mark kibort

ouch!!
Maybe just need a wide HP curve so you can just use 2nd 30 to 80mph.

Captain_Slow

I'm focusing on the video's message about large diameter hollowed-out wheels with low profile tires are more for looks than performance. I don't disagree with what you've been saying. It's true that rotational inertia and the linear (translational) inertia are NOT a factor in fuel economy for a steady state --- except for the detail of it takes a lot of gas to get it moving, or change it's velocity (Three ways to change velocity: speed up, slow down, or change it's direction....thus the success of the ultra-light Spider 550 back in 50s against American heavy metal -- good example of Porsche brains over brawn).

I like it...think I'll use this one when explaining it to my classes

mark kibort

Yes, and sorry I wasn't more clear when referring to inertia. I though I was always implying "rotational inertia, not transitional".
so, the video clearly points to the rotational inertia being a factor in fuel economy, and clearly by the points brought up here, its NOT. yes, starting out and accelerating , it can be..... but it also might not, as that rotational inertia, is stored KE that can be released if you let off the gas and decelerate to almost cancel out.... use the brakes and you lose that balance and MPG would suffer. but on and off the throttle on the hyway, with no use of brakes, would keep rotational inertia out of the factors of fuel economy.
so , I guess the final point here is that wheel weight has no more effect than that same weight as it was in your car, and its a rounding error at best for gas mileage efficiency. rolling friction due to larger size and rubber compound differences, as well as tread design, and aero frontal area drag, is THE main factors to consider.

Glad you liked the flywheel analogy...... you sparked me to think of that one.
What classes do you teach?

RedPanda

tell me if i got anything wrong, but this is my view point on how things work.

wheel weight will have a reverse relationship affect 0-60 acceleration, but has minute affect on speed

wheel size (not rim size), has a linear relation with speed, but has minute affect on acceleration

top speed is affected by the weather and road conditions mostly, with little wheel size. acceleration is just the means to get to top speed (ie engine)

mpg is a mix of both, smooth acceleration and constant speed, with good aero, good weather, good roadd and good tire grip. acceleration and stopping is the worst thing for mpg.

braking is more weight distro of the tire and brake size, and partially your speed and car weight and inertia.

car weight affects acceleration and deacceleration, little affect on speed.

mark kibort

weight and inertia are basically the same... weight, or "mass" has a resistance to be moved, and that's its inertia. F=ma.

wheel and tire inertial have a relationship to transitional inertia (non rotating), which is based on its size and weight distr. generally, 2x the effect for tire, 1.5x the effect for mass on the rim.

top speed is dictated by HP to push through rolling friction plus aero drag
because it has no time limit.
0-60 is a time based goal, so its all about HP-seconds vs mass, rolling friction and a little aero drag. again, the wheel and tire mass only effects this in exactly the same way that that weight would, if in the car..... times a rolling inertia factor.

I think you got it!

this is impossible.... if you lost 4-6mph, it wasn't due to wheel weight. the 1/2" wider tires would cause more aero drag, but not enough to effect that much MPG. the total 5lbs per wheel weight, or 20lbs overall, is just like you had a bag of groceries in the side seat... would that lose you 6mpg? no, it wouldn't have the rolling friction enough to do that. so, what is left, aero drag of the 1/2" wider tire, where a 1/4 of it is facing on coming air flow? don't think that would do it either. rolling friction of the tire tread design or compound.....?? probably mostly responsible.... and mostly responsible, was how you were driving and conducting the tests. (along with a few other possible causes).

wheel diameter is not one of them..... the video message is wrong too.

Captain_Slow

Physics, Earth Science, and dual-enrollment Geospatial Analysis (GIS) course through James Madison University.

EDIT: I should point out that only the GIS course is through JMU. I have a physics endorsement on my VA teacher license, but my degrees are in geology and paleoflood hydrology/watershed science. I did a little bit of work in the San Juan mountains working on paleoflood reconstructions (lots of detective work and some HEC-2 computer modeling) and hydrometeorology of flooding at elevations above about 7500 feet in the Animas River basin, near Silverton....fun times.

RKD in OKC

Okay, how about this one for mpg...

Had an 80 GMC Jimmy, full size 4x4 like a blazer. It had a 305 V8 and 32" off road tires. It got 10 mpg and did not have the power to go over 60 mph on the highway.

I swapped out the 305 V8 small block for a 400 V8 small block with 2.02 heads (the big valves), RV cams, Edelbroch intake, headers, and Rotchesther 4bbl carb. Everything else the same. Went from 10 mpg to 19 mpg and had no problem making it past 60 mph.

geschwindig

...And that's called, "Volumetric Efficiency" !

Great discussion! I was wondering when a physics teacher would chime in.

Kevin

mark kibort

Yep!!

ha ha ha...... guess this one is a little obvious.... unless you changed the wheel and tire sizes to something like 25" road tires (spedo error ), you probably had a real poor running 305 engine.

Nice! I feel honored you are going to use my little analogy.
Let me know when the HP/Torque topics come up in class! Still looking for the best analogy to start off that discussion!

Captain_Slow

Wheels and gears are levers. Mechanical advantage can make a dinky tractor apply enough force (as long as the tractor has enough mass) to push over a large tree. But it does the WORK very SLOWLY. Doing work slowly means it is LOW POWER. On the other hand a 928 does WORK very QUICKLY, but will never be able to push over a tree....the gear "levers" in the transmission will not apply enough force due to mechanical advantage of the transmission (nor does the 928 have enough mass). But the 928 does a lot of work very quickly...and this is why it has inherently more power than an tractor. So it's wrong to think of tractors and pull dozers as powerful when pushing over trees. They have large forces applied via mechanical advantage. To be powerful work must be done quickly. Work = ForcexDistance. Power = Work/time. I once had a football lineman in a class who could climb a flight of stairs one step at a time (to maintain constant velocity....skipping steps allows for acceleration and then we need calculus) so fast that when calculating how much work he did divided by the time....it equaled 0.98 HP. Not bad!! Here the key was he had a lot of mass, and thus a lot of weight to overcome (FORCE), and he was very quick.

"weight and inertia are basically the same... weight, or "mass" has a resistance to be moved, and that's its inertia. F=ma."

You're half right above. Weight, mass, and inertia are commonly misunderstood. It's common to say "The Earth weighs much more than a person". In reality (no joke)...the weight of the Earth is the same as your own weight - relative to you. Nothing has weight until it is gravitationally interacting with another object (technically this means every object in the universe is interacting gravitationally with all other objects in the universe...but things that are very far apart don't really feel the two-way tug-o-war between them). So...when you stand on a bathroom scale you get a weight number. How can we let the Earth "see" what it's weight is? Flip the scale over! It does not matter that it's also true that the Earth weighs the same as each of your classmates (so easy to know who's cheating on these questions..lol). Weight is like magnets pulling on each other (the formulas are even the same, but with some different symbols). Stick a force meter between two touching masses and we get the force of attraction between them (their weight relative to each other - it's a two way pull, and both feel the same force).

What most people mean to say is "The Earth has much more mass than a person".

OK...so if an astronaut is tasked with "Weighing an object in space" what are they to do? Scales don't work in free-fall around earth. In space astronauts use "inertial balances" to measure the MASS of materials in space. Traditional lab balances also need gravity to function, so they won't work in space either. The inertial balances literally apply a known force to shake the object and an accelerometer measures the acceleration that results. Then m=F/a.

So weight and inertia are not the same thing. In a car on Earth, the weight is a force vector pointing down. But the earth pushes back up with an equal force called the normal force. So the weight of our 928s is cancelled out by the normal force. But NOTHING can cancel out the INERTIA (which most physicists consider to be just another way of saying MASS). Simply put, when anyone is talking about the effects of the "weight" of a car on it's handling and performance, they are really talking about it's mass, or possibly moment of inertia or center of mass/gravity.

One thing for sure...the Porsche engineers working on the 928 were thinking about all of this stuff VERY carefully and thoroughly. In fact, I believe the use of aluminum in the 928 body was not primarily to lower the mass of the car. If this were true, they'd have made the rear quarters out of aluminum. I think they used aluminum up front simply to get the 50/50 mass distribution. Also, a massive car is MUCH comfortable when cruising at high speeds. A more massive car resists vertical motions...forcing the suspension soak up the road imperfections -- thus, the 928 is way more comfortable for a long trip than a 911 built back then. Porsche was not trying to make it "light", quite the contrary. They wanted to make a rather massive and comfortable GT handle and perform as well as possible.

Enough rambling... at least I ended with something relevant to the 928

mark kibort

Yes, agreed! Relationship of weight/mass and inertia.... I was generalizing. Its really about the mass and inertia being interchangeable for our discussion, but really about how the mass is distributed in a spinning object that determines its rotational inertia.

But, as far as your tractor comparison.... hmmm, maybe half right there too, mainly because a tractor and a 928, commonly have the same HP. One is geared to do a lot of work very slowly, and the other does less work very fast. 'doing work slowly" doesn't mean lower or low HP, comparatively. a lot of force over a small distance = a little force over a long distance. (same work..... W = F x d). So, its not "wrong" to look at tractors as powerful.... they do the same work at the same rate as the same powered car either running at top speed (power= Fv) or accelerating, ( power = rate of change of KE). since HP is the rate of doing work, you can very easily see the comparisons by looking at the trade offs. That's like your football Linebacker on the stairs. if he weighs 300lbs, and runs up the stairs in 10 seconds.... if the 150lb wide receiver runs it in 7 seconds, he might have required the same power. This is why vipers and Porsche 911s can run equal lap times and accelerate the same at any part of the race track because their power to weight ratio is identical, and both have the ability to use their max power at any speed on the track.....
(500hp Porsche vs 500hp viper, but engine torque values being one 2x of the other.)

yardpro

although we agree on most of this, the going up grade will indeed use more fuel..

there is some energy stored in the heavier spinning mass, BUT even though it will maintain velocity better than a lighter wheel, it still looses velocity and if climbing a long grade it will indeed require more energy to maintain the same speed as a lighter mass...

this is very similar to the torque hp debate.... a heaver spinning mass delivers power through torque, where a lighter spinning mass is horsepower...
both deliver energy and you cannot have one without the other as they affect each other

mark kibort

Its not like the HP torque debate because there is power both with a heavy or light spinning mass..... one fast one slow, both will provide same force at any same velocity. The Hp torque debate , is always a debate ONLY because people forget that the force for acceleration is at the tires, not at the engine in the form of torque. HP will clearly indicate what the force is at the tires at any vehicle speed. (engine torque means absolutely nothing for acceleration at a slow or fast vehicle speed... without an engine speed attached to it)

as far as keeping a constant speed of a rotating mass I think we (hopefully) all agree, with no change of speed, a heavy vs light wheel of the same size will NOT effect gas mileage. spinning or not. the force needed by the car to maintain speed is only to fight friction and drag. to accelerate it, it acts as if 2x is sitting in the car..... still a non factor in the area of fuel efficiency.

Now, when you accelerate It costs more because there is a higher rate of change of KE. climbing a hill, at a constant rate of speed, is changing the KE because you are storing KE in the form of potential energy.
I guess the question is, will a spinning wheel at 1000ft, have more KE + PE, then a non spinning wheel (just an equal weight round mass) at 1000ft?

In other words, if you had a 100lb tire and 100lb block of iron tied to it, and you spun that tire up to 1000rpm, it would have a KE associated with it , and so would the block of iron . if that block of iron with the spinning tire was then to climb a hill with speed not changing, the total KE would only go up in the amount that the elevation added the energy needed to lift the weight of the 200lbs to that 1000ft elevation.

I don't think it will have any more energy, or you would need any more energy to lift the spinning mass. But, let me know what I might be missing here if you think I'm wrong.

yardpro

it IS like the hp/torque debates that are so famous here...

and captian slow is correct with the tractor analogy. power, specifically horsepower is a measurement of work done in a specific time frame... specifically the amount of energy to lift 500 pounds one foot in one second.

torque is the force applied to an object. there is no time factor,

you can produce the same 10,000 ft lbs of force from a 1hp motor or a 1000 hp motor. yes the larger hp will deliver it faster, but they will still deliver the same force.

a tractor will deliver power to a tree through torque and gearing, not hp...most tractors are low hp....

our bobcat t200 was one of the larger units made, only one larger was the t300.... they are 10K lb skid steers... they only had 90hp. they could just about push a house over......

worf928

I guess I missed the news that CA repealed the Laws of Thermodynamics.