Rotating Mass – Marketing Fallacy?

[joseph hollinger]

[Scott McFadden]

Here’s another way to look at this.

Put your kart (or anything else with wheels) on the floor. Mark a point on the tire sidewall. Now push the kart forward and track the “absolute” course of the mark.

The mark does not rotate at all, it just moves in a basic sine wave pattern. Add as many marks as you want at any point on the wheel sidewall sprocket or brake rotor. None of them will rotate they will all follow the sine wave course differing only in magnitude. Now consider that each mark represent some mass, it can be as small or as large as you choose. Its mass remains constant, the forces trying to move it forward remain constant accept for the up and down vectors which (did you guess) are equal and opposite to one another and therefore cancel themselves out. Force/mass = acceleration so acceleration will be constant.

Effectively then the combination of all the “marks,” lets say all of the mass of the rotating components creates a single vector in the same direction that the kart is travelling. So reducing that mass given the constant force of our motor, will result in greater acceleration, BUT we have to add the same mass to the back of the seat in order to make wait. We’ve gained nothing for all intents and purposes.


Inertia is irrelevant as Newton’s laws of motion apply the same to bodies at rest or in “uniform motion in a straight line”
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Scott

[Scott McFadden]

[quote="joseph hollinger"] The log accelerates more slowly in the case where
it has to rotate.

I think you need to support this statement. I believe it to be an incorrect assumption.
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Scott

[Steve Frank]

Scott McFadden wrote

3. we’re relying on one motor with constant power

A kart motor (or any internal combustion engine really) does not make constant power. Power changes as a function of RPM and torque. A lighter flywheel takes less torque to accelerate, meaning the the engine has more of its power available to do other work (accelerate the kart in our case).

As far as hubs and wheels go, consider the radius of gyration. IE, take a bicycle wheel and hold it by the axle with both hands. Now have a buddy give it a good spin and try to turn the wheel left and right. See how much force it takes to change the direction of the spinning mass? Now do that with a passeger car wheel. Guess which one is more difficult to change the direction.

Happy racing this weekend!

Steve Frank

[Greg Mason]

In the "Real" karting world, I think you end up getting more from strength, and reliability of rotating stuff, than you could ever hope to get from less rotating mass. What good, is a liteweight chain, if it stretches, and falls off during the race? What good is a Magnesium liteweight wheel, if it cracks when you get up on the curbing, and the tire goes flat, because the magnesium is so brittle compared to aluminum? I guess you can get to put more lead in a more strategic place, but only if you're not as fat as me!
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www.Shifterkart.biz

[Scott McFadden]

[quote="Steve Frank"]A kart motor (or any internal combustion engine really) does not make constant power. Power changes as a function of RPM and torque. A lighter flywheel takes less torque to accelerate, meaning the engine has more of its power available to do other work (accelerate the kart in our case).

Well know I did specify my assumptions which are always necessary in order to make a reasonable analysis. But as you've mentioned it what you say of course is true, right up until the point where you establish a direct connection between the engine flywheel and the ground. This in effect is what you do on all direct drive karts and once you have engaged the clutch fully in any other type of kart. Once this mechanical link with the track is established considering the mass of the flywheel in isolation of the total mass of the kart (which is constant) is leads to an incomplete analysis at best.

The Gyroscopic forces are again irrelevant given my original and reasonable assumption that it was sufficient for the purposes of this argument to assume we should initially stick with linear motion of the kart as a whole. However in my last post I did touch on this issue, and knowing a little bit about gyroscopic forces from my aircraft background, there are two things necessary for any significant gyroscopic forces to be generated 1. high RPM or 2. mass or 3 large diameters none of which we have on a kart even without getting into the fact that the force your talking about is only the difference between the standard rotating parts and the lightened ones available.

Come on ……somebody challenge this with some credible arguments. Or are you prepared to just dismiss it, because hey, if it didn’t make at least a little bit of a difference, why would the manufactures flog it so?
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Scott

[joseph hollinger]

[quote="Scott McFadden"]

[joseph hollinger]

[Larry Andrews]

[Larry Andrews]

[Tim Doll]

[Mike Goebel]

[Larry Hayashigawa]

This following should provide an insight to what effect rotating inertia and kart weight has on acceleration. Its pretty intuitative.

Put kart on stand. Start motor. Now floor the throttle. See how quickly the tire accelerates. Now think, will my kart tires accelerate that hard on the track. NO! Conclusion: Rotation inertia is microscopic compared to the weight of the kart.

If you are still a disbeliever, then go to the next step. (not sure if this is safe)

Put your kart on the stand, start motor. Turn on your data ack system (MyChron), get your beacon flash the sensor to start the clock, bring the throttle up to 40 mph and then floor the throttle, let off the throttle at 50 mph and flash the sensor again to stop the clock. (I would not do this on the stand but you are free to do so)

Now put the kart on the ground. Do the same experiment, maybe at the track? Bring the kart up to 40 and then stand on the throttle and let the kart go to top speed (hopefully greater than 50 mph).

Your time from 40 to 50 will be significantly longer in the kart than on the kart stand. It will be pretty obvious that rotating inertia is tiny compared to the mass of the kart. If you want, use the math function (take the derivative of speed) in Race Studio to compute acceleration from speed and compare the two.

[Scott McFadden]

[quote="joseph hollinger"]Fundamentally, you have to appreciate that there is energy stored in
flywheels. That energy is independent on their translational (linear)
movement.

You add an unnecessary degree of complication by talking about inertia because all we’re dealing with here is a change of speed. Whether from zero to 60 or from 60 to zero the kart as a whole has inertia once its moving and the inertia is a function of the total mass and the force applied. You cannot separate the energy stored in the rotating parts from that stored in the non-rotating parts. Well you can if you want but it proves nothing. Whether the energy is stored in the wheels tires brake rotors or the linear motion of the kart it is still a function of the mass.

Weather mass is accelerated rotationally or in a straight line the force required to achieve a given acceleration is the same. your basic F=ma

Your argument suggests that because one part of the kart is rotating, its mass can store more energy than a non rotating part of identical mass, and this is simply not possible as long as the relationship between them is fixed ie. your flywheel ceases to be a flywheel in terms of a pure energy storage device, as soon as you connect it to the ground, because now rotational and linear motion are directly connected, one cannot happen without the other.

when the kart is on the track, and there is no wheel spin, your flywheel energy is no longer independant of the linear movement as you assure.
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Scott

[joseph hollinger]