35 replies to this topic

[paulk]

I've read a couple of threads on the site about the vx and different aspects of downforce and wonder what sequence of modifications would the experts here employ to try to increase downforce? My understanding is that the front of the car generates downforce whereas the rear generates lift - I would assume that this isn't desirable and that steps should be taken to minimise rear lift as much as possible. My feeling would be to: Check the rake of the car front to back and increase if possible. Combine this with an overall lowering of ride height - this should increase the speed at which air passes under the car and create low pressure. I'd also fit a smaller number plate that doesn't protrude into the airflow entering the underbody region so that a smooth flow can be maintained. Increasing underbody flow would then necessitate the use of an efficient diffuser to bring that high-speed low pressure air back to atmospheric. The rear underside of the vx already curves but I think that the diffuser should be as long as possible maybe replacing both rear underbody sections in as large a curve as possible with the side walls being as long as practicabley possible for a road car. Rear wing - don't know how efficient the existing vxt rear wing is but would presume that a better shaped bigger wing will be needed to trim the car after the other mods had been made. I know Thorney produces some aero bits which obviously fit into my scheme but just wondered what other peoples views were. Has anyone done any proper testing when trying to modify the aerodynamics of a vx

[x-isle]

Firstly a question, do you use your car mostly for track days? If not, don't bother, it ain't gonna make too much of a noticable difference anyway. Secondly, Lotus spent a bit of time designing the down force on the car, which seems to be pretty good as it is. IMHO, chucking shopping trolley handles on the back is just plain stupid and Chavvy. Sorry if offends anyone that's already done it, just my opinion. (I'm only moody because I've not gone out tonight and there's another stooooopid arsewipe program on TV. Something to do with washed-up tw@ts falling over on ice skates! -Fcukwits!)

[paulk]

x-isle - I agree - not looking to chav up my car. Just looking to optimise the car - got geo/ ohlins etc - but if the aero can be improved (especially getting downforce from under the car which doesn't involved introducing a load of drag which is often associated with big wings) then why not. Don't get me wrong I like the idea of a good looking diffuser, but they can be made to be functional as well - high end sports cars seem to manage this trick quite well. I would't stick a non-functional diffuser under my car.

[amrit]

x-isle - I agree - not looking to chav up my car.

Just looking to optimise the car - got geo/ ohlins etc - but if the aero can be improved (especially getting downforce from under the car which doesn't involved introducing a load of drag which is often associated with big wings) then why not. Don't get me wrong I like the idea of a good looking diffuser, but they can be made to be functional as well - high end sports cars seem to manage this trick quite well. I would't stick a non-functional diffuser under my car.

Generally the car is producing lots of lift. All the add-ons (splitters, spoliers and diffusers) are there to reduce the amount of lift produced. Less lift implies that the car has more weight planted on the tarmac which equates to greater traction and hence a quicker laptime. I'm sure I've posted this before but it's hard to know what impact a diffuser has without testing. As you say, the aim is to reduce the amount of air under the car - hence the front spoliers and splitters.

Now if someone has access to a wind tunnel we could get the answer.

Edited by amrit, 20 January 2007 - 10:57 PM.

[swerve]

x-isle - I agree - not looking to chav up my car.

Just looking to optimise the car - got geo/ ohlins etc - but if the aero can be improved (especially getting downforce from under the car which doesn't involved introducing a load of drag which is often associated with big wings) then why not. Don't get me wrong I like the idea of a good looking diffuser, but they can be made to be functional as well - high end sports cars seem to manage this trick quite well. I would't stick a non-functional diffuser under my car.

Generally the car is producing lots of lift. All the add-ons (splitters, spoliers and diffusers) are there to reduce the amount of lift produced. Less lift implies that the car has more weight planted on the tarmac which equates to greater traction and hence a quicker laptime. I'm sure I've posted this before but it's hard to know what impact a diffuser has without testing. As you say, the aim is to reduce the amount of air under the car - hence the front spoliers and splitters.

Now if someone has access to a wind tunnel we could get the answer.

I dont think the VX produces masses of rear end lift.

I always thought the diffuser aided lateral cornering stability not downforce

I'm not sure one has ever been in a wind tunnel, although JT did extensive trackwork to test the wing/diffuser package from TMS

[Jesse]

The Speedster/VX220 Turbo was testet in a windtunnel by the german sportauto magazin. Downforce Front closed softtop: 221 Nm Downforce Rear closed softtop:- 186 Nm Downforce Front open: 254 Nm Downforce Rear open: -142 Nm all at 200Km/h

[Code Monkey]

So the best option, rather than putting a wing on the back it to take off the roof, and enjoy the sunshine.

[paulf-cam]

So the best option, rather than putting a wing on the back it to take off the roof, and enjoy the sunshine.

Not according to those figures it isn't!

Downforce Rear closed softtop:- 186 Nm
Downforce Rear open: -142 Nm

More downforce with soft-top on

[speedyK]

So the best option, rather than putting a wing on the back it to take off the roof, and enjoy the sunshine.

Not according to those figures it isn't!

Downforce Rear closed softtop:- 186 Nm
Downforce Rear open: -142 Nm

More downforce with soft-top on

No, you've misread it.

The figures are: rear downforce closed -186Nm (that's minus 186NM, i.e. 186Nm of lift ), which is reduced to -142NM of lift with the roof off.

But, of course, the drag increases when open, thus reducing topspeed.

The figures (in weight) for the VXT closed (again from Sport Auto measured at 200kmh) are:

Front 23kg downforce
Rear 19kg lift

In comparison, for the Exige S2, they measured:

Front 18kg downforce

Rear 26kg downforce – i.e. 45kg more downforce than the VXT

Edited by speedyK, 21 January 2007 - 03:38 PM.

[paulf-cam]

So the best option, rather than putting a wing on the back it to take off the roof, and enjoy the sunshine.

Not according to those figures it isn't!

Downforce Rear closed softtop:- 186 Nm
Downforce Rear open: -142 Nm

More downforce with soft-top on

No, you've misread it.

The figures are: rear downforce closed -186Nm (that's minus 186NM, i.e. 186Nm of lift ), which is reduced to -142NM of lift with the roof off.

Oh right, doh! Thats a pretty bad way to indicate data though, ought to have read:

Lift rear closed: 186Nm

Similar kind of thing to saying "i was travelling down the road backwards doing -30mph"

[Winstar]

I dont think the VX produces masses of rear end lift.

I always thought the diffuser aided lateral cornering stability not downforce

I'm not sure one has ever been in a wind tunnel, although JT did extensive trackwork to test the wing/diffuser package from TMS

A diffuser creates down force on a car by creating an area of low pressure under the car effectivly sucking the car to the floor.

[paulk]

So does anyone produce a diffuser that will produce any downforce (and can be proved to do so) and still look marginally attractive? I really like the idea of a diffuser as generally they are capable of producing free downforce ie without the associated drag that wings tend to produce By the way thanks to everyone who has contributed so far - it's good to have so much aero information in one thread P

[subseamac]

has that thread been continued anywhere? I recently posted some ideas on redhairsucks "exige engine cover" thread about incorporating vortex generators at the trailing edge of the roof bar cover as well as VG's under the car roughly at the front axle to work with a rear diffuser

[robfenn]

I've never had a look, has the VX got a flat floor?

[speedyK]

I've never had a look, has the VX got a flat floor?

Yes!

[c18gmr]

I second that

[subseamac]

I hope nobody got hurt as a result of these although that seems unlikely given the damage! To get back to the subject of aerodynamics, you can see where the flat panel with the 2 naca ducts under the engine is. Immediately after that the last panel (under the exhaust etc) continues at a fairly steep angle. There are two points where maximum downforce occurs: right at the front immediately under the nose and again at the edge of the flat engine undertary and the angled undertray. The ideal configuration is to (1) bring that last edge as far ahead of the rear axe as possible - we can't do that, (2) ensure the angle of the existing diffuser (the last bit of undertary) is no more than 14 degrees or so (3) ensure the rear diffuser clears the back of the car sufficiently to reduce drag and in the event somebody is also using a wing, interact with the wing properly. That last point is something we can presumably work on, i.e repalce the existing diffuser with one that has the correct angle and would extend beyond the rear of the car. Another possibility is to install a couple of vortex generators roughly on the front wheel axis, ideally two long pieces approx 20-25mm tall and angled at approx 20 degrees in order to break the airflow attached to the bottom undertray and increase the efficiency of the rear diffuser. Ideally the cars should be 5-8cm to the road surface which is totally impractical for non-circuit use. It would be very interesting to produce a rear diffuser design that works with front vortex generators mounted on the undertary near the front axle, as well as vortex generators mounted on the trailing edge of the rollbar cover at the back of the roof. VG's could be made of plastic (you can buy them ready made for the latest Evos) while the diffuser wouldn't need to be made of anything too fancy (pressed aluminium) ideas?

[speedyK]

I hope nobody got hurt as a result of these although that seems unlikely given the damage!

Not much
>How NOT to do it<
>The end... and new beginning!<
– but enough to cause me to buy the Exige Cup

To get back to the subject of aerodynamics, () ideas?

Thorney has played with diffusers, as did the racing team a couple of years back – try a search for diffuser.

[subseamac]

crikey you don't do things by half ... Read most threads I could find on the site. I don't think there is one that goes through the aerodynamics of the car systematically. I've looked at windtunnel tests for the eco speedster that was altered to have half the drag of our VX's but that's a bit beyond our means (e.g. reduced frontal area massively by changing windscreen and dropping height of the car by 20cm, etc). there's also some windtunnel testing stuff for elises on the reverie site, but nothing else I can find.

[paulk]

Hey Subseamac I guess that was the kind of reply I was looking for originally. It can be that difficult to produce a 14degree upswept diffuser that actually works - just having having vertical vanes to guide the airflow I wouldn't imagine to be all that effective. I would imagine that bending and welding of ali int the kind of diffuser we have in mind might be pricey paul