Downforce.....

[Zak33]

....is a complex thing.

And I was talking to some race drivers on Friday, and what came up sparked a thought, that we should chat about

It's about spoilers, and where they join the body, and what effect they have on the shell.

Example....put a spoiler on the back, oabove the boot lid, BUT have very long "feet" on it, that go into the boot itself, and go under the rear seats BEFORE it touches the body shell. This spolier is at the rear, BUT its effect is a force on the centre of the shell, way ahead of the rear axle.

Which means we need to consider the axles as axis.

If you put a spoiler on the rear of the car, big old spoiler, way back....at high speed it LIFTS the front of the car a tad...you LOSE front downforce while gaining read, because the rear axle acts as a point of rotation.

Same for the front, and critically, same on the roof. Short roofed cars, with spoilers above the rear window, ones that have an effect, often bear down on the CENTRE of the car.....

think about it.....it's like having "leverage" on a long pry-bar....the area of effect can be a long way away from the actual force.

Blinding subject, or WHAT?



What does everyone think?

[Big RICHARD]

You been sniffing something?

Aerodynamics is a strange subject. Look up 'Bernoulli's principle' for more aerodynamic delight!

[Butuz]

So i suppose ideally you want the rear wing giving downforce at far towards the back of the car as possible, BUT you need the front bumper / splitter to be producung downforce too in order to compensate understeer?

Butuz

[Rave]

Example....put a spoiler on the back, oabove the boot lid, BUT have very long "feet" on it, that go into the boot itself, and go under the rear seats BEFORE it touches the body shell. This spolier is at the rear, BUT its effect is a force on the centre of the shell, way ahead of the rear axle.

Yeah but the effect would be exactly the same, the force from the spoiler would try to rotate the car around the point where the 'feet' touched the shell. If anything it'd be worse than having the spoiler on the bootlid because there it's the rear axle that's the pivot point.

What you need for balanced front/rear downforce is a spolier on the roof. <reads on> Oh, you said that already.

[Zak33]

Yeah but the effect would be exactly the same, the force from the spoiler would try to rotate the car around the point where the 'feet' touched the shell.

ah you see I thought that too...but apparently not! The leverage created by the length of the arms, AND the position they contact the shell changes it.

Imagine a dragster style spoiler, WAY out the back.....past the back wheels....past the rear of the car...

but connect it to the car in FRONT of the front axle.

Not the rear..the front.....it would compress the entire car using the front axle as a pivot point.

OK..bit excessive, but interesting thought.

Having the same spoiler on a HUGE 6 foot arm, that connected to the rear bumper (pretend its strong enough) has a different effect to having a short arm and it connecting to the floor infront of the fuel tank.

My mind is boggling

[dkmech]

Never did think about this but it makes perfect sense (advantage of a degree in Mechanical Engineering ).

P.S. Remember F1 cars reducing downforce at the front to reduce oversteer. You remove downforce but the gain in balance makes the car better.

[ikonia]

I too race cars and on new tracks get quite involved with the team with regard to setup and in certain situations areo setup.

After delving in more and more, it makes me laugh harder and harder every time I see a guy with an escort or a 206 or something with a silly lego style spoiler just plonked on the back, it must cause no end of problems

[schmunk]

The leverage created by the length of the arms, AND the position they contact the shell changes it.

Hence the massive late-60s Formula 1 wings-on-sticks which were promptly banned (for tending to fall off and hurt people)

[5lab]

im not buying this. the longer the arm for a spoiler, the more twising force there is

provided all things are strong enough , if you have a spoiler like this (imagine only the top line affects aerodynamics)

     _
    /
o---o

it would have the same forces as one like this

     _
 /--
o---o

because while the forces are applied to different points in teh car, the latter has much more of a twisting force than the former. it doesnt matter where you put the bars, as long as they are strong enough. the actual for-aft position of the spoiler affects it, but NOT how it is attatched to the car. the force would go into the chassis at a different point, yes, but they type of force is different, so the actual force is the same

try this. make a car out of lego (techno if possible, normal lego might not have strong enough joins), and put a spoiler on the back only attatched to the back. press down on it. the front of the car lifts up right? now keep the spoiler in the same place but adjust the rods further forwards. teh same force will still have the same affect. the only reason to adjust where the rods go is for strengthening/lgihtening purposes.

[glenthemole]

surely its where the force "originates" from that matters,, eg the spoiler, not where the spoiler joins the car.

If you had a spoiler above the rear boot and attached directly to the boot, it would be exactly the same as if the spoiler in the same position were attached with some long poles to the front of the car. (assuming nothing bends)

[Rave]

surely its where the force "originates" from that matters,, eg the spoiler, not where the spoiler joins the car.

If you had a spoiler above the rear boot and attached directly to the boot, it would be exactly the same as if the spoiler in the same position were attached with some long poles to the front of the car. (assuming nothing bends)

Nope, because the other variable is where the pivot points are compared to where the force is acting. In Zak's extreme example, a spoiler at the back connected by a very long pole to a point in front of the front wheels would pivot the car around the front axle- which would give a very different effect to connecting it behind the back wheels.

[5lab]

no, i'm fairly sure it wouldnt. you see with the pole you have much more of a twisting motion which would infact still orientate around the REAR wheels, not the front wheels. if the spoiler is behind the rear wheels, the rear wheels will be the pivot, not the front. if it is between the wheels then neither/both (depending how you look at it) will act as a pivot, and if you put it infront of the front wheels then they will be the pivot.

if you put a big enough wing behind the rear wheels, and hooked it up via a bar to the front bumper, then drove fast enough, the front would lift off the ground.

[5lab]

ok, i've thought of another way of demoing it. get a hole punch. rest it on 2 pens (which are, for now, acting as teh wheels, the hole punch is our body). make sure the rear pen (at the end where you push down) is an inch or 2 inside the front. now push down on the bottom part (below the handle) of the hole punch. the front will lift up (this demonstrates a spoiler bolted straight on). now push down on the handle (you might need to put something where the holes go to stop the handle just going down. you will see that once again, the front lifts up. this is the equivilent of the spoiler attatched tot eh front of the car..

i'm sorry, but i'm 99% sure that zak is wrong on this one..

[glenthemole]

If you sit on a see-saw, with your mate on the other end , it balances. If you get some pole and make a seat directly above where you were sitting which is attached to the see saw at the other end, the see saw will still balance.

Its the point of action of the force that matters, if this doesnt change, there is no change in any resultant forces/moments.

[5lab]

If you sit on a see-saw, with your mate on the other end , it balances. If you get some pole and make a seat directly above where you were sitting which is attached to the see saw at the other end, the see saw will still balance.

Its the point of action of the force that matters, if this doesnt change, there is no change in any resultant forces/moments.

oooh thats a good example.

[Rave]

if you put a big enough wing behind the rear wheels, and hooked it up via a bar to the front bumper, then drove fast enough, the front would lift off the ground.

Right, this is a wing, so the forces it's producing (in this instance) are going down (thanks to the downforce effect) and back (thanks to drag). Now if the wing is connected in front of the front axle and exerting a force that goes down and back, explain to me exactly how that's going to cause the front wheels to lift up? It would cause the rear suspension to compress more than the front which would give the car a nose-up attitude, but the wing itself could never cause the front wheels to lift off the ground.