Wheel weights

[Crash Enburn]

I think that we are looking at two effects here: 1) the rotation of the car as it goes through the transition, and 2) the steadiness of the car on the flat (no wiggles).

The wheel weights are expected* to be effective in this on both counts. Since the weight surrounds the axle, the very point about which the car is rotating, they help in minimizing the amount of energy spent in rotating the car.

Also, much like a tightrope walker's balance pole, the weights are outset from the body to hopefully provide better stability.

*This thread is saying that some guys had to put a lot more steer than normal on their cars when they ran the tungsten Puma weights. It could just be that the tungsten weights changed the CoM such that the DFW wasn't getting enough weight on it to drive it, and so needed more steering to compensate. With or without the wheel weights, I think that you should still be shooting for a CoM of 1/2-3/4" — which could very well mean only one row of weights behind the rear axle. YMMV.

 

[down4derby]

This is the reason I feel the lighter weights are more forgiving. With heavy wheel weights if and when your dfw moves away from the rail it will be harder for your car to settle again. There is a balance for a perfect weight and I believe we are getting there.

 

[Fumbles]

I just built a car that required me to try using the full round tungsten wheel weights as there was limited space to put weights elsewhere. I'm hoping that it is stable as I can't really remove the weights and substitute anything else as then it wont weigh 5.0 ounces.

 

[bracketracer]

Also, much like a tightrope walker's balance pole, the weights are outset from the body to hopefully provide better stability.

Crash, I know you're not the first person to make that statement and I mean no offence to you. I'm directing this comment at the statement and not at the person making it.

I have never been able to understand how that quality benefits a PWD car, given that we have two rear wheels spaced fairly far apart running on a smooth aluminum track. Totally different setup compared to a person standing on a tightrope with both feet close together.

 

[Crash Enburn]

The wheel weights are expected* to be effective...

Also, much like a tightrope walker's balance pole, the weights are outset from the body to hopefully provide better stability.

These were the initial ideas on the utility of the wheel weights, the veracity of the idea may or may not be.

I look at the weights as an opportunity to condense the mass closer to the rear axle, as well as drop the CoM lower (closer to the track) -- lower is better as far as gravity is concerned.

 

[bracketracer]

I look at the weights as an opportunity to condense the mass closer to the rear axle, as well as drop the CoM lower (closer to the track) -- lower is better as far as gravity is concerned.

But having the mass clustered tighter to the rear axle may very well be the cause of the instability if it makes it easier to unload the DFW coming out of the transition. As the DFW sees less load your effective drift gets closer to zero so the gremlins can take over control.

A racer might be able to lower the COM with the Cub weights, but I would argue that the original weights raised the COM when installed as intended with the notch down.

But like anything else in pinewood derby racing, you have to give it a try and see if it works for you!

 

[B_Regal Racing]

A racer might be able to lower the COM with the Cub weights

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[pony express]

The wheel weights have been a mystery. Some are faster than others. The Puma II should be faster, but so far have been disappointing. Trying drilling the axle holes at a different level should help. It's not always better to get the weight lower. The roll center would be at the center of the weight system essentially. It's tougher getting the roll center with the wheel weights. I suspect the puma II roll center is much lower than with usual cubes. That would mean installing the axles at 4/32 would give a roll couple not the same as the roll center which wil linduce roll in the car. The roll couple should be the same as the roll center for stabilioty. That's why drill the holes at 6/32 slows the car tremendously. Lowering the weights is not always the best way to go. It's a trade off as always in pinewood derby cars.

 

[B_Regal Racing]

"Roll center" vs "roll couple"?

I have never heard of these terms, nor do I understand the difference. Could you explain, not necessarily the advantages/disadvantages of weight height, but what these terms mean specifically? This sounds intriguing...

 

[Bullet]

I will take a stab at this. I believe PE is meaning that there is a moment arm introduced when the center of mass is different than the axle centerline. This could cause a moment arm between the too points, the car would be trying to rotate around the axle line while weight would want to rotate around the vertical COM. The distance between these two points would the the moment arm, or the "couple." Sound about right?

 

[pony express]

In physics terms it is the moment arm. In racing and car alignment it is referred to as roll center and the roll couple. It always boils down to physics. We need to keep the roll couple as close as we can to the roll center. The vertical location of the axles will determine how close we are to the roll couple. The roll center is determined by the weights of the car. If the axles are in the dead center of the back weights you are very close to the roll couple.

 

[B_Regal Racing]

Given what you said, the Puma Cub Weights really have no chance of being *fast* when used in a league car, because they always hang below the axle. There is no way to raise the roll center up the the height of the axles.

Of course, fast is a relative term as most of the Pros could beat me using a stack of quarters on their car vs. the weighting system of my choosing. I use the term fast in the sense that the same builder created two identical cars, with identical preps, with the identical axle drill locations, and with identical weights on all three wheels; the only difference in my hypothetical example would be the vertical location of the weight placement.

 

[bracketracer]

Given what you said, the Puma Cub Weights really have no chance of being *fast* when used in a league car, because they always hang below the axle. There is no way to raise the roll center up the the height of the axles.

If you're using the cub weights make sure to use the upper shields also.

 

[GravityX]

Given what you said, the Puma Cub Weights really have no chance of being *fast* when used in a league car, because they always hang below the axle. There is no way to raise the roll center up the the height of the axles.

If you're using the cub weights make sure to use the upper shields also.

Sounds aero....^^^^^

 

[down4derby]

PUTA weights! From DIRTY MONKEY! Those guys are brilliant!!!!! I just threw up in my mouth.

 

[down4derby]