. Mar 18, 2016

### jerromyjon

I meant to say that, but I didn't know how until you said it.