Derby Talk is a forum for Pinewood Derby, Awana Grand Prix, Kub Kar Rally, Shape N Race Derby, Space Derby, Raingutter Regatta and other similar races where a child and an adult work together to create a race vehicle and a lot of fun and memories
A late up date for the project, and also another scenario...
We decided to go with 2 cars on a 2 lane track with a 5' running surface (total of six foot length track)...
What we used:
We used 1 pine-pro block and 2 sets of pine-pro wheels turned on a lathe only to smooth the running surface and 2 sets of axles prepped in the same manner...
We thinned the block into (2) 1/4" slices and drilled the axle holes at 3/4" from each end on both the blocks...
The total weight for each car (#1 red) was 1.2 ounce and (car #2 BLUE) was 1.1 ounces...
Track has a 5' running surface pin to finish with a blue lane and a red lane...
We ran red car in the red lane 3 times at 2', 3' and 3'6" starting heigths for a total of 9 runs...
The blue car was also running in the blue lane at the same time...
Test was to see if a higher starting heigth made the car go faster...
Well as we all probably suspected, they did go faster starting at a higher starting heigth...
If it were up to the kids in the school she would have won the grand prize, cause all the kids were really fascinated with the project, but the adult judges thought different, She won her class , but not the school winner...
Overall first and third place for the school were third graders and overall second was a 4th grader ...
Down a five foot track was well under a second at the 3'6" starting heigth but thanks to the micro wizard we were able to see the final times...
ANOTHER scenario!!!!!
The red car in the red lane weighing in at 1.2 ounces beat the blue car weighing in at 1.1 ounces all three runs starting at 2 foot and red also won all three runs starting at 3 foot heights...
But the blue car won all three runs staring at the 3 foot 6 inch starting heigth ...
I thought maybe she started the cars different or something on the 3'6" starting runs...
So after she went to bed I again did the test...
The red car again won at the 2' and 3' starting heights and again the blue car would win when starting at 3'6" starting heigth...
Running friction varies with car body weight, so its effect should just about cancel out ... if the frictions are the same. If not, the higher friction car has a disadvantage.
Wheel rotational inertia losses vary with speed, but not with body weight. Thus, wheel rotational inertia loasses are a higher percentage of total energy for the lighter car, and the result is a disadvantage for the lighter car, which decreases as the potential energy of the cars increases.
These two functions may "cross over". It is possible that the example you showed exhibited just that crossover.
Stan
"If it's not for the boys, it's for the birds!"