How are all the budding chassis Designers fairing?

[Kevin M]

Matt,

I agree that if the car is tracked, you want to address front-end lift. Just because it is not extreme until 190 MPH doesn't mean it isn't happening at lower speeds. If a car has lift, it has lift at all speeds. You won't notice as much of a change at 150 MPH as you would at 190 MPH, but you will still have some downforce instead of lift if you make the changes. This can show up as better balance in cornering, which people frequently don’t attribute to the addition of downforce, because the old lift situation wasn’t drastic and so they didn’t realize how much it was throwing the cornering balance off. This is one of the advantages of the adjustable rear spoiler. It gives you the ability to tune high speed cornering behavior by adjusting the height.

A flat under tray will help with drag, but won’t do a lot for downforce unless the car is equipped with diffuser tunnels (In which case it is mandatory). If you have a tube frame car, an under tray will clean up the airflow (although at a cost of extra weight for the aluminum sheet). If you have a monocoque car, you already have a smooth underbody.

As for cutting an aperture to vent air, it would be too late. The high-pressure air would still be present under the first several feet of the nose and would cause significant lift before it got to the vent. You need to keep the air from ever getting under there to begin with.

Kevin

 

[Howard Jones]

The problem with the front end of the car not producing downforce is a very simple thing to fix. The problem is, to do so changes the entire look of a GT40.

Have a look at the yellow peril on the GTD club websight. The front of that car is just about what you end up with if you leave the radiator in the front and address the rest.

To fix a GT40 nose you must draw a line from the leading edge of the nose FORWARD and down, not towards the rear. At the lowest point of the forward reaching nose you must add a spliter.

Then at the sides in front of the front wheels the bodywork must not work its way inward towards the centerline from the top of the wheelarch as it sweeps downward. It should continue straight down from the top of the wheel arch towards a point that connects to the slopeing nose and tie into the spliter.

This alone will prevent most of the air infront of the car from getting under the car. With the goal being to get all the airflow up over the top of the car and in contact with the body without generating turbulance.

The other place to be fixed is the lower sections of the side pods. The sidepods should continue straight downward from the bottom of the door to the lowest point along the sill not curve inward and under like a GT40. This prevents airflow comming off the sides of windshield (high pressure) and airflow along the side of the car from flowing to the low pressure area along the side of the car behind the rear wheels. Once the airflow is into this area it continues on under the car.

These two things are clearly seen in the design of every modern full bodied racecar. Once these things are done the only thing left to do at the front is to move the radiator so the nose can be even more sloped downward from the lower windshield line towards the leading edge of the nose.

You then put the radiators in the the side pods and exit the air right in front of the rear wheels where it has nearly no time to slip under the car before is has been pulled into the low pressure area behind the car. This also keeps the radiator exaust off of the rear wing. A clean airflow is a high downforce airflow when it comes to airfoils. This all ends up looking like a 962.

The rear of the car behind the rear door opening line is pretty good by modern standards, but it would be good to seal the entire bottom of the car instead of an open bottom engine room like our GT40s. We might be able to pull in enough air from the top of the rear bodywork above the headers with NACA ducts into the engine room and since the bottom is inclosed it will not end up under the car. Thsi will inclease pressure in the engine room and force the hot air to exit out the rear of the bodywork behind and below the tail spoiler like the GT40.

As to creature comforts and driving long distance. If the seating area is large enough, including foot room, the seats are comfortable, air temp is kept at about 75 degrees, and there is enought room between the seats so that the people in the car are not on top of each other then thats about all that can be done. Ride quality will be a function of spring rates and shock dampening to a large extent. Both of these can be adjusted by the owners as they see fit. I like go carts whereas others like cadilacs. It's all in the shocks, springs, and rollbars.

The AC system is going to be just at a 100 pounds, I think we must and can live with the extra weight with our 2400 pound target. This is why I started at 2400 instead of 2000. This will of course include insulation.

 

[Paul W]

Howard, made me think of the VBM - anyone know if the molds are still around ?

 

[Richard]

Good aero stuff guys.

In addition, you can exit some radiator hot air out of the front wheel wells/arches if you have too much for good reattachment over the windscreen, but may have to be careful about increasing drag at the sides through flow separation around the car. Not much of a problem with a road car if the sides of the car are draggy anyway (ie big wheel arches, mirrors, vents etc) in the quest for a really cool looking machine. Minimising side drag (for want of a better expression) lead to the really ugly slab sided sports racing cars of the 80s and 90s.

In principal, a flat undertray should be included with as big a rear diffuser as possible, noting the gearbox will take up a fair amount of room at the back. But, if we find we are limited with the amount of front downforce available because any more makes too much drag, we will need to tune down the rear diffuser/spoiler. One way to keep good (fairly) smooth airflow under the car, reduce the downforce of the diffuser, and add cooling air to blow the engine bay is to use inverted NACA ducts near the transition from the flat floor to the diffuser (ie behind the seats). Everything comes at a cost though, because the extra air going out of the rear mesh will add drag by interfering with the reattachment of the air between the diffuser and our (well designed) spoiler, unless it is carefully vented and accelerated to smoothly join the airflow at the upper surface. Like suspension, in practice aero will always be a compromise. Particularly on a road car.

You can also use some of the air coming in under the front of the car by sloping part of the undertray up at the front across a proportion of the width of the car. This increases front downforce, but the air has to be managed as it makes its way towards the diffuser. You can use its own increased pressure to vent some out of the sideskirts with vanes or vortex generators... To complex for our budget? Maybe, but you can get the basics without a wind tunnel by using transducers to measure the relative movement of the shock absorbers as speed increases to work out approximate front and rear lift ratios, and then play around with aero ideas until the balance is good. Too much drag? Make more power! (sorry).

Howard, I will concede 2400lbs (1090kgs) until I finish my car as Australian Design Rules compliant and have it on the weighbridge. Unfortunately, I do have a sneaking suspicion your weight estimate will be pretty close to the fact once the climate control, power windows and comfy interior are added...

 

[Kevin M]

Paul,

The VBM was basically the beginning design used by Factory Five for their GTM. They have changed the rear end a little, but I think you will find the similarity striking. Check this link:

http://www.factoryfive.com/table/ffrkits/GTM/GTMkit.html

Kevin

 

[Howard Jones]

Paul as you can clearly see in your picture, If we move the radiators to the rear the nose can be droped at least 3 or 4 inches. This would have a huge effect of both front downforce and overall drag. Both in the correct direction.

The radiators would take their input from in front of the rear wheels and exaust between the inside of the rear wheel and the body pannel then out the rear of the car.

This will require twin radiators I'm sure and create packageing problems with thr AC condensor and engine and transaxle coolers but in a full size mockup I think we would be able to find room back there.

If we ran out of room at the rear we could place the engine and transaxle coolers side by side in the nose down low. The would present a much smaller frontal area that the water radiator would.

The airbox on top of the car is something I forgot about. Here is our solution to engine room cooling and cold air intake to the engine.

The over all shape of this car in just about exactly what I had in mind. If it was scaled up aproximately 10%, and with a proper cabin inside, we can see that it could work very well.

 

Would we have a problem pumping viscous fluid like oil from the back of the car to the front and back again, especially with small bore piping?

And in my humble opinion, those side intakes on the VBM look really crap, the front end looks tasty, but the wheel arches need return material at the edge to produce a lip - looks a bit NASCAR without the finishing.

Thanks for your comments Richard, some really good ideas flowing - what a difference not having the nay-sayers makes! I know that the radiator would obviously stifle air flow through the front of the car and up through the bonnet (hood), but, as you suggested, if we split the air intake at the front into three sections, just like the GT, VBM, we could as you suggested shift that air through into the wheel arches and over the brakes. As you suggested, that exiting the air thought the sides may create more drag on the sides, especially if we had mirrors etc (I'd assume they'd be mandatory here for ADR compliance, Richard?), could we vent the wheel arch through a louvred vents behind the mirrors, would that create a venturi affect and drag the air out of the arches? Or would it have an adverse effect on the air flowing around the sides and top of the car?

 

[Howard Jones]

If you have a look at 917's you will see a rather large oil cooler down very low at the leading edge of the nose. The distance's involved in pumping oil up and back add overall weight to the car with additional oil volume and plumbing hardware. But a high volume pump can surely handle the added load(distance).

At this point if we are going to dicuss oil system layout we should consider a dry sump. More weight and plumbing but some real benifits during those track days. And we can really lower the motor which means we can lower the rear bodywork over it and clean up air flow to the spoiler. Again more downforce guys.

 

[ QUOTE ]
Matt,


A flat under tray will help with drag, but won’t do a lot for downforce unless the car is equipped with diffuser tunnels (In which case it is mandatory). If you have a tube frame car, an under tray will clean up the airflow (although at a cost of extra weight for the aluminum sheet). If you have a monocoque car, you already have a smooth underbody.

Kevin

[/ QUOTE ]

I have a flat uderside of my car that ends at the engine fire wall. If giving the car a 1" rake I get 120 mm of water vacuum in the possition of the shifter, at 90 mph. The entire surface under the car is 32000cm2 that has to be muliplied with some average suction force as it will be less along the sides of the car.
Compare to the original flor designe that probabley creates lift I concider the flat floor worthwhile.
Goran Malmberg

 

[Kevin M]

Goran,

Your car is a modified Pantera, and if I remember correctly, it has a very aggressive front spoiler with a splitter and fairly aggressive side skirts?

I wouldn’t think a GT40, with it’s rounded lower sides and no skirts, would see as much downforce in street trim. Also, I don’t think you could rake it a lot and still have it be streetable.

That said, did you measure this on you car before you fitted the spoiler and side skirts? If you were able to get something near that much from a Pantera without side sills and with a streetable spoiler, I may have underestimated what is possible here.

Kevin

 

When it comes to "Supercars" I have a list here. The cars listed is driven by the same experienced race car driver, so the times shows pretty well the performanc of the car alone. The driver has tested ove 300 cars on the same track!

It is a high speed slalom course starting of with a 75 metre straight, a 90 dgr right bend followed by a 200m straight. Then we got a slaom course followed by 300 meters traight, slalom cours again followed by a new 200 meters straight and a 360 dgree turn and back again. Slalom speed is say 70 mph for a fast car.

Radical prosport 1,14,84 Slicks
Maxda rx7 racing car 1,17,36. Slicks
Formel Renault 1,17,50 Slicks
Radical S3 1,20,06 R tires
Opel Speedster 290 hp 1,21,29 R
Bmw M3 csl 480hp 1,22 31 Slicks
Audiquattro A2 480 hp 1,23,20 Slicks
MitsubishiEvo 450hp 1,23,50 R
Corvette DRM 586 hp 1,24,44 Slicks
Porscheturbocup 400hp 1,24,84 R
Misubishi evo V1 1,25,03 R
Porsche 996 Carrera 98 1,25,62 R
Lingenfelter Corvette 1,26,28 R
Ferrari 360 Modena 1,27,00
Alfa GTA 250 hp 1,36,00 Comparsion for a regular car.
Allright, the list is looonger. But the Opel speedster just needs a little hotter engine to beat anything "supercar" but Radical.
Goran Malmberg

 

[ QUOTE ]
Goran,

Your car is a modified Pantera, and if I remember correctly, it has a very aggressive front spoiler with a splitter and fairly aggressive side skirts?

I wouldn’t think a GT40, with it’s rounded lower sides and no skirts, would see as much downforce in street trim. Also, I don’t think you could rake it a lot and still have it be streetable.

That said, did you measure this on you car before you fitted the spoiler and side skirts? If you were able to get something near that much from a Pantera without side sills and with a streetable spoiler, I may have underestimated what is possible here.

Kevin

[/ QUOTE ]

Kevin, I am awhere that my car has a agressive spoiler-splitter. But the side skirts is mostley Pantera GT5 design that dosent hang down lower than the underside. So it is no real "tunnel".
I have done a lot measurments, but there is allways something missing. One is standard Pantera underside pressure. But I know that with totally level underside (no rake) there is Zero pressure-vacuum. There are a image of the underside on my site http://hem.passagen.se/hemipanter/
klick Aerodynamics.
Goran Malmberg

 

[Kevin M]

Goran,

Interesting list. Did they ever test an Ariel Atom? It's about the only streetable (marginally) car I can think of that might be able to beat the Radical on a course like that, other than maybe a really hot Lotus 7 clone. No question, if you don't have something like a Mulsane straight to deal with, small and light frequently beats big and heavy. Collin Chapman proved that enough times.

It would be very interesting to test a GT40 for rake effects. Does anyone happen to have a car with suspension height sensors fitted? If so, a couple of runs, one flat and one with 1" of rake would be very interesting. If it comes close to Goran’s Pantera, it would certainly be an easy improvement to implement.

Kevin

 

[ QUOTE ]
Goran,

Interesting list. Did they ever test an Ariel Atom?
Kevin

[/ QUOTE ]

No, they didnt test Ariel Atom.

I measured the underbody pressure with a hole in the floor with a welded on pipe, 2mm inside hole. A hose conected to the pipe is leading to a can with coluoured water. With a scaled transparent tube I can see the if the water is rising or beeing lower in the can.
The can is wented throught a pitot tube on the roof of the car to get the reading against outside barometic pressure.

The can can also be connected to a small sensor thet can be located at different spots like the front to measure splitter overpressure.

Goran Malmberg