Aero downforce?

[ZERO25]

Can you get too much, causing drag on lower hp motors?

[grt74]

throw the cog to her (gear)

[CCHIEF]

Definitely, crate/lower HP cars really benefit from reduced drag, especially at bigger tracks. And they usually produce lowest lap times with highest mid corner speed....usually higher than a Super late. IDK.. lowering gear ratio to overcome drag limits top speed, but viable when track slows down. Not much of a change in ratio although.

[fastford]

more rear aero down force will allow for less static weight though , i remember when the 602 class first came out , RJ was killing them with 45% rear weight because with that big rear spoiler he didnt need the static rear weight to hook it up and was right at minimum weight on scale , most everyone else at the time wanted 55% rear or more and was pulling a lot more weight... just my 2 cents...........

[MasterSbilt_Racer]

more rear aero down force will allow for less static weight though , i remember when the 602 class first came out , RJ was killing them with 45% rear weight because with that big rear spoiler he didnt need the static rear weight to hook it up and was right at minimum weight on scale , most everyone else at the time wanted 55% rear or more and was pulling a lot more weight... just my 2 cents...........

Down force is never bad. Drag can be if you lack the HP to overcome it.

[ZERO25]

Down force is never bad. Drag can be if you lack the HP to overcome it.

Ronnie Lee killed them one year at Brps Nesmith races with a 8" spoiler. Where do you draw the line?

[billetbirdcage]

Ronnie Lee killed them one year at Brps Nesmith races with a 8" spoiler. Where do you draw the line?

The drag squares with speed increase, but a short formula is drag goes up 4 times if the speed was to double. So 100# of drag at 60MPH goes to around 400# at 120MPH.

However as the drag increases it takes more and more HP to overcome that drag, it's roughly at a 8:1 ratio. Every time the speed doubles, the horsepower required to overcome the drag forces is increased by about eight times.

Now keep in mind that when talking speeds under a 100MPH the HP requirement is quite low, typically the drag on a LM while high still only takes around 400HP to run in the 130 to 140MPH range. Now that doesn't mean it's going to get there quickly, but gives you an idea of what your dealing with. Typically with a super you are not concerned with drag and will accept any extra drag to increase downforce, but that is because we have way more power then the speeds we run to actually hurt you much.

If you have a over abundance of traction and limited power then reducing drag can be an option least as far as acceleration but at a cost of maybe cornering speed.

[95shaw]

Not detracting from billet's answer.

Is the drag number derived from the frontal area alone, or are there other factors?




Basically, I am asking, can drag be reduced, without changing frontal area?

Thanks.

Btw, great post, billet.

[RaceEngineer]

Not detracting from billet's answer.Is the drag number derived from the frontal area alone, or are there other factors?Basically, I am asking, can drag be reduced, without changing frontal area?Thanks.Btw, great post, billet.Anything catching air adds drag. Like the spoiler.

[billetbirdcage]

Not detracting from billet's answer.

Is the drag number derived from the frontal area alone, or are there other factors?




Basically, I am asking, can drag be reduced, without changing frontal area?

Thanks.

Btw, great post, billet.

Drag is from entire car. Example a LM, then take the spoiler off the car. You haven't changed the frontal area (least of how I would refer to frontal area - especially if it was a regular car with front and back windshields) but will reduce the drag considerably.

Typically but not always (just rare) that you can reduce drag and increase down force. So most of the time to deduce drag your going to have a loss of down force.

Drag can be shown as a drag coefficient or in pounds of drag. While I'm far from an aero engineer, here are some typical numbers in drag coefficient you might see on various cars:

Best Stock auto's: .23 to .28 is really good
F1 car: Rumored to be as low as .7 to as high as 1.5 depending on the car and track configuration its set for.
LM: in the .5 range give or take some.

[95shaw]

I guess I see frontal area as anything that can be seen from the plane opposing the direction of travel. (anything the wind directly sees. Spoiler on a dirt lm would definetely be that.)
Spoiler hid behind the "greenhouse", not so much. The side of a lm if the car if skewed from rear steer would fall into frontal area.
Maybe I'm looking at it wrong.

[Kromulous]

Speaking of drag, and aero, anyone seen the spoilers with the bubble or concave section on the backside of the spoilers? Opinions?

I seen some CFD anylisis on a SLM on you tube, i think i have posted it before but they surmised that the Drivers helmet was a massive killer of air on the left side of the spoiler.

Also how the angled back portion of the fuel cell helped the air rise up behind the car and help with the massive suction or low pressure are behind the car.

Thought it was interesting, and Billets .5 drag number, wow...

[95shaw]

Hmmmm.

Maybe an aero reason for where some guys have their head?!! ;)

[Matt49]

A racing buddy and I were chatting recently and he said, "Aero is more important now than ever." To which I replied, "Yeah, so is gravity."

My point being that before humans understood what gravity was, things were falling to the ground at the exact same rate. Our understanding of things doesn't make them more important. It just means we know how to better manage the inputs to the outcomes we want.

[RaceEngineer]

A racing buddy and I were chatting recently and he said, "Aero is more important now than ever." To which I replied, "Yeah, so is gravity."My point being that before humans understood what gravity was, things were falling to the ground at the exact same rate. Our understanding of things doesn't make them more important. It just means we know how to better manage the inputs to the outcomes we want.You used to not be allowed to take advantage of it as much. Now, a 4 year old car is junk because of the decking.

[ZERO25]

A racing buddy and I were chatting recently and he said, "Aero is more important now than ever." To which I replied, "Yeah, so is gravity."

My point being that before humans understood what gravity was, things were falling to the ground at the exact same rate. Our understanding of things doesn't make them more important. It just means we know how to better manage the inputs to the outcomes we want.

My college physics professor said it is absolutely necessary that every person should know how to measure the rate of gravity.....if not, they should not be allowed to live!

I believe most racers know how, but don't really understand it!

[billetbirdcage]

I guess I see frontal area as anything that can be seen from the plane opposing the direction of travel. (anything the wind directly sees. Spoiler on a dirt lm would definetely be that.)
Spoiler hid behind the "greenhouse", not so much. The side of a lm if the car if skewed from rear steer would fall into frontal area.
Maybe I'm looking at it wrong.

I guess my response wasn't as understandable as I meant it to be.

I would agree frontal area is the area the air would see, but I was referring to the spoiler on a car with windshields to explain my point. At a front view the rear spoiler may not add any frontal area, but would increase drag. Which is what I was referring to as to answer what I think your question was on if drag can be changed without altering the frontal area.

Maybe a simpler way to describe this is:

Take a LM and chop the nose off right in front of the front tires and instead of a sloped nose you have a blunt flat plate. You could have the exact same frontal area, yet the drag would be higher on the blunt front end.

Or another way is take a LM with say 900 square inches of frontal area and compare to another car or even a flat piece of plywood with the same 900 sq inches and the drags would be considerably different.

I think that would answer your question unless I didn't understand your question right. I think the main reason they figure frontal area is to get the drag coefficient into a number. Most of the data I've seen doesn't use the drag coefficient number but the drag in pounds instead. I think the drag coefficient is more drag vs surface area when comparing vehicles and for racing isn't overly cared about or all that useful.

Again NOT an aero engineer so. . . .

[billetbirdcage]

If you research some F1 aero stuff, part of the reason the front wings are so wild looking is they are directing air around the front tires to reduce drag and increase down force from the open wheels. The front lower part of the front tires (rear was well) actually increases lift so they are reducing that by moving air around the tires and they run in sort of a wake or void of air there instead of the air hitting the tires directly. It's pretty crazy when you see some in depth air flow on them, a lot of it is over my pay grade, lol.

[RaceEngineer]

Even on a SLM, a lot of the spoiler is not frontal area.

[95shaw]

I see your point, billet.
If increasing the angle on the nose, the gain is front downforce, at the expense of more drag.
No free lunch, lol.
Lose the spoiler, less drag at the expense of rear downforce.
Less body skew , less drag down straights, at expense of side force helping in the corner.


Wouldn't the spoiler being obscured by some other part, be a wash in terms of frontal area?

Obscuring the proper portion of spoiler could be benificial, yes?

[RaceEngineer]

I see your point, billet. If increasing the angle on the nose, the gain is front downforce, at the expense of more drag.No free lunch, lol.Lose the spoiler, less drag at the expense of rear downforce.Less body skew , less drag down straights, at expense of side force helping in the corner.Wouldn't the spoiler being obscured by some other part, be a wash in terms of frontal area?Obscuring the proper portion of spoiler could be benificial, yes?It's a wash, yes. But it is a different surface.