Beyond CoM: Other performance predicting metrics

Secrets, tips, tools, design considerations, materials, the "science" behind it all, and other topics related to building the cars and semi-trucks.
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Vitamin K
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Beyond CoM: Other performance predicting metrics

Post by Vitamin K »

Hullo pineheads,

So I've been thinking, which, like scraping cobwebs from an attic, tends to come up with a bunch of stringy mess that I, now and again, attempt to pluck ideas from.

One thing I'm wondering about is what kind of useful metrics can we come up with that predict the performance of a pinewood derby car, or at least inform our build decisions.

One example is Center-of-Mass. By selecting a particular target, this informs weight placement and toe-in settings, and gives us a rough idea of energy transfer down the track. But I wonder if there are any others that might also be useful.

For example, CoM can tell us where the (wait for it) center of the weight distribution falls, but it can't actually tell us the exact placement of the weight, since, depending on the weighting of the front end, the position of the rear weight could be further forward or further back.

Perhaps another useful metric to track would be weight at the rear axle? Assuming all the cars weigh the same (5oz), this would be a flat number, as opposed to a percentage. Is there a certain weight target to shoot for?

Perhaps tracking the weight on the front wheels would be valuable too. Too little, and perhaps you're sacrificing your stability. Or at least need to increase your steer.

Another thing maybe worth looking at would be the maximum dimensions (in any direction) of the ballast weight. Too much spread would seem (to me) to indicate a larger rotational section, which might slow transition from the curved part of the track to the flat part.

One thing I'd love to do would be to come up with a list of these values and then record them across a number of cars and then compare these metrics with car performance.

Well, that's all I have for now. Thanks for reading my brain percolations. :scratching:
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davet
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Re: Beyond CoM: Other performance predicting metrics

Post by davet »

I've heard others say they have specific numbers they aim for but I've only seen one person post their exact numbers. Here's ours from last year's Scout season. I got them by using 2 identical, calibrated scales. I used the COM Visualizer to get the accurate COM number. We came in 7th place out of over 3,100 cars. League racers may have way different numbers but I think we'll try to shoot for similar this year. Last year was our first railrunner and we used advice from guys here and elsewhere.

Wheelbase: 5 5/8"
COM: .725
Tungsten behind rear axle: 12 cubes totaling 2.04 oz
Tungsten ahead of rear axle: 9 cubes totaling 1.53 oz ( around .10 oz tungsten putty added ahead of rear axle for tuning)
Left side DFW, weight: .65 oz
Right rear wheel weight: 2.55 oz
Left rear wheel weight: 1.84 oz
Total weight of car: 5.04 oz at weigh in
Drift set at: 4" over 4 feet.
Stock BSA axles with bent rears at 1.5 degrees.
Stock BSA wheels
Oil
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pwrd by tungsten
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Re: Beyond CoM: Other performance predicting metrics

Post by pwrd by tungsten »

1) Rear axle as far back as possible. >>>Required<<<
2) COM in the 5/8 - 3/4 realm. First time builders can move to 1"
3) Graphite / lubrication
4) 3.6 ounces of added weight (or more)
5) Drilled axle holes. Rear holes drilled at an angle (perhaps 2.5 degrees)
6) Rail ride
7) Lift one front wheel
8) Wheel and axle prep.
W Racing!!!!
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