Down force problem

[J4P RX]

Originally posted by Mr_7
If you think about it its not going to affect it at all im wondering if your being sarcastic

It most certainly will affect it.  How I dont know, otherwise I would be an aerodynamicist on a load of money making racing cars and stuff :p

Its more likely to affect engine cooling though.  Theres bound to be a few vented FD bonnets out there that do more harm than good.  Its all about air speed and high and low pressure areas.

[MikeLMR]

Originally posted by wookie

 Or instead of a day at a wind tunnel buy a CFD programme and play to your hearts content without making anything. Then when you figure out what you want, go ahead and make it/have it made.

don\'t forget the high end cad system that can cope with free form surfaces oh and better get a degree in aerodynamics too so you know how to use that CFD system and what to do with the data it chucks back at you !  I\'d say it would easilly be cheaper to hire a wind tunnel and make some wing / splitter mock ups

[zevans]

Originally posted by Mr_7
If you think about it its not going to affect it at all im wondering if your being sarcastic

So you don\'t think moving significant airflow from UNDER the car to OVER the car (and possibly changing the degree of turbulence too) will change the lift / downforce on the car?

I am starting to think this is a troll...

OTOH if you are getting to speeds where downforce matters on UK roads, you probably need a RoadAngel before a spoiler... and you need a wheelbarrow to carry your cojones.

[Mr_7]

Well for starter i have a road angel and secondly i dont have a plate you work it out if you cant use the potentail of your FD sell it buy soimethin u will be happy with. speeds of 150+ occur every weekend you have no point and no leg to stand on your just ruining a perfectly good thread. and i know that bonnet lourves would not make a difference unless your bonnet is like a siv

[rx7veilside]

let me know which roads u are doing this 150+ every weekend so i can keep out of the way for when u have ur accident..and please dont tell me u are such a good driver it is safe for u to do this speed

[mikeeech]

Originally posted by Mr_7
Well for starter i have a road angel and secondly i dont have a plate you work it out if you cant use the potentail of your FD sell it buy soimethin u will be happy with. speeds of 150+ occur every weekend you have no point and no leg to stand on your just ruining a perfectly good thread. and i know that bonnet lourves would not make a difference unless your bonnet is like a siv

Mr sarcastic back again...
Now I speak with no detailed knowledge of areo dynamics so someone please correct me where I am a bit off beam but...

Down force is affected by the velocity of air passing under the car relative to the velocity of air flowing over it. As most cars are not flat at the top then the air will have a tendancy to flow faster over the roof line and this will inherantly provide lift at speed. All any areodymics expert can to, without providing positive pressure with Front and rear spoilers is to reduce this lift.

Now to speed up air flow under the car the ride height is commonly dropped. this has the effect of push air under the car at a higher rate with highest velocity, and hence negative lift, coming as the ride height approaches zero. This makes it tricky to negociate speed bumps on street cars but this principle was used on F1 cars in the 80\'s and 90\'s to increase downforce. This resulted in the death of Aryton Senna of course...

So consider the frontal area of the car. It has an intake for the radiator which allows air into the under bonnet cavity for cooling and intake, however not all of this air is used and it has to come out somewhere. Presumably by cirulating around the engine and belhousing before passing into the airflow passing underneath the car. As speeds increase and the airflow naturally passing under the car increase then it becomes harder for this air to pass out into this fast flowing stream and when it does it creates turbulance that increases drag and will, inevitably slow down this air flow thus creating lift.

Now it seems to me that a vented bonnet with vents correctly designed will act as a venturi sucking air out above the car thus reducing the total amount of air underneath the car. This would have the effect of reducing the natural lift generated at speed by creating a "vacuum" for want of a better word. (see ferrari f50, f60 etc etc)

As an aside you will of course have notice on all f1 cars that cooling air exits above the body work and not underneath it. (including the exhausts!)

Now onto rear spliters. These this are designed to improve the airflow from the rear of the car by reducing eddies and hence drag, thereby affecting top speed. Without the benefit of a larger smooth undertray the affect on lift will be limited.

Like most things on street cars it\'s as much to do with looks and personal preference, as a street car is inherantly compromised. If you want loads on downforce then I think a RX7 is not your thing! Even a F60 is compromised.

I have removed my rear wing and have not had a problem with a light rear end but then I don\'t do 150mph around corners where I would need the grip. but again it\'s horses for courses.

[Mr_7]

mikeeech

thank you for sharing your knowlege this post is not meant to start a fight but instead for me to learn what i can by researchin this topic, your last post has tought me quite alot i satnd corrected and therefore apologise to you for what i said in the previous post.

The little other school girl comments from other members leaves alot to the imagination like should they have bought a french chav mobile and joined saxosportsclub.com?

if any of you would like to go for a drive out just a cruise or blat or meet il be happy to get to know some of you as i am a new member and do not want to start by making enimies. i attended the ace cafe meet and will be attending the reading meet tomorrow

[maximtaylor]

I\'ll see you there tonight Mr 7, I\'m the bald one in the silver FD

Max

[maximtaylor]

Here\'s a good solution for rear downforce. I have a sneaking suspicion that it may create a little turbulence and adversely affect the handling.

[iaint]

Originally posted by mikeeech
Now it seems to me that a vented bonnet with vents correctly designed will act as a venturi sucking air out above the car thus reducing the total amount of air underneath the car. This would have the effect of reducing the natural lift generated at speed by creating a "vacuum" for want of a better word. (see ferrari f50, f60 etc etc)

Hard to tell on an FD where the air that enters the engine bay goes to without vents but the undertray certainly keeps it from exiting underneath too quickly.  A few guys running without them had big cooling problems.  From what I remember of the underside of mine most of the air will leave at the bottom-rear of the bay on either side.  

Adding vents to the bonnet might have a small effect on air-pressures but not enough to create significant \'vacuum\' as all road cars are far too high.

Iain

[Initial-D]

I have a vented bonnet too! Some may say I\'m going for looks but it\'s just not true... it\'s all for downforce I tell ya!;)

We should get a wind tunnel day going and see what some of these options really do for the cars apart from making em look nice, would be quite interestuing and may be able to get a feauture in a mag too perhaps??

[scooby_si]

i have a vented bonnet too & wots more if you can supply the tunnel i have the wind http://www.smileycentral.com/?partner=ZSzeb008_ZS\' target=\'_blank\'>http://smileys.smileycentral.com/cat/10/10_15_6.gif\' border=0> lol
Si

[Mr_7]

Yes a wind tunnel day would be good but do they advertise such a day to joe public? i know suzuki cresent motorbikes do but never heard of car ones.

P.s that would solve the downforce problem maybe veliside should put it into production on the basis it increases it lol

[MikeLMR]

you can hire MIRA\'s wind tunnel for the day, not sure how much it costs like. Maybee a mag like practical performance car might do a feature ?

http://www.mira.co.uk/site/what_research_facilities.asp

EDIT ... piccy of the CFD screenshot looks very much like an NB mx5

[the lemster]

in the words of the great philosipher...

heres my 10 cents coz my 2 cents is free.

we all know that F1 cars can drive upside down at speeds of 130 ( or whatever it is) and this is all due to down force provided the the \'inverted wing\' on the back.

the problem is that there are three forces in play here and it seems to me they are getting confused...
firstly as I mentioned is downforce

secondly is drag

think of a plane... ( firstly I must state that my misses works for Airbus)  this has lift provided by the wings and secondly very low drag co-efficient provided by the aerodynamic shape of the plane.

thirdly is thrust...

if the drag goes up you need more thrust  to get it off the ground, and if the lift goes down you need more thrust or lower drag.

now on a car you want it to stick/push down to the ground, hence we have the problem of lift/drag/thrust (BHP).

now I am sure that even the FD was subjected to CFD ( thats computer fluid dynaimcs fro those that dont know ) plots and wind tunnels to maximise the effect of these three elements, and I am certain that the downforce as standard is such that it is in the capabilities of the car, or 15% over, depending on the margin of safety, so If you go and put another spoiler on the back, some big whale tail or something this will effect two of the three elements ie. drag and downforce( DF).  for straight line speed you want as little DF as possible. and as little drag as possible.

splitters between the back wheels will reduce drag by making the flow of air out of the back of the car smoother , but this will also make the down force greater, or have a greater effect as the drag will ahve dropped, adding an inverted wing will increase DF and will have a negative effect the amount of trust needed to get to a certain speed... as it will change the airflow over the top of tha car... increasing hte surface area it has to go over...

to an end... all of this make no difference when going round a corner as the direction of the forces invloved changes and the whole car pitches and so the airflow under the car will alter as will the flow over the car so this whole thread seems a little pointless!!!