Aerodynamics discussion

[mobybrz]

Originally Posted by plucas

Real world testing can defiantly be used. Coast down test aren't perfect but if done right can get very good results. I always just forget to mention it since I do computational analyses

I would be very interested in helping with some real world coast testing and compare this to computational results.

I would definitely be interested in that. I think a simple system of a front and rear ultrasonic ride height sensor along with a GPS speedo would probably be sufficient for basic correlation work.

Originally Posted by EarlQHan

To be honest, I think you guys are overthinking a very simple matter. These are low-powered, production-based, closed-wheel cars. The aero effect is more than likely negligible for a 7.5" wide wheel vs. a 9" wide wheel. The tire is the important part, and assuming you make a similar increase in tire width, the additional grip will overcome the drag and inertial forces in lap times. And that's the thing that really matters here; sometimes it's really easy to lose sight of that.

Agreed, I had a reading comprehension problem.

Originally Posted by Captain Insano

The original person that asked about this was not talking about a static car. u/Josh was asking if the proposed road test would be ok to determine drag difference of using a wider wheel.

Sorry, I misunderstood the discussion point you were making. I thought it was more global about coastdown testing not just the wheels portion I was just trying to through out some info on the general process. I am in total agreement with you an EQH that the aero difference of wider tires would be insignificant compared to the extra grip.

[GTerran]

Just discovered this thread. AWSOME!


anywho.. if anyone is wanting to do the cost down test, i have a stock areo BRZ with 17x9 RPF-1, w/ Pilot super sport 245/40-R17. would be willing to try something in the LA area some time.


Also, I have known a lot of campuses to use 'virtual super computers' to complile a lot of off the shelf or built PCs from around the world to take very large calculations cheaply.... anyone know how to set one of these up? could really crank out some aero data that way :B

[infinite012]

Originally Posted by mobybrz

EDIT: SAE J2084 lays out a procedure for coastdown testing. if anyone wants to spend $80 http://engineers.ihs.com/document/ab...RXCAAAAAAAAAAA

I can get that from work (I'm actually looking at it now...). Who wants to touch me?

[Captain Insano]

No problem mobybrz. Really I was just pointing out the tests could be useful if that is your only means to test a change, just be very careful with results as there are so many variables at play. I actually enjoyed the link to the article about the closed tunnel so thanks for posting that.

EarlQHan, totally agree. If I had to pick between traction and drag for that discussion of the wheel/tire combo, I would care much more about traction and weight change. Last, I would care about drag increase of wheel. RPF1 wheels (15-17lbs) of any normal size for our car are much lighter than OEM wheels (20 lbs).

[plucas]

Originally Posted by infinite012

I can get that from work (I'm actually looking at it now...). Who wants to touch me?

Get it and post up a link so all can see. I would be interested in checking it out.

[infinite012]

Oh herro prease

[mobybrz]

Originally Posted by infinite012

Oh herro prease

nice, that'll make some good bathroom reading today!

[infinite012]

Originally Posted by mobybrz

nice, that'll make some good bathroom reading today!

My mechanical engineering degree is screaming at me saying I should understand at least 10% of what is being said in that spec. My attention has wandered off to trying to schedule my dealer appointment for the crickets.

[u/Josh]

Originally Posted by Captain Insano

Wouldn't overall vehicle and rotating weight play a large role in the propsed test, not just drag? If the wheels had a decent amount of weight difference, I woudl think even over 1 lb per wheel/tire, that might be affecting the overall time it takes vehicle to slow down during the test. So I'm saying you have at least two variables at play in this test. And really more, wind speed changes that occur in real life, even just from minute to minute. Things like that.

Not saying it's a bad idea, just pointing out you might not be 100% confident any one thing you are trying to measure (drag) is all that is affecing the car during the proposed test.

This is correct, although my assumption is that the moment of inertia of the replacement wheels is close to that of the stock wheel and any difference would be negligible when compared to the inertia of the vehicle. Of course it won't be perfect, but engineering is all about making assumptions. A quick calculation of the energy stored due to the linear velocity of the vehicle and the energy stored due to the rotational velocity of the wheels (and any other rotating components if you would like) should show this assumption to be quite good for any reasonable wheel choice.

Originally Posted by EarlQHan

To be honest, I think you guys are overthinking a very simple matter. These are low-powered, production-based, closed-wheel cars. The aero effect is more than likely negligible for a 7.5" wide wheel vs. a 9" wide wheel. The tire is the important part, and assuming you make a similar increase in tire width, the additional grip will overcome the drag and inertial forces in lap times. And that's the thing that really matters here; sometimes it's really easy to lose sight of that.

I would agree, except I believe @CSG Mike has mentioned in the track forums that he thinks a stock powered car is faster around a track on 8" wheels than 9" wheels.

Originally Posted by EarlQHan

Edit: Simplest solution I can think of is stretching the OEM tire on a wider wheel. Some wheels, like the RPF-1 might be as light as stock, making the weight similar. I imagine the inertia isn't too different. Do a coast down with each. Not exact, but it can be used as a reference if you want to know how a wider wheel alone will affect the car.

This is a great idea, and made me think of a similar idea. Just compare the stock wheels/tires to the stock wheels/tires with a large spacer. I don't know how large of a spacer could safely be used (even if temporarily for this test), but this would eliminate a few key variables.

Originally Posted by infinite012

Oh herro prease

I hope my responses don't sound argumentative. You all made very good points. I think peer review is very important, and this forum is great for that.

[EarlQHan]

1. You can use a bolt-on spacer (15mm should be okay), no real world issue. (Some Subaru guys might go "blah blah" about the wheel bearings, but if it's just for a test, it's not staying on very long)
2. The spacer will put the OEM wheel closer to the fender which has an effect, but then you're not considering the air under the car. The wheel is still the same width so it does not have the same frontal area a 9" wheel would.
3. Better have a damn good timing system. I'm willing to bet the difference will be so minute that recording errors will make the biggest difference in times.

[johnnie]

Any possibility of you guys modelling the effect of removing the large exhaust backbox?

It's suposed to improve airflow and I was wondering if leaving a big gap would have a noticeable effect as most aftermaket exhaust mfs just remove it in favor of two smaller silencers.

[Gunman]

It's nice to read some good discussion on the internet about aero. I've worked as a design engineer for 3 different race teams, mostly mechanical design, but also a lot of surfacing for CFD and wind tunnel testing. I've worked with some very good aerodynamicists, and the only sure thing about air flow, is that it has a mind of its own

Question for plucas...out of curiosity, are you using actual Scion CAD data (via SEMA tech transfer) or did you scan a car?

Good reading discussion, keep it up!!

[plucas]

Originally Posted by Gunman

It's nice to read some good discussion on the internet about aero. I've worked as a design engineer for 3 different race teams, mostly mechanical design, but also a lot of surfacing for CFD and wind tunnel testing. I've worked with some very good aerodynamicists, and the only sure thing about air flow, is that it has a mind of its own

Question for plucas...out of curiosity, are you using actual Scion CAD data (via SEMA tech transfer) or did you scan a car?

Good reading discussion, keep it up!!

Airflow as a mind of its own for sure and that is why cfd is so interesting. Since the flow is almost always turbulent around a car at speeds that matter, part of cfd is trying to quantify the chaos of the fluid.

We tried to get SEMA to allow us in the tech transfer, but we aren't an established enough company with products And since we do not have a scanner YET, the car was modeled using very good blueprints I found.

[mav1178]

For an overly simplistic look at what is a diffuser, take a look here:

http://www.mulsannescorner.com/diffuser.htm

Long story short, it won't do much on the FRZ/BRZ if the airflow in front of the diffuser (under the car) is not cleaned up.

-alex (my first post, yay!)