Prius 75-91mpg

[Nathan jones]

I dismantled the mirrors and cannot find an easy way to remove the motors. Currently they are 800g each but will be given attention next year

[Ecky]

You might be surprised to learn, but I did some math yesterday and it turns out the inertia of a wheel actually goes down as the aspect ratio (and thus overall diameter) goes up, because it spins more slowly. So, in effect, a 165/80r14 tire is "lighter", inertially speaking, than a 165/40r14.

Going more narrow is always a win, but don't compromise safety.

There's also gearing to consider. I know a Prius has "infinite" ratios, but going up or down in size may have unintended consequences.

[Vman455]

Originally Posted by Ecky

You might be surprised to learn, but I did some math yesterday and it turns out the inertia of a wheel actually goes down as the aspect ratio (and thus overall diameter) goes up, because it spins more slowly. So, in effect, a 165/80r14 tire is "lighter", inertially speaking, than a 165/40r14.

You mean angular momentum? Rotational inertia is independent of speed.

All that as a prelude to saying, I don't get that result using representative tire dimensions and weights from Tire Rack--for the 185/55R14 and 185/75R14 I compared just now (they don't list any 165/xxR14), the larger tire still has higher angular momentum....

[Ecky]

Originally Posted by Vman455

You mean angular momentum? Rotational inertia is independent of speed.

All that as a prelude to saying, I don't get that result using representative tire dimensions and weights from Tire Rack--for the 185/55R14 and 185/75R14 I compared just now (they don't list any 165/xxR14), the larger tire still has higher angular momentum....

Maybe I'm confused by the terms, but the math works out that the energy stored in a spinning tire is very slightly less in the tire with a larger diameter, all else being equal, if you want to be traveling at the same speed. It's counter-intuitive but the reduction in RPM is worth more (slightly) than bringing the mass closer to center and spinning it faster.

Angular momentum is likely the term I wanted.

[Nathan jones]

Thanks, they will be reduced from 65 to 45mm. Only for weight, car height and acceleration. Width will be slightly increased from stock as they are now

[Ecky]

Originally Posted by Nathan jones

Thanks, they will be reduced from 65 to 45mm. Only for weight, car height and acceleration. Width will be slightly increased from stock as they are now

Unfortunately I think this may undermine what you're trying to achieve.

By going smaller, you *effectively* make the wheels heavier, because they have to spin at higher RPM for a given road speed. This is normally close to a wash, but by going wider at the same time you're definitely going to cause the wheels to hold more angular momentum, even if they weigh less on a scale.

Most cars would "feel" like they accelerate more quickly with smaller tires, but it would be because they're now geared lower. I don't know if this applies in a Prius.

You would achieve your goal of lowering the car, but you'd also likely hurt both aerodynamics and rolling resistance (all else being equal) with a smaller, wider tire.

[Vman455]

Originally Posted by Ecky

Maybe I'm confused by the terms, but the math works out that the energy stored in a spinning tire is very slightly less in the tire with a larger diameter, all else being equal, if you want to be traveling at the same speed. It's counter-intuitive but the reduction in RPM is worth more (slightly) than bringing the mass closer to center and spinning it faster.

Angular momentum is likely the term I wanted.

I've mulled this over for a few days now (and gone back over my physics notes!). When you say, "all else being equal," are you including mass? The tires I used for my calculations weigh, according to Tire Rack, 13 lbs for the 185/55-R14 and 16 lbs for the 185/75-R14, which is quite a bit different. Considering the energy of a rolling tire/wheel, remember that rotation is not the only component we have to consider; rolling = rotation + translation, and Krolling = (1/2)mv^2 + (1/2)Iw^2. For that taller tire in my example, I is larger because of the larger radii (I is a function of the squares of the inside and outside radius for the "disk" of the tire tread), but its lower angular velocity, w, is not enough to counteract that increase and its overall angular momentum is larger. Plus, the higher mass means the kinetic energy of translation is higher as well, all of which results in a higher kinetic energy of the larger tire.

I might play around with this some more using other tires and their masses; this might be enough to split into a new thread.

[Nathan jones]

Originally Posted by Ecky

Unfortunately I think this may undermine what you're trying to achieve.

By going smaller, you *effectively* make the wheels heavier, because they have to spin at higher RPM for a given road speed. This is normally close to a wash, but by going wider at the same time you're definitely going to cause the wheels to hold more angular momentum, even if they weigh less on a scale.

Most cars would "feel" like they accelerate more quickly with smaller tires, but it would be because they're now geared lower. I don't know if this applies in a Prius.

You would achieve your goal of lowering the car, but you'd also likely hurt both aerodynamics and rolling resistance (all else being equal) with a smaller, wider tire.

Hi yes, the wheels may weigh a very small amount more/equal, yes the gearing is changed and aerodynamics is changed. Lowering the car increases the pressure above and reduces the pressure underneath, helping to hold the car down. Pressure drag is also increased but a small sacrifice for race situations. I dont think over 107mpg is possible with this chassis/engine/surface area

n.b unless the night temps are above 30c and theres no traffic

[redpoint5]

So this thread was about reducing weight, but we still don't have an answer of how much weight was lost.

[Nathan jones]

The weighstation is closed down but will find one at some point. Quarter lights and boot to be replaced next month