Physics Question for those much smarter than myself

[THEYTOOKMYTHUMB]

Which is probably most of you.

Something I was pondering while performing the mindless act of mowing the other day. Can anyone here write up the equation necessary to determine the run time difference between say a drilled and undrilled RC10 chassis? For instance if the weight of the car were the only variable and all other properties equal: Running surface, tire size, battery voltage, battery capacity, amp draw of the motor and anything else that would enter the equation. Just curious...

[abailey21]

I think you also have to factor in running conditions because you can't control those. You'd need identical laps so you'd probably need one of the best drivers in the world for consistency

[THEYTOOKMYTHUMB]

I think you also have to factor in running conditions because you can't control those. You'd need identical laps so you'd probably need one of the best drivers in the world for consistency

On paper we can assume a completely controlled environment where we can make any one or multiple parts of the equation perfectly equal, you're right though.

[LTO_Dave]

E = RC10²

[THEYTOOKMYTHUMB]

E = RC102

I should have known that was it!

[Coelacanth]

I'll take a stab at this. As mentioned, you would need to find a way to keep all variables constant, EXCEPT for weight. The only way to limit all the myriad variables re: track conditions, you'd need to devise some sort of treadmill that you could run the car on, while keeping the car stationary. The rear wheels would push the treadmill belt when accelerating, while the weight of the car would rest on all four tires. Then, calculate the run-time until a specific point, such as when the car speed drops down to 5 mph.

Then, add or remove a set amount of weight; i.e. 10 grams, and re-charge the same battery, and run the test again. Record the change in run-time.

Repeat the test in 10 gram increments.

Graph the run-time results by weight and you'll begin to see a linear (most likely) correlation; this will give you a ratio - x/y.

You could then estimate run-time as a function of weight by multiplying by that ratio.

Easy-peasy...

[THEYTOOKMYTHUMB]

True, but couldn't it be worked out theoretically in a mathematical equation? It's been a looooong time since I've done any physics (and hope to keep it that way! ), but I thought you could figure out things like that by entering them into a formula of some kind. A very long formula mind you...

I may be way off the mark on this whole thing. My poor old brain has been thru a lot in the last 20 years!

[GoMachV]

As mentioned, a world class drive might feel it and experience that extra little bit of run time, but for the average racer he would blow any gains in a bad corner or two. Back then you didn't have batteries and motors that would make 4 minutes with a heavy finger so they tried to lighten them up. Don't get me wrong, it helped, but not nearly as much as it hurt the resale value lol!

I don't think a formula would work unless you can factor in a consistent driver and that's the problem!

[jwscab]

you could generate a formula for steady state loading of the battery pack. essentially, you are looking at force/energy equations. the battery energy is some particular value in watt/hrs based on the pack configuration(voltage, chemistry, and capacity). based on that total energy available, you have to determine either a steady state uniform power dissipation, or a constant uniform acceleration, or a blend of both. since both of those calculations will be determined by the battery capacity, the horsepower of the load, and the overall mass the load device has to move, you can keep everything constant except the mass to figure out what you want. The more simplistic view will be to look at how these formulas calculate out, and look at how the energy and mass are related. if they are linear (1:1), then 10% mass reduction will equal 10% run time improvement. If they have some quadratic relationship, then it will follow that relationship, so on and so forth.

since there are many other variables that we haven't even scratched upon, the effects of weight savings may be far outweighed by everything else.

[THEYTOOKMYTHUMB]

I see what you're saying. Barely! But I see it. Your linear reference got my brain going in the right direction.

It's kinda weird to me still that power is so unlimited in today's racing. I think that, more than anything else is my reason for not being able to stay interested in racing. One of the funnest parts for me was trying to balance maximum power and 4 minute heats.

[Coelacanth]

True, but couldn't it be worked out theoretically in a mathematical equation?

My experimental conditions above would generate your equation, and would at least be completely valid. Anything else would be theoretical and nothing but a mere guess--and results would almost certainly be invalid for any real-world extension.

I have a B.Sc. in Science and I'm well-versed with the Scientific Method; any calculation or experimentation that doesn't follow that Method would have questionable and invalid results.

http://www.sciencebuddies.org/science-fair-projects/project_scientific_method.shtml

The important detail is being able to limit all variables EXCEPT the one you wish to change; i.e. weight. There's no way to eliminate a whole host of variables out at a track. Driver adjustments, changing track conditions with each pass, different routes, changing weather conditions, etc...it's impossible. You need to create a totally synthetic test-bed to make an experiment on run-time vs. weight valid and fair.

[THEYTOOKMYTHUMB]

True, but couldn't it be worked out theoretically in a mathematical equation?

My experimental conditions above would generate your equation, and would at least be completely valid. Anything else would be theoretical and nothing but a mere guess--and results would almost certainly be invalid for any real-world extension.

I have a B.Sc. in Science and I'm well-versed with the Scientific Method; any calculation or experimentation that doesn't follow that Method would have questionable and invalid results.

http://www.sciencebuddies.org/science-fair-projects/project_scientific_method.shtml

I guess it would have been a far better Science Fair project than the one I faked in 8th grade!

[SFC K]

My head hurts just reading this...

[Phin]

Wouldn't the first thing you'd need to define be the power output of the motor?

If you know how many watts or horsepower the electric motor produces you can determine the power to weight ratio for the undrilled and drilled chassis. Those numbers could then be divide by voltage to determine amps and the amps compared to the Ah rating of the battery to determine run time.

[slotcarrod]

And this helps us how Thumbs? (X2 on the headache! )

You should be doing what you do best. Show nice girl pics, good jokes, good food and beer! Stick to the things we love you for!