Prius vee Coasting

I cannot answer your question however one thing I'd like to point out.

The V5 having 17" wheels does not increase rolling resistance.
In fact t increases inertial mass. Think of a figure skater spinning. She pulls her arms in and she spins faster. Smaller wheels take less energy to make them turn fast than larger wheels. Steel is more dense than rubber, so having steel out at the 17" or 16" diameter means more inertial mass than having 15" wheels.

Think of wheel size yielding forces on the car.
You have 2 main forces Rolling resistance and inertial mass.
I would say rolling resistance is better in the 17" wheels than the 15"
I would also say that inertial mass is worse in the 17" than the 15"

I'd venture to guess that at coasting speeds rolling resistance is a primary force while inertial mass is a secondary force so you see the car coasts pretty well.
At highway speeds inertial mass becomes a primary force while rolling resistance becomes a secondary force that is overwhelmed and hence the proposed slightly worse economy with the 17" wheels.

I guess I possibly did answer your question though I do not have data to back this up. This is just a back ofthe napkin engineering analysis of the situation.

 

The only problem I see with the above analysis is you are just working out the mass and not the resistance due to tire construction, design, and materials. Unfortunately there are not many LRR tires in 17" sizes so comparing rolling resistance between 2 difference vehicles with different tires and wheels is an exercise in futility.

In general 17's will have more rolling resistance due to mass, distance of mass from the hub, tire design and materials etc. Thus in nearly all cases you will find that 17s decrease coasting/gliding distance if all other factors are kept constant. Increase the width of the tire and you will observe further increase of rolling resistance.

M8s may be experiencing increased rolling capacity due to mass and gearing design in the v vs. the GenII.

 

Well, I can certainly understand why LLR tires would make a difference and I did not claim to do a full analysis and I really don't want to.

I didn't know that there are less 17" LLR tires than 16" or 15". Thats good to know.

If you make that variable equal I think my engineering perspective is right.

If you take a tire that is a total diameter of 23" for example with a 15" rim and one that's total is 23" with a 17" rim, then the greater inertial mass towards the outer diameter will inherently cause it to want to slow down more than with the 15" rim inside.

Also, with rubber compound and all that being equal, a tire with a lower profile side wall will have a lower rolling resistance than that of a higher sidewall.

 

Agreed!

 

Except at steady speeds, mass moment of inertia has no effect on fuel economy. If you aren't braking hard enough to use the friction brakes, it has close to zero effect in variable speed driving. And unless you use the brakes, you get back all the energy you used to accelerate the tire/wheel mass moment of inertia when you slow down.

 

xs650, I may be misunderstanding some of your point, but I may just disagree a bit.

I agree that if it takes x amount of energy to rotate a given tire, then that same tire will give back that same amount of energy when you try to slow it down with brakes or friction system.

However, due to system losses it wont be equal.
I'd say if it takes 100 units of energy to accelerate a 17" tire to some speed, then when you recover that energy you'll only recover say 70% of that (may be higher or lower, I do not know the Prius efficiency). So you lost 30% of 100 units in my example, so net loss is 30 units.

If it takes 80 units to accelerate a 15" rim tire to the same speed, and you lose 30% to system losses then you lose 24 units of energy, vs 30. So, it still results in more loss.


Additionally, I disagree that mass moment has no effect at constant speed. Or maybe I'm mislabeling the force I'm referring to as mass moment of inertia.

If you rotate a tire at 50 MPH it takes a given amount of energy. Say again for example talk, 100 units with a 17" rim in there. It keeps taking that 100 units over time for as long as it remains there.

If you put that same overal diameter tire with a 15" rim that weighs less, it might take 80 units of power to keep it turning at 50 mph.

As long as the tire rotates it'll keep using the same amount.

If you stop applying the rotating force to both tire examples above, initially the 17" rim will slow down faster than the 15" rim because at speed the inertial moment is a primary force.
As the tire gets slower and slower the friction of the bearings will become a primary force and so while initial slowing may be more measurable in the 17", the slower end of the slowing down will less less different as the bearing system will be more equal in both.

Remember, when a car is running along at 50 mph, the tires aren't just spinning at 50 mph. Each tiny part of tire at the outer diameter is going from a dead stop (while in contact with the pavement) to 100 mph at top dead center, and back to a stop again. That's a lot of forces at play.

 

Wait, I thought the inertial moment is directly proportional to the mass and well as the square of the radius? Assuming the density of the 17" and 15" are equal, both the mass and radius are larger on the 17", so doesn't that mean it will have a bigger inertial moment and thus slow down slower?

 

You raise a good point. I don't have time to research it right now. Interesting. I equate that back to the figure skater that more mass away from the center of rotation would cause it to slow faster. But I can see your point being valid as well. Not sure which it is.

 

I see your point too.. and I can explain your point: with a larger radius, a wheel doesn't need to spin as fast to go the same distance as a smaller wheel. But that doesn't mean the larger wheel doesn't have more momentum.