JK's Track Tool

[JK]

Unfortunately, I have no up-to-date pics in the moment. Only a few which are about 3 weeks old.

What you can see there is the modified rear hood attachement to make room for the diffuser.

The diffuser itself is made from aluminium plates. The top is formed after an airfoil, calculated with a little program to fit in the available space. The aluminium plates are currently being laser-cut and hopefully ready the next days.

The diffuser starts close behind the lower suspension arm and goes from there up to the bottom edge of the rear hood. It will be mounted with a brace at this diagonal bar, going up from the hood mount.

To stabilize the outer plates, we currently plan to simply mount an aluminium bar to the bottom of the diffuser and fix it somwhere in the middle. When the diffuser works good, I will think about something less ugly

The middle section under th gearbox and the opening under the engine will be covered with a removable aluminium plate.

 

[Jimmy P]

Re: New Member

JK, Waiting for more......Any thoughts on colour for the car?

 

[JK]

Re: New Member

I love this color scheme. I saw once a small photo of this car somewhere and found this one here in the forum.

 

[JK]

Re: New Member

So, after the diffuser channels are ready, here is a first fitting test.

Now they have to be fixed somewhere and the sides and the openings in the underbody have to be sealed with some panels.

 

[Mark]

Re: New Member

No expert here, but shouldn't the airflow be channeled out in a lower plane? Your diffuser looks to me as tough it will cause uneccessary drag by guiding the air upward as it exits the car? Or is that a good thing?

 

[JK]

Re: New Member

Hi Mark,

guiding the air upwards is really the intention here. This is a key factor of producing downforce.

I am not an expert here, but I have been reading long discussions and many articles about this topic.

An airfoil produces lift, because the path, the air has to travel along, is longer at the upper side than on the underside. This produces a kind of rotation of the air, which is mainly responsible for the lift. This rotation is illustrated in the picture below (source: http://www.diam.unige.it/~irro/animation/prosmo030.gif).

For a car, everything is reverse, we want to produce downforce. So the path of the air over the upper side of the car has to be shorter than the path under the car. When the air under the car comes out horizontally and merges with the air from above at near street level, than you produce lift.

Therefore it is important to direct the air from under the car upwards to produce downforce. A spoiler will do the same, but the drag of a diffuser is lower.

Not that I'm too concernded about drag, because the engine produces about 600 hp and I will hardly reach the possible top speed with my gearbox. Maybe, I add a spoiler later, first I want to see what the effect of the diffuser is. I will do measurements of the spring travel (is this the correct word?) of all wheels and after heavy averaging, one should see an effect of the aerodynamics for different speeds. We will see .

 

[JK]

I'm currently waiting for my CAV GT40. It gets assembled here in Germany.

It has a 427 cui small block and a 5 speed ZFQ gearbox.

The setup is for having fun on the track, so I have reinforcements here and there (clutch, drive shaft, etc.).

 

[Mark]

Copied form our discussion in the New Member Section:

JK wrote;
guiding the air upwards is really the intention here. This is a key factor of producing downforce.

I am not an expert here, but I have been reading long discussions and many articles about this topic.

An airfoil produces lift, because the path, the air has to travel along, is longer at the upper side than on the underside. This produces a kind of rotation of the air, which is mainly responsible for the lift. This rotation is illustrated in the picture below (source: http://www.diam.unige.it/~irro/animation/prosmo030.gif).

For a car, everything is reverse, we want to produce downforce. So the path of the air over the upper side of the car has to be shorter than the path under the car. When the air under the car comes out horizontally and merges with the air from above at near street level, than you produce lift.

Therefore it is important to direct the air from under the car upwards to produce downforce. A spoiler will do the same, but the drag of a diffuser is lower.

Not that I'm too concernded about drag, because the engine produces about 600 hp and I will hardly reach the possible top speed with my gearbox. Maybe, I add a spoiler later, first I want to see what the effect of the diffuser is. I will do measurements of the spring travel (is this the correct word?) of all wheels and after heavy averaging, one should see an effect of the aerodynamics for different speeds. We will see .

Sounds reasonable JK. It will be interesting to see how you get on with it.
Mark

 

[JK]

So, the posts are a little bit mixed up now, but thanks to Ron, they are now where they should be

I'm really looking forward for the first test drive. Or, maybe not for the very first one. The first days, I will drive around a little bit in the evenings, close to the garage, to see whether everything holds together.

But in the last week of October, I hope I can make some serious test drives. First for me, to get comfortable with the car, and then for measurements with the data aquisition system.

First weekend in November then, I will have the first track day with the GT40. Nothing serious, of course, but I'm curious about the time I'm able to produce with my little rocket

I will post pictures and videos as soon as I have them.

Next thing to fit at the car will be a front splitter. Good solid woodwork at the beginning, to see how it works. Later I will see whether I use a fancy carbon plate or something

This is what I want to have at the car and test it this season. So, effectively the next few weeks.

 

[David]

I think you may have got the theory a bit mixed up. A diffuser is *not* a wing on the bottom of the car to direct the flow up.

The idea behind a diffuser is to create the venturi effect, essentially increase the volume passed a restriction, the speed increase corresponding to a pressure decrease and that pressure decrease corresponds to downforce.

I can't tell from your pictures but it looks to just be a section at the back of the car just starting from nothing behind the rear suspension. Sorry but I don't think its going to do anything effective if it just starts there. It needs to integrate with the underbody to be effective and much longer and shallower. eg see this ferrari.

 

[Dave Bilyk]

Dave (fastdruid) has it,
Here's a reasonable explanation of the diffuser, and front splitter. Diffuser (automotive) - Wikipedia, the free encyclopedia
Essentially the diffuser works by speeding up the airflow beneath the underbody, further reducing the pressure (read up on Bernoullis theorem), so it's not the diffuser per se that generates the downforce, but the effect it has on the underbody flow. The reason it speeds up the airflow is by the venturi effect, and by decelerating the flow more gradually to mix gently with the slower moving air behind the car, there is less resistance / back pressure, and so the air slips underneath the car more efficiently.

Diffuser (automotive) - Wikipedia, the free encyclopedia

Dave

 

[JK]

O.K., sorry for starting this discussion here, this was not my intention. I've been reading through pages of similar discussions, when I tried to find out what I can actually do, to improve the aerodynamic downforce of my GT40.

I will start with another wikipedia citation :

(Lift (force) - Wikipedia, the free encyclopedia)

"It is amazing that today, almost 100 years after the first flight of the Wright Flyer, groups of engineers, scientists, pilots, and others can gather together and have a spirited debate on how an airplane wing generates lift. Various explanations are put forth, and the debate centers on which explanation is the most fundamental."
— John D. Anderson, Curator of Aerodynamics at the National Air and Space Museum

I know about the venturi effect. But at first, my answer was pointing to the question, whether it is a good thing, guiding the air upwards behind the car. And yes, it is.

I don't want to stress too much the notion of common misunderstandings in aerodynamics because I'm really not an expert in this topic. But, what I have understood so far is that mainly responsible for creating lift (or downforce vice versa) is a circular airflow around a wing, a table leaf or even a car.

I personally like this picture

When you look at it upside down in case of a car you see, that the circulation causes the air to be faster on the underside (of a car).

The venturi explanation of the diffuser has on big catch. The situation under a fast moving car is really not comparable to air streaming through a pipe or something like that.

On one side of the "pipe" you have the street at which the car is driving and you have air in between, having initially the same velocity as the ground, namely zero.

And on the other side you have the underside of the car moving at high velocity. When the car moves over the ground, it accelerates the air by friction of its surface. But also the friction above the surface of the ground will slow down the air. But when you move with the car, you see that the ground increases the relative velocity of the air under the car and will by this increase the circular flow around the car.

So the intentions for the aerodynamic measures at my car are first to close the underbody to have the air travelling the whole way under the car to get any airflow at all.
I want to create an angle of attack by making the front suspension a little bit lower than the back.
And finally the diffuser should continue a path with a closed surface for the airflow and support the angle of attack by the geometrical shape of the car.

I know it would be better to have them longer. But this is the maximum what fits into this car. I hope that this is better than nothing. O.K. it will be better than letting the air travel through the gearbox mount and the rear hood attachment

So, sorry for the bad english. My english is generally not very well, it is a complicated topic and it's late here now and I'm tired

P.S.: some references

The Origins of Lift
2.972 How An Airfoil Works
Aerodynamics of Winged Flight

 

[Cliff Beer]

Re: New Member

Hi Mark,

guiding the air upwards is really the intention here. This is a key factor of producing downforce.

I am not an expert here, but I have been reading long discussions and many articles about this topic.

An airfoil produces lift, because the path, the air has to travel along, is longer at the upper side than on the underside. This produces a kind of rotation of the air, which is mainly responsible for the lift. This rotation is illustrated in the picture below (source: http://www.diam.unige.it/~irro/animation/prosmo030.gif).

For a car, everything is reverse, we want to produce downforce. So the path of the air over the upper side of the car has to be shorter than the path under the car. When the air under the car comes out horizontally and merges with the air from above at near street level, than you produce lift.

Therefore it is important to direct the air from under the car upwards to produce downforce. A spoiler will do the same, but the drag of a diffuser is lower.

Not that I'm too concernded about drag, because the engine produces about 600 hp and I will hardly reach the possible top speed with my gearbox. Maybe, I add a spoiler later, first I want to see what the effect of the diffuser is. I will do measurements of the spring travel (is this the correct word?) of all wheels and after heavy averaging, one should see an effect of the aerodynamics for different speeds. We will see .

JK, I hope you don't mind me saying, but those are really short diffusers there (you probably realize this...). In other words, yes, the goal is to get the airflow moving upwards, but with the radius your working with there I'm not so sure you're going to keep the flow attached. I would guess that it's going to detach and turn into regular old turbulence same as if you didn't have diffusers. You might have seen other diffusers that actually start ahead of the lower suspension where the lower suspension actually passes through the diffuser body. Something to consider at least.

Related, one little trick I've seen used well to good effect is to not use parallel slab sided diffusers - you can manipulate the flow rate and pressure right at the turn to help keep the flow attached with non-parallel/curved diffuser sides. Just my $.02.

 

[Dalton Veitch]

While all the above is relevant to the handling of the airflow beneath the car, I thought historically the major aerodaynamic flaws with the GT40 at serious speeds were the front end lift (front winglets sometimes fitted) and the large frontal area (cockpit narrowed on later adaptations).

 

[David]

Dalton, The winglets (or to use their correct name dive plates) while creating a small amount of downforce in the case of the GT40 I think are likely to have a major secondary effect. While I'm only guessing I would have thought they would improve the airflow around the front wheel area and may setup a vortex down the side of the car which would reduce air loss from underneath, down side of that theory is that the underside of the original GT40 is terrible for airflow.

JK, I think you are focusing far too much on how aeroplanes work and not how cars work, a major part of the aerodynamics of a car is the ground effect that doesn't affect a plane (Ekranoplan excepted). You dismiss ~30 years of F1 and sports car aerodynamics, why do you think F1 teams go to the expense of creating rolling road wind tunnels?

I found "Competition Car Aerodynamics - A Practical Handbook" by Simon McBeath a very good primer on the subject. You may also want to read "Race Car Aerodynamics: Designing for Speed (Technical including tuning & modifying)" by Joseph Katz

 

[Dave Bilyk]

I believe Dalton is right that the front end lift is a major issue, the winglets have an effect, but they are simplistic in nature and small in area. What has a substantial effect on front end lift is the front splitter because it promotes a lower pressure beneath and a higher pressure area above that extends further back, acting over a much larger area to make it effective. Small pressure changes using subtle elements like the splitter over large areas are most effective.

fastdruid is right also, you need to focus on the car aero, not wing and plane aero, the fundamental workings of the diffuser are more to do with the proximity of the bottom of the car with the ground, and that adds an extra dimension to the problem that is more complex.

So by all means read up on aerofoils, but as fastdruid says, without also reading articles and books specific to car aero, you will never come to understand what makes it tick. I like your questioning approach, keep it up, but also take the advice of others, and you will get to where you want to be.


Dave

 

[David]

One major point is that by changing the front you affect the rear and vice versa. You really have to think holistically.

You have the advantage at least of starting with a replica, as I previously stated, the underbody on the original is shocking from an airflow POV, the reinforcement folds on the bottom disrupt the smooth airflow you want and then there is a big hole for the engine and the gearbox hangs out the back. See the underside of one from the racing icons website.

That's bad for both drag and downforce.

Have a look at Mulsanne's Corner, technical analysis of contemporary sports prototype racing cars specifically the underbodys. There are a lot of images of serious downforce cars, now while you're always going to struggle with the GT40 without making it something else entirely you can at least learn what works.

Before I ended up buying a kit I was considering a scratch build with my own bodywork, as such I spent quite some time considering aerodynamic changes without fundamentally changing the look of the GT40. Its tricky as while the GT40 was designed from an aerodynamic POV it was before they knew much so quite frankly some of it is shocking.

Personally I'd have said the first thing to do is tidy up the front end airflow, ensure all air that goes through the front goes through the radiator and then out of the front vent(s). Most replicas seem fairly shocking in this regard, the intention being that the front vent(s) is/are more cosmetic than functional, it should *seal* against the radiator (and the clip should seal to it in front) but most seem to leave the front section open and let the air flow out of the rad under the clip. If you want the brake coolers to work as well rig them up too.
If yours has the front structure open, you'd want to close off the sides too.

Beyond that you start making changes that change the look, a splitter and airdam would be a start, the way the nose of the GT40 curves in at the front there is a large amount of drag from the front tyres, you want to make the air flow round rather than into the tyre. This is obviously a mustang and fairly subtle but you would want something similar but probably longer on the GT40

It would probably be ideal to have both diffusers into the front wheels, if you see this image again.

You can just make out how the body angles up ahead of the front wheels.
(and the rears but that's for another post)

Ideally you then want somewhere for the high pressure air under the front arches to go, you're a bit stuck without major surgery however. If you want the best then chop vents above the tyres and open up the back. Trouble is that takes you into the sills which contain the tanks...

One way is you could do something similar to this, extend the front clip down to a horizontal extension to the floor and fair round the rear arch.

Or do something like the RX-8 and put horizontal vents,

This is non-stock RX-8 but shows both above tyre vents and rear vents


Once you have the front sorted you then need to start to think about the back.

 

[JK]

Re: New Member

JK, I hope you don't mind me saying, but those are really short diffusers there (you probably realize this...). In other words, yes, the goal is to get the airflow moving upwards, but with the radius your working with there I'm not so sure you're going to keep the flow attached. I would guess that it's going to detach and turn into regular old turbulence same as if you didn't have diffusers. You might have seen other diffusers that actually start ahead of the lower suspension where the lower suspension actually passes through the diffuser body. Something to consider at least.

Related, one little trick I've seen used well to good effect is to not use parallel slab sided diffusers - you can manipulate the flow rate and pressure right at the turn to help keep the flow attached with non-parallel/curved diffuser sides. Just my $.02.

Hi Cliff,

it is unfortunately nothing but the truth, the diffusers are really short. But no way to make them longer, without changing the monocoque, which is not an option here .

I was fiddeling around with the profile quite a time, and a numerical calculation (actually a rough approximation, to be honest) shows evidence that it might work. A starting point for the profile was a standard NACA airfoil with an angle of attack of about 15 degrees and I made the calculation, including the ground effect.

But every theory is good, as long as it is not related to reality. To increase my chances to keep the flow attached also in reality, I will add in any case a vortex generator in front of the diffusers.

I have two Ideas for this in the moment. One causes a little bit bigger effort and will be something for next year.

What I will most probably do is to put simply a small serrated plate in front of the diffusers. With that attached, I would bet the flow keeps attached.

I will see, maybe I manage it to get some smoke under the car during a test drive (no jokes please ) to see what will come out of the diffusers.

And I will see, whether the diffusers work or not with the longitudinal suspension sensors.

I will think about other diffuser constructions, when the actual ones turn out to be a major failure

 

[Dalton Veitch]

I agree with you, David, re starting at the front then relating this to the whole car.

 

[JK]

While all the above is relevant to the handling of the airflow beneath the car, I thought historically the major aerodaynamic flaws with the GT40 at serious speeds were the front end lift (front winglets sometimes fitted) and the large frontal area (cockpit narrowed on later adaptations).

imho, the main problem with the front is all the high pressure air that gets under the front hood.

We are putting a good solid splitter under the nose and sealing all the entrances for air at the nose.

I am quite sure this will solve the problem with front end lift.