Best Weight Configuration

[Jewel]

What do people think about weights?

What shape do people prefer, cylindrical, spherical . . .etc?
Do you want the weight along the length of the car or along the line of the wheels? Is a light car body good and then all the weight is concentrated in one place?

[Darin McGrew]

What do people think about weights?

I think one of the most important things to consider is the ability to adjust the weight easily at registration. I've seen too many cars ruined by the rough handling required to remove weight at the last minute. And extra weight added at the last minute to bring a car right up to 5oz sometimes comes loose during the race.

One of the cleanest ways of adjusting the weight that I've seen is the use of small screws in a recessed area on the bottom of the car. If your derby allows you to remove weight, but not to add weight, then just start with extra screws already in the car, and remove them one at a time until the weight is low enough.

[Yeah, I know it's not really the kind of response you had in mind...]

[Jewel]

Good point. I have found a simple way to adjust weight is to use a screw with a stack of small washers that are about a tenth of a gram, and then remove and or add washers to bring your car into weight.

Another easy way to deal with weight is to use modeling clay or moldable lead, and then just add or subtract from this continuously variable substance in a pocket in the car.

But my real question is how to minimize the rotational inertia of the car. The rail design is nice because you can make the front of the car very light and lose little energy as it bumps and goes through the curved section in the track. You just want to make sure you have about 1 once on that front guide wheel.

I tried to raise the weight vertically one year. I put my weight on a screw and varied it up and down. A high weight may save some energy going through the curve but you lose stability. I concluded low is good. Also when your car hits a bump from the side your weight traces a larger angle and thus moves further, losing more energy.

I was thinking a sphere as low as possible for stability and low inertial losses from any direction. Make the body of the car as light as possible, a very thin rail might be good. How could you suspend the spherical weight and still be legal? A cylinder might be better in you have a really smooth track and don’t have to worry about side to side stability.

[gpraceman]

What do people think about weights?

What shape do people prefer, cylindrical, spherical . . .etc?
Do you want the weight along the length of the car or along the line of the wheels? Is a light car body good and then all the weight is concentrated in one place?

It all depends on your car design. With some low profile cars we've drilled holes, used cylindrical weights and sealed it up with wood putty. With others we have routed out a cavity on the bottom and used flat weights. For tall cars, we've built them in slices, with all possible material removed inside to make a weight cavity. This also aids in shifting the COG to the rear by removing material in the front. Whatever your design, it is best to have some easy way to add/remove weight come race day. This should be factored into the design before even one cut is made.

Lately, we've added the weight before we finish the car, leaving it a couple of tenths of an ounce under the limit. Then come race day, we do the final weight adjustment. In this way, we reduce the possibility of marring the finish, due to the handling required to add the weight. I've seen some finishes ruined due to the handling (and clamping in a vice) required to make room for weight, on race day, since people didn't think ahead. Finishes can get damaged, but you can also mess up the wheel alignment with the extra handling.

For weight concentration, we pack it close together and as close to the rear axle as we can to shift the COG just forward of the rear axles.

In another thread I mentioned that I like the lead strip weights that I get from a local hobby store. They are graduated, easy to cut and adhesive backed so making last minute weight adjustments is easy. But since they are lead, we do follow that up with a coating of hot glue to encase the lead if we do not have a covered compartment to stuff it into.