Rotational Weight
 

Now, I know this subject has been discussed before in other weight reduction threads, but I had a random thought and was curious as to what you all thought about this.

Rotational weight, especially in the reference to performance vehicles, usually is considered separately from standard... "sprung"... weight reduction. I know that most members here have seen only slight improvements from standard weight reduction in their vehicles, and when accounting for rotating weight, the general consensus seems to be that it only really helps in city/stop-and-go driving.

My question is, has anyone tested the effectiveness of decreased rotational weight on combined mileage numbers?

The reason(s) I ask is because the benefits of lowering rotational weight are noticed most prominently in the periods when the vehicle is accelerating, and one of the few things that I remember from high-school physics is that acceleration can come in many forms. For instance, as I am driving up an incline, I am technically accelerating, and I notice the 5 to 10 mpg drop as I am doing so. This is true whenever I am driving up a hill... both in the city or on the highway. Another instance in which my car is technically accelerating is when I am making changes of direction. Sure, much of the "pure" highway driving that I do only involves straight lines, but still I often find myself "accelerating" around curves and corners.

Furthermore, it seems to me that most of the ABA testing that people do only includes relatively straight roads with little to no inclines. To properly test this, it seems to me that you'd almost have to test this modification on curving roads with varying inclines. Obviously, the consistency needed for ABA testing would be more difficult to maintain, but I believe it is possible.

I have been made more curious about this because I know of parts currently available for my car that could potentially lower the rotational weight by 60-90 lbs (depending on how extreme I wanted to go). Now, most of the people who have these parts aren't concerned with changes in fuel economy, so they haven't been tested for that. But in one case, a modification that reduced the rotational weight by 15 lbs resulted in a 3 mph increase in the car's 1/4 trap speed. So it is certainly having an effect in terms of ease of acceleration.

Now I understand that in terms of economy, these modifications might never pay for themselves, but if I am on a quest to have my cake and eat it too, they might be worthy of investment. If nothing else, I could bite the bullet for the team and properly test the efficacy of these modifications in terms of economy... Just a thought.

For 1/4 miles time each pound of rotating mass you remove is said to be like removing up to 8 pounds of more static mass.

Since the EPA claims that every 100 pounds of weight reduction improves fuel economy by a few percent you would have to think that the same would be true for rotating mass. It would be very effective (1lb rotating = 8lb static) in stop and go driving but would act more like static weight reduction when the vehile speed remains static such as when traveling at highway speeds.

And see, this is exactly what I'm talking about. I keep hearing that rotational mass will act more like static mass at highway speeds, but I think that is an incomplete statement. 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)? I'm not convinced that rotational mass would continue to have the same effect as static weight in those cases.

That doesn't count for the rotation tho.

You said it your self, you're only changing speed by 5-10mph most of the time. The faster you are going the less that 5-10mph change will matter.
The more the RPMs your wheel or drive train changes the more this mod will be worth. The RPM change in the rotating mass is the only accelerating that matters here.

Jesus, how heavy is the stock wheel/tire combo on the EvoX? Also, one thing to consider is that most lightweight aftermarket combos are going to be wider than stock. 9-10" wheels wont help your aero much.

That said, yes, anytime the velocity of the car is changing, rotational inertia is working against you. You will never recover all of it by coasting so lowering the amount of inertia stored in the wheel/tire/brake combo would be beneficial.

I'd like to see someone do an ABA test with the same tires/wheel specs but somehow make one combo much heavier... weld a 10lb weight to each wheel or something.

...or, how about the gyroscopic forces you're fighting because of that rotating weight, especially in banked turns?

@ ProDarwin: The stock wheels are over 22 lbs each, as are the stock front brake rotors. The stock tires are almost 26 lbs, and the stock tire/rim combo weighs 48-49 lbs. The driveshaft is well over 40 lbs, and entry-level replacements knock almost 15 lbs off of that (high-end replacements drop the driveshaft weight down to about 10-15 lbs). There are also random pulleys and other modifications that will drop a pound or so here or there. I already dropped over 3 lbs per corner just by switching to my current tires.

@ Old Tele Man: That's kind of what I was referring to. If a 3,500 lb car is traveling 55 mph North bound and then must take a corner that shifts the direction East or West bound (still at 55 mph), wouldn't that require more energy than just a 5-10 mph acceleration?

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

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

The extra inertia is only a liability if you don't use it for forward movement. If you coast down to a stop without braking, it's neither good nor bad. If you have to brake for a turn or a light, higher rotational inertia is bad because you're throwing it away. If you can change your driving to avoid braking you'll recover most of the possible losses.