Undercar Aerodynamics

[ChawenHalo]

brilliant car's perfectly stabble at 150 MPH though, i can assure you. front end's way less gittery and steering less light in feel than any car I've driven at those speeds. The fast ratio of the steering rack on the coupe is more of a worry in long hight speed corners, slight direction changes and cambers. Needs calm steady imputs...

 

[exdos]

brilliant car's perfectly stabble at 150 MPH though, i can assure you. front end's way less gittery and steering less light in feel than any car I've driven at those speeds. The fast ratio of the steering rack on the coupe is more of a worry in long hight speed corners, slight direction changes and cambers. Needs calm steady imputs...

Thanks.

With the AC Schnitzer Racing suspension and other suspension mods on my Z4MC, I've driven at 155mph and found the car was very stable and a great improvement on the OEM suspension. BUT, it still doesn't handle as well as my modded Z3MC which remains "flat" around corners, as below.



Whereas my Z4MC still has a tendency to lateral bodyroll with all its suspension mods, as below.



My Z3MC has AC Schnitzer Front splitters and roof wing plus various other under car aerodynamic parts that I've made, including a rear diffuser. So it strikes me that the difference in handling is now largely due to the aero parts on the Z3MC. In my quest to get my Z4MC to handle as well as does my Z3MC, I've started with the underside of the car first. The look of Z3MC is actually improved with the fitting of the AC Schnitzer parts, whereas I think that the OEM Z4MC is a thing of real beauty and that sticking anything onto the painted upper bodywork will spoil its appearance, and so I'm loathe to do it, if I can avoid it. I want to properly assess the "state of play" before I add a splitter and wing. I find it really interesting doing the research and trialling ideas so I expect that this will be a work in progress for sometime yet.

 

[jimmybell]

How does the ride height (and therefore centre of mass) compare between the two Exdos?

I say that because the Z4 looks higher in the pics above (maybe that's just me...), but could potentially mean more body roll forces working against the suspension. Just a stab in the dark. Equally if you're comparing them directly - what's the difference in suspension set-ups?

 

[exdos]

How does the ride height (and therefore centre of mass) compare between the two Exdos?

I say that because the Z4 looks higher in the pics above (maybe that's just me...), but could potentially mean more body roll forces working against the suspension. Just a stab in the dark. Equally if you're comparing them directly - what's the difference in suspension set-ups?

Here's a photo of the 2 cars side by side. I have them both as low as I dare for practicality and I've settled on the heights with measurements rather than guesswork. As you can see the roof of the Z4MC is lower than the Z3MC by about 2 inches, which suggests that the centre of mass is lower.



The Z3MC has KWV3 with independent adjustment for bump and rebound all round, which is a front coilover set up. The Z4MC has AC Schnitzer Racing suspension which has progressive springs and combined bump/rebound adjustable all round. Both have uprated ARBs (the Z4MC being much bigger than the Z3MC) both have 10mm spacers all round, both have Strut braces. As you can see, at the time this photo was taken, I'd already done aero mods on the underside of the Z3MC as well as the ACS roof wing and front splitters.

 

[exdos]

I've now had a chance to use my pressure washer to see how the aero parts I've made might function when driven.

These photos show the way that a water spray which I directed longitudinally and adjacent to the inside of the front wheel exits from behind the wheel and forms a vortex as it travels away from the rear of the wheel. This follows the published information that a wheel in motion produces 2 "jetting vortices": i.e. one from air passing the inside of the wheel and one from the outside. It is clear that my front wheel deflector does, in deed, deflect the inner jetting vortex, as I intended it to do. However, I don't know what will happen when the deflected inner vortex meets the outer jetting vortex. The fan-like wet patch to the side of the car suggests that the combined jetting vortices will take air from under the car, thus producing low pressure under the car as hoped.









When I directed the pressure jet at an angle of around 45 degrees under the cill at about 50cms in front of the rear wheel, because of my deflector inside and in front of the rear wheel and the vertical diffuser strakes attached to the silencers, the spray exiting the rear of the car is restricted to flowing besides the inside of the rear wheel and under the adjacent silencer, only. Therefore,if the front wheel deflectors cannot prevent the re-entry of air under the cills before the rear wheel, then the rear deflector and rear diffuser strakes, will ensure that the air-flow tavelling the length of the car, longitudinally, from front to rear, will be minimally affected, thus reducing the tendency to turbulence beneath the car. As such, the air travelling under the car should be as least turbulent and the fastest possible. It's interesting to see that the spray exiting the rear doesn't seem to rise higher than the lip of the bumper. If my rear wheel deflector and diffuser had not been present then water sprayed under the car from the same position would exit from the rear of the car across the full width of the bumper, and not restricted as it is now.







This photo shows the way that the air travelling over the upper surface at the rear of the car.

 

[ChawenHalo]

I know what you mean, the Z4MC does sometimes feel as its "hanging" on its tyres and even on its stiffest settings there is some roll before the tyres start to screech and that rear end is working very hard in off power / lift off. From memory you're on PS2's? Do you not have lower profile rubber on your Z3? Who runs Michelin SS or track focused rubber on this forum? How do they differ in feel apart from grip?

I've tried a couple of single sweaters on track and it would be great to get some of that "sucked down" feeling. Just came back from a 850km round trip across the Pyrenees to La Rioja- long 120 kph (speed limit) corners up and down hill and it can be quite unerving Also I keep seeing the beautiful flat under car of Caymans and 997's at my locals and it makes me jealous!

 

[exdos]

I know what you mean, the Z4MC does sometimes feel as its "hanging" on its tyres and even on its stiffest settings there is some roll before the tyres start to screech and that rear end is working very hard in off power / lift off. From memory you're on PS2's? Do you not have lower profile rubber on your Z3? Who runs Michelin SS or track focused rubber on this forum? How do they differ in feel apart from grip?

I've tried a couple of single sweaters on track and it would be great to get some of that "sucked down" feeling. Just came back from a 850km round trip across the Pyrenees to La Rioja- long 120 kph (speed limit) corners up and down hill and it can be quite unerving Also I keep seeing the beautiful flat under car of Caymans and 997's at my locals and it makes me jealous!

On my Z3MC I'm running 18" wheels with 285/35 rears whereas OEM is 245/40/17, so the actual profile on those wheels is greater than OEM. In the photo of my Z4MC on track, the car was on the OEM Contis, and despite the critical comments of others on here, I really couldn't fault the tyres; grip wasn't a problem and it didn't feel to me that the car was rolling over the tyre walls during cornering, because the slip of the tyres was very predictable. I'm now on Michelin PSS, (265 rear, 235 front) but not been on track with them yet.

I find that if a car has too much bodyroll that the feedback I receive from the movement of the car is detected by the movement of my shoulders, whereas if the car remains flat with lateral loading, I detect this through my hips, which gives me far superior spatial awareness/control, so that's what I'm wanting from the handling of the car.

 

[exdos]

UPDATE

I took the car to The Ring last week with the aeroparts shown earlier in this thread. I'm still here, so at least there was no positive lift and the car didn't overturn! My subjective opinion of the combined effect of these parts was that drag seems reduced somewhat, in that when lifting off, the car doesn't seem to slow down quite so fast and it certainly didn't slow the acceleration. I'd recently fitted Michelin Pilot Super Sports all round, whereas, previously, I'd been running the OEM Contis, and I have to say that I agree with the opinions of others that these are an excellent tyre and definitely better than the Contis. I found that the car took the high speed corners (such as, Schwedenkreuz, Fuchsrohre and Metzgefeld) better than it had done previously without the aeroparts and that the Michelin PSSs gave the grip. If using the MPG is anything to go by, I found that my MPG on the 1,000 round trip was about 1mpg better than last year which suggests less drag, and my MPG around the Ring was 1.5mpg less (used more fuel), which suggests that I was able to push harder from the effect of better grip at higher speeds, some of which I expect was gained by the aerodynamic effects. On the last past of the return journey home along the motorway, it was raining and I took particular note of the spray behind the car through the wing and rear-view mirrors, and there seemed to be minimal spray behind the car which suggests that the pressure of air leaving the underside was around ambient pressure with reduced turbulence in the wake, which is what I would hope for.

I really should use R compound tyres on the car, when at The Ring which would take the performance to another level, and I certainly would if I could transport a spare set inside the car, but with a car like the Z4MC this is impossible, and since I have to drive considerably more road miles than track miles, it seems a waste of money to use R Compound tyres for mainly road use on the full trip to The Ring and back. Then again, my interest is to try to make my Z4MC as competent a road car as I can, which will perform very well on track with a few mods, rather than a track-focused car.

If anyone would like to lend/rent or sell cheap a full set of R Compound tyres on rims for next year, I'd be very interested.

 

[ChawenHalo]

UPDATE

I took the car to The Ring last week with the aeroparts shown earlier in this thread. I'm still here, so at least there was no positive lift and the car didn't overturn! My subjective opinion of the combined effect of these parts was that drag seems reduced somewhat, in that when lifting off, the car doesn't seem to slow down quite so fast and it certainly didn't slow the acceleration. I'd recently fitted Michelin Pilot Super Sports all round, whereas, previously, I'd been running the OEM Contis, and I have to say that I agree with the opinions of others that these are an excellent tyre and definitely better than the Contis. I found that the car took the high speed corners (such as, Schwedenkreuz, Fuchsrohre and Metzgefeld) better than it had done previously without the aeroparts and that the Michelin PSSs gave the grip. If using the MPG is anything to go by, I found that my MPG on the 1,000 round trip was about 1mpg better than last year which suggests less drag, and my MPG around the Ring was 1.5mpg less (used more fuel), which suggests that I was able to push harder from the effect of better grip at higher speeds, some of which I expect was gained by the aerodynamic effects. On the last past of the return journey home along the motorway, it was raining and I took particular note of the spray behind the car through the wing and rear-view mirrors, and there seemed to be minimal spray behind the car which suggests that the pressure of air leaving the underside was around ambient pressure with reduced turbulence in the wake, which is what I would hope for.

I really should use R compound tyres on the car, when at The Ring which would take the performance to another level, and I certainly would if I could transport a spare set inside the car, but with a car like the Z4MC this is impossible, and since I have to drive considerably more road miles than track miles, it seems a waste of money to use R Compound tyres for mainly road use on the full trip to The Ring and back. Then again, my interest is to try to make my Z4MC as competent a road car as I can, which will perform very well on track with a few mods, rather than a track-focused car.

If anyone would like to lend/rent or sell cheap a full set of R Compound tyres on rims for next year, I'd be very interested.

Nice one, that's really interesting. One question: if the undercar aero is really efficient would not the spray coming out the back be higher and longer? If the aero is working does that not mean a lower pressure at the back of the car (sucking it down) and therefore "aspirating"? Or has the work you've done just to reduce drag coef?

 

[exdos]

Nice one, that's really interesting. One question: if the undercar aero is really efficient would not the spray coming out the back be higher and longer? If the aero is working does that not mean a lower pressure at the back of the car (sucking it down) and therefore "aspirating"? Or has the work you've done just to reduce drag coef?

If the under car aero is really efficient (i.e. reduced drag), then the air exiting the rear of the car will be at ambient pressure with minimal turbulence in the wake and the lack of much spray behind the car suggests that this appears to be the case. The lowest pressure of the air travelling under the car will be at the front, where the aperture is smallest, because the air will be travelling fastest at this point. As the space increases towards the back of the car the air will slow down and will eventually revert to ambient pressure at the back.

In order to produce low pressure under the whole of the car (i.e. to prevent aerodynamic lift or get downforce at the rear of the car) I will need to work on the aerodynamics on the upper surface of the car as in the third diagram below. The objective would be to make the air travelling under the car take a much longer path before it reunites with the air passing over the car, so that the air passing on the underside must move faster, thus at lower than ambient pressure, so that the whole of the car acts like an inverted wing. To do this, the air leaving the boot lid must be made to kick upwards with a spoiler or a wing.






As it happens, I've just done some simple experiments with various configurations of metal strips to act like wicker-bills (Gurney Flaps) like the OEM lip spoiler to see how this might work.


The photo below shows the spray pattern over a flat plate:





The photo below shows the spray pattern over a flat plate with a 10mm strip at the end:





The photo below shows the spray pattern over a flat plate with a 25mm strip at the end:





The photo below shows the spray pattern over a flat plate with a 70mm strip at the end:




From the photos above, it would seem that a wicker-bill stuck at the end of the Z4MC's boot lid, which is between 10mm to 25mm would create the desired airflow to make the airflow under the car travel further to reduce aerodynamic lift or produce downforce.

I thought that I would see how a rear wing (something like the AC Schnitzer design) might work, so I placed the same metal strips used as wicker-bills in the above photos, but this time, I placed them 40mm above the horizontal flat plate.


The photo below shows the spray pattern over a flat plate with a 25mm strip raised by 40mm:





The photo below shows the spray pattern over a flat plate with a 70mm strip raised by 40mm:





It appears to me that a wicker-bill type of lip spoiler (like the BMW OEM lip spoiler) functions more effectively than a raised wing, and that a wicker-bill will also produce less drag.

I think that with my undercar aero parts and a boot lip spoiler, the car would produce a "rooster tail" spray pattern in the wet, which would then indicate that I would have reduction in aerodynamic lift/ downforce with a minimal drag penalty.

 

[Sae]

Good reading as always, just as a reference, what would you estimate the speed of the water from the jet wash?

 

[exdos]

just as a reference, what would you estimate the speed of the water from the jet wash?

The speed of the water immediately exiting the nozzle will be very high (I've seen it suggested it's in the order of 225mph or so), but as the water fans out the further it goes from the nozzle, both the speed and pressure will diminish considerably. All I'm really trying to do is replicate something akin to a smoke trail in a wind tunnel and I hold the nozzle in the best position to demonstrate airflow at a reasonable motoring speed. Moving the nozzle closer, or further, to the wicker-bills I was testing (i.e. changing the speed of the airflow) doesn't seem to change the pattern of flow by much, unless the nozzle is extremely close to the wicker-bill, so I assume that the test is a reasonable demonstration.

 

[ChawenHalo]

Nice one, that's really interesting. One question: if the undercar aero is really efficient would not the spray coming out the back be higher and longer? If the aero is working does that not mean a lower pressure at the back of the car (sucking it down) and therefore "aspirating"? Or has the work you've done just to reduce drag coef?

If the under car aero is really efficient (i.e. reduced drag), then the air exiting the rear of the car will be at ambient pressure with minimal turbulence in the wake and the lack of much spray behind the car suggests that this appears to be the case. The lowest pressure of the air travelling under the car will be at the front, where the aperture is smallest, because the air will be travelling fastest at this point. As the space increases towards the back of the car the air will slow down and will eventually revert to ambient pressure at the back.

In order to produce low pressure under the whole of the car (i.e. to prevent aerodynamic lift or get downforce at the rear of the car) I will need to work on the aerodynamics on the upper surface of the car as in the third diagram below. The objective would be to make the air travelling under the car take a much longer path before it reunites with the air passing over the car, so that the air passing on the underside must move faster, thus at lower than ambient pressure, so that the whole of the car acts like an inverted wing. To do this, the air leaving the boot lid must be made to kick upwards with a spoiler or a wing.

Ah OK so you're in effect making the airflow smoother from the front to the rear reducing drag. As regards to downforce, my suspension was set up to make the front lower than the cack (which is already the case OEM, to widen the gap between the rear underside to increase the ACS diffuser efficiency with a low pressure (suction). The lower front of the car effectively restricts the air flow under the car.

 

[exdos]

Ah OK so you're in effect making the airflow smoother from the front to the rear reducing drag. As regards to downforce, my suspension was set up to make the front lower than the cack (which is already the case OEM, to widen the gap between the rear underside to increase the ACS diffuser efficiency with a low pressure (suction). The lower front of the car effectively restricts the air flow under the car.

That's right!