New Pricing: A2 Wind Tunnel

[Sumner]

Sum, somewhere we got the info that open spinning wheels were a 1.16 to 1.21 depending on size.  Larger were the lower drag but still a little worse than a brick.  But I think A2 is on it with the baseline and then try to improve.  You are stuck with Open wheels in the lakester class so their drag doesn't really matter.  All you can do is try to get the best from the part you can change. 

Well if you know somewhat what the Cd of your open wheels are then you might be able to get an approximate estimate for the Cd of the car if you know the speed and the rear-wheel HP.  Now I realize there are a lot of if's there, but I'd still like to try and figure some of that out. 

Dave the body is 6 inches high at the side and center of the wheel and the body is more than 12-15 inches away.  I realize that a wind tunnel would be a good deal, but is probably going to be out of the question for someone working on the budget I and a lot of others are working with.  We do appreciate all that you have shared here.

Stainless I've seen those numbers before for a wheel, but it also seems I've seen way lower like the .4 I mentioned that would be in the neighbor hood of the sphere or half sphere, but it doesn't look like they take the rotation into account.  I would think the rotation would also be effected the amount of tread on the tire.

c ya,

Sum

[Stainless1]

If you have open wheels then, yes, you are stuck with them unless you try various sizes (widths, heights) and different covers (flat vs. convex).  A wheel alone test will tell you about the wheel and which one is best, but what is that really telling you?  The car runs as a system and all the pieces/parts interact and talk with each other.  A solid axle will affect the wheels aero different then a control arm front suspension and any combo in between.  Also depending on how far away the wheel is to the body, the streamlines will not be parallel to the flow.  So testing the wheel alone is not the best representation of what is happening on your specific car.  And I would be careful with posting Cd # of a wheel.  You need to be really specific when you post 1.16 – 1.21 … Is that Cd or CdA?  What is the reference area if that is CdA (ft^2 or m^2)? Where did that # come from (a book, independent test)?  Aerodynamics is not as simple as if you have this Cd for one wheel you can just add 3 more to get the drag on the wheels for your car.  A parachute is something like Cd = 1.3 and a flat plate is close to Cd = 1.0 so those numbers you came up with seem really high for a Cd and that is why I am asking you to be more specific.

A2, let me try this again...
I am not an aerodynamisist and have never advertised I am one.  I just race a lakester.

Sum, somewhere we got the info that open spinning wheels were a 1.16 to 1.21 depending on size.  Larger were the lower drag but still a little worse than a brick.  

When we converted the Bockscar from a streamliner to a lakester we talked to a couple of aero folks, one was the guy that originally designed the car.  One of them told us spinning wheels in open air are worse than a brick (your 1.05) and would be a substantial drag penalty.  We were planning to use Motorcycle wheels and tires and he said you are looking at about 1.16 CD for an open wheel 28 X 4.5 frontal area.  We could improve that number with inner and outer moon discs.  We were also given the wheel configuration, wide front, narrow back.  Several years later we found the tires we currently run and called again since we were loosing 45 Sq inches of frontal area per tire.  Again told smaller tires had more drag because they must be spun faster and create more turbulence.  Expect 1.21 CD for smaller wheels.  Some improvement over that for moon discs.  All I know is the car went 10 MPH faster with the small wheels.    no other changes.  Can a smart aero guy tell anything from that info?

But I think A2 is on it with the baseline and then try to improve.  You are stuck with Open wheels in the lakester class so their drag doesn't really matter.  All you can do is try to get the best from the part you can change.

Running a lakester your wheels must not have any streamlining outside the inner plane of the narrowest tire other than wheel discs.  So you can only work with wheel configuration, and body.  You have 4 open wheels generating turbulence. Not much you can do there, that is why lakesters are generally slower than streamliners.  So you can hide your steering inside the wheel, and the tie rods behind or inside an aero axle.  I'm sure all of us could improve our speeds by improving our aero, it is easier than improving HP in most cases.

So to be specific, we were given the numbers from the "hip" of a LSR Aero guy who is well known but unfortunately no longer with us.  Take 'em or leave them, to a lakester they don't really mean anything other than you have drag from the wheels.
We have always used the big white dyno and the big white wind tunnel to help us determine how fast we will go.  We take ideas with us and make changes when possible to try our ideas. 
We are just plain old landspeed racers, doing what we can to go fast... 

[Rex Schimmer]

Sum,
Dave makes a great point about the interaction of the wheel/tire assembly and the rest of the car. In a recient issue of Race Car Engineering, their resident aero guy, Simon McBeath, did some wind tunnel work with a Formula V, which as you know, uses the standard VW trailing arm front suspension. This has a pair of tubes, one above the other that the trailing arms are connected to. They look pretty "un-aero". They tested the standard set up for a base line then added some fairing around the tubes and the rest of the suspension. Almost no change in Cd and the Cl actually increased! All do to the interaction of the wheel/tire with the suspension and with the cars body. It ain't obvious!!

I see these kinds of aero tweaks the type of thing that the A2 tunnel could really help you with.

A little bit more regarding the VW suspension. In the next issue Simon did a little more analysis and it appeared that one of the reasons for the increase in the Coefficient of lift, Cl, was that the air coming past the front of the car was actually traveling upward when it passed the faired suspension parts which gave the aero fairing an angle of attack and produced lift and the accompaning drag. Simon theroized that maybe the most aero shape for the suspension tube fairing would be for them to be mounted with a nose down attitude so that they were inline with the air flow. Not something you would think about when you were taking the first try. And on this note look at the axle fairings on Seth's new car, they are angled down. Interesting.

Rex

[Rex Schimmer]

Stainless,
I think your aero guy is off when it comes to the CD of wheel/tires. If you look in "Race Car Aerodynamics" by Joseph Katz, you will see that he gives the Cd of wheel/tires based upon their width/Diameter ratio.

Width/Dia               Cd
.28                        .18 (.272)
.50                        .40 (.95)
.612                      .48 (.76)

The number in the ( ) is the Cd of the NONROTATING wheel/tire.

Also if you look in "The Leading Edge" by Goro Tamia he list the Cd of some very thin and large diameter wheel/tires, basically racing bicycle tires, which probably have a W/D ratio of less than .1 and the Cds are in the .04 area and these numbers were generated by the MIT for their sun racer.

Rex

[Rex Schimmer]

I am on a roll here! Just ran across this pic of the wheel/tire fairing on the no. 857 AMGR of Kuntz/Jackson. Pretty neat. This is the kind of thing that you find out in the wind tunnel if it is worth it.

Rex

[Stainless1]

Rex, or Joe has it wrong....  just mentioning what we were told in 84, not sure where he got his info, but it does not really matter, we put the car together, went to the salt, ran it and the rest is history....    The car has changed a lot over the years, all changes tested at the salt.  Here is where we started, hard to say where we will finish... 

[1212FBGS]

i think the A2 guy is right and wrong... right fron the standpoint that you can refine the body without the wheels spinning but wrong in implying you can get results without the sum of the whole package... you can get you lakester as dialed in as possable but as soon as you spin those tires all your hard work may just go out the window... especially if the wheels are close to the body for the wash to screw up your nice new bodywork...  if at all possably test as you will be running... it may be more money getting into a tunnel that has a rolling road like swift but it wont be waisted money like if you test without....kinda like peeing in the wind ....
kent

[Stainless1]

Kent, like most LSR types we have spent a little time pissing in the wind.   
We have installed numerous temporary mods at the salt to see if they deliver any advantage.  So lakester guys take note your wheels add drag, they may change your aero when they spin, and the only way to know how fast your car will go is to take it to Bonneville, pray for good salt, good conditions and race. 
I think our chief LURKER Jack D once said "theoretical cars only set theoretical records"... 
Just go racing and prove your calculations.  If it was easy we would all have a hat like Sparky!

[Sumner]

..................A little bit more regarding the VW suspension. In the next issue Simon did a little more analysis and it appeared that one of the reasons for the increase in the Coefficient of lift, Cl, was that the air coming past the front of the car was actually traveling upward when it passed the faired suspension parts which gave the aero fairing an angle of attack and produced lift and the accompaning drag. Simon theroized that maybe the most aero shape for the suspension tube fairing would be for them to be mounted with a nose down attitude so that they were inline with the air flow. Not something you would think about when you were taking the first try. And on this note look at the axle fairings on Seth's new car, they are angled down. Interesting.

Rex

Porkpie gave me some ideas for the side pods that will run along the sides of the car from the front axle to the rear to help reduce any lift associated with them that even though it is going to be more work I'm going to try.

I've felt that some of the success for Seth with the original lakester was due to the wheels/tires being away from the sides of the body and it influenced me enough to try and emulate it with some changes.

c ya,

Sum

[Dr Goggles]

Without wasting too much ink...the Rev and I decided that compared to the frontal area of axle the benefit of getting the wheels away from the body was huge...no point having the body mixed up in the turbulence created by them...might as well have a box if that was the case...er ,and it meant we didn't have to narrow the rear....

[A2WindTunnel]

i think the A2 guy is right and wrong... right fron the standpoint that you can refine the body without the wheels spinning but wrong in implying you can get results without the sum of the whole package... you can get you lakester as dialed in as possable but as soon as you spin those tires all your hard work may just go out the window... especially if the wheels are close to the body for the wash to screw up your nice new bodywork...  if at all possably test as you will be running... it may be more money getting into a tunnel that has a rolling road like swift but it wont be waisted money like if you test without....kinda like peeing in the wind  ....
kent

Like I said, there are ways to simulate the turbulence of the wheels spinning in a non-spinning application.  Is it perfect? No, it's a simulation, and for indy car teams to have done this for many years I can't imagine all those engineers would consider it "peeing in the wind".  I'm pretty sure they found some value in testing this way or why waste the $$ and time?  Is any wind tunnel perfect? No.  A wind tunnel is a testing laboratory with a Controlled environment to keep variables to a minimum and able to give precise, accurate, repeatable data.  Even rolling road wind tunnels have limitations and compromises.  I am on here to help you guys learn about wind tunnel testing and the importance of aero.  I think 1212FBGS is missing my point.  The point is you are not looking for absolute values no matter which wind tunnel you test at, and a "simulation" of the wheels spinning is not the best, but you will still see what changes help and hurt the car.  You also need ask your self this.  If a rolling road wind tunnel is the only way to test, then why would non-rolling road wind tunnels exist?  Because you can still learn useful information without the wheels spinning and race teams have done this since the 70's before they had rolling road wind tunnels.

I am not on here by any means to say that A2 is the only wind tunnel, and everyone should test here . However, A2 is a great wind tunnel for $390/hr when compared to other options.  I did put the list off all my competitors (Full Scale Car) on this thread for all of you to see what options are out there and the costs associated to see if they might better benefit your racing program.  I did say peviously that I do not have info on which tunnels test motorcycles or costs, but welcome any info that someone is willing to share.  There is one FULL SCALE rolling road wind tunnel in North America (wind shear) at $4500/hr (Swift is not a FULL SCALE wind tunnel for cars).  Another one is AeroDYN, which has an active boundary layer control system (simulated moving ground plane) with spinning wheels, automated ride heights, and is where nearly 100% NASCAR teams test at $1600/hr.  If you think the add $$ is worth the wheels spinning for your car, and the only way you can get useful data, then I welcome you to book some time at either of them.  We have NASCAR Cup, Nation Wide, and Truck teams in A2 testing without wheels spinning and the teams seem to find the information learn very valuable and useful (these teams have been to AeroDYN, full scale and scale rolling road tunnels).  I have also had several open wheel car teams in that found the information valuable, and the LS open wheel cars and motorcycles that have tested did go faster after the information learned in A2 (for whatever that is worth).

Track testing is another option and can be done, but you have many variables to keep track of, and one variable you cannot control on the track is the natural conditions.  If you make one pass early then make one late, what are the changing conditions between the two? : head wind, cross wind, temp, salt conditions, engine, tires, etc...  There are engineers at all the auto manufactures that have made careers out of track testing and even they will tell you it is not a simple thing to do.  You do need to validate what you learn in a wind tunnel with track testing, but just putting something on at the track and looking at your speed is not the whole picture.  Just a 5mph head wind can affect your drag at top speed.  So does that mean that the piece you put on didn't work?  Did it add downforce and added traction made the change in speed or was it drag related?  These are just a few questions that you need to ask yourself when track testing.  Plus the time/$$ it takes to track test.  Say it takes you 4 hours to make two passes where the conditions changed and you are not sure how to interpolate the data.  In a wind tunnel you could make 25 runs and test 25 different things and know exactly how much of an affect it had on drag, downforce front/rear, side force front/rear.  How many trips/$$ to the salt would it take to test 25 things in “ideal” conditions to find the best configuration for your car/motorcycle?

Here is a video of A2 conduction a bicycle track test at Lowe’s Motor Speedway.  They did not talk about all the variables that we monitored because they didn't have time and it would probably make the video really boring to those non-nerds.  But it did show that it can be done and it correlates with what we learned in the tunnel.
http://www.testrider.com/fly.aspx?layout=player&video=1

I think this might need to be split to 2 treads: cars and motorcycles.  What do you guys think?  We have been talking about open wheel LS cars and then a motorcycle question gets thrown in which is two different set of answers when it comes to rolling road tunnels that can test a car or a motorcycle.  I don't want the car guys to think they can test in a scale tunnel, and at the same time I don't want motorcycle guys to think that there is only one rolling road wind tunnel in North America they can test.

[krusty]

Maybe wind tunnel testing without spinning wheels isn't the perfect solution, but Dave is correct when he says you can accomplish  A LOT in the tunnel.  Although all of the cars I've taken to the various tunnels in the US and Canada were closed wheel, we were certainly able to quantify minute changes in one run, due to the controlled environment. Having AeroDyn (and A2) here in the area  spoiled us; we used to have to haul to Marietta, GA, or Detroit, or Ottawa, Canada to use a tunnel. Testing scale models is equally expensive as extreme accuracy must be maintained.  Coast-down testing is really difficult as it is near impossible to duplicate conditions run-to-run. As a veteran of the eastern side of the GM Proving Grounds in Mesa, I know that maintaining fluid temperatures and wind speed is hard to do. Evaluating small changes to the shape of the underside of a Cup car is difficult (and laying on the hot asphalt out there to do it is no fun, either). That's why the Ganassi guys built their coastdown area in an abandoned mountain tunnel in Pennsylvania - to control the variables. Unfortunately, I've heard that it is also hard on equipment. Perhaps some enterprising racers on the West Coast would be interested  in licensing Gary's A2 design out there.  Our new build (an open wheel car) will be in the A2 (hopefully May 09).  No, I am not an aero guy or an engineer - I was just one of the guys riviting and taping  the changes on the cars. vic