Fast Cars Under 5oz.

[Splinter Sprinter]

This past Saturday we raced and did well (undefeated in den and then all wolf dens) until the pack finals. My son’s car was beaten by two cars. (they placed 1st and 2nd over-all) At the spaghetti supper that evening we had all the cars on display and I was able to look over a few. The two that beat us seamed to be under 5 oz’s. Nothing special in design, the usual coins and washers glued to the top of the car. Alignment and wheel prep seamed to be just OK, with only one wheel canted on one of the cars, but the big thing that stood out was the lack of weight in the cars.

Our design was a 3 wheel wedge with the center cut out and weighted 11/2 in front of the rear wheels, 1oz on the front of the car, a total 5oz’s. The track was rough (we couldn’t use the middle lane because the cars would hit a bump and cross over to the other lanes)

Question- with a rough track and cars possibly not aligned perfectly, how could much lighter cars run faster over a good 5oz car?

This would have been a good time to try out Grandpa’s flex car.

[terryep]

Splinter Sprinter, did you notice if these two cars were ahead at the bottom of the hill or did they overtake your car on the straight away?

Terry

[Stan Pope]

Just how much "too light" do you think these cars were?

A while back, I analyzed my choice of scales for inspection by looking at the effect of various magnitude errors on race outcomes. Here is the kind of stuff that I found:

The piece of information that I was looking for was that checking weight to 0.01 oz would limit outcome variance to 0.02 inches. By comparison, checking weight to 0.1 oz would allow outcome variances to reach 0.2 inches!

Now, to your concern:
Here's some info for 32' (28' start-finish) Piantedosi tracks:
Weight -- Lose by sec. -- Lose by inch
5.00 -- 0.000 -- 0.000
4.99 -- 0.000 -- 0.019
4.90 -- 0.001 -- 0.195
4.50 -- 0.006 -- 1.059
4.00 -- 0.014 -- 2.369
3.00 -- 0.038 -- 6.212

Now, these are margins that can be offset by minor to major car faults. I think that it would be unthinkable that an otherwise perfect 3 ounce car would win against all of the other cars in the pack, although it would win its share of races. It is more likely that an otherwise perfect 4 ounce car would win against all of the other cars in the pack, but still ...

For comparison, how many times have you seen in a pack race the second fastest car at least a car length slower than the winner? This probably is not applicable in this case, since your car wouldn't have been that much slower.

If you did not actually put the cars on a scale, but hefted them by hand, then there is a strong possibility that their apparent density fooled your senses. (A rail car "feels" heavier than it actually is; a car with lots of wood, i.e. a half ounce or so , feels lighter, even though they both weigh in at 5.00 oz.)

[PWTom]

I think Stan hit the point with the apparent density. It is amazing what the impact of that can be on people. My son built a rail car this year and weighted it with 4 oz's of tungsten. The clustering of that much mass in the rear of the car made it feel so much heavier that everyone that held it swore my son cheated and was running a much heavier car.

By contrast, another boy had a car with nearly all of the weight in wood (large body with fenders, wheel cowls, etc). When he came to weigh in, I would have sworn the car weighed no more than 3 ozs. It was dead on for 5 ozs!!

The concentration of the mass makes a huge difference is how the car feels. You really need the scale to make the determination of weight.

[Splinter Sprinter]

OK, here is the skinny on the light weight cars.

I went and looked at the pictures of the cars, counted all of the coins, Pine Car stick-on weight segments, and other various Lego pieces and rebuilt them on my scale at home. It was pretty easy to do, there is not a lot of hidden mass in a quarter, just weigh all of the components. We have everything right at home, even 12 to 15 different unweighted car bodies to pick from.

1st Place Car 4.2 - 4.4 oz’s
2nd Place Car 3.9 – 4.2 oz’s

The second place car had to beat my son’s car twice in a row to win second, and it did by about half an inch to an inch each time.

Terry to answer your question-where the cars were holding their speed on the 4 section 32ft wooden track was after the third section (the flats) with the whoop-tee-doos at each joining end. The track is 15 years old, the joints are so bad on the third section you can easily see that it’s high at the joints and creates whoop-tee-doos at each joint. We couldn’t even use the middle lane because the cars would launch off it into the bleachers. They seemed to maintain their speed where as the whoop-tee-doos took something out of his car. Next year we will have to study my son’s Hot Wheels cars that are designed for those sorts of loop-to-loop tracks, before we build his car.

Question- Could the two-sided foam tape holding the Pine Car stick-on weight segments isolated the weights that much to be a deciding factor? You know how much was written in other threads about isolating weight if you were able to do so.

[terryep]

Isolated weights may have taken some of the "edge" off the impacts I don't know. There is no need to allow a track to get that bad. Ours had some bad joints but a few hours of work was all that it needed. Surely someone would step forward and offer to fix it up.

Terry

[Darin McGrew]

By contrast, another boy had a car with nearly all of the weight in wood (large body with fenders, wheel cowls, etc). When he came to weigh in, I would have sworn the car weighed no more than 3 ozs. It was dead on for 5 ozs!!

A number of my cars have been like this. People who pick them up are surprised that they really weigh 5oz. And they're really surprised when I tell them how hard it was to get the design under the 5oz weight limit. The first version of "Calvin and Hobbes" was over 10oz. The first version of this year's "Nemo and Friends" was over 9oz. Both are bulky low-density designs that feel very light.

[Stan Pope]

OK, here is the skinny on the light weight cars.

Okay. The light weight is based on estimate, albeit, an informed estimate. You haven't mentioned any objective measures, such as comments by the inspection crew or others who actually weighed the cars, etc.

Have you eliminated possibilities such as a hole drilled and filled with a section of 5/16" bolt? The "trim weights" may have been just that... weight added after "completion" to bring the weight up to 5 ounces. I used that strategy when I didn't have precision weighing equipment at home. I'd use a "diet scale" or a makeshift balance with a bag of pennies to get the weight up around, but below, target, then add trim weight at the last minute.

The track is certainly very rough! Wow. If you think it should be improved, start the ball rolling early, so that everyone involved knows that the track will be "different from last year." Don't expect those who know how to negotiate a rough track to be overly enthusiastic about the "improvement," although "launching cars into the crowd" certainly seems sufficient justification!

It sounds as though you have zeroed in on the really rough spots as the source of the performance differences. We don't know how much of the weight was "sprung" versus "unsprung," but it sounds as though less that 20% or 30% of the weight is even partially isolated. More precision is needed to make detailed computations. But we do know that sprung weight reduces the energy expended in negotiating the bumps by spreading the time over which the weight is moved by the bump. Foam tape provides a bit of springiness. On that track, I'd take advantage of it if it were allowed.

The weight-time comparison that I gave earlier suggests a 1 to 2 inch difference for typical cars that are equally well prepared but weighted as you estimated. It doesn't take much to accumulate a disadvantage of 1 or 2 inches. Early energy loss magnifies the effect of energy loss. You're describing three separate episodes ... three bad joints. I can see these as easily accumulating the deficit, even with very minor differences in weight isolation.

So, I think that you have solved the problem, regardless of actual car weights. Now you have contingency plans for next year!

[dknowles67]

If the track was really rough, could lane equity have played a part?
Did your son's car lose twice in the same lane?
Maybe one lane was substantially slower.
On a rough track, there is also quite a bit of randomness thrown into the equation. Perhaps if the cars had raced each other 100 times each, the outcome would have been different. It sounds like you used the Double Elimination method. The fastest car can have 2 unlucky runs, and not win the race.

[SPEEDBUGGY]

DON'T FORGET ABOUT THE DIFERENCE IN WOOD DENCITY.ONE KIT COULD WEIGH MORE THEN THE OTHER LIKE IN ANOTHER TOPIC.

[twojakes]

Stan Pope wrote:


Now, these are margins that can be offset by minor to major car faults. I think that it would be unthinkable that an otherwise perfect 3 ounce car would win against all of the other cars in the pack, although it would win its share of races. It is more likely that an otherwise perfect 4 ounce car would win against all of the other cars in the pack, but still ...

. . .

Apropos of Stan's comments, I built two cars for our sibling race last month, one for my son (age 3) and one for my daughter (age 6). I weighted my daughters car to 5 ozs and my son's to 4oz, and tried to make them the same otherwise, with the intention that my daughter win between the two since my son will get the chance to build cars when he is a scout. I only spent about 30 minutes per car, and let the kids paint them and decorate them with stickers.

Wouldn't you know it, for the final heat of the siblings' race my son's 4 oz. car drew the "fast" lane and edged out my daughters car for the sibling trophy! (Ugh). I thought my wife was going to skin me alive. Next year I'm not going to add weight to his car since he won the race this year, but it still goes to prove Stan's point.

Good luck racing, and learning.

TJs