Aero with no inner fenders

[AkumaNoZeta]

Ok, lets say you cut off the inner fenders because of a tube framing or something. If you have your heat exchangers ducted, a underbelly pay the runs to the firewall with cut outs for the tires to rotate all the way around without hitting it and vents in the fenders some small ones on the hood. Would that help or hurt the aero not having the inner fenders there or would it actually help it because it makes it easier for the air to reach the fender vents? I think if you have the front of the tires blocked off effectively by the airdam and fender than it shouldn't hurt it too much to leave it open to the inside of the engine bay would it? I've always thought of these as seperate areas but making them into one area is a lot for my brain to work out and I don't know where to start.

[johnc]

Wheel wells tend to be low pressure areas and are used as outlets for front diffusers, radiator exhaust, and engine bay exhaust. Liners are used in modern cars to improve the ducting and flow from the high pressure areas mentioned above to the wheel wells. The inner liners on my 350Z have a lot of specific molded openings for the purposes I mentioned.

[JMortensen]

I don't think he's talking about the fender liners. I think he's talking about the inner fender, or the area between the strut top and the frame rail. The problem with removing it on a Z is that it is a unibody car, so that is structural. If you built a cage that would support the strut towers then I think some/all of that sheet metal can be removed. If you get into purpose built racecars, a lot of them have no inner fenders, or just a shield to keep whatever gets thrown off the tires from hitting anything important under the hood. Without inner fenders, the low pressure around the wheel well can then be used to reduce the pressure under the car, which will act on the bottom side of the hood and fenders to increase downforce.

[thehelix112]

Where did the theory that wheel wells are low pressure areas come from may I ask? That sounds like an extremely broad statement concerning something very complicated. It also sounds very wrong.

 

Look at every prototype car, they all try to keep the area behind the wheels as open as possible, as when its closed, it tends to act as a scoop and is a build-up of high pressure air.

 

When you start talking about prototypes that run the underside of the front diffuser to exit behind the front wheels, you need to realize that its somewhat more complicated than that and the entire front section of the car is designed to support this. The channel between the wheel guards and the nose section will slope upwards and then exit well behind the front wheels as the air getting pushed around the front of the car and running down the side will be moving quickly (low static pressure). They then run the underside of the front diffusers upwards and exit in these channels, so that the entire front diffuser is actually a wing, with endplates (the sides of the channel) to boot. E.g first picture here: http://www.mulsannescorner.com/bentleylmgtp03-2.html

 

Also on the page, note all the lourves and an additonal dedicated wheel well vent for the purpose of ``enhancing downforce''.

 

Unless I'm missing something, I'm pretty sure the wheel wells are very much high pressure areas.

 

 

If you cut out the inner guards in a zed, I think you'd need to know what you're doing and test quite a bit to avoid making things worse aerodynamically. If it was me, I'd be venting the wheel wells as much as possible, trying to work in a very small front diffuser/wing, and ensuring all the air flowing into the engine bay is ducted for entry and exit, and that the exits are placed appropriately.

 

Aerodynamics makes my head hurt, does any of that make any sense?

 

Dave

[JMortensen]

Where did the theory that wheel wells are low pressure areas come from may I ask? That sounds like an extremely broad statement concerning something very complicated. It also sounds very wrong.

 

Good point Dave. I think the "low pressure around the wheelwells" ASSUMES that the car has been designed to have low pressure around the wheelwells. NASCAR does this, they have a front fender that extends past the tire, so creating a lip in front of the tire which creates a turbulent low pressure zone in the wheel well area and this constitutes a large part of the downforce that NASCAR's have. I had linked to a circle track article about this previously but I can't seem to find that old CT page anymore, but suffiice it to say that they talk about putting a concave shape in from the nose to the wheel opening for the purpose of getting more downforce by evacuating more air out from under the car.

 

With the front of the tire exposed (eg spook) the tire itself generates quite a bit of lift, and I honestly don't know what happens in terms of pressure around the wheel well area, but I can't imagine it being a good thing.

 

When you start talking about prototypes that run the underside of the front diffuser to exit behind the front wheels' date=' you need to realize that its somewhat more complicated than that and the entire front section of the car is designed to support this. The channel between the wheel guards and the nose section will slope upwards and then exit well behind the front wheels as the air getting pushed around the front of the car and running down the side will be moving quickly (low static pressure). They then run the underside of the front diffusers upwards and exit in these channels, so that the entire front diffuser is actually a wing, with endplates (the sides of the channel) to boot. E.g first picture here: http://www.mulsannescorner.com/bentleylmgtp03-2.html

 

Also on the page, note all the lourves and an additonal dedicated wheel well vent for the purpose of ``enhancing downforce''.

 

Unless I'm missing something, I'm pretty sure the wheel wells are very much high pressure areas.[/quote']

Bad point Dave. If the wheelwells were high pressure areas, the last thing you'd want to do is have your diffuser exit or wing there. That would kill the flow through the diffuser or on the underside of the wing. Having a diffuser exit into a vacuum will help to pull air through the venturi in front of it. Just like the rear diffuser exits into the low pressure wake of the car, the front diffuser should do the same.

 

The reason that you see all the louvers and vents is because you can get even more neg pressure by using the Bernoulli effect to use airflow over the fender to increase suction. Louvers don't necessarily vent high pressure. They CREATE low pressure.

[thehelix112]

Jon,

 

My second point was that in prototypes at least the front diffuser has next to nothing to do with the wheel well.

 

I agree with your argument that a diffuser exitting into high static pressure would kill its flow, however I don't believe this is the case with prototypes. As I said the channel exits well to the rear of the wheel well on the side of the car, at which point the air flow is fast and hence low static pressure.

 

Good point re the application of louvres and vents. It sounds like we agree their purpose is to reduce the statis pressure in the wheel well. Be this to take it from high to reasonable, or from low to lower; I have no idea.

 

Dave

[JMortensen]

Well now that I've actually looked at your link, maybe I can make more sense...

 

I think the reason the exit is so far back there is to keep the flow connected, a sharper curve there would probably hurt the effectiveness of the diffuser. The reason for the triangular bit between the back of the wheel and the diffuser exit is to keep the flow reasonably attached so that you can use the low pressure from the high velocity air to enhance the front diffuser, not to separate the high pressure of the wheel well from the low pressure of the diffuser exit. There is low pressure in both areas, they're just using airflow along the side of the car and maximizing the low pressure in both.

 

If you take a look at an older IMSA car or a Spec Racer Ford, they have no triangular wedge in between, just a wide open space from the rear of the tire back out to the full body width of the car.

 

Spec racer Ford: http://upload.wikimedia.org/wikipedia/en/7/7c/SRF.jpg

 

Here's one in between:

http://www.conceptcarz.com/view/photo/191824,13482,0,0/1981-Porsche-935_Photo.aspx

 

Pike's Peak Celica: http://www.carsbase.com/photo/photo_full.php?id=36966

 

I would guess that we would be better off looking at the NASCAR example and not the Bentley example, since NASCAR does a lot with a little and very crude aero (no tunnels or flat belly pans, etc) while Bentley does a lot with a lot and has a lot fewer design constraints.

 

EDIT--Look at the huge vents on the Pike's Peak Tacoma (and the rear tunnels) DAMN!!!

http://www.ultimatecarpage.com/pic.php?imagenum=4&carnum=3266

[thehelix112]

Yep. I interpret all the pics you linked (which were awesome btw) as trying to reduce the buildup of high pressure area in a normal wheel well by venting the rear, as per my initial: `...try to keep the area behind the wheels as open as possible...'.

 

I'm a bit fixated on front diffusers, mainly because nothing I've ever read has satisfactorily explained how they work, hence my theory on getting them to act much like a wing. I would also guess that none of the cars you link to have front diffusers?

 

I completely agree with picking where you take your influences from, but I think there is potentially things to be learnt from all classes of racing. And there is nothing to say that techniques used on more exotic cars couldn't be used on a zed, given enough development time/$.

 

I dunno, I guess when you start talking about full tube-frame cars and chopping out inner guards, I think you're into the realm of at least considering these things.

 

Dave

[JMortensen]

Yep. I interpret all the pics you linked (which were awesome btw) as trying to reduce the buildup of high pressure area in a normal wheel well by venting the rear, as per my initial: `...try to keep the area behind the wheels as open as possible...'.

OK. I think that's where the problem is. You're right that these areas are venting, but not specifically to reduce high pressure, they generate low pressure. Have you seen the exhaust based PCV systems? Same thing there. They use the flow of the exhaust to create a vacuum in the crankcase.

 

I'm a bit fixated on front diffusers, mainly because nothing I've ever read has satisfactorily explained how they work, hence my theory on getting them to act much like a wing. I would also guess that none of the cars you link to have front diffusers?

I don't know if any of those cars has a front diffuser. Have you read "Race Car Aerodynamics"? It has a pretty good bit on front diffusers. It says that the front diffuser works like the bottom side of a wing in ground effect, and the top side of the car forms the top of the wing, the pressure differential between the two gives your lift value.

 

Likewise on the NASCAR it is also a pressure differential that gives downforce, only it is the pressure differential on either side of the hood and fenders, which is a much less efficient way to make downforce.

 

I completely agree with picking where you take your influences from, but I think there is potentially things to be learnt from all classes of racing. And there is nothing to say that techniques used on more exotic cars couldn't be used on a zed, given enough development time/$.

 

I dunno, I guess when you start talking about full tube-frame cars and chopping out inner guards, I think you're into the realm of at least considering these things.

I didn't mean to say that you SHOULDN'T look at the Bentley, only that the NASCAR stuff would likely be more useful.

[thehelix112]

I don't know if any of those cars has a front diffuser. Have you read "Race Car Aerodynamics"? It has a pretty good bit on front diffusers. It says that the front diffuser works like the bottom side of a wing in ground effect, and the top side of the car forms the top of the wing, the pressure differential between the two gives your lift value.

 

I have read RCA indeed, but it was a year or so ago, and my memory was that Katz did not explicitly say what you said above. Maybe it did and I'm mis-remembering, but I recall it took me a while to come to that conclusion. I'll go back and re-read it as soon as I can I reckon.

 

Thanks for the input.

 

Dave

[JMortensen]

I have read RCA indeed, but it was a year or so ago, and my memory was that Katz did not explicitly say what you said above.

Dammit, wrong book. I meant to say Competition Car Aerodynamics.

[thehelix112]

I've read that as well. I don't recall it mentioning front diffusers in any decent depth, but its been longer since I read that one. Will re-read as well.

 

Dave

[AkumaNoZeta]

I'm not going to keep the strut suspension, I'm going to a Mustang II with coil-over front suspension whenever I get around to replacing the front frame rails so the inner fenders would just be excess weight after that. So if you have the air dam go further out than the tire like on the NASCARs would a belly pan make that approach less effective since you said it was done to create a pressure difference between the hood instead of the top and bottom of the car? I agree with the aerodynamics making my head hurt, especially when talking about those full out race cars like the IMSA prototypes.

[JMortensen]

I've read that as well. I don't recall it mentioning front diffusers in any decent depth, but its been longer since I read that one. Will re-read as well.

I went back to find this quote for you and couldn't find it. I think I had come to this conclusion on my own and had remembered it being stated more explicitly.

 

On p.61 you get this:

The forward upper surface shows a small positive (upward) change in static pressure indicating the airdam causes some additional lift over the bonnet. The underside however shows a large area of negative (vertically downward) change in static pressure, indicating additional suction on the underside caused by the airdam. This extends roughly halfway along the car, and then changes to a slightly positive value, indicating some lift under the rear after the airdam was fitted. The net result is the gain in downforce we see, which is concentrated at the front."

 

And on p.69:

Figure 4-14 compares the static pressure coefficients with and without the front diffuser, and it is apparent that after 'fitting' the diffuser the high-pressure zone (red) above the splitter has remained pretty much unchanged. However, there has been a significant decrease in static pressure under the splitter, evidenced by the larger zone of darker blue which also extends under the forward part of the diffuser.

 

Put two and two together...

[johnc]

Where did the theory that wheel wells are low pressure areas come from may I ask? That sounds like an extremely broad statement concerning something very complicated. It also sounds very wrong.

 

Turbulent flow around a cylinder (the tire) generates low pressure behind the cylinder. Also, pressure is generally low inside a vortex tube (the spoked wheel center). In addition you have the laminar flow along the sides of the vehicle acting as the extractor (or the exhaust pipe in Jon's PCV example above).

[tube80z]

Where did the theory that wheel wells are low pressure areas come from may I ask? That sounds like an extremely broad statement concerning something very complicated. It also sounds very wrong.

 

While I don't have any theory to add to this I can tell you I vented my inner fender panels and used this to extract the radiator flow. I used a smoke bomb to check the air flow and found that it mostly went out the wheel wells.

 

I do agree the rear of the fender could be altered to make this approach work even better.

 

C

[ckolander]

On a slightly related topic, what about removing the plastic fender liners??? Will that negatively impact anything??

[drmiller100]

one way I think about it is to think about air flow speed.

 

The tops of the fenders are often clear out in the windstream, like the front of the hood on a 240.

So air is going really fast past it, uninterupted. Because of Bernoulli, this will be a relatively low pressure area, and creates lift.

 

At the base of the windshield the air basically stacks up and quits moving. This becomes a high pressure area.

 

When you put a splitter on the front of the car, you are trying to get most of the air to around the car, creating high speed areas on the sides of the car, which creates low pressure areas on the sides of the car where it really doesn't matter.

 

Under the car gets weird. You are trying to get no air under the car, with a huge area. the ground actually "pumps" air out the back of the car.

 

Those that try to build front diffusers are ambitious. The idea is to create smooth flow and cause air to go under the car and accelerate, causing a low pressure area. This is just like an airplane wing works, except you are using it in ground effect with the car as half the venturi and the ground as the other half.

I tried it on 2 cars, and am pretty much convinced I would need a full scale windtunnel with moving floor to get it to work.

[capt_furious]

I tried it on 2 cars, and am pretty much convinced I would need a full scale windtunnel with moving floor to get it to work.

 

 

You mean like this?

[drmiller100]

yeah, like that!!!!!

 

So, back to the OP's question. One issue with a 240z and a V8 is getting under hood air out from under the car.

A front splitter or air dam obviously helps that a bunch because it pushes air around the front of the car instead of through it and under it.

This is also why v8z's can have problems cooling - the pressure inside the engine compartment can be almost as high as the pressure in front of the radiator.

Another symptom you can test with is to leave the hood unlatched and drive the car at 60 mph. When you get it right, the hood stays down. If the hood comes up some, then obviously the pressure under the hood is obviously higher then the air above it.

 

I looked pretty hard at putting vents into the inner fender walls of my 260, but eventually decided the strut tower support is already pretty flimsy, and drilling holes isn't going to help it.

 

I am pretty well convinced if a race S30's didn't have inner fenders, and air was allowed to bleed out behind the front tires, the car would cool better and have more downforce.

 

To maximize this, keep the fenders behind the front tires tucked tight towards the center of the car, and let there be a pretty good gap between the front tires and the fender behind the tire.