Under Tray?

[[email protected]]

Hi

 

How much does the under tray actually do? I understand the principle of it, surely it only has an effect at high speeds?

 

Cheers

[Steve J]

If you drive fast enough into a lake you can skip across it like a stone

[chorton_1]

Hi

 

How much does the under tray actually do?  I understand the principle of it, surely it only has an effect at high speeds?

 

Cheers

<{POST_SNAPBACK}>

 

 

How much it does depends on who you talk to. I don't think it is at all potent, due to the ride height being quite large.

 

Either way, you do need to be going briskly (e.g. at the top end and beyond of legal road speed).

[Ladders]

I dont think it does much for ground affects - apparently you would need edges running down each side and a lower ride height to produce this.

 

The rear diffuser does have an effect as your not reccomended to drive without it as the rear can produce lift without it at speed.

 

Here's some of the Lift figures as measured by Caic off SELOC which you might find interesting!

 

S1

Config Drag  Front Lift  Rear Lift

#1      0.455  0.057      -0.051

#2      0.438  0.074      -0.051

#3      0.436  0.089      -0.057

#4      0.401  0.085      -0.059

#5      0.452  0.044      -0.053

#6      0.456  0.038      -0.087

#7      0.460  0.001      -0.058

#8      0.473  0.045      -0.127

#9      0.472  -0.016      -0.090

 

(negative lift co-eff indicates downforce)

 

where configurations are:

#1 = No roof

#2 = Soft top fitted

#3 = Hard top fitted

#4 = Side windows fitted

#5 = part blanked radiator inlet

#6 = underbody side skirts

#7 = increased chassis rake

#8 = rear gurney flap on spoiler

#9 = front splitter

AFAIK each of the configurations were measured independently,

i.e. #3 is hard top no side windows, #5 is blanked inlet with

no roof or windows, etc.

 

 

As you can see HT fitted creates more lift at front, but not much more DF at rear.

My car will now see 35mm sideskirts and a special wingdiffuser. I've given up the front gurney as it adds to much drag (but also a lot of DF). The car is to light at rear with the front gurney. Then it'll have the Swedish wing to suck air at the rear and a front flat splitter made of plywood. I choosen to try a wing as it's easier to adjust than using a rear gurney.

[Steve J]

Dave, what's the lift measured in?

[Ladders]

You mean as in what Units? I'm not sure to be honest Steve, might be worth PMing Caic about it?

[Steve J]

Hopefully not grammes

[Ladders]

Inches I should think!

[dobbo]

Surely speed is relevant to lift figures ??

 

Also, not sure how long the rear clam would last if you drove it fast without the rear diffuser.

[chorton_1]

All those numbers are coefficients, i.e. unitless. The right two columns are coefficients of lift for the front and rear, and the first column is coefficient of drag.

 

Lift = Coeff Lift x Air Density X Velocity Squared

 

Drag = Coeff Drag x Air Density x Velocity Squared x Frontal Area

 

And don't ask me to do the math.....its Fri afternoon FFS!!

[Steve J]

Very interesting, but still doesn't give a real world measure.

 

If it was measured at say 60mph, and gave the downforce in kgs then that would mean much more to us laymen

[chorton_1]

I knew that was going to be the next reply......

 

I'll try to put a spreadsheet together.

[Steve J]

Oh and for an S2 please

[chorton_1]

Right then, here is a first (rush) draft.

 

You can change the speed in mph to see the effective change. I'm pretty happy with the numbers; the Exige was advertised as having 80kg of downforce at 100mph, so the numbers in the spreadsheet are in the ballpark (given that the Exige has a proper rear wing and a lower ride height).

Elise_Lift.xls

[chorton_1]

PS The Series 2 numbers aren't included, as I believe the lift numbers aren't available in kg, only shirts. Rest assured though that they register very high on the 'shirt-lifter' scale.... :)