Front Downforce

[doodlepugg]

Judging by how heavy everything in the roadster is, I should think losing weight is pretty easy on them. Mx5's seem like there's nowt left to strip in comparison  s:D s:D
As/when I flat floor mine, I expect I'll just vent the bonnet in a way to suck air out. With the aircon rad removed and a twin core rad that should be plenty cooling.

[Eastsoutheast]

That carbon bonnet looks really good but would not really suit my car. I like the idea of a subtle mod that retains as stock a look as possible. Seems there is a lot to think about before proceeding but if done well could prove to be a really decent mod.

[Jrichards20]

I would be extremely cautious about modifying the aerodynamics. Has Sferrari had his car in a wind tunnel that shows it is causing greater downforce. I would be worried that creating a vent in the bonnet may actually lower the downforce created by upsetting the windflow essentially pushing down over the bonnet holding the car to the ground. As mentioned above the vents in the elise are there for airflow for the radiator as it is mounted flat. I'm not an aerodynamics engineer but just some more food for thought if you were thinking of this, would it help, or possibly hinder your downforce depending on the placement of the vent.

[Anonymous]

im no expert but I thought sucking air from below the car would be better than from the front, but I can also see merit of moving the max amount of air from the front and pushing it over the car. Air movement on stock with mostly be forced under the car after going through the rad which is the wrong side. but as said im no expert on this, just looking at it from a logical point

[Jrichards20]

I'm no expert but I thought sucking air from below the car would be better than from the front, but I can also see merit of moving the max amount of air from the front and pushing it over the car. Air movement on stock with mostly be forced under the car after going through the rad which is the wrong side. but as said I'm no expert on this, just looking at it from a logical point

If this was the case, I would not expect the venturi floors to start from the front of the car. In my opinion, again same as you not an expert, the air under the car would be better of served being directed to the rear of the car and using a diffuser to increase down force.

[jonty]

Taking air from the front and sending it up and over the top is best. Lotus use the horizontal rad because of the bonnet outlet location, not the other way around. When unducted air exists under the car from the engine bay it is slow and low energy, which makes the(already unstable) airflow under the car turbulent and blocks the air coming under the front bumper, causing lift. If you eliminate this exit path you make a net downforce gain (even if in absolute terms it is still in lift). If you combine with a flat floor you can get into real downforce.

Downforce and lift is all about air speed. Low speed = high pressure and vice versa. If you have high speed flow over a surface it tries to pull the surface - if this is over the bonnet, body etc, you get lift; if this is under the floor or a wing this gives downforce. The air going over the bonnet creates lift anyway -  watch some videos from A2 Windtunnel and you'll see the smoke trail is compressed as it speeds up over the bonnet (on every car), even when the wand is about a metre above the car. This means it is a good point to put the radiator outlet duct, as this low pressure over the bonnet will pull air out of the rad, aiding cooling even more. Forget about air particles bouncing off the car or wing, or whatever, and start thinking in terms of flow speed, and you'll start to imagine the airflow better.

One of the things with car aero that sometimes takes a bit of time to get your head around is that the diffuser is not really there to make downforce; it is actually to decelerate the air coming under the floor so that it is slow enough to join with the air coming around the sides and over the top of the car - think of it in terms of speed again; if the airflow out the back of the diffuser is faster than free stream you have lower pressure in the diffuser, and so the free stream air (going slower - therfore higher pressure) will try to get in from behind and cause the floor to stall due to the blockage at the back. This means the airflow right the way to the front bumper is slowed, and so you won't make as much front downforce compared to if you used a diffuser to balance the speed/pressure. Group C cars with the dirty great big tunnels worked by the floor making huge front downforce, and then the equally enormous rear wing balanced this. The tunnels/diffuser are there to make front biased downforce.

For this 'macro aerodynamics' you don't need a wind tunnel, you just need to be able to imagine what the flows are doing!

[doodlepugg]

Fairly accurate description. Nice to be in a community where people are keen on fluid dynamics. Race car aerodynamics books make for great reading, and are well worth investing in. If I didn't want to keep the frunk, I'd make a vented area not too dissimilar to an elise, and make ducts to the front brakes, too.
As it stands, I *may* make two (slightly more subtle) vents on each side of the frunk, nearing the rear of the bonnet, when I make the venturi floor.

What was said about diffusers is kind of correct, though it's not to *slow* the airflow, it is to expand the space so that the slowing airflow has somewhere to go without causing high pressure, while eliminating flow separation which causes drag.

Interesting trivia: Did you know there is more drag at the rear of the car than the front? This is caused by the vacuum created by an object moving through air. So large frontal area is not the biggest concern with high speed vehicles, but in fact how large the rear of the car is. Note how low to the ground LMP race cars rear ends' are  s:) s:)

[Jrichards20]

Ok, I think everything I have posted so far has been corrected, so another thing that entered my mind that hopefully someone can tell me that it doesn't matter either. If you are increasing the speed of the air through the radiator, in normal driving condition it will be fine, but during higher speeds, if the airflow through the radiator is increased too much, surely that will reduce the cooling effects as the air passing through would not be able to "absorb" the heat (not entirely sure of the physics here, but in my idiots guide that's what I interporate). Hence why some radiators have a slight forward lean, to slow the airflow down before entering the rad to "absorb" the heat and then have designed the vent on the outside to draw it out?

[mrzwei]

I suspect the cooling effect is proportional to the mass of air not the speed thus if you want more cooling you fit a larger radiator (or one with a greater cooling area).

Long time since I've done physics though   s:lol: :lol: s:lol:

[jonty]

Nice to be in a community where people are keen on fluid dynamics.

I'm dead keen on it; I blitzed this stuff at uni, but then went to work for Mercedes HighPerformanceEngines, so although F1 there was no aero involved!

As it stands, I *may* make two (slightly more subtle) vents on each side of the frunk, nearing the rear of the bonnet, when I make the venturi floor.

do it... do it... do it...

What was said about diffusers is kind of correct, though it's not to *slow* the airflow, it is to expand the space so that the slowing airflow has somewhere to go without causing high pressure, while eliminating flow separation which causes drag.

Nope, I maintain my position, as this was the point of my comment!  s:) s:)   The air does not get (appreciably) expanded or compressed in sub-sonic aerodynamics; the changes in pressure are due to speed changes. Air in the diffuser is slowed because there is a greater volume to fill, not because you have X number of molecules going from Y to 2*Y cross sectional area... yes through the throat of the diffuser flow is sped up and so there is a local decrease in pressure/increase in downforce, but the structure exists to get the free stream and underbody flows at a compatible speed/pressure difference to rejoin. Once you get your brain around this you will never look at rake/tunnels the same again...   s:lol: :lol: s:lol:

**EDIT** I'll correct myself and say I do agree about the flow separation - it keeps the flow energised so it doesn't stall and block the floor, but I maintain the main function is 'pump' the rest of the floor, not just to make make downforce as an isolated element

Interesting trivia: Did you know there is more drag at the rear of the car than the front? This is caused by the vacuum created by an object moving through air. So large frontal area is not the biggest concern with high speed vehicles, but in fact how large the rear of the car is. Note how low to the ground LMP race cars rear ends' are  s:) s:)

Indeed, it is a cool area of development!

[jonty]

I suspect the cooling effect is proportional to the mass of air not the speed thus if you want more cooling you fit a larger radiator (or one with a greater cooling area).

Long time since I've done physics though   s:lol: :lol: s:lol:

bang on  s:) s:)  

Ideally you also want as thin a core as possible, since in the second half the air has a much smaller temperature difference due to the heat already absorbed... but we can't run a 5ft wide rad, so compromises are made!

Slanted angles are just about reducing frontal area and where the exit flow is going. An F1 car would cool better if the rads were vertical, but it would have more drag (and look  sshit shit sshit )

[Anonymous]

so stupid terms all air wants to be over the car, so the air that has to go through the front for the rad needs to go up and over, with a rear diffuser not to create downforce but to actually slow down the air from underneath so it is back to same speed as the bit over the top. now my guess is that you want side skirts to stop air from getting under or out, but to be directed from front to back?

[1979scotte]

Your all nice lads I'm sure but you need to get out more.  s;) s;)

[Anonymous]

Your all nice lads I'm sure but you need to get out more.  s;) s;)

scott you have seen my car, I need to put it back in one bit before I can get out, its just when I read threads like this I go that's nice and pull it back apart again lol

[martin_b]

[quote="jonty"}

Slanted angles are just about reducing frontal area and where the exit flow is going. An F1 car would cool better if the rads were vertical, but it would have more drag (and look ****)[/quote]

Perhaps surprisnly F1 cars have a CD of around 1.0 ( depends on the track and the latest set of rule changes) normal cars, between .25 and .35, those bicycle wheeled solar powered endurance cars .07

[mrzwei]

Yeah, the problem for F1 cars is the massive drag generated by the open tyres and suspension. That is overcome by the power of the engines and designs that generate serious downforce.

[doodlepugg]

I'm far too tired to make any logical reply! You actually need airflow under the car to utilise ground effect (downforce from the underside of a car) so much so, infact, that if a race cars' splitter hits the floor under braking then it will lose all downforce.
Another fun fact: You can gain far more downforce from ground effect than any wing. This is why F1 cars had restrictions placed on floorpan areas (and to stop cars taking off!) and they now have the raised noses.

Side skirts are indeed desirable to maintain attached flow, and keep the pressure as low as possible under the car.

Getting distracted, so doubt I've made any sense hahaha

[Eastsoutheast]

WOW my mind is well and truly blown and I bow to the amount of technical knowledge some of you have.

Out of interest is my car (for handling terms) better with or without the plastic nappies under the car? Since fitting the zero manifold and Janspeed system my rear nappy is in the garage and the front one (assuming there is one as standard) is missing.

Thanks

[jonty]

I think they are more there to stop road crud getting up into the engine bay rather than an aerodynamic device, so it prob makes very little difference from a downforce point of view

[SFerrari]

All I can say at this stage is, after having very well observed what has been done on sport/race cars and using my "engineer" knowledge, the difference on the road is night and day. Before creating the duct and vent, the car was jumping right to left on the road, it was way dangeroulsy floating, jumping... Now the car is grounded, stuck to the road, considering the car is very very light (specially on the front end car).
Now, I'm sure the setup could be improved, specially if stronger material had been used (fiber glass, carbon fiber...), in order to gain in rigidity (unlike the plastic I've been using), so it would increase the downforce gain I beleive...

[doodlepugg]

Fluid dynamics is brilliant fun to play with, as there are so many variables.
As for the under trays, I've not actually looked at them. They will, if designed right, reduce drag under the car considerably. So it is a good idea to keep them all fitted.

Another fun fact: General rule of thumb for testing splitter strength is to stand on it. The forces a splitter encounters at speed exceed that of the weight of average Joe. That's why all the plastic fantastic splitters they stick on mx5's make me laugh. And they wonder why they fall off on track/motorways.

 s:D s:D

[jinxedkitten]

Another fun fact: General rule of thumb for testing splitter strength is to stand on it.

Hahah, I remember Matt demo-ing that on T400X at dingday   s:lol: :lol: s:lol:

[SFerrari]

One more pic from above :

[doodlepugg]

Aaahh I see what you've done now. Literally a straight up exit. Fair enough, it's a cool idea! I am a tad pedantic about low drag and the such.

I think someone asked earlier in the thread about if air flowed too fast through the rad would it work effectively. In one of my books on race car aerodynamics it does actually say that you want to slow the air around the rad (or area you want to cool) for maximum effect, and accelerate it away. The most efficient way being to create a kind of venturi, with the rad in the middle, high-pressure/low-velocity area.
I'm hoping to move into a house close to my work, so I can walk... When this happens, expect some made aero experiments to come from me   s:twisted: :twisted: s:twisted:

[SFerrari]

In front of the rad, I've installed some plastic panels ducting all the air going through the front bumper, so all the air incoming is going through the rad, then it exits almost vertically (110 or 120°). The upper and thiner plastic panel above the exit is just there to make the junction with the hood once in place. Behind those panels is an aluminium structure to reinforce the whole setup (you can see the rivets).
I aimed to create something as close as those (not putting the same amount of money in R&D   s:mrgreen: :mrgreen: s:mrgreen:    s:mrgreen: :mrgreen: s:mrgreen:  ) :