Inertial and Vibration Losses?

[Jewel]

How big a factor are the suspended designs shown in the Go Ask Grandpa website? I have not thought of isolating the weight of the car with foam, I have usually just used five-minute epoxy on brass weights to firmly fix the weights into the body.

I also do not have the correct physical intuition on the physics of this factor in Pinewood design. How does isolating the cars mass help and or creating a sophisticated suspension? It seems that there might be multiple plateaus of car performance to pursue.

[Stan Pope]

We need a "real engineer" in here to talk about how energy is lost or is temporarily stored and then returned when various obstructions are encountered.

For instance, if there is a "whoop-de-doo" (a short raised section) in the flat area of the track, there is loss due to the increased friction from elevated G force on the car as it starts up the incline. There is also conversion (storage) to potential energy as it rises above the finish line. The energy taken to change direction is probably lost and probably depends on the weight distribution (due to moment arm).

Darn. I slept through that class!

[MaxV]

On a smooth track, I don't believe the suspended weight will provide any benefit. In fact, it could detract, as some energy in the system will go into moving the suspended weight.

However, on a rough track the suspended weight may help the car track better (won't get jolted around as much), just like the suspension on an automobile helps it track better on a rough road.

I have built 2 cars similar to the one you saw at go ask grandpa. They are at:


Standard Wheelbase


Extended Wheelbase

[Jewel]

If the car hits a small whoop t’ do as Stan puts it what happens?

Well the mass of the car travels upward and you lose potential energy, and then it travels back down gaining it back. However, if the mass does not rise as far then less potential energy is lost, and then less is gained back. However there are additional frictional losses with this movement, so if you do not move the mass as far then you have less net losses.

This is the kind of intuition I need to make sure makes sense. A flexy type car body has less losses over a rough surface. I would also think that high frequency vibrations in very stiff car might be another form of loss creating greater friction. However these two elements might be considered as separate issues.

If you have a long rough track these issues may be more significant. Putting a high frequency absorbing foam over a very light car body might be an interesting idea to damp out vibrations in the car especially if you used the flexy design. The purpose of the flexy design is to isolate the mass of the car, so you have to be careful to make sure it is raised above to get the advantage if the intuition above is correct.

[Darin McGrew]

When a rigid car hits a small bump, its mass moves up. It requires energy to move the car up. Where does this energy come from? It comes from the car's kinetic energy (i.e., from its speed rolling down the track). And there is no mechanism to convert this energy back into forward-motion kinetic energy, so it is essentially lost.

When a suspended car hits the same small bump, then only part of its mass moves up. There is still no mechanism to convert this energy back into forward-motion kinetic energy, but less energy is lost than if the entire mass of the car had to move up.

Also, a bump on the left causes the mass of a rigid car to roll right, and then roll back left. (And vice versa for a bump on the right.) This causes additional energy losses. This is another advantage a suspended car has.

Note that this only applies to small bumps, not to any "whoop-de-doos" built into the track. All cars rise the same amount when they reach the "whoop-de-doo" so there is probably no significant difference between rigid and suspended cars.

[Jewel]

Well put Darin.

It is starting to become clear. Would a really flexible body be necessary to gain these advantages of independently suspended wheels? The weights are one or two ounces per wheel and the speeds are maybe 10-20 feet per second so the car would need to be pretty flexy I would think so that body roll on one side of the car would not translate over to the other side of the car.

The driving lead weight that holds most of the energy in the car would be nice to have suspended as well so that minimal kinetic energy would be lost moving it. Would a damped spring work well to hold the weight? Is foam or something like sorbathane the way you want to go to damp out motion in the main weight?

[Stan Pope]

What is the energy cost of damping an oscillation?

It is good to minimize the "unsprung weight" of the car. Right? Does damping offset that, and, if so, how much?

Vertical oscillation of the car body does not change the average friction between the axles and wheels, but I think that is hurts the elapsed time by pulling the decelerations earlier in the run.

Hmmm ... time to head back to the physics texts!

[gpraceman]

Having a foam mounting may provide other benefits to dampening vertical movement of the weight. It seems that it would allow the weight to move backward a small amount as the car accelerates, thus storing some potential energy that would later be regained during the deceleration in the straightaway. The adhesive foam would try to pull the weight back to its normal position as it tried to return to its unstretched state. Assuming this scenario makes sense, these effects would slow the car during the car's acceleration, but would provide some extra umph as the car decelerates. What do y'all think?

[Stan Pope]

It seems that it would allow the weight to move backward a small amount as the car accelerates

Your tongue is firmly pressed into your cheek. Right?

[gpraceman]

Your tongue is firmly pressed into your cheek. Right?

Maybe I'm crazy but what may seem like small effects can surprise you. When looking at Michael Lastufka's DOE experiment data, I am not so fast to discount the effects of something I thought small, which was aerodynamics. In his first experiment to weed out factors to test, out of 11 factors, aerodynamics was in the top 5.

I would have originally thought that the amount of flex of the wood in a flex design would be so small as to be negligible, since the wood is still relatively stiff even if thinned.

[Darin McGrew]

When looking at Michael Lastufka's DOE experiment data, I am not so fast to discount the effects of something I thought small, which was aerodynamics. In his first experiment to weed out factors to test, out of 11 factors, aerodynamics was in the top 5.

It's been a while since I studied his data, but IIRC aerodynamics was one of the factors where doing something/anything had a significant effect, but there wasn't going to be much difference between different aerodynamic designs.

I would have originally thought that the amount of flex of the wood in a flex design would be so small as to be negligible, since the wood is still relatively stiff even if thinned.

On the other hand, you don't want too much flex...

Several years ago, one dad entered a car in our All Comers division that replaced the front wheels with skids made from large safety pins. We didn't have any rules that clearly prohibited such a design (we do now), so we let him run.

His races were something of a [junk] shoot. When the car worked as intended, it won easily. The skids had less friction than a pair of wheels, and the car glided over small bumps easily. But most of the time it lost by a large margin because the car started bouncing around too much.

[Darin McGrew]

Having a foam mounting may provide other benefits to dampening vertical movement of the weight. It seems that it would allow the weight to move backward a small amount as the car accelerates, thus storing some potential energy that would later be regained during the deceleration in the straightaway.

Stan may have been too subtle, so here goes...

The weight is providing the accelleration. This isn't like driving home with a trunkload of groceries, where the groceries shift forward when you brake and backward when you accellerate. Gravity is providing the accelleration, and it accellerates the entire car at the same rate.

There is going to be an effect, but it's somewhat the reverse of what you described...

If you mount the weight so it can shift forwards and backwards, then it's going to shift forwards at the starting gate. For the car as a whole, the effect of gravity is countered by the force of the starting gate. For the weight, the effect of gravity is countered by the force applied by the suspension. The weight will move forward until the force from the suspension matches the force of gravity.

Once the gate opens, the car and weight move forward. The difference between the two is that the car is subjected to minor friction forces (minor compared with the starting gate), and the weight is subjected to equal forces from the suspension. So the weight moves back closer to its neutral position.

So if anything, the moving weight acts to push the car forward a little at the start, at the expense of a small amount of extra energy that might have been gained if the weight had been securely anchored and had dropped a little further.

[gpraceman]

The weight is providing the accelleration. This isn't like driving home with a trunkload of groceries, where the groceries shift forward when you brake and backward when you accellerate. Gravity is providing the accelleration, and it accellerates the entire car at the same rate.

There is going to be an effect, but it's somewhat the reverse of what you described...

If you mount the weight so it can shift forwards and backwards, then it's going to shift forwards at the starting gate. For the car as a whole, the effect of gravity is countered by the force of the starting gate. For the weight, the effect of gravity is countered by the force applied by the suspension. The weight will move forward until the force from the suspension matches the force of gravity.

Once the gate opens, the car and weight move forward. The difference between the two is that the car is subjected to minor friction forces (minor compared with the starting gate), and the weight is subjected to equal forces from the suspension. So the weight moves back closer to its neutral position.

So if anything, the moving weight acts to push the car forward a little at the start, at the expense of a small amount of extra energy that might have been gained if the weight had been securely anchored and had dropped a little further.

It is nice to see that you and Stan are not beating up on me too much. It does seem true then that any fore-aft movement of the weight would not help, but hinder.

[Barga Racing]

How big a factor are the suspended designs shown in the Go Ask Grandpa website?

We built 2 of the flex cars to run in our state fair PWD. The cars were pretty fast but not fast enough. My son was 5th overall and my daughter never made it out of her heat. She raced the eventual winner in her heat and barely lost (by about an inch). Her next race she won but her car slammed the end of the track and the weight broke loose. I had put it on double sided foam tape with a bead of silicone on both sides as to reduce the tendancy to want to roll when it stopped. After gluing the weight back on her car did not look near as fast, it is possible it got knocked out of alignment, which was not perfect to begin with. Two races later she lost again and the weight came loose again. This time the tape actually seperated. Also I should mention that this was an aluminum track that was pretty smooth. As for the alignment not being perfect, we cut the cars so thin that they were really flexible. When you pushed the car forward the gaps where the cuts were made would open in the front and close in the rear and when you pushed it backwards the same thing happened opposite. We set it up so all wheels migrated to the axle head when pushed forward. I new this was not optimum and even tried to talk the kids into running the other cars, (ones we made quick in case the flex cars were not allowed). But they wanted the suspension cars because they were neat. So if you build one be aware of making it too flexible as well as how rough the stop is going to be.
I also emailed "Grandpa" to thank him for the design, let him know how we did, and to tell him about this new forum. I told him that with his physics background he would be a perfect addition. He emailed me back and said he was still interested in PWD cars and had offered his help to this forum. I am really surprised he has not posted yet as much as his website and observations have been discussed lately. It would be good to hear from him.

[gpraceman]

I also emailed "Grandpa" to thank him for the design, let him know how we did, and to tell him about this new forum. I told him that with his physics background he would be a perfect addition. He emailed me back and said he was still interested in PWD cars and had offered his help to this forum. I am really surprised he has not posted yet as much as his website and observations have been discussed lately. It would be good to hear from him.

An invitation was extended. I hope he will register and contribute his knowledge and experience. I think it would be of great benefit to us all.