Wheel wells and skirts

[heavy85]

Been thinking about how to get a few more tenths and realize aero is a cost effective tool - especially after playing with wing angles and seeing a very measurable improvement. I'm trying to understand what to do with the wheel wells and side skirts (if any). I currently dont have any fender liners and am thinking of doing something in this space. What comes to the top of my head is to add a 'floor' between the bottom of the fender (rear of front tire) and the frame rails then cutting a big vent into the lower side of the fender just behind the tire. My thinking is this would force the air out the sides and limit what is allowed to flow underneath. Think of the Hemi-Panter for example. Second topic, the skirts, and I see from the wind tunnel testing that the skirts hurt if anything. Why was that as it seems to be a potential plus? Last topic is what to do behind the rear tires. Mine are completely open but I'm thinking the rear valence is a parachute and should be addressed. Again, any ideas here?

 

Thanks

Cameron

[JMortensen]

I think you'll get A LOT more bang for the buck from something that is out in the airflow like a wing than you will from a fender liner so a bigger/better wing might still be the best bet for the moment.

 

As to fender vents, I've got some thoughts on that one. Plenty of cars try to force air out from underneath through an open area behind the front wheel. Everything from Spec Racer Fords to Pikes Peak cars do it. Is it worth the hassle though? What air are you trying to exhaust? If your trying to get air out and you have a really low airdam, it might be wasted effort. If you're trying to exhaust the radiators out the sides you need to cut holes in the walls in the engine compartment walls and run ducts from the rad to the wheelwell, and then run ducts to your fender vents then it makes sense to look at the potential problems there. If a car drives up next to your exhaust hole on the side of the car, this can seriously affect the downforce on your car. An alternative if the purpose is for rad ducting is to go out the top, this makes the ducts simpler and makes the car less dependent on clean airflow to the sides. If you want to vent the fenders, putting vents right on top of the fenders or behind the wheels or both will still work to pull out any air that happens to get in and would have less reaction to a wheel to wheel situation. I'm not sure if the Speelab Vette that I think you referenced by the builder's username on a different forum (http://www.hemipante...rdic%20Supercar) uses those vents for the radiator also, or if they just use them to create downforce.

 

Test #13 in which the skirts showed an increase in lift were done with a BRE chin spoiler. My guess is that with a chin spoiler what looks to be 5" off the ground too much air was getting under the car, and that the skirts were trapping that air, increasing lift. None of those cars tested had a radiator duct that kept air from going around the radiator and under the hood and car. It may be that with a lower front end or less air getting under the hood or a diffuser in the back to pump the air through, that result might be different. The side skirts should have serve the function of keeping the low air pressure under the car separate from the higher pressure air outside. If you don't have low pressure air underneath the car, then side skirts shouldn't be used.

 

With regards to the rear, I think there are a couple options. 1. Diffuser. Whether or not it makes sense to have a fully formed diffuser on a Z would have to be tested to find out. I think as has been pointed out in other discussions here and elsewhere, having a higher rear end with an upward slope from the rear axle area to the back of the car does help. 2. Cut the rear valence off the car entirely, NASCAR style. I don't think there is any doubt that this would have an effect, the question is would it have an effect great enough to justify doing it. 3. Put a bunch of holes in the bottom valence. I have pics of a Lotus Europa race car that has the whole back end swiss cheesed and a spoiler on the rear deck. I think it looks great and the car certainly hauled ass, but I remember posting it here and the reaction was not universally supportive if you get my meaning.

 

Those addressed the rear of the car, but your average diffuser doesn't address the area behind the tire that you mentioned. I think maybe the idea there should be something like ZR8ED's rear fender vents or a big ol hole in the back of the fender like the California Z rear flares, or a protruding lip in front of the rear tire in conjunction with a good fender liner to try and suck air from out of the wheel well area. Again, it would be an interesting thing to try out at the wind tunnel and see what kind of difference it makes and whether it was worth the hassle.

Edited October 11, 2010 by JMortensen

[bjhines]

For the rear diffuser you can locate the fuel tank and mufflers to completely fill in the rear valence area like on the stock car.

[260DET]

As Jon indicates aero often has to consist of a complete package, sometimes just adding one component makes things worse. I'm a big fan of bonnet/hood vents to get air out of the engine compartment and over the top and to the side of the car both to reduce drag and to improve cooling. The better the air is exhausted out of the engine compartment the less radiator opening is required too, less drag again.

[heavy85]

My airdam is relatively low but there is still some air that goes underneath plus brake cooling air that is piped into the wheel needs to go somewhere. Reason it comes to mind is the area behind the front wheels is sloped down under the car with the fender on the side to trap the air and direct it under the floor ... causing lift. At least that's my theory. This is where the blue windtunnel car with the fender liner and skirt angling out would push this air out the sides, creating a negative (relative to air over the top of the car) pressure behind the skirt. I dont know if this would be negative relative to atmosphere but at least lower pressure than air going over the top, creating downforce. This is where a skirt design and / or the floor behind the tires w/ the cut-out in the fender makes a lot of sense to me.

 

The Camaro tank angles up nicely and fills the center of the valence very well, but the sides directly behind the tires are just a direct shot all the way back. This is where I wonder if holes in the valence or fender liners with a lip to trip the air hitting the front side of the rear tires would help get the air out before it 'piles up' in the valence? Maybe it's insignificant, I dont know.

 

Realize this is a balance which is why I asked. I've got the basics covered (wing, airdam, blocked front end, hood vents, etc) but want to take lap times down a notch and think aero may be a cost effective way to make some gains. I'm between 0.3 and 0.5 sec off two separate track records in my class so I'm close but need just a little more. Sure a $1,000 mac daddy wing would trump any of this but that's not real cost effective, at least for what I'm looking for.

 

Cameron

[JMortensen]

Care for some other suggestions?

 

How about a big lip in front of the front wheels? That would create a bigger low pressure area around the front tire and should suck whatever air is in there without the ducting. Could do that front and rear. How about a little spoiler in addition to the wing in back? Would help to pull air out from under the car, and lots of cars use a spoiler and wing together, Or my air dam splitter combo thingy. I'd love for someone else to guinea pig that idea...

[260DET]

If you want to cut holes in your inner guards/fenders that will vent air from the engine bay into the wheel wells, which of course is a low pressure area. May create even less drag than bonnet/hood vents and may also help to ensure there is more pressure on top of your proposed 'fender floors' than below.

[heavy85]

Already have a strip in front of front tires but still wonder if the rears even matter in the grand scheme of things. I like the spoiler and wing idea. Never thought of it really. Jon - where have you seen this used effectively? Another way to think of it, short of an effective splitter is there any other practical way to get more downforce on the front end? This is really just a sounding board so keep the thoughts coming.

 

PS - I'll let Jon develop the air damn splitter thingy.

[JMortensen]

Already have a strip in front of front tires but still wonder if the rears even matter in the grand scheme of things. I like the spoiler and wing idea. Never thought of it really. Jon - where have you seen this used effectively?

Lots of racecars use both. Daytona Prototypes, LeMans type cars, just about every Porsche that has a wing on it has a spoiler underneath it, etc. I think the idea is to help get the air up from underneath the car headed in the right direction to help get more flow under the wing. I expect you could run more angle without stalling the wing, in a way similar to a dual element wing. Here is a Z car that has both: http://www.youtube.com/watch?v=7TwOBR5Yep8&feature=player_embedded

 

DP: http://www.raw-foota...DP/6/6-1253.jpg

Porsche: http://www.theautoch...066281.1-lg.jpg

Vette: http://www.z06-corve...ette-C5R-06.jpg

 

Another way to think of it, short of an effective splitter is there any other practical way to get more downforce on the front end? This is really just a sounding board so keep the thoughts coming.

 

PS - I'll let Jon develop the air damn splitter thingy.

Splitter is probably the easiest solution to front end downforce. It's in free airflow so a little change makes a relatively big difference. I do think my splitter/airdam will be ugly and will look like the car has a hellacious underbite, but I also think it will work and will be less pitch sensitive than a regular splitter, even if it produces less overall downforce than a splitter alone.

Edited October 15, 2010 by JMortensen

[johnc]

Take of the big aero items first. Wheel well work is an incremental item. Get as much downforce in front and back as possible.

 

1. A front undertray with diffusers dumping to the front wheel wells.

2. Flat bottom the car including an engine pan and something to cover the rear diff and lead into a rear diffuser.

3. A rear diffuser that starts to open up immediately behind the diff. A custom fuel cell can with a 14 degree up angle works well. Cut out the rear valance. Add vertical strakes. Move the muffler forward or up to get it out of the way of the diffuser.

4. Move the wing off the back of the car so it can work as a pump for the diffuser.

 

These kinds of mods are where the big aero gains come from.

[DrSideways]

+10 to what John said. It looks like you want to do 1/4 mile racing. Really with any competition you need to focus on the large gains first. The stock Z Car is VERY dirty on the bottom. It has a REAL big frontal air intake. Taking care of these first and then look at the rest.

 

Any air into the engine bay needs to get out.. Less in = less that needs to be evacuated.....

 

Any air over the top of the car needs to be smooth flowing. The ends of the car air flow need to have sharp edges for the sir the break free from. Not that I would EVER condone ANY rules encroachment:)but some creative quarter panel end work work.... may just may enhance some air flow. Not that I would do any such thing.....

 

Sounds easy doesn't it???

 

Focus on keeping the car low and not letting air under there in the first place. Watch some old (for some of us;)Warren Johnson down track vids on how the car gets lower as it goes down track.

 

Hope this helps,

 

Alan

[ZROSSA]

A couple of thoughts......

 

A spoiler has been proven to improve the areo profile of the s30, it reduces lift and improves the Cd as well.You could use a wing as well, but it would depend where you put it. Up high and above the spoiler at you could get a gain, down low to drive a diffuser it could be detrimental.

 

Most of the aero improvment discused will only work if the car is low......really low! We are talking 2 to 3 inchs of the ground. Will your suspension work at that ride height?

 

I was watching Bathurst last weekend (Aussie Taxi Racing)and was wondering about the way that they cut the area out behind the front tire. Under braking it looks as though they are trying to get maximum downforce by driving the air out of the wheel well. As far as I can tell they lose it(downforcce) when they get off the brakes, mind you, they look very soft as well.

 

So where does my gewurztraminer induced rambeling lead........ down force needs a super stiff very low set up that will more then likely be out side what the chassis is set up to handel. I did some things on my zed to help in this area but I doubt I cant tell the differance. The rear is cut out and molded into the tank, the back muffler will be the same shape as the petrol tank "diffuser", the radiator is ducted and vents into the low pressure area in the hood. I still drive on the road. The lowest part of the spoiler is about 4" above road level. For track and rally work I will make a flexable/removable spliter and run this as close to the ground as possable.

 

Dont forget, If you cant tidey up the air flow under the car......dont let the air in!

 

On to the Viognier.....

 

Douglas

Edited October 17, 2010 by ZROSSA

[tube80z]

It sounds like the basics are covered here. If you are starting to generate downforce and want to be able to run softer main springs you may look into a third spring. For our cars the easy way to do this is a Z-bar. I think I would focus on keeping the high pressure air in the engine compartment from dumping under the car any way you can. Take a look at pics of ALMS GT2 cars as they have a lot of ideas you can poach (splitters, full length floors, wheel vents, diffusers, etc).

 

Cary

[JMortensen]

Dont forget, If you cant tidey up the air flow under the car......dont let the air in!

I think I'm with you on this part. The underside of a Z is pretty dirty, we all agree there. I think the easiest solution is to not let air under the car as you said. NASCARs make a lot of downforce and they don't have undertrays, they just have airdams and fenders designed to suck air out of the engine compartment and a big spoiler on the back to balance it out. I've found stats like 1500 lbs for the old style cars, and this site says up to 1750 lbs for the COT. http://www.livescience.com/technology/070215_nascar_aero.html We don't know if this is a short track high downforce config, or a high speed config that is getting these numbers, but they're still at least marginally useful for a very coarse comparison to a Z car.

 

Katz says that "aerodynamic forces such as drag and downforce increase with the square of speed. So if a car develops 100 kg at 100 km/h, the downforce is expected to increase by 4 (to 400 kg) at 200 km/h and to 900 at 300 km/h." He shows a formula to figure this all out on p.51 of his book Race Car Aerodynamics. While I do truly suck at math, I think that this equation would hold true whether one is using metric or SAE measurements since the scales are linear. The windtunnel testing shows a max total downforce of 108 lbs at 100 mph, so I think we could expect 440ish at 200 mph. We can compare this to the NASCAR figuring on ~1750 lbs at 200 mph, as this gives the most generous estimate in comparison to the Z testing. This means that we're falling WAAAAAY short of NASCAR's numbers, and they do what they do without undertrays, diffusers, etc. That's not to say that they aren't complicated, but I think that we can take some of their basic concepts and apply them to the Z and improve the numbers a lot.

 

Combine that with some of the CFD testing shown in McBeath's book where a splitter was added to a NASCAR profile to see the change and you get some ideas about where to spend the time and effort. McBeath just added a 100mm lip in front of the airdam with literally nothing at all behind the airdam and showed a 10% gain in downforce. He says that downforce maxed out at 150mm but didn't quote an increase in terms of either percentage or force. He then added a diffuser that went back to the front axle (still completely ignoring what was going on after the front axle) and gained another 3.9%. Side note--If you compare the diagrams 4.11 and 4.14 and look at the blue section of low pressure behind the air dam and created by the diffuser, they look pretty similar. Maybe I stared at those pictures too long, but that is exactly where the airdam on the splitter idea comes from.

 

Regardless, I think it's pretty easy to see that there are still BIG gains to be made while only focusing mostly on the top of the car, without getting too caught up in the more difficult work on the underside. Problems with the underside are numerous. Need to control the height in order to prevent the car from having variable downforce based on ride height and pitch, need to route exhaust so that it doesn't intefere with the airflow, need to do something about the control arms to keep them out of the airflow, etc. To an extent those same issues apply to wings and spoilers on top, but they aren't as susceptible.

[JMortensen]

Got pissed off at theorizing on the splitter/airdam combo for years with no resolution. Emailed Simon McBeath. Will let you all know if he responds and what he has to say...

[heavy85]

Studying what little details I could find it looks like the C6.R Corvette GT2 car uses many of the same principles as NASCAR that Jon mentioned. Particularly seeing the airflow over the wheels and shape of the fenders. You can see the 'fender floors' used there as well but I cant tell where they lead to??

 

http://www.racecar-engineering.com/articles/sportscar/385795/corvette-c6-r-gt2-technical-review.html

 

Cameron

 

PS - I'm really liking the wing + spoiler idea, now need to figure out what that spoiler may look like. My search fu is weak and I cant find the old article where the 'ideal' Z spoiler angle and length was determined. Anyone have a link to that?

[JMortensen]

I don't think you necessarily want the "ideal", because a NASCAR style spoiler with a Gurney on it will probably make the most downforce but I think it would screw up the airflow on the bottom side of the wing and make it less effective. I'd use the 911 as the example to follow, because it has the long sloping tail like the Z. If you look at the Porsche (and the Vette for that matter), they have a fairly narrow spoiler that sits in the middle of the body. I think the reasoning may be tied in with something you said on another thread. You said you had flow separation on the bottom of the wing in the middle but not on the sides. Assuming that I'm right about the function of the spoiler under the wing, that it turns the airflow to make it easier for the flow to stay attached to the bottom of the wing, then that seems perfectly logical. Put a spoiler in the middle but don't extend it to the sides because the center is where the flow is the dirtiest and it's needed to make the wing work. If you're using it to try and pump air out from the bottom of the car, a bigger one is probably better.

 

You could put little drips of oil on the leading edge of the wings all the way across to see how the flow changes just like they do on real racecars. I know they have some flourescent dye they use, but maybe a little food coloring or something might substitute.

Edited October 17, 2010 by JMortensen

[heavy85]

Looking at the Porsche example I'm contemplating starting the spoiler at the base of the rear window instead of all the way back on the rear edge. Could easily tie it into the existing wing supports. Maybe match it to the angle of the bottom of the wing? This would all be to help support air getting under the wing plus any side benefit of the spoiler itself. I'm not sure though since the air wasn't (not sure if it is now or not) staying attached I wonder if there just isn't much air down there in the first place due to the shape of the Z? If so VGs may help. Still more pondering to do.

 

Cameron

Edited October 18, 2010 by heavy85

[JMortensen]

I like the idea of starting the spoiler up higher. There is a guy in central CA I used to autox with, Rich Breshnahan, who had a spoiler like that probably 8 years ago .It was like a whale ttail type of spoiler that he made from a piece of Lexan and some angle aluminum. He moved it up to comply with the "cannot extend beyond the shape of the vehicle when viewed from above" rule. It was crude, but I recall him saying that it worked. Maybe someone has pictures. I searched and couldn't find any.

[Leon]

Here's an idea, start it even earlier, Cossie style!