Internal Body Aerodynamics

[vette427sbc]

Thanks, guys! Working on, disassembling and modifying these types of cars has really helped with the design and engineering of my Vette and other classics cars/ hot rods that we build here. It’s a great learning experience and keeps us up to date with new materials, techniques, etc...

The jet ground plane is interesting. I wonder if that phenomenon is the intention of those downward vents? They aren’t just on these performance cars, I’ve noticed them on a lot of other cars in here too.

Another trend I’ve noticed is that only Toyota/Lexus takes advantage of vortex generators. They are all over their cars. Multiple on the taillights, and very often on the side view mirrors (to prevent wind buffeting with the window open I assume). Haven’t noticed them on Tesla’s or other hyper mileage type cars...

 

[phantomjock]

Took a bit to find the search term, but this works: jet ground plane in cross flow.

Very Interesting - and I'd bet that image is for a "Hover" jet? Any link for a jet that has forward motion? I recall from designing a hovercraft (Uni-days) that the tough problem we faced was sustaining the jet underneath while moving at speed. To keep the air bubble underneath the hovercraft (including skirts et al) we needed to have a lot of air to support at speed - more than at rest.

Clearly what we'd want here is to provide is negative lift under the chassis - to make downforce and this might be a way move the air out from underneath. The pics Chris shared show some interesting ways to do that. Thanks!

This seems to be an area to work with out a lot of external "stuff" - splitters skirts, wings, etc. (but they look so cool).

Cheers - Jim

 

[rtj]

It is for jet in hover with crossflow. You can see they drew the crossflow velocity with the boundary layer type profile ( zero at the ground). Not exactly the same as the car, but analogous. Also, velocity ratio for the car will probably be closer to 1.

But, they made them (extra cost) so there must be a benefit, like forcing air to go around the tire. Just guessing.

 

[COLE SWEAT]

Make new rocker moldings out of thin aluminum which are 1 to 2 inches longer. This will act as side skirts keeping flow out from underneath. Lower is always better for side skirts.

Creative use of black paint could disguise how low they are.

Also, the spare tire area is a mess, a diffuser would be too obvious for your goal, at least a smooth under belly would help.

We're on similar wavelengths here. Several years ago I pulled the stock rocker panels off and made 2" lower/wider replacements with Lexan sheet, and painted them semi-gloss black. I like the looks better, and so far no one has seemed to notice the change.

I had an aluminum bellypan on the car for a year or two, but my unscientific perception of the setup convinced me that the weight (about 30#) was hurting me more than any actual aero improvement was helping.

I've also looked into doing something at the rear, but so far I can't justify the weight of a diffuser. My current direction is to reduce the slight understeer I have, and ignore the rear until it becomes the limiting factor during cornering.

Are you running a rear sway bar?

 

[69427]

Make new rocker moldings out of thin aluminum which are 1 to 2 inches longer. This will act as side skirts keeping flow out from underneath. Lower is always better for side skirts.

Creative use of black paint could disguise how low they are.

Also, the spare tire area is a mess, a diffuser would be too obvious for your goal, at least a smooth under belly would help.

We're on similar wavelengths here. Several years ago I pulled the stock rocker panels off and made 2" lower/wider replacements with Lexan sheet, and painted them semi-gloss black. I like the looks better, and so far no one has seemed to notice the change.

I had an aluminum bellypan on the car for a year or two, but my unscientific perception of the setup convinced me that the weight (about 30#) was hurting me more than any actual aero improvement was helping.

I've also looked into doing something at the rear, but so far I can't justify the weight of a diffuser. My current direction is to reduce the slight understeer I have, and ignore the rear until it becomes the limiting factor during cornering.

Are you running a rear sway bar?

No. I'm running the '84 Z51 rear spring, so I've got plenty of rear roll stiffness. I've been focusing my efforts at changing the front suspension geometry to improve the front grip.

 

[rtj]

6 yrs old.

[ame]https://www.youtube.com/watch?v=J7Yb_6wW0uk&t=35s[/ame]

 

[rtj]

[ame]https://www.youtube.com/watch?v=s8dp9m6c_ro[/ame]

 

[COLE SWEAT]

RTJ
This has nothing to do with what your talking about, but
Have you put the body back on your chassis? I did the trailing arm adjusters that eliminates the shims and now the that it’s done I’m wondering if the body is going to fit?
Do you know if it does?

 

[rtj]

RTJ
This has nothing to do with what your talking about, but
Have you put the body back on your chassis? I did the trailing arm adjusters that eliminates the shims and now the that it’s done I’m wondering if the body is going to fit?
Do you know if it does?

Should fit. I did the original frame with the body on. The second frame, got sold. I retired early, then decided to go back to work. I sold some stuff preparing to move (the frame) and then stayed here. The frame I had bugged me a bit, because it really wasn’t good enough to spend a lot of hours on. So selling it was maybe a good thing. Start with a better one if I try again.

Start a thread on your upgrade. I’m interested to see.

Edit: the only interference is taking the bolts out or putting them in, I cut the fender flares.

 

[rtj]

Another big dollar super (hyper?) car, but I’ve never seen the canards (vortex generators on steroids) before. He says they give big drop in drag.

[ame]https://www.youtube.com/watch?v=gkmjOR55DwU[/ame]

 

[rtj]

[ame]https://www.youtube.com/watch?v=5PoDNZFK-5M[/ame]

 

[rtj]

1 yr old video. Another big $$ car with lots of aero.

[ame]https://www.youtube.com/watch?v=kBu8oD69IZQ[/ame]

 

[rtj]

Tribute race car from Porsche. Very low production numbers. This is long, haven’t watched it all yet, but those mirrors are unique. Looks like it has lots of aero features.


[ame]https://www.youtube.com/watch?v=oAxR5qoeECM[/ame]

 

[rtj]

I like the rear diffuser.

[ame]https://www.youtube.com/watch?v=bpX3gA40IFI[/ame]

 

[vette427sbc]

Another McLaren 720s... with some notable bits and pieces compared to the last one...
Under car strakes that move air out from under the car to the side just after the front wheels are very pronounced. Probably around 1.5” tall.



The rear wing is active, and also an air brake I assume, judging by the massive hydraulic rams. The hydraulic feed lines are -4AN, compared to a typical Porsche 911 active spoiler with is -3 or even smaller (im referring to a 996 chassis, don’t know about the newer ones). My guess is to be able to change wing position quicker

 

[vette427sbc]

I thought the undercar strakes were interesting considering how large and efficient the rear diffuser looks. Is it a general rule that removing as much air out from underneath is best? Even when that air can be used to generate downforce from the diffuser? These cars also have the opposite of a typical rocker panel “skirt”. They are very rounded vs. flat, like the C7Z skirts and most other track oriented super cars

 

[phantomjock]

Is it a general rule that removing as much air out from underneath is best?

Yes. Think Chaparral-Sucker car. Lower pressure under body creates downforce.
​

Even when that air can be used to generate downforce from the diffuser?

A diffuser does generate downforce, generally aft of the rear wheels. The undesired-unintended consequence is stiffer springs to counteract the"lifting" of the front end due to a "fulcrum-like effect.

The "hottest" prototype cars (and F1) use tunnels for a similar effect as the strakes seen here. The tunnels help move the low pressure area more forward to get balance in downforce.​

These cars also have the opposite of a typical rocker panel “skirt”. They are very rounded vs. flat, like the C7Z skirts and most other track oriented super cars

The rounded skirts(I think) are being use to shape generated vortex sheet to reduce drag around the wheels/tires.
Hers is my best guess of what the streams might look like:



Yellow are teh streams - sorry takes too long to make curves of vortex! Red arrows downforce; wing, splitter, "rocker/skirts", and blue the very slippery angle of the front wind screen.

Hope this makes sense - can't say its CFD - just one guy's opinion/idea of what might be going on.

Cheers - Jim

BTW - I did run the SR with side skirts (not authorized - but if you ain't cheatin"...) and the car was very well balanced and like riding on rails. Removed them and have ran with splitters and a wing. Now have 2 tunnels cut into the side pods and they work well. Now in the rebuild am adding a spoiler in lieu of wing. We'll see how that all goes. Still considering strakes similar to the M720.

 

[BBShark]

Don't know anything about the aerodynamics but I like that car!

 

[rtj]

That large vertical looking “wing” behind the front wheels is probably responsible for the side skirt area (rocker area) being so different from the c6 or c7.

Jim is right, vortex dominated flow down the sides.

Well it speaks volumes about how much trust your customers have in your work. Big $$$$ there.

 

[vette427sbc]

Hope this makes sense - can't say its CFD - just one guy's opinion/idea of what might be going on.

Cheers - Jim

BTW - I did run the SR with side skirts (not authorized - but if you ain't cheatin"...) and the car was very well balanced and like riding on rails. Removed them and have ran with splitters and a wing. Now have 2 tunnels cut into the side pods and they work well. Now in the rebuild am adding a spoiler in lieu of wing. We'll see how that all goes. Still considering strakes similar to the M720.

Your CFD makes total sense! Thank you for the explanation.

That large vertical looking “wing” behind the front wheels is probably responsible for the side skirt area (rocker area) being so different from the c6 or c7.
Jim is right, vortex dominated flow down the sides.

Again, this makes much more sense :thumbs:

Well it speaks volumes about how much trust your customers have in your work. Big $$$$ there.

Don't know anything about the aerodynamics but I like that car!

These are pretty wild cars... I typically like “raw” cars; lots of hp and a well balanced chassis. Not much more. I think this up there with my favorites though! And I definitely get an interesting variety here. I think people find more trust in someone to work on their car when they see that I’m also an enthusiast and not just working a job to pay the bills. This McLaren had just under 400 miles on it :shocking:

 

[rtj]

Teaser vid.

[ame]https://www.youtube.com/watch?v=JttSo9WiiM8&app=desktop[/ame]

First half is aero related.
https://www.motor1.com/news/261384/mclaren-720s-rocket-science-video/

Details much easier to see with white paint.
[ame]https://www.youtube.com/watch?v=LemZQaRs6kU&app=desktop[/ame]

 

[phantomjock]

BTW - both are excellent videos- THX rtj!

Continuing with the underbody areo on the M720.

I sourced the following image showing the vanes and splitter on a 520 - similar underbody thinking.



Note the mini-diffuser built into the splitter in front of the tires. These can be very powerful. One outfit did some downforce testing trackside and found these splitters gave a 200% increase in downforce of the splitter. Nice.

Also note that the "tire diffuser" is located below the duct work -- heaps of downforce I'd bet.

Also note these vanes are made as a single piece and mounted. Similar on the 720? Makes the install easier - one set of fasteners and sets the relationship of one vane to the other. Based on testing-or just smart intuition?

Here is a paint splatter test result. Not all the air leaves - just enough...




There are several great ideas here that can apply to the C3s and keep it "subtle" at the same time.

Cheers - Jim

 

[rtj]

https://www.motor1.com/news/261384/mclaren-720s-rocket-science-video/

This guy claims to be a rocket scientist, but I suspect he may be an actor. Granted he does claim to be an aerodynamicist, but I would think a rocket scientist would not claim “laminar” flow over the front tires. It is a spinning disk! There is no way in hell that can be laminar in the true sense of the word (by definition).

I think they are creating big vortices to keep the flow somewhat attached and go where they want it. Sort of like the F1 cars we’ve been looking at.

Edit
In fact the cfd plots they throw on the screen briefly after he says that, shows the flow rolling up.

Bill Nye science guy maybe?

Yes this was bugging me since yesterday

 

[rtj]

Actually, you could steal a lot of those ideas for a crazy c3. Given a lot of time and ambition (I’m kind of low on those right now). But a set of inter coolers at the back for some rear turbos would not be very big and could grab that rear scoop flow.

 

[phantomjock]

Hey you talkin' bout me?

This guy claims to be a rocket scientist, but I suspect he may be an actor. Granted he does claim to be an aerodynamicist...

The underbody strakes are a real keeper, like the tire air curtains many posts back, - as is full belly pan. Flow directing with diffusers built into splitter, some cool suction or boost in the rear deck... and on, and on, an on it goes. So many mod possibilities -- so little time/money/ pick one.

Cheers - Jim

 

[rtj]

[ame]https://www.youtube.com/watch?v=car9JbNR2JA[/ame]


[ame]https://www.youtube.com/watch?v=CTjbgKWcP4Y[/ame]

 

[69427]

BTW - both are excellent videos- THX rtj!

Continuing with the underbody areo on the M720.

I sourced the following image showing the vanes and splitter on a 520 - similar underbody thinking.



Note the mini-diffuser built into the splitter in front of the tires. These can be very powerful. One outfit did some downforce testing trackside and found these splitters gave a 200% increase in downforce of the splitter. Nice.

Also note that the "tire diffuser" is located below the duct work -- heaps of downforce I'd bet.

Also note these vanes are made as a single piece and mounted. Similar on the 720? Makes the install easier - one set of fasteners and sets the relationship of one vane to the other. Based on testing-or just smart intuition?

...................


There are several great ideas here that can apply to the C3s and keep it "subtle" at the same time.

Cheers - Jim

I'm trying to figure out what's the purpose of some items I'm seeing there.

1) Do those small diffusers add any downforce, or are they just shaped that way to feed cooling air to the brake cooling scoop attached to the lower suspension link?

2) What does that J shaped piece do, other than perhaps direct air to the low pressure rearward side of the tire?

Just trying to understand what I'm seeing.

 

[phantomjock]

My guess/analysis-estimate, is the design and placement is to: (Front to Rear)

1. Add some flow/cooling (without adding any ducting) to the brake caliper (very clever vane they've added to the A-Arm - eh?)

2. Move air to the rear of the tire - and out the side (rear-facing) scoop - feeding energy down the side to the rear devices

3. Generate downforce at the area created by the "throat" formed between the left and right vanes. I understand the design was developed by an engineer experienced with CAD, CFD, and worked with engineering for NASA, and others. The flow-vis pattern shows a lot is on the underside of the car on the outside of the vanes. There is a low pressure area that is sucking the flow-vis up. I have read that this was run at 100 mph but have not seen any data from pressure taps. Would really like that!
​

The McLaren apparently has a lot of High Speed understeer at and above 100 mph. These vanes have been suggested to eliminate that understeer - adding more front wheel grip (downforce enabled). And, the results -- 2 seconds faster on a 2 mile course!

Downforce aft of the front axle is "better" balance than forward, just as forward of the rear is better than aft. Reducing pitching moments is a good thing.

Here is a good graphic you'll enjoy:


Regarding ride height. These mount with the same ground clearance as air curtains, so would get scrapped off at the same time as other underbody enhancements.

Hope that helps. (BTW - I really gotta shout out to Chris for bring this subject in. THANKS Chris! Even though it gives me more ideas to work...)

Cheers - Jim

 

[phantomjock]

Just had another cup of coffee, and thought I'd add some more.

Here is a graphic of the M720 overlaid with the vane downforce distribution:


When you look back at the comparison graphic, there is about a 200% increase in downforce from those vanes. Wow - that is significant. Also, you'll note the stock underbody sort of balances the front downforce equally around the front axle, the modified aft of the axle.

The "modified" (vanes installed) has a HUGE increase aft of the front axle - with little to no change in the rear - so I'll retract the added energy thoughts to the "rear devices" I posited earlier.

This seems like a very easy and inexpensive (and stealthy) means of generating some significant downforce we can use on our cars. And note there is very little increase in frontal area, certainly less than we see in many "air dams."

If anybody has an idea where the CG is on the M720, I'd like to know that too. Useful to look at center of pressure, center of gravity, and moment of inertia all at once.

I'm going for another coffee.

Cheers - Jim

 

[vette427sbc]

I'm going for another coffee.

Cheers - Jim

Put the next cup on my tab... this is all such valuable and interesting information. Thank you!