Rotational Weight

[Ken Fry]

Originally Posted by Ladogaboy

Isn't it more accurate to say that rotational mass will act more like static mass when the velocity stays constant? What about highway speeds through winding roads or up an incline (i.e., when the velocity is changing)?

The velocity is not changing when you go up an incline, unless the speed changes. Velocity changes only at the transition from level to incline.

[Ladogaboy]

Originally Posted by jamesqf

You're not going to see much decrease in rotational inertia from changing driveshaft weight, though.

The 15 lb reduction in weight from the driveshaft is what resulted in the +3 mph 1/4 mile trap speed, so it does have some effect.

Originally Posted by jamesqf

Well, does it? Surely you've gone around curves while coasting: does it slow you down?

Yes, and depending on the situation, sometimes significantly so.

[Ladogaboy]

Originally Posted by Ken Fry

The velocity is not changing when you go up an incline, unless the speed changes. Velocity changes only at the transition from level to incline.

So velocity might not be the right word to describe what I am talking about. So what force, specifically, is causing my car to require more energy to climb an incline than it does to maintain the same speed on level ground? For example, my car gets ~ 19 mpg cruising up a ~ 4% grade at 65 mph but gets ~ 28 mpg cruising at 65 mph on level ground. What's causing my car to burn that extra 30% energy?

[California98Civic]

Originally Posted by jamesqf

Remember that the inertia is the product of the mass times the distance from the axis of rotation, and a driveshaft is long & skinny. Same applies to wheel+tire combos. It's possible that a lighter weight wheel+tire would actually increase the rotational inertia, if it put more of the weight further from the hub.

Amazing to come inside and see this thread. I just today dropped about 40 lbs static weight by switching from my 14" wheels and 185/65s to the 13" VX wheels with 175/70s. I do think that there should be a benefit to such reduction in rotational mass, especially in city P&G, but also on the freeway running from 55-65 MPH over and over in P&G cycles. Just my 2 cents.

[Ladogaboy]

Originally Posted by California98Civic

Amazing to come inside and see this thread. I just today dropped about 40 lbs static weight by switching from my 14" wheels and 185/65s to the 13" VX wheels with 175/70s. I do think that there should be a benefit to such reduction in rotational mass, especially in city P&G, but also on the freeway running from 55-65 MPH over and over in P&G cycles. Just my 2 cents.

Nice! You'll have to let us know what kind of differences you see after the switch.

[California98Civic]

Originally Posted by Ladogaboy

Nice! You'll have to let us know what kind of differences you see after the switch.

Will do! I'm only 85 miles into this tank (out of my usual 600+ miles). Two factors against an immediate improvement: (1) the newer tires have much more tread and (2) I had to take off my wheel skirts to change the wheels. I also need a good formula for calculating the effect of smaller wheels on the ODO's miles driven for my FE calculations.

[some_other_dave]

Originally Posted by Ladogaboy

Rotational weight, especially in the reference to performance vehicles, usually is considered separately from standard... "sprung"... weight reduction.

Pedantry follows:

Sprung mass is anything that is supported by the suspension springs. The seats, the engine, almost all of the car is sprung mass. Some of the things that make up that sprung mass also rotate, such as the engine's crankshaft, the flywheel, etc.

Unsprung mass is anything that is not supported by the springs of the suspension. Things like the wheels and tires, the brake calipers, and so on. Obviously much of that mass does rotate (wheels, tires, brake rotors), but some of it does not (brake calipers, the housing of a live rear axle).

Some parts are partly sprung, like the springs themselves, suspension arms, and drive shaft(s).

Car makers generally try to reduce unsprung mass as much as they reasonably can, as it tends to have a sizeable effect on the ride quality of a vehicle.

-soD

[California98Civic]

Originally Posted by some_other_dave

Car makers generally try to reduce unsprung mass as much as they reasonably can, as it tends to have a sizeable effect on the ride quality of a vehicle.

Meaning that reducing unsprung mass improves the ride quality, but how and in what ways? Traction and comfort maybe?

[Ladogaboy]

Originally Posted by some_other_dave

Pedantry follows:

Sprung mass is anything that is supported by the suspension springs. The seats, the engine, almost all of the car is sprung mass. Some of the things that make up that sprung mass also rotate, such as the engine's crankshaft, the flywheel, etc.

Unsprung mass is anything that is not supported by the springs of the suspension. Things like the wheels and tires, the brake calipers, and so on. Obviously much of that mass does rotate (wheels, tires, brake rotors), but some of it does not (brake calipers, the housing of a live rear axle).

Some parts are partly sprung, like the springs themselves, suspension arms, and drive shaft(s).

Car makers generally try to reduce unsprung mass as much as they reasonably can, as it tends to have a sizeable effect on the ride quality of a vehicle.

-soD

You are 100% correct. I did not mention the distinction only because I wanted to focus specifically on the rotational mass. I should have used "static" as the counterpoint. Most of the mass I am talking about is also unsprung, but you are right, some of it, technically, is not (e.g., the driveshaft which is actually supported by the body/frame).

There is also quite a bit of unsprung mass that can be removed from the car, but since those parts tend to be more costly to replace and still act much more like the static mass (albeit to a greater degree) than rotational mass, I'm less interested in addressing them.... for now.

[kurzer]

Originally Posted by California98Civic

Meaning that reducing unsprung mass improves the ride quality, but how and in what ways? Traction and comfort maybe?

both! and more. the lighter wheel can follow the road surface quicker. better comfort and better traction, more grip, less tyre abrasion and so on...