35 replies to this topic

[cyberface]

I'm not an engineer but what's with the lift / downforce being measured in Nm (newton metres) ???? Surely lift (for the exact shape of the car) will be purely newtons i.e. a force in the upward / downward direction? The Lotus boys point out that the VX has positive downforce at the front and lift (negative downforce) at the rear, however the VXT in particular has more mass at the rear, giving greater mechanical downforce. Since the Exiges etc. are faster on track (supercharged Exige == VXT weight-wise) then the aero must make a difference. So a clever reduction in drag along with neutral or positive downforce at the back would make sense... Then again I like the mobile back end of my VXT Can a proper engineer here tell me why newton-metres are being used for lift / downforce? Just sounds wrong to me.

[Baron Von Scubadaddy]

I 'm having tat make me a diffuser if any one wants to put my car in a wind tunnel I have no objections

[andyroo]

I thought the problem with rear downforce on the VX was the step down for the rear windscreen? What is the Elise rear downforce like? If it is similar to the VX, then it goes to show that the Exige performs better because of the wing and the smooth bootlid/roof transition. An Exige style bootlid would look so on a VX

[subseamac]

yes an exige type cover would improve aerodynamics but causes problems with heat extraction unless you start fiddling with removing the boot? What I've been thinking about are 20mm delta wing shaped vortex generators at the edge of the roof bar that will reduce drag and create more airflow nearer to the top of the engine cover - you win on both counts: less drag and more cooling. Only issue is that I think they look a bit naf. do a google on mitsubishi vortex generators and you'll see what I mean, like a series of 10 or so delta shaped 20mm tall plastic pieces sat on the roofbar (looks a bit geeky..)

[AndyVXT]

yes an exige type cover would improve aerodynamics but causes problems with heat extraction unless you start fiddling with removing the boot? What I've been thinking about are 20mm delta wing shaped vortex generators at the edge of the roof bar that will reduce drag and create more airflow nearer to the top of the engine cover - you win on both counts: less drag and more cooling. Only issue is that I think they look a bit naf.

do a google on mitsubishi vortex generators and you'll see what I mean, like a series of 10 or so delta shaped 20mm tall plastic pieces sat on the roofbar (looks a bit geeky..)

Well if you are feeling brave enough to mess with your car there are some on flea-bay. Stick on jobbies that can be attached to any car as required. Not sure how much these would actualy help reduce the lift though as they work by trying to maintain the boundry layer attached to the rear screen etc of a saloon as it curves down from the roof. The VX has a vertical rear screen and hence there is nothing for the boundry layer to actually be attached to. I guess there may be drag reduction benefits from better flow control as the flow seperates from the roof. The pdf link from the ebay seller to a mitsubishi tech document has all the interesting science. Really all of this is just interesting discussion unless someone can get access to a wind tunnel or writes some pretty swept up CFD.

[framauro]

I'm not an engineer but what's with the lift / downforce being measured in Nm (newton metres) ????


Surely lift (for the exact shape of the car) will be purely newtons i.e. a force in the upward / downward direction?
...
...
Can a proper engineer here tell me why newton-metres are being used for lift / downforce? Just sounds wrong to me.

Not a proper engineer, but a 'bit' of a physicist. Nm's are obviously a measure of torque, so presumably the downforce / lift is measured in Nm's to indicate the force of rotation about a lateral axis through the centre of mass of the vehicle.

This makes sense in that if the car were longer, and the exact same rear wing / aero were applied to a car with same mass further back (further away from CoM), then the torque forces would be greater. (Nm = Newtons x Metres = Force x Distance) And to get the same torque applied, a longer car would need a less drastic wing = less drag = higher max speed. And of course it does not necessarily need to be 'a wing', but any aero enhancement device.

Hence we see 'long tail' versions of cars such as the Eco Speedster and Porsche 962. LOOKY

[manus]

Hence we see 'long tail' versions of cars such as the Eco Speedster and Porsche 962. LOOKY

Don't forget the long tail Lotus GT1  Lotus_GT1.jpg   124.37KB   1 downloads
(for sale at the moment in the Netherlands)

[mbes2]

Bump... some good reading from a old post...

[dw1]

Yes and some great pics of the car at unusual angles. My highly tuned arse dyno and sphincter valve registered a slight improvement in reduced lift when using the Jimsan diffuser at speeds above 100mph. A big, high rear wing with massively sticking out diffusers are about the only thing that would edge us towards positive downforce, but then on roads it is not to everyone's taste.

[techieboy]

Bump... some good reading from a old post...

So you've now discovered your new wing will do chuff all then?

[mbes2]

lol Im yet to test the new wing.... I know it wont do bugger all on normal road use, its on track if I will feel any difference.... Roll on next year... Ive got other mount ideas, if I need to make it higher... its all about playing ... this is why its fun...

[efrenlotus]

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

Do not understand this

¿ Someone I can specify (if you have come to publish these data in sport auto) the difference in downforce means going with a roof and go homeless ?

Do both in kg... I have no idea of ​​comparing forces in nm.

Here I have the original scan (sensing with the roof closed, but not specified):

[mbes2]

Or something like : (kinda) The low relative flat face of the only moderately luftwuderstandsbeiwert 1.61 from 0.38. the relevant air resistance index (c xA) amounts to 0.61. At the front axle, which is charged with static kilogram legiglich 349, there is at 200 km / h and a closed roof skin a healthy output con least 221 newton roof astonishing LIMITATION MAY increased to 254 newton. the drive to the rear axle is reduced in the open state also underscore won 186 to 142 newton

[efrenlotus]

Applying a simple rule of thumb that data to me I get this: Downforce Front closed softtop: +23kg Downforce Rear closed softtop: -19kg Downforce Front open: +19,6kg Downforce Rear open: -23,5kg Right? Or am I totally wrong?

[Nev]

Wow, firstly, this is the first genuine metrics I've ever read on this matter, thanks for resurecting the thread Mark. Secondly, the lift/downforces seem fairly low IMO, though clearly they will increase at roughly a square factor to the speed. Thirdly, NM is a unit of force (torque is force BTW), hence it's use to measure lift/downforce. Fourthly, and most surprising to me (as I percieved the opposite when driving at high speed), the lift seems to come at the rear of the car whereas I thought the front of the car seemed light at high speed. Clearly my bum-o-meter was wrong !

Edited by Nev, 16 December 2011 - 08:41 AM.

[dw1]

Nev, you were probably travelling at light speed in your beast so no wonder the from felt light.