Aero/Downforce expertise required

[phoenix]

At a recent test on a newly built GT type car we were very surprised by the amount of downforce apparently generated by the front splitter/body combination. Surprised mainly because it is a late 70's/early 80's design and has no ground effect or anything as sophisticated as that yet the amount of downforce that I have calculated was .55 G at 130 mph and, if my maths is right, it will be around 1.03 G at 180 mph. I can not post attachments, otherwise I would be able to demonstrate the downforce with a picture. The result of losing ground clearance was that a slight off onto the grass ripped the front splitter off and caused damage to both the front and rear bodywork. I need to calculate new spring rates to allow for the downforce at speed but I need to validate my finding somehow as I don't want to end up with a bigger collection of springs than Demon Thieves following an experimental route. Therefore I want to establish as accurately as I can the downforce being generated.

We are not in the position to afford wind tunnel testing to confirm this by measuring downforce at the contact patch. The downforce has been calculated from measurements of body clearance to the road at speed and static. I am not certain how to approximate the centre of pressure and apply any leverage values (two examples: distance of the splitter to the front axle line, or centre of pressure to the front axle line) to the calculations which, if they apply, could seriously affect the calculated values. As there are lots of areas I am not sure of which may make my calculations inaccurate I wondered if I could find any expert help and guidance here.

Please, if you can, HELP!

[GordonG]

How are you measuring the body/road clearance on the move? (maybe by ear - it's easy to tell when clearance becomes 0!).

A better measure woudl be suspension deflection, which you could get with an entry level data logger attached to potentiometers linked to the suspension. Though I've never used it to measure downforce, I've run Vauxhall throttle position sensors with simple linkages to measure suspension position, and they do seem to be sufficiently robust.

If you can measure how much the suspension is compressing at speed, you can then either calculate the downforce at the wheel, or more simply load up the car in the garage until you get the same reading and meaure how mugh weight it takes.

I'd be concerned measuring body position as body panels may deflect down with drag, for instance. Suspension movement will more accurately reflect the effect on the tyre contact patch, which is the most important thing in racing.

I run a sports racer style body but at a mandated 75mm ride height and with fairly stiff suspension, and an off on to the grass will usually cause damage even at that height - dont expect to be able to go off road without damage!

G

[trikesrule]

I use the seals on the shock shafts to measure suspension movement. This would probably work if you had smooth road to run upto speed on........trikes

[AU N EGL]

I run a 2.5" front splitter and 1" rear spoiler. Also have 750# front springs and 600# rear springs. My car with me is 3025 lbs

Sounds like you may not have enough rear downforce to conterbalance the front spiltter.

for anything larger the 2.5" splitter, as in a 4" splitter you will need to put a 72" wing ( or body width) on the back, side skirts for to keep the under car air stable in corners and extractor hood too allow air out of the front part of your car.

[phoenix]

Thanks for your input guys.

How do I post pics? It would be so much clearer.

The front splitter is (was!) about 2 inches deep (front to back). We have a full rear wing about 70 inches wide. The rear of the car was also down considerably, so although are unlikely to have achieved perfect aero balance, the pitch of the car was relatively unchanged from static.

Of course suspension pots and data logging are the way to go I am sure, but at the moment I am trying to get in the ball park. We felt sure that the design was for 'anti lift' front and rear, not for actual downforce, so we have been caught by surprise. If any of you guys have figures you have worked out for your cars I would appreciate and indication of what you have found. i.e. is the amount of downforce we are seeing relatively high? Or quite reasonable?

Our measurements are from trackside pictures. With 6 megapixel photos it is easy to blow up and measure - we have reference dimensions marked (like a ruler) on the body to work from. The downforce was calculated at a particular spot on the track with the splitter clearance at zero (wear marks to prove later!) in a head-on photo and the speed known on the section of the track from datalogging.

Point taken about body movement, however the splitter is stiff enough and braced to the chassis (triangulated) and so can only move a matter of a few millimetres.

Point also taken about going off the track - it was a test and the driver was under stict instructions not to overdo it - to avoid complication like we eneded up with. But, he decided he knew better! If he had listened to me and waited until we had adjusted the running height, having seen the pictures from the first session, he could have given it a bit more on the second outing, and even had he visited the grass then the end result may not have been as painful or expensive! Mind you, I had also asked him not to use more than 5000 rpm in the first session and he used 6000 on 3 occasions on his out lap alone! He denied it of course, but the data logger and the in car cam don't lie!

[AU N EGL]

are your photos on a wed site? if not use something like photobucket.com to host your pictures.

Rember dont make them too big or they dont load well.

Here is my car (500 pix x 260 pix ) Last winter testing

Speed almost 100 mph just prior to a heavy braking zone

[GORDON STREETER]

As trikes said about shock absorber shafts I have used cable tie's on them . Very simple but cheap and easy but effective. You can also get a good idea on how the air is moving about on your car by taping bits of wool on to different parts of the car and get a mate to drive beside you and video it .
You would be surprised what it shows you in slo mo . I wouldn't advocate doing this in rush hour traffic , best on a track day. You dont need to drive that fast to see it work . Also probably a few thousand pounds cheaper than a wind tunnel !!

[AU N EGL]

Originally Posted by GORDON STREETER

As trikes said about shock absorber shafts I have used cable tie's on them . Very simple but cheap and easy but effective. You can also get a good idea on how the air is moving about on your car by taping bits of wool on to different parts of the car and get a mate to drive beside you and video it .
You would be surprised what it shows you in slo mo . I wouldn't advocate doing this in rush hour traffic , best on a track day. You dont need to drive that fast to see it work . Also probably a few thousand pounds cheaper than a wind tunnel !!

Could you put the car in a garage and use big fans to blow air past the car??

[GORDON STREETER]

I have a massive old space heater rather like an industrial fan, that I use for cooling the racer in the workshop (just like a rolling road fan)
But I don't think it would give a very true picture of whats going on .
If you had a big enough space and a big enough fan you could make your own wind tunnel be it very crude !

[Locost47]

If you've got approximate front and rear downforce loadings from your suspension/body measurements then you can work backwards to calculate the centre of pressure in the lengthwise axis of the car. You can't narrow it down in spanwise or vertical directions without additional and much more complicated measurements, but they don't matter a great deal anyway.

To calculate the centre of pressure from the data you have taken:

LF= (L/2) + (My/WB)
LR= (L/2) - (My/WB)

where LF is the apparent front lift or downforce, LR is the rear, L is the overall, My is the overall pitching moment about the centre of wheelbase and WB is the wheelbase length. The + & - are interchangeable depending on the directions you take as positive for each of the forces & moments

Now L = LF + LR, so you can put your measured numbers into the formula to get L and then work out My by re-arranging the formulae above.

Then,
XCoP = 100*My / (L x WB)

Where XCoP is the position of the centre of pressure due to lift along the longitudinal axis of the car, given in terms of a distance forward of the centre of wheelbase and expressed as a percentage of the wheelbase length itself.

I may have got the signs wrong somewhere in here (the coffee hasn't sunk in yet this morning) but as long as you're consistent in the directions you choose to be positive and the units you use, then it should all make sense.

Hope that helps.

[Notso Swift]

Originally Posted by GORDON STREETER

As trikes said about shock absorber shafts I have used cable tie's on them . Very simple but cheap and easy but effective.

That only shows the limit of compression, hit a ripple strip at speed and that will be the figure you get.

[phoenix]

Originally Posted by Locost47

If you've got approximate front and rear downforce loadings from your suspension/body measurements then you can work backwards to calculate the centre of pressure in the lengthwise axis of the car. You can't narrow it down in spanwise or vertical directions without additional and much more complicated measurements, but they don't matter a great deal anyway.

To calculate the centre of pressure from the data you have taken:

LF= (L/2) + (My/WB)
LR= (L/2) - (My/WB)

where LF is the apparent front lift or downforce, LR is the rear, L is the overall, My is the overall pitching moment about the centre of wheelbase and WB is the wheelbase length. The + & - are interchangeable depending on the directions you take as positive for each of the forces & moments

Now L = LF + LR, so you can put your measured numbers into the formula to get L and then work out My by re-arranging the formulae above.

Then,
XCoP = 100*My / (L x WB)

Where XCoP is the position of the centre of pressure due to lift along the longitudinal axis of the car, given in terms of a distance forward of the centre of wheelbase and expressed as a percentage of the wheelbase length itself.

I may have got the signs wrong somewhere in here (the coffee hasn't sunk in yet this morning) but as long as you're consistent in the directions you choose to be positive and the units you use, then it should all make sense.

Hope that helps.

Fantastic - thanks!

[AU N EGL]

Bolting on aero parts is relativly easy. Fine tuning these parts to increase aerodynamics is takes a lot of work.


Here is some basic aero terminology: http://aerodyn.org/Annexes/Racing/hlifts.html


Physics of Racing: http://www.miata.net/sport/Physics/index.html