Is it feasible, that a 7oz. car could beat a 10 oz. car?

[Tom]

Our scout district is having an "Ultimate Pinewood Derby Race". The weight limit is 10 ounces and we can use speed wheels and axles. Last year was the first year for the race, but we missed it. We were told that the heavier 10 ounce cars lost out to cars weighing around 7 ounces. With everything being equal, how could a lighter car beat the heavier car? or Why would someone enter a 7 ounce car in a 10 ounce limit race? We're very suspicious of this information and wonder if anyone has had any experience with a race like this.
Thanks in advance for your input,
Tom

[sporty]

I here that 8oz to 8.2 oz is the best to be at.

Anything over and the car will be slower.

just comes to the force and friction place on the wheels.

Heavier is not always better.

Likely what you heard was true.

Sporty

[Darin McGrew]

With everything being equal, how could a lighter car beat the heavier car?

For starters, we don't know that everything else is equal. But ignoring that...

The advantage of more weight is that the car has more energy to overcome forces (e.g., aerodynamic drag) that are not proportional to the car's weight. But at some point, those forces become relatively minor, and additional weight doesn't help much. This happens sooner for low-profile designs with a small aerodynamic cross-section.

One disadvantage of more weight is that the structure of the car must be able to support it. Imagine a derby car that weighs a few tons, because Dad is a retired Starfleet engineer with access to depleted unobtainium. The plastic wheels would crack. The metal axles would bend. The wood body would flex and snap.

Well, the same kinds of failure can happen at a smaller scale, with less weight. At the highest end of competition, perfect alignment is key. It doesn't take much force to knock a wheel out of alignment. And ultra-light wood car bodies (which allow more control over the distribution of the car's mass) cannot support as much weight without flexing, which would affect the car's alignment.

Also, at some point, adding more weight forces you to make your car thicker (increasing its aerodynamic cross-section) or to move your car's center of mass forward (reducing its ratio of potential energy to mass). These disadvantages can offset the advantages of the additional weight.

[pack529holycross]

C'mon...daren... be reasonable....


anyone whos anybody knows that when unobtainuim is allowed the wheels are ALWAYS made out of transparent aluminum...

....I cant believe you forgot that... get it together man! im a derby chairman, not a bricklayer!


Nicholas

[GravityRacer]

100X

...and I'm a simple country physicist.

[sdupont1]

Go 10oz, use razor wheels with bearing, the bearings will handle the load better than speed axles.

[pwdarchitect]

Our scout district is having an "Ultimate Pinewood Derby Race". The weight limit is 10 ounces and we can use speed wheels and axles. Last year was the first year for the race, but we missed it. We were told that the heavier 10 ounce cars lost out to cars weighing around 7 ounces. With everything being equal, how could a lighter car beat the heavier car? or Why would someone enter a 7 ounce car in a 10 ounce limit race? We're very suspicious of this information and wonder if anyone has had any experience with a race like this.
Thanks in advance for your input,
Tom

Yep. The lighter car was very fast and well made.
Through track testing, Found out that there is a break point to which your weight added didn't give you any added speed, or reduction in Time since the race is afterall a timed race.

Someone would enter a lighter car if they knew that the light car could and would do better based on the track testing and other variables.