Aerodynamics discussion

[djliquidsteele]

GREAT!! So where are we at on hoods? I want to get going on the underpaneling but want to make sure engine, transmission, and diff temps don't increase dramatically. I already figure i'll need the cusco diff cooler or something like it. Can a transmission get too hot?

[plucas]

Originally Posted by djliquidsteele

GREAT!! So where are we at on hoods? I want to get going on the underpaneling but want to make sure engine, transmission, and diff temps don't increase dramatically. I already figure i'll need the cusco diff cooler or something like it. Can a transmission get too hot?

Give me an idea of what you are looking for hood design wise. I can get some base models made. Also are you running a stock radiator setup?

Yes a transmission can get too hot

[u/Josh]

Originally Posted by Hancha Group

We just got done creating a new, and more accurate CFD model for the FRS. This will be used for our development of the car. Below are some post-processing pictures of a stock FRS with Rota Grids.

Any simulations comparing the aerodynamic drag of stock wheels vs. a wider wheel? I know there is some debate in the track section of the forum whether or not the car is faster with 9" wheels.

[djliquidsteele]

Originally Posted by plucas

Give me an idea of what you are looking for hood design wise. I can get some base models made. Also are you running a stock radiator setup?

Yes a transmission can get too hot

I'm interested to see if the Seibon DV Style and TS style will hurt anything aerodynamically and if it would help cool the engine.

[djliquidsteele]

Also, would the APR mirrors help reduce some drag. And with this, would the TRD side vent canards like on the Griffon 86 help reduce drag over the mirrors even more?

[Hancha Group]

Originally Posted by u/Josh

Any simulations comparing the aerodynamic drag of stock wheels vs. a wider wheel? I know there is some debate in the track section of the forum whether or not the car is faster with 9" wheels.

I'm sure Paul can model this when he has more free time. Right now, he's working on finishing some CFD work for a GTI that's competing in One Lap. As for the wider wheel, you can add a wider tire, increasing grip, which will likely offset any increase in weight, drag, or lift.

Originally Posted by djliquidsteele

Also, would the APR mirrors help reduce some drag. And with this, would the TRD side vent canards like on the Griffon 86 help reduce drag over the mirrors even more?

1/2*p*v^2*coeff*a

Aero mirrors decrease the area, so as long as the coefficient doesn't significantly increase, the drag force will be reduced.

Paul will have to answer the hood question, as I am not as knowledgeable.

- Earl

[plucas]

Originally Posted by u/Josh

Any simulations comparing the aerodynamic drag of stock wheels vs. a wider wheel? I know there is some debate in the track section of the forum whether or not the car is faster with 9" wheels.

I have not compared the differences however I can. The problem with this is that is will not be a black and white answer. It will depend on the track. What really needs to be done is to run cfd analyses in both configurations and use this data to run vehicle simulations at different tracks.

Originally Posted by djliquidsteele

I'm interested to see if the Seibon DV Style and TS style will hurt anything aerodynamically and if it would help cool the engine.

To help cool, you will need ducting. Airflow is the most important aspect to heat rejection of the radiator. It shouldn't hurt anything, but wouldn't know for sure without an analysis.

Originally Posted by djliquidsteele

Also, would the APR mirrors help reduce some drag. And with this, would the TRD side vent canards like on the Griffon 86 help reduce drag over the mirrors even more?

Yes they would as the area is less. Coefficient of drag is probably similar. The vent canard is interesting. If it works, it should reduce the drag over the mirrors.

[u/Josh]

Originally Posted by plucas

I have not compared the differences however I can. The problem with this is that is will not be a black and white answer. It will depend on the track. What really needs to be done is to run cfd analyses in both configurations and use this data to run vehicle simulations at different tracks.

I know, but it would at least be a start. If your testing shows the increase in drag is negligible then the debate is over. Of course we would have to agree on a definition of 'negligible' and that may take us back to running simulations at various tracks.

Side note, is there any reason real world testing couldn't be used here? I am thinking of the following test procedure:

1) find level, straight road
2) accelerate to a safe max speed (say 85 mph)
3) put the car in neutral and time how long it takes to slow down by some amount (say to 65 mph)
4) repeat on the same road in the opposite direction
5) repeat steps 1-4 with wider wheels/tires

From here you could back out the drag coefficient of each configuration. Of course this would also include other friction losses but if the testing speeds are high enough those other losses should be negligible relative to aerodynamic drag.

Obviously this data still wouldn't conclusively answer which setup is faster on a given track, but again, it is a start.

[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.

[BRZ NA]

Originally Posted by OrbitalEllipses

Well that settles that...

@ayau Check it Aran...

Can it be fit with stock exhaust?

[mobybrz]

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.

http://aspeckt.unitbv.ro/jspui/bitst...4030-Paper.pdf is a reasonable starting place.

You are obviously correct about other things impacting the slowing of the vehicle. Those would include rotating inertia, mechanical drag, and aerodynamic forces. those would all be then further impacted by other parameters such as sloped roads, road crown, wind, etc. The procedure however can still be broken down into a characterizable phenomenon.

Mechanical drag - generally very linear and constant. dependant on the total mechanical friction of the drivetrain.
Rotating inertia - agian, very consistant, and not really all that variable. The impact is easily calculated.
outside forces - can be adjusted by running repeated tests in both directions on a given piece of road. This will cancel out wind and slope.
aero forces - follow a power rule and will look very different from the other forces at varying speeds.

One would need to perform a fairly large number of coast down tests on a known circuit at a variety of speeds in both directions. Then, a regression analysis could be performed which should make it possible to separate out the aerodynamic forces mathematically from other forces. I should note, that unless you have an instrumented suspension, you will afaik only be able to extract the force of drag. Extracting downforce would require knowing and measuring suspension defflection. Once a baseline analysis was performed it would be fairly simple to perform further tests with modifications and basic data collection.

This type of testing is so common/helpful in motorsports that Chip Ganassi actually bought an old tunnel in Pennsylvania (http://www.racecar-engineering.com/a...f-laurel-hill/). These tests are highly useful to establish correlations between CFD, wind tunnel, and real world testing which is a serious problem.

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

[Captain Insano]

I am not saying it would be useless, only that with all those variables simultaneously in play you might draw erroneous conclusions. Besides the external elements, if the car did appear to have more drag but the wheel tire combo was lighter than oem, was it more drag of the 9" wheel or the weight change affecting inertia? On the flip side, if wheel tire combo had what appeared to be less drag and it was heavier was it due to weight or drag. See what I mean? And you might have in reality more drag on that wider wheel, but it actually appears to have less overall drag because the weight is heavier and having a more dramatic affect on the test (inertia versus drag) on proposed coast down from speed test.

Your article about the tunnel actually supports what I am saying. It is a very controlled environment.

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.

[plucas]

Originally Posted by u/Josh

I know, but it would at least be a start. If your testing shows the increase in drag is negligible then the debate is over. Of course we would have to agree on a definition of 'negligible' and that may take us back to running simulations at various tracks.

Side note, is there any reason real world testing couldn't be used here? I am thinking of the following test procedure:

1) find level, straight road
2) accelerate to a safe max speed (say 85 mph)
3) put the car in neutral and time how long it takes to slow down by some amount (say to 65 mph)
4) repeat on the same road in the opposite direction
5) repeat steps 1-4 with wider wheels/tires

From here you could back out the drag coefficient of each configuration. Of course this would also include other friction losses but if the testing speeds are high enough those other losses should be negligible relative to aerodynamic drag.

Obviously this data still wouldn't conclusively answer which setup is faster on a given track, but again, it is a start.

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.

[EarlQHan]

Originally Posted by mobybrz

http://aspeckt.unitbv.ro/jspui/bitst...4030-Paper.pdf is a reasonable starting place.

You are obviously correct about other things impacting the slowing of the vehicle. Those would include rotating inertia, mechanical drag, and aerodynamic forces. those would all be then further impacted by other parameters such as sloped roads, road crown, wind, etc. The procedure however can still be broken down into a characterizable phenomenon.

Mechanical drag - generally very linear and constant. dependant on the total mechanical friction of the drivetrain.
Rotating inertia - agian, very consistant, and not really all that variable. The impact is easily calculated.
outside forces - can be adjusted by running repeated tests in both directions on a given piece of road. This will cancel out wind and slope.
aero forces - follow a power rule and will look very different from the other forces at varying speeds.

One would need to perform a fairly large number of coast down tests on a known circuit at a variety of speeds in both directions. Then, a regression analysis could be performed which should make it possible to separate out the aerodynamic forces mathematically from other forces. I should note, that unless you have an instrumented suspension, you will afaik only be able to extract the force of drag. Extracting downforce would require knowing and measuring suspension defflection. Once a baseline analysis was performed it would be fairly simple to perform further tests with modifications and basic data collection.

This type of testing is so common/helpful in motorsports that Chip Ganassi actually bought an old tunnel in Pennsylvania (http://www.racecar-engineering.com/a...f-laurel-hill/). These tests are highly useful to establish correlations between CFD, wind tunnel, and real world testing which is a serious problem.

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

Originally Posted by Captain Insano

I am not saying it would be useless, only that with all those variables simultaneously in play you might draw erroneous conclusions. Besides the external elements, if the car did appear to have more drag but the wheel tire combo was lighter than oem, was it more drag of the 9" wheel or the weight change affecting inertia? On the flip side, if wheel tire combo had what appeared to be less drag and it was heavier was it due to weight or drag. See what I mean? And you might have in reality more drag on that wider wheel, but it actually appears to have less overall drag because the weight is heavier and having a more dramatic affect on the test (inertia versus drag) on proposed coast down from speed test.

Your article about the tunnel actually supports what I am saying. It is a very controlled environment.

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.

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.

IndyCar reference isn't relevant in this case, now more than ever, because it's a spec series. They try to make gains by reducing drivetrain loss. I know because my old roommate is an IndyCar engineer and they'd do things like make brake dynos and diff dynos and all other sorts of ridiculous things to gain .01 s.

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.