Motormouth

iTrader: (44)

haha, I was about to post that reponse! well said.

XKR

iTrader: (1)

chebosto .....

The person that invited me to the high speed even ( where the beast ran 190) drives a car like the pic below....matter of fact...he is the one that invited me to attend the SSC with him. His Aston Martin is a SC 800 + whp V12. He is keeping the body stock as possible to stay under the radar as I am. He did a few 197 mph runs but will be over 200 mph the next time we meet... look at his rear wheels. His car came stock with 285's like the one in the pic.....he now runs 315's .....



I spoke to him about your concerns.....because he is a long time racing pro.His reply was... You cannot look at a car with deep contour and say that the wind would affect the way the car handles at high speeds until you either drive it at such speeds or take it to a wind tunnel. If the sides of the car was flat ( as one of the pic you posted) then thats a different story. The addition or removal of a side view mirror can cause a dramatic change in where the air travels....

So I guess I will see what happens when we get to the win tunnel.

I really appreciate all the help and advise i have been getting from you guys besides Sharif and Sam...especially Terry...I receive tons of PM's a day with questions and advise....Keep it coming.

BTW...Tom....call Sharif and ask him that question...You know which one (Is it finished yet).

Chebosto

iTrader: (43)

here's a link,
http://autospeed.com/cms/title_Modif...5/article.html

take it with a grain of salt, but from the data on the volvo,



the wheels correlate to nearly 20% percentage of total drag on the car, where as the floor, exhaust and axle only make up 10.7%... albeit this is a production volvo, but % wise, relatively the same imho for most production cars.. and that's with OEM sized wheels and widths.. XKR is going much wider, and with more aggressive offset which will put it even more in the airstream. so i would estimate his wheel drag would be much higher than normal.

so they're spending alot of time, and money, on the flat underbody, which only concentrates on 10% of the total car's drag coeiffient..

they should be concentrating on both areas, since they're doing nothing on body lengthening, or reducing the pressures cause by air into the engine bay..

XKR

iTrader: (1)

^^^^ And thats exactly what he said....if it were a flat sided car ( IMO..Like the Volvo) or the pic of the white car you posted..then yes. But a car with contour needs to be tested in a wind tunnel or driven at high speeds to see if anything needs to be adjusted....Thats what we have been doing...running it at high speeds to see how the car sits. We first took it to 120...then 140...and up ..We kept increasing the speeds to see how the car reacts. If the car got light...we would have stopped.

The reason for the flat panel...especially at the rear....is because of the wide open space where the stock muffler use to sit....I am getting a umbrella effect as per the 3 drivers that drove it over 180 and the racing shop that examined the car. They feel the air will rip the rear bumper off if the gap is not fixed.

Thanks for the Link...very helpful

rcdash

iTrader: (18)

That was an interesting article. My interpretation of that data was that it's air turbulence generated by movement of the front wheels that contributes to significantly higher drag than one might expect. In the example in the article, air was re-routed rather than tucking in the wheels (since it's not the wheels themselves generating drag, but rather their movement).

Regardless, taking the car to the wind tunnel that can measure drag is a great idea. What type of equipment will be on hand for analysis of drag of various components?

XKR

iTrader: (1)

Raj...I have 4 brain cells left...dont make me have to use them...

My understanding of the article was the same.

We will either use Sharif's hookup for the windtunnel or Terry and I will have to find another spot to have it tested. we will find out more about the equipment they use when we setup up an appointment... Matter of fact...I think Sharif said that it was in your area of the woods. Maybe you can show up

Motormouth

iTrader: (44)

the only thing I know of the issue is that for better aero the rims or any portion of the tire should not stick out. that is why generally race car's wheels are not pushed out to have a flush offset. even just a MM further inside:







but compromise is always made as they try to maximize the width for handling and how big of a deal it is, I do not know. nor do I know about the difference between flat and curved sides.

The McClaren F1 GTR LM version added a front wheel arch to cover the added width of the rim:



and the rear:

Sylvan Lake V35

iTrader: (23)

I might be wrong here but I thought the MAJOR goal of the rear diffuser(in this case) was to create additional down force so no rear wing was needed, not necessarily reduce drag although it would certainly be a secondary function. Obviously the less drag the better but thats what 1000whp is for to over come the drag and the diffuser makes sure the 1000whp is going to the ground. From what I recall the other goal was to keep it a stock looking as possible to me thats no wild fenders or big flares. that being said there is some great info going on here by some educated guys keep it up

XKR

iTrader: (1)

You just reminded me of something else that was said to me.....

I did mention to the speed shop that I wanted to go to the wide body kit...and he mentioned what would have to be done to counter the changes...thats why I changed my mind.

This is the stock body to the white porsche you posted. I agree if you intend to pull the body out that far...you will need to make adjustments. The 997/GT3 wheels stick out a few cm's more than mine. On my 997TT....it come out more stock.

XKR

iTrader: (1)

You are correct....Yes the first plan is to get downforce....but the speed shop feels that the open space left behind when the stock muffler was removed, will not hold up long at those speeds....it will rip the rear bumper off.

Also....if air got trapped in there and then desided to escape at the wrong time...that could be a control problem.

Sylvan Lake V35

iTrader: (23)

I see and understand and agree with that, I was refurring to all of the talk about wheels sticking out and correcting that. It seems the drag caused by those "issues" was never really anything that concerned you due to the fix for that will likely be a widebody or fender flares that were not really an option if keeping the stock body to fly under the radar. Yet it seems the focus has shifted to the wheels and the drag they caused, just trying to get the focus back to the underbody

Driven1

iTrader: (2)

In for vid in the wind tunnel.

I would REALLY like to see that. I would love to take my car in one....but NASA aint cheap and that's the closest one to me that I know of. lol

The underbody is WAY more important than the wheels although I completely understand the wheels comment. I've noticed a difference at high speeds (mainly 140+) from my 20" Savini's to the 18" Enkei's I run for the track. It's definitely smoother with the 18's (Im not referring to road smooth).

I have done both the Vizage Type A (FRP w/ reinforcements and no diffuser) and now the Amuse with the front diffuser and I can say there is a difference with the Amuse and diffuser. Significant?...well to be honest...not really. The longer more pointed nose on the Vizage I think helped quite a bit and it had an angled lip at the bottom. With the Vizage I took the side vents/fog light holes and piped them to the brakes...what this ended up doing (I think) is changing the flow around the front wheels b/c the positive air flow coming out of a concentrated hole (brake duct) pushed the flow away from the wheels it seemed. Food for thought although with the location of your oil and trans coolers....it may not be possible.

I actually just went down and talked to my metal fabricator and Im thinking of doing something with that exhaust area underneath as well.

P.S. I love this thread. Gets me thinking.

str8dum1

iTrader: (11)

The Audi R8 has some serious undertrays


There's a few windtunnels in NC
http://www.windshearinc.com/
http://www.aerodynwindtunnel.com/ only $1650 an hour. Thats not bad.
http://www.a2wt.com/ us cyclists use this one, but not a rolling platform..

IIQuickSilverII

iTrader: (13)

^^thats pretty cool.... but it seems liek they also addressed the issue cheboso pointed out with the wheels...bah..i am sure XKR can jsut buy another set of wheels and stop the debate

XKR

iTrader: (1)

Yeah...I will have to work with the stock body....I am sure it will be fine.

Yeah....wait to see what we come up with for the rear before you go getting something made for the rear....You may like what Terry and the gang comes up with.

Funny Jorge ....Not way I am going to change these bad boys

ttg35fort

iTrader: (2)

The primary reason we started on the underbody project was to increase down force. Any additional reduction in drag will be a bonus.

Based on the article provided by Cheston, the drag from the engine is indicated as being from “engine bits” hanging down. We will eliminate that altogether with the front portion of the underbody. Also, we will be eliminating the drag from the exhaust, some of the drag from the rear axle, and the drag under the car will be reduced.

Assume we reduce the drag from the floor and axle each by 50%. Our resulting drag reduction will be about 8.8% based on these figures. Not bad… Of course, our car is different and our actual drag reduction will vary.

To put this into context, I had my car up to about 155-160 mph at Sebring. I was running about 500-550 whp at the time, and it was still accelerating hard. I only backed off because the instructor started tapping my hand to let me know that was fast enough for a newbie my first time out.

Now, Mike will be running significantly more HP, but his wheels stick out a bit whereas mine are pretty well tucked (19 x 9.5F, 19 x 10.5R, both with 30 offset). Of course further drag reductions would be nice to reduce the stress on the motor, but I don’t see that he is going to have a problem achieving the speeds he has set for his goal so long as everything well structurally secured.

Assume my car would have acheived 175 mph (it would have at least reached that for sure), and mike wants to hit 200 mph. 200^2 / 175^2 = 1.31. Let's say I had 550 whp at the time (I know it was less than that because I turned the boost down 3 psi and it was a hot day), Mike would need 720 whp to hit 200 mph assuming that the drag reduction we are getting with the underbody and the fact he won't be using a rear wing offsets the additional drag of his wheels sticking out a tad bit. I beleive that he will be running 800 whp at a minimum, so his HP should be adequate to get him there.

Just my $.02.

EDIT: I forgot, I should have said "FRANKLY, his HP should be adequate to get him there." lol

XKR

iTrader: (1)

Well said...

If it turns out that area is causing drag...it would be an easy fix from what I am told to modify the chargespeed and rear a little so that its not obvious that a stock car was modified. Brian ( the guy with the Aston Martin) said I need to shut up

ttg35fort

iTrader: (2)

You are going to need to modify the CS bumper anyway, so it should not be a problem. Also, since you don't stick out much, you might be able to have the shop use a fender roller to push out the fenders a little bit. I think that will work.

On a side note, I keep opening up the drawing for concept #4. The more I look at it the more I like it. It's now my favorite one. THANK YOU CHESTON FOR THE ADVICE!!! I have to go down to Japtrix to measure my mufflers to see how far I can move them up. If I can get enough clearance with mine, you will have no problem since your mufflers are smaller.

ttg35fort

iTrader: (2)

^

Mike, I first said use a "fender" to push out the fenders. I meant "fender roller".

Motormouth

iTrader: (44)

the amount of power XKR is pushing really should negate the need for all this concern about wheel offsets. of course it'll be more stable and quieter but really the HP should do the trick. I am surprised the rear diffuser is even needed to be honest.

The bugatti isn't a very aero car and look what it did with 1k hp!