the energy change is proportional to the change in angular momentum, which is proportional to moment of inertia and angular velocity. For a given setup, at a fixed roller rpm (pure load mode), those variables are constant; assuming we have not gone nuclear, E is conserved.

dgz was mixing in a linear motion explanation. It is true circular motion is accelerated motion, but the accelartion vector is perpendicular to the tangential velocity, hence the use of the angular velocity concept.

 

You may be right, but I'll have to noodle on this a bit.

An object spinning at a constant rotational speed experiences angular acceleration on its mass, but it is cancelled by a centripetal force pulling the mass back to the center. I believe that angular momentum is conserved, i.e. a spinning top in outer space will spin forever until it encounters an object or force that tells it not to.

 

>>Even under the constant speed of load mode, there is acceleration occuring.
>>Since the wheels, tires, driveshaft, flywheel.... are circular, the angular velocity requires acceleration.

This is not true for a constant velocity situation. Physics site Play around at this site. It is EXCELLENT.

>> That means energy is constantly being applied to maintain the angular velocity.

This IS true, but for a different reasonthan acceleration.

>> since a constant speed does require acceleration, because energy is lost through friction, heat, sound....

NOW you've got it exactly right. It's the friction, heat, sound etc that cause the required energy input for a constant velocity.

But, why do these change more than a teensy-tinsy amount when changing the weight of a tiire?

>>Therefore it will have an affect, albeit a lot less than on a inertial dyno.

 

>>You may be right, but I'll have to noodle on this a bit.
>>
>>An object spinning at a constant rotational speed experiences angular acceleration on its mass, but it is cancelled by a centripetal force pulling the mass back to the center. I believe that angular momentum is conserved, i.e. a spinning top in outer space will spin forever until it encounters an object or force that tells it not to.

Andy,

You are correct, except you are stating that a certain load is placed on the top and the top is left spinning in a vacuum. The wheel is not left spinning on the roller, the roller is applying a force and the engine is applying a force. If the top you are talking about in space is heavier it would take more force to get it moving. In space there is no opposing force (or at least not one we want to talk about, since there are gravitational forces, and it is not a perfect vacuum). Placing an opposing force (or a load) requires the engine to constantly be applying a force.

jlm,

I did mix up my analogy, and they are constant, but not constant between two different objects that have different weights or distributions.

 

Jeezy Pete!
Someone just go to the dyno and slap on some big heavy 19" wheels and then swap to the 15" lightweights.

 

Yeah, my head hurts

 

dgszweda1 wrote:
Neglecting drag forces (which should be pretty insensitive to the mass of the wheels, all other things being equal) a heavy wheel spinning in space should spin forever. If a heavy wheel is connected to a heavy roller in space, and they are both spinning, they should spin forever. If a light wheel is connected to a heavy roller in space, and they are both spinning, they should spin forever.

Bisch wrote:
Someone did. Check out the Nissan article I linked earlier in this thread. Cliff Notes version: Heavy Wheel and Light Wheel dyno'ed just about the same.

 

If you reduce the rotational diameter of the wheel/tire, there has to be slightly higher torque. It would be gear reduction of sorts.....no? ....assuming both wheels are same weight.
If you put 50 pound, 10 foot diameter wheels on the MINI (bear with me a moment), you would by no means have a torqy little machine. But, if you put 50 pound 15 inch diameter wheels on the MINI, it would surely be torqiererer.

I know, the original question refers to weight alone. So, if you compared two tire/wheel combinations both weighing 50 pounds each.
One has a very heavy mass near the hub, with ultralight tires...
One has an ultralight hub, with very heavy tires...

Which one requires more energy to rotate? Are they equal?

 

Heavy hub, light tires, would accelerate faster than light hub, heavy tires.

Changing the outside diameter of the tire acts like changing the gearing, plus there is the effect of the rotational inertia effects as addressed above.

 

>>Heavy hub, light tires, would accelerate faster than light hub, heavy tires.
>>
>>
Would that translate into more/less measurable torque? Power?

 

>>Neglecting drag forces (which should be pretty insensitive to the mass of the wheels, all other things being equal) a heavy wheel spinning in space should spin forever. If a heavy wheel is connected to a heavy roller in space, and they are both spinning, they should spin forever. If a light wheel is connected to a heavy roller in space, and they are both spinning, they should spin forever.
>>

Andy,

I am not disputing the fact that they will spin forever if we neglect all drag forces. I am disputing that they will take more force to get to spin forever. The heavier the weight the more force it takes to spin it. Although last time I looked I didn't see a dyno in space, unless you include the exercise bike on the space station.

 

dgszweda1,

You are confusing acceleration of the spinning object with maintaining a fixed speed of the spinning object. Once the object is spinning, it doesn't take any power to keep it spinning, except that used to counteract drag.

If power is being supplied by the engine into a spinning wheel, and the roller is absorbing that power, then none of the engine's power is going into ACCELERATING the wheel, or the roller.

 

>>Dynos measure torque at a FIXED RPM;

You're right if you're talking about brake dynos, but inertia dynos depend on accelerating a drum of a known mass.

I've never actually seen a brake (also called eddie current, methinks) dyno operate, so I'm not sure if they pause at certain RPMs.

Inertia dynos also tend to be more optimistic in their estimates, or at least that's been the trend in magazine tests.

HTH, Jeff

 

Whew im glad i stayed out of this one....lol as also crank weight, rod weight, piston weight, etc etc....

 

friedduck wrote:

Load dynos can operate in several different ways:

- As inertia dynos, simply using the acceleration of the known mass of the rollers to compute horsepower.

- As hybrid inertia/load, whereby a load is applied to simulate the power that is consumed on the road by drag forces. This is the "normal" running mode of a load dyno. It looks just like an inertia dyno in operation, but the duration of the pull may be longer or shorter.

- As a pure load dyno, dial in 3,000 rpm, accelerate at WOT. Once the rollers get up to speed, the rpm will be held steady at 3,000 rpm. If you sprayed nitrous for example, the rpm would remain at 3,000 rpm, but the power used by the dyno would skyrockect.

caddman wrote:
Well, flywheel crank mass are rotating mass, so once they start spinning, they'll keep spinning in the absence of drag forces. Rod weight and piston mass are reciprocating mass, so every time you hit TDC, they need to reverse direction. Reciprocating mass is pure evil. Rotating mass can be tamed.

 

>>dgszweda1,
>>
>>You are confusing acceleration of the spinning object with maintaining a fixed speed of the spinning object. Once the object is spinning, it doesn't take any power to keep it spinning, except that used to counteract drag.
>>
>>If power is being supplied by the engine into a spinning wheel, and the roller is absorbing that power, then none of the engine's power is going into ACCELERATING the wheel, or the roller.

Maintaining a fixed speed requires force especially to counteract drag. The whole space scenario needs to be thrown out, because it is not realistic to the discussion. The drag that is occurring is the load of the roller. To maintain a fixed rpm of the roller as more power is applied by the car, the roller must counteract with more drag to maintain the fixed rpm. They are both exerting force against each other. The car's wheel and the dyno's roller. They both exhibit the same force to counteract each other to ensure that the dyno is running consistent. Not only is drag occurring from the roller but all the other moving parts on the car are presenting drag to the engine. That is why the dyno doesn't calculate real horsepower figures of the engine, but only at the wheel, that is because a counter force is occuring throughout the drivetrain (i.e. pistons, flywheels, valves, gearbox, oil, crankcase oil, brake discs, wheels, tires......) All of these require force to accelerate as well as to maintain constant speed, since there is still drag occuring across of these parts and energy is being taken away from the process (i.e. heat, sound, gravitational forces, wind resistance [the tires are still moving through the air]....).

Since we know energy is being lost between the engine output and the foce applied at the rollers, we know it is being taken away by other components. Since most of the car is at a standstill, energy is very minimally being taken away from those parts (maybe some vibrational losses and sound losses..). That means the moving parts are taking away the rest of the energy. We know that move force is needed to not only accelerate a heavier object, but also to maintain the speed of a heavier object (on earth not in space), then the heavier wheels and tires will take more energy not only to acclerate but to maintain speed. You can test this yourself. Take a piece of string and tie a metal bolt at the end and swing it around in a circle. Doesn't take much to get it going in a circle, and also not much to keep it moving in the circle with a constant speed. Now take a piece of chain and connect a 20lb wheel at the other end and start swining it around. Takes a lot to get it going, and a lot of energy to keep it moving in the circle around your body. This is extreme differences, but still shows the point. We will not see this dramatic of a difference between a few pounds here and there on the wheels being read on a dyno, but it will still exist. The question was will it have an affect. For the most part you will probably not see the results. But you would if you put a 15" 11lb rim on and then a 21" 40lb rim. How much would you see between the stock 16" and 17" rim, probably not much, because of the accuracy and controls of the dyno are not so exact.

 

>>dgszweda1,
>>
>>You are confusing acceleration of the spinning object with maintaining a fixed speed of the spinning object. Once the object is spinning, it doesn't take any power to keep it spinning, except that used to counteract drag.
>>
>>If power is being supplied by the engine into a spinning wheel, and the roller is absorbing that power, then none of the engine's power is going into ACCELERATING the wheel, or the roller.

Maintaining a fixed speed requires force especially to counteract drag. The whole space scenario needs to be thrown out, because it is not realistic to the discussion. The drag that is occurring is the load of the roller. To maintain a fixed rpm of the roller as more power is applied by the car, the roller must counteract with more drag to maintain the fixed rpm. They are both exerting force against each other. The car's wheel and the dyno's roller. They both exhibit the same force to counteract each other to ensure that the dyno is running consistent. Not only is drag occurring from the roller but all the other moving parts on the car are presenting drag to the engine. That is why the dyno doesn't calculate real horsepower figures of the engine, but only at the wheel, that is because a counter force is occuring throughout the drivetrain (i.e. pistons, flywheels, valves, gearbox, oil, crankcase oil, brake discs, wheels, tires......) All of these require force to accelerate as well as to maintain constant speed, since there is still drag occuring across of these parts and energy is being taken away from the process (i.e. heat, sound, gravitational forces, wind resistance [the tires are still moving through the air]....).

Since we know energy is being lost between the engine output and the foce applied at the rollers, we know it is being taken away by other components. Since most of the car is at a standstill, energy is very minimally being taken away from those parts (maybe some vibrational losses and sound losses..). That means the moving parts are taking away the rest of the energy. We know that move force is needed to not only accelerate a heavier object, but also to maintain the speed of a heavier object (on earth not in space), then the heavier wheels and tires will take more energy not only to acclerate but to maintain speed. You can test this yourself. Take a piece of string and tie a metal bolt at the end and swing it around in a circle. Doesn't take much to get it going in a circle, and also not much to keep it moving in the circle with a constant speed. Now take a piece of chain and connect a 20lb wheel at the other end and start swining it around. Takes a lot to get it going, and a lot of energy to keep it moving in the circle around your body. This is extreme differences, but still shows the point. We will not see this dramatic of a difference between a few pounds here and there on the wheels being read on a dyno, but it will still exist. The question was will it have an affect. For the most part you will probably not see the results. But you would if you put a 15" 11lb rim on and then a 21" 40lb rim. How much would you see between the stock 16" and 17" rim, probably not much, because of the accuracy and controls of the dyno are not so exact.

 

Nobody is doubting that drag forces are involved. What is being compared is the power needed to spin a heavy wheel at a constant speed compared to spinning a light wheel at a constant speed. Once the wheel is spinning, the mass of the wheel has nothing to do with the power being used to overcome the drag forces on the wheel.

A metal bolt and a string compared to a wheel on a chain are completely different systems, even ignoring the mass.

BTW, you may want to check the setting on your browser - your last responses were duplicated.

_________________
SHOW ME THE NUMBERS!

...How Fast is Your MINI?...My Mods...

 

I agree 100% with Andy's last post.

Nice summary.

 

I haven't had the patience to read all of these posts, but the conclusion should be that:

-an inertia dyno will show different results for different weight (mass) wheels; and
-a load dyno, at a steady state measuring power at a given RPM, would not.

Regards,

Jeff

 

Yup.

 

https://mbworld.org/forums/c63-amg-w...orsepower.html

 

I miss Andy. Lots of great info and the inventor of the HAI and the anti yo-yo AGS or whatever the vacuum routing was with the bypass valve.