I'm really confused.

Gravity is the same for all masses - 9.81m/s/s. More mass = more force but same acceleration (for a given amount of drag).

Therefore lighter cars, with less rolling resistance, and enclosed bodies (less aerodynamic drag) perform better.

Otherwise why would soapbox cars be as light as possible with younger drivers - to keep the weight down, because it's quicker.

because terminal velocity is at the equilibrium between the force caused by drag and the gravitational pull => all else being the same more mass = higher terminal

Quote from tristancliffe :

I'm really confused.

Gravity is the same for all masses - 9.81m/s/s. More mass = more force but same acceleration (for a given amount of drag).

for the same reason that a 700kg race car that has as much mass as possible closest to its center of mass, is faster than a 700kg race car that has its mass spread out.

smaller moment of inertia.

when a car goes down a hill, the dynamic energy it has (weight * height*) is distributed to kinetic energy of its center of mass BUT also to the wheels in the form of rotational motion.

so in the end, you have the same amount of dynamic energy divided to forward motion and rotational motion.

Erm, but for a car accelerating in a straight line down a gradient, MOI doesn't come into it accept for the rotating parts. Which is why soapbox cars also have light wheels and small brakes... The lower non-rotating mass (the sprung mass mostly) still wants to be as light as possible to reduce rolling resistance, which increases with normal load.

im not sure im following you here

I've been thinking that I've not been following you either... Everyone seems to think that a heavier gravity powered car will be quicker over a run than a lighter one, and that just isn't the case.

Quote from tristancliffe :

Otherwise why would soapbox cars be as light as possible with younger drivers - to keep the weight down, because it's quicker.

Actually I think that the car itself and the driver are as light as possible so it can be easier to distribute the ballast and not compromise handling.

Read #2 here:

http://www.ehow.com/how_2086565_race-soap-box-derby-car.html

And a full analysis here:
http://www.youth.net/nsrc/sci/sci036.html

Specifically, there's also mention of soap box cars in section V of the above.

And best of all, part of what is probably the only useful series of videos on youtube:
http://uk.youtube.com/watch?v=9lvNofoUYwI

And yes, keeping the moving parts as light as possible, specifically the wheels, is generally a good idea - plus making them as aerodynamic as possible (narrow and covered).

Quote from tristancliffe :

I've been thinking that I've not been following you either... Everyone seems to think that a heavier gravity powered car will be quicker over a run than a lighter one, and that just isn't the case.

ok

car on top of hill. it has the same dynamic energy (energy due to its position, i'll call that quantity DE) with any other car of the same weight at the same point. right? no matter how the weight is distributed... (that is, car A (wheels included) = car B (wheels included))

all clear till now?

when both cars go to the bottom of the hill, both cars have lost some energy due to friction. we ignore that for the moment.
now
car A:
kinetic energy due to motion + kinetic energy due to rotation (of wheels) = DE

car B:
kinetic energy due to motion + kinetic energy due to rotation (of wheels) = DE

now... if the wheels in the second car are HEAVIER (which means the actual body of the car is LIGHTER) they need more torque to turn. because they have greater moment of inertia. which means that between the two cars, more energy goes to the wheels of the car B than the energy that goes to the wheels of car A... so in the end, car B has a smaller percentage of the initial DE converted to kinetic energy.

http://uk.youtube.com/watch?v=tcs93mPn91E

http://uk.youtube.com/watch?v= ... 5nuJY&feature=related
just watch this

Quote from george_tsiros :

http://uk.youtube.com/watch?v=tcs93mPn91E

http://uk.youtube.com/watch?v= ... 5nuJY&feature=related
just watch this

neither of which has anything to do with soapboxes unless youre daft enough to put on heavy wheels

Quote from tristancliffe :

Gravity is the same for all masses - 9.81m/s/s. More mass = more force but same acceleration (for a given amount of drag).

Sorry to be pedantic but Gravity differs accross the globe - it can be as low as 9.79 ms/s up to 9.81...

so it HAS to do with soapboxes
lol

anyway i wrote what i wrote to explain to tristan what i was writing before. no need to get so aggresive. shees

but that was never what the discussion was about... the question ultimately is whether aerodynamic or mechanical drag has more weight in a soapbox... iirc tristan is the only one here with the knowledge necessary to use a cfd sim so get to work trist

Quote from Shotglass :

iirc tristan is the only one here with the knowledge necessary to use a cfd sim

Which is quite ironic given the circumstances. :P

indeed... im fully expecting him to skew the results in his favour

I'm still waiting for lerts to show up and tell him he has no grasp of basic physics to seal off his decline into dementia.

I do have an old copy of Gambit and Fluent here, but haven't used them in a while... And my slightly not-legal version of Solidworks might have basic CFD, but I gave up with Solidworks long before getting that far

A bit more info about this car... Performance claims sound impressive but then of course they're only claims...

Quote from evo website :

Caterham meets McLaren F1? That’s the sort of route your mind takes on encountering the Aspid. Revealed at the London motor show, the car is a showcase for Spanish R&D company IFR Automotive.

IFR’s aim is to make cars and their components as light as possible, which is why the 700kg Aspid, with its 400bhp supercharged 2-litre four-pot, can reach 62mph in a claimed 2.8sec and 100 in 5.9.

Its chassis is made from an ‘aluminium extrusion composite’ called Alexcom with panels between spaceframe tubes. The hollow, blade-like wishbones are also also aluminium extrusions.

Inside, where there is much visible carbonfibre, the instruments are concentrated in a single steering-wheel-mounted screen called Unidrive. It’s the visible part of a wiring and electronics system claimed to be much simpler and lighter than those of most modern cars, employing only a quarter as many microprocessors.

Despite this, Unidrive will enable the driver to adjust the car’s suspension settings and engine map, and can also accommodate telematics and multimedia systems.

Quote from Michael Denham :

A bit more info about this car... Performance claims sound impressive but then of course they're only claims...

They say it's meant to be as light as possible, but it also 'accomodates a multimedia system.' And why the silly headlights, if they want a really light car?

Quote from hrtburnout :

They say it's meant to be as light as possible, but it also 'accomodates a multimedia system.' And why the silly headlights, if they want a really light car?

Well don't ask me, I didn't design it :P I was just referring to the acceleration claims being impressive. But yes they're obviously trying to separate themselves from the 7. Personally if I were to do something like this, I'd have it totally stripped out and just make the performance as extreme as possible. But still, while there is a lot of negativity surrounding this car, I always think it's good when companies make a new car that's potentially fun and exciting. Sure, it may not be to our particular tastes, but we should be celebrating the fact that there are people out there with a good enough sense of humour to make a 700kg car with 400hp that will out-accelerate a McLaren F1 to 100mph... Yeah it's ugly but I'd rather they made this ugly but potentially fun car than not make anything at all.