Rotational Weight

[Eggman]

I've been interested in discussions here on rotational weight and can't help but think that *sometimes* (not always) claims of advantages are overstated. I get the concept that less mass to move means reduced inertia to overcome for acceleration. All rotating parts - pulleys, wheels, flywheels, etc.

For example, I don't know if there is a measurable difference in fuel consumption between a Mirage fitted with factory standard steel rims and one with factory standard lightweight aluminum alloy rims. I know there are many combinations of tire and rims that can cut weight. Would a smaller rim be a better choice than a larger rim? These days larger rims are considered an 'upgrade.'

For racing, I don't doubt there may be a benefit. For economy, I'm not sure the expense will be covered by the gains.

So help me understand this. Any and all who are better versed in this subject share your knowledge and experience. Thanks.

[Loren]

Reducing rotating mass helps acceleration (and deceleration). It's great for improved responsiveness in either racing or just street driving. The engine revs more freely. Rev matching downshifts is a snap. And if you've lightened anything on the other side of the clutch (from pressure plate to axles, brakes and wheels), the car will even brake quicker.

But, it honestly doesn't do a lot for fuel economy in most cases. I guess it depends on what kind of driving you do. For pure pulse & glide, it would probably help. Less mass to accelerate. But, also less mass to have inertia to keep the car moving.

If you're cruising, less rotating mass really doesn't help you much. It's just the inital acceleration that is "easier", and could use a little less fuel.

I'm going to bet that if you search on EcoModder, somebody has probably done documented testing on something like a lightweight flywheel and its direct effects on fuel economy.

I'm sure it would make a difference. But, if it costs $3-400 (plus all the labor or time to install it), it would take a LOOOONG time to recoup your money if the goal is simply to save money on fuel.

I guess a cheaper and easier experiment would be to play with wheels. My experience has been that on a sub-2400 pound car, you can FEEL the difference in wheel weight as little as about 3 pounds per wheel. Less than that, you can still sort of feel, but 3 pounds... yeah, it's there. And one of the things you'll notice when you go to lighter wheels is that they don't coast as far. Less inertia. But, HEAVIER wheels take notably more brake pedal pressure to come to a stop. Again, more inertia.

I didn't spend enough time doing any baseline testing on my car with the stock wheel/tire package when I bought it. And now I've modded the front control arms to the point that I can't fit 14" wheels on the front, so there's no going back for me. I've got a wheel and tire package that weighs about 30 pounds. (probably more than the stock steelies because they're so wide) And they're "anti-LRR" tires. I can still see an indicated 47-48 mpg on the display, maaaaybe up to about 50 if I try really hard and keep speeds below 45. But, I'm not getting more than that. Some of that is wheel/tire weight. Some of it is larger diameter tires. Some of it is wider tires. And some of it is non-LRR, grippy performance tires. I don't run crazy high pressures, either. The tires and suspension are firm enough without adding the harshness of high tire pressure. I set them in the high 30's for autocross, and just leave them there.

[Eggman]

So what would be one's list of priorities for reducing rotational weight? I would guess

1. Wheels
2. Flywheel, clutch & pressure plate
3. Pulleys
4. Any others?

[foama]

Changing OEM steel to OEM aluminium rims is pointless for economy if the wheel and tyre size becomes larger.

According to the 2014 Mitsubishi European brochure, a 14 x 4.5J steel rimmed Mirage with standard 165/65R14 tyres
is rated at 8% less fuel consumption
than exactly the same car but with OEM aluminium 15 x 5J rims and 175/55R15 tyres.

[Loren]

So what would be one's list of priorities for reducing rotational weight? I would guess

1. Wheels
2. Flywheel, clutch & pressure plate
3. Pulleys
4. Any others?

If you're getting hardcore, you can do lighter brake rotors, axles, driveshafts... crankshaft. Anything that rotates can be lightened.

Ever heard of aluminum or carbon fiber driveshafts on RWD cars? That's a good one.

But, everything is a compromise. Lighter often means more fragile. They get away with it on an aluminum driveshaft by making it a larger diameter to tolerate the same torque as a steel driveshaft. But, they can be very "tin can" like. If you get a dent in a thin-walled aluminum driveshaft, it can crush and twist just like a soda can.

Brake rotors are another one. If you look at purpose built autocross cars, you will occasionally see things like seriously swiss-cheesed rotors. They'll fit the smallest brakes they can (opposite of the usual performance trend towards larger brakes) and then mill the rotors to minimum thickness and drill them full of holes. Net result is a brake rotor that weighs half as much as the original. They can get away with it because an autocross run is usually only 40-60 seconds long, and you only brake hard a couple times. So, the brakes don't have time to get overheated. Even with very little mass left to absorb the heat. But, if you tried to do the same thing on a track car, you'd only be good for maybe one lap around the track before smoking your brakes! And if you did it on the street, you'd likely get into serious pad fade after just a few "moderate" stops. But, for an all-out autocross car... it works.



You can get aluminum or titanium lug nuts. (but, you have to be SUPER careful about keeping them clean and torquing them properly)

Lightweight pistons and connecting rods, and subsequently lightened counterweights on the crankshaft.

It's always a constant battle between strength, weight, durability, and cost. Stamped steel and cast iron are always the cheapest, and generally very strong and durable... but, aluminum and other materials can be significantly lighter... at greater cost.

[Eggman]

Changing OEM steel to OEM aluminium rims is pointless for economy if the wheel and tyre size becomes larger.

According to the 2014 Mitsubishi European brochure, a 14 x 4.5J steel rimmed Mirage with standard 165/65R14 tyres
is rated at 8% less fuel consumption
than exactly the same car but with OEM aluminium 15 x 5J rims and 175/55R15 tyres.

I always wondered that. Thanks for posting this.

[Eggman]

If you're getting hardcore, you can do lighter brake rotors, axles, driveshafts... crankshaft. Anything that rotates can be lightened.

Ever heard of aluminum or carbon fiber driveshafts on RWD cars? That's a good one.

But, everything is a compromise. Lighter often means more fragile. They get away with it on an aluminum driveshaft by making it a larger diameter to tolerate the same torque as a steel driveshaft. But, they can be very "tin can" like. If you get a dent in a thin-walled aluminum driveshaft, it can crush and twist just like a soda can.

Brake rotors are another one. If you look at purpose built autocross cars, you will occasionally see things like seriously swiss-cheesed rotors. They'll fit the smallest brakes they can (opposite of the usual performance trend towards larger brakes) and then mill the rotors to minimum thickness and drill them full of holes. Net result is a brake rotor that weighs half as much as the original. They can get away with it because an autocross run is usually only 40-60 seconds long, and you only brake hard a couple times. So, the brakes don't have time to get overheated. Even with very little mass left to absorb the heat. But, if you tried to do the same thing on a track car, you'd only be good for maybe one lap around the track before smoking your brakes! And if you did it on the street, you'd likely get into serious pad fade after just a few "moderate" stops. But, for an all-out autocross car... it works.



You can get aluminum or titanium lug nuts. (but, you have to be SUPER careful about keeping them clean and torquing them properly)

Lightweight pistons and connecting rods, and subsequently lightened counterweights on the crankshaft.

It's always a constant battle between strength, weight, durability, and cost. Stamped steel and cast iron are always the cheapest, and generally very strong and durable... but, aluminum and other materials can be significantly lighter... at greater cost.

Drilling that many holes in the rotor has to have a significant impact on it's ability to slow the car! That's wild. I hadn't considered all the other stuff.

[Loren]

Drilling that many holes in the rotor has to have a significant impact on it's ability to slow the car! That's wild. I hadn't considered all the other stuff.

Like everything else, it's complicated. There's less surface area, so less friction with the brake pad. But, that can be countered by using a larger pad or a pad with a greater coefficient of friction. The diameter of the rotor is also a factor. If you're clamping around the perimeter of a brake rotor that's 12" diameter vs 10" diameter, even if everything else is the same, your "lever" is longer with the 12" diameter, so you're creating more braking torque.

But, the big thing that brakes have to do, and this is the true purpose for going to larger brakes, is that they have to dissipate heat. You're turning momentum into heat, and the brake rotors have to deal with that.

Reducing brake rotor mass, no matter how you do it, is reducing the heat capacity of the braking system.

So, the swiss-cheese rotors pictured above probably work just fine... once or twice! But, don't ask them to work REPEATEDLY. They'll get hot in a hurry, and then you'll have nothing.

[Top_Fuel]

Changing OEM steel to OEM aluminium rims is pointless for economy if the wheel and tyre size becomes larger.

A larger wheel/tire doesn't necessarily mean you're going to take a fuel economy hit. In most cases it does because most people end up doing one or more of the following when changing to larger wheels:

1. They buy a heavier wheel
2. The larger tire is usually heavier
3. Often times the larger tire used is not a Low Rolling Resistance tire

...a 14 x 4.5J steel rimmed Mirage with standard 165/65R14 tyres is rated at 8% less fuel consumption than exactly the same car but with OEM aluminium 15 x 5J rims and 175/55R15 tyres.

That's true. But keep in mind the following...

• A 165-65-14 Enasave on a 14 x 4.5 steel wheel weighs 26 pounds.
  A 175-55-15 Yokohama Avid S34 on a 15 x 4.5 aluminum wheel weighs around 31 pounds.
  That's a 5 pound (19%) increase in weight.

• Low Rolling Resistance (LRR) tires make a difference. The 165-65-14 Dunlop Enasave is classified as LRR, but it looks like the 15" tires used by Mitsubishi (Yokohama Avid S34) may not be (according to the Tire Rack).

For the record...

I have averaged 51mpg for almost 50,000 miles using light-weight 15 x 6.5 wheels and Low Rolling Resistance tires. And I don't do any super-crazy hyper-miling stuff.

[Basic]

For pure budget economy, the easiest, cheapest and most effective thing you could do is take your Mirage and strip the interior out. It will make it louder and overall a much less enjoyable experience, but I guarantee you will see a difference for ZERO invested cost.