The Mustang Forum for Track & Racing Enthusiasts

Taking your Mustang to an open track/HPDE event for the first time? Do you race competitively? This forum is for you! Log in to remove most ads.

  • Welcome to the Ford Mustang forum built for owners of the Mustang GT350, BOSS 302, GT500, and all other S550, S197, SN95, Fox Body and older Mustangs set up for open track days, road racing, and/or autocross. Join our forum, interact with others, share your build, and help us strengthen this community!

Ask AJ Hartman - Aero Questions

I think one of the thing we really should investigate is what happen when a tire is POKING OUT A LOT

some says the tire spats will help with drag deflecting the air that would otherwise impact the rotating tire and also with DF (.with better air extraction from under the car)
Someone says tire spats increase.DF but with major drag.
Also.how much tire would u cover?
50% 75% 100%?


many test are done with tires practically completely cover by the fenders.
Would be nice to.test a car with and without SPACERS and tirespats.
 
194
146
Exp. Type
Autocross
Exp. Level
20+ Years
So Cal
I think one of the thing we really should investigate is what happen when a tire is POKING OUT A LOT

some says the tire spats will help with drag deflecting the air that would otherwise impact the rotating tire and also with DF (.with better air extraction from under the car)
Someone says tire spats increase.DF but with major drag.
Also.how much tire would u cover?
50% 75% 100%?


many test are done with tires practically completely cover by the fenders.
Would be nice to.test a car with and without SPACERS and tirespats.

Those spinny things in the airstream are never good. One job of a front wing endplate on an open-wheel car is to get the air away from the tire. I do not think that it's much of a drag penalty (if any), because the drag penalty of a spinning tire is massive. So, I'd cover as much of it as you can, but no more than necessary to get the air around the tire (how's that for hedging bets?).

If there's one thing that I have *absolutely* learned about testing aero is that whatever you think might be happening probably isn't. The only way to really know without going to a tunnel is to test it about 1/2 dozen times until you're convinced one way or the other. If you have suspension position sensors, it's a little easier. If you have load cells, it's a lot easier.
 
Those spinny things in the airstream are never good. One job of a front wing endplate on an open-wheel car is to get the air away from the tire. I do not think that it's much of a drag penalty (if any), because the drag penalty of a spinning tire is massive. So, I'd cover as much of it as you can, but no more than necessary to get the air around the tire (how's that for hedging bets?).

If there's one thing that I have *absolutely* learned about testing aero is that whatever you think might be happening probably isn't. The only way to really know without going to a tunnel is to test it about 1/2 dozen times until you're convinced one way or the other. If you have suspension position sensors, it's a little easier. If you have load cells, it's a lot easier.
tell me more about load cells pls.
 
I think one of the thing we really should investigate is what happen when a tire is POKING OUT A LOT

some says the tire spats will help with drag deflecting the air that would otherwise impact the rotating tire and also with DF (.with better air extraction from under the car)
Someone says tire spats increase.DF but with major drag.
Also.how much tire would u cover?
50% 75% 100%?


many test are done with tires practically completely cover by the fenders.
Would be nice to.test a car with and without SPACERS and tirespats.
Who says it helps with drag? We've chatted on FB about this exact thing. I have yet to see a wind tunnel result, or a CFD result where they don't increase drag so I'm skeptical that a tire blocker will decrease drag. I would love to see a setup with data where it may cause I love to learn. On the flip side, they are easy Df generators and generally work well, but the amount of Df they add vs. the amount of drag is on the lower side of things, making them not that efficient of a setup.
 
Who says it helps with drag? We've chatted on FB about this exact thing. I have yet to see a wind tunnel result, or a CFD result where they don't increase drag so I'm skeptical that a tire blocker will decrease drag. I would love to see a setup with data where it may cause I love to learn. On the flip side, they are easy Df generators and generally work well, but the amount of Df they add vs. the amount of drag is on the lower side of things, making them not that efficient of a setup.
Yep i was referring at u for "tire spats increase.DF but with major drag." :)

I received other input from other people when i went to daytona to put some tire spats covering 50% of the tire to decrease the amount of air interfering with the tire in rotation. Which is hard to measure in a static wind tunnel.

But again I'm learning!

So far as we planned ,
tire spats on when I'm in a lower speed track. And off in a higher speed one.
But i would love to learn more on a dynamic and not static test.
 
Last edited:
Also would you not need a rolling road dyno in a wind tunnel to really model the impact of tire spats and/or splitter end plates because of the impact of the rotating wheels?

Also AJ could it be possible that tire spats do not increase overall drag because the tire sticking out prior to the addition of the spat created the same amount or more drag than the spat itself? Add the spat which is say 50lbs drag but as a result you lose 50lbs drag that the tire was previously producing? So on a net basis drag is the same after the addition of the tire spat?
 
Yep i was referring at u for "tire spats increase.DF but with major drag." :)

I received other input from other people when i went to daytona to put some tire spats covering 50% of the tire to decrease the amount of air interfering with the tire in rotation. Which is hard to measure in a static wind tunnel.

But again I'm learning!

So far as we planned ,
tire spats on when I'm in a lower speed track. And off in a higher speed one.
But i would love to learn more on a dynamic and not static test.
So did you test this? It would have been easy enough to do an A-B test with and without the tire blocker and look at your top speed data, especially at Daytona.
 
Also would you not need a rolling road dyno in a wind tunnel to really model the impact of tire spats and/or splitter end plates because of the impact of the rotating wheels?

Also AJ could it be possible that tire spats do not increase overall drag because the tire sticking out prior to the addition of the spat created the same amount or more drag than the spat itself? Add the spat which is say 50lbs drag but as a result you lose 50lbs drag that the tire was previously producing? So on a net basis drag is the same after the addition of the tire spat?
To the first part, it would simulate real life a little better yeah. But if a tire blocker creates drag without the tire rotating, I have an extremely hard time believing that it would behave so much differently, that the car would reverse trends and now be making less drag. So until I'm proven wrong on this either by myself or others, I'll stand behind tire blockers increasing drag almost no matter the size. And heres why, which also answers the second part of your question. I think were all in agreement that a tire blocker sticking straight out sideways to block air hitting the tire creates a lower pressure on the tire face and in the wheel well by creating a nice outwash there. That forward facing high pressure zone doesn't go away, its just moved forward to the tire blocker, instead of the tire face. So drag from total frontal area hasn't changed. Now back to the lower pressure on the tire face and in the wheel well. This new lower pressure zone wants to be filled, so more upstream air from under the splitter (which is the big contributor to their effectiveness in helping the car make more downforce), and through the radiator (we've seen pressure drops across a radiator when doing tire blockers and canards which supports this) wants to fill it and has to go somewhere. So a combo of the splitter being worked harder (more air under that then hits the under side of your car) and more air through the radiator (that then hits everything in the engine bay and has to go somewhere) are what contribute to the drag increase. The whole car and package need to be taken into account. Not just the small sliver of blocking a tire without any other effects.
 
So did you test this? It would have been easy enough to do an A-B test with and without the tire blocker and look at your top speed data, especially at Daytona.
I did not. Was too focused to not kill my self lol.

And mostly because I'm learning. So i assumed it was the right thing to do.but you are right. I should have.
I'll try to test at willow springs where speeds are high enough.

Let me ask the last question. We have talked about it but i think it is nice to share it with the group.
How much is too much for a tire spats and canards? I mean
In the case of Sal (fabman) the tire spat completely stop the flow of canards . In mine it is ~40%.
During the wind tunnel you tested the combo.
Any recomandations?

Screenshot_20220511-073458.png
 
To the first part, it would simulate real life a little better yeah. But if a tire blocker creates drag without the tire rotating, I have an extremely hard time believing that it would behave so much differently, that the car would reverse trends and now be making less drag. So until I'm proven wrong on this either by myself or others, I'll stand behind tire blockers increasing drag almost no matter the size. And heres why, which also answers the second part of your question. I think were all in agreement that a tire blocker sticking straight out sideways to block air hitting the tire creates a lower pressure on the tire face and in the wheel well by creating a nice outwash there. That forward facing high pressure zone doesn't go away, its just moved forward to the tire blocker, instead of the tire face. So drag from total frontal area hasn't changed. Now back to the lower pressure on the tire face and in the wheel well. This new lower pressure zone wants to be filled, so more upstream air from under the splitter (which is the big contributor to their effectiveness in helping the car make more downforce), and through the radiator (we've seen pressure drops across a radiator when doing tire blockers and canards which supports this) wants to fill it and has to go somewhere. So a combo of the splitter being worked harder (more air under that then hits the under side of your car) and more air through the radiator (that then hits everything in the engine bay and has to go somewhere) are what contribute to the drag increase. The whole car and package need to be taken into account. Not just the small sliver of blocking a tire without any other effects.

thank you very much for taking the time to explain this!
 
194
146
Exp. Type
Autocross
Exp. Level
20+ Years
So Cal
the amount of Df they add vs. the amount of drag is on the lower side of things, making them not that efficient of a setup.

Ya, but it's a GT car. Any downforce you can get, regardless of efficiency, is usually worthwhile on the watch, especially since that big lump between the front tires means we need to generate quite a bit on the front of the car.

I've got some real numbers on wheel arch gurneys somewhere. I'll try to find them.
 
194
146
Exp. Type
Autocross
Exp. Level
20+ Years
So Cal
OK, I found the spreadsheet. I believe this was for a 1/4" or 3/8" fender gurnery which started at the forward bottom of the wheel arch and went to axle centerline height. The car was the original Riley DP.

The wheel arch gurney added 19# to the front and took 17# from the rear. Drag was under the noise floor to measure. So, on this car, it was really more of a tool to shift balance rather than something which actually added to total downforce. We ran some version of them almost all the time, but, at some place like Daytona, they were either very small or removed altogether.
 
I did not. Was too focused to not kill my self lol.

And mostly because I'm learning. So i assumed it was the right thing to do.but you are right. I should have.
I'll try to test at willow springs where speeds are high enough.

Let me ask the last question. We have talked about it but i think it is nice to share it with the group.
How much is too much for a tire spats and canards? I mean
In the case of Sal (fabman) the tire spat completely stop the flow of canards . In mine it is ~40%.
During the wind tunnel you tested the combo.
Any recomandations?
That stinks. For someone that wants as many answers as you, you missed a golden opportunity there. And again, we've chatted about this. I feel like you think there is one, easy, perfect for every situation or track solution and there simply just isn't. Sals setup is set for pretty much maximum Df working within space constraints and a rule set which dictates what is possible sometimes. If he was taking that car to Daytona, I'd tell him to take it all off. A tight twisty track, I'd run it how he posted above. Some middle of the road track on Df vs. top line speed... pull some parts off and test.
 
194
146
Exp. Type
Autocross
Exp. Level
20+ Years
So Cal
If he was taking that car to Daytona, I'd tell him to take it all off. A tight twisty track, I'd run it how he posted above. Some middle of the road track on Df vs. top line speed... pull some parts off and test.

I'm always surprised at how much a little extra drag around Daytona kills lap time. You feel like a little extra downforce will help in the kink, bus-stop or braking (and it does), but it's just never enough to make up what you lose on the banking.
 
OK, I found the spreadsheet. I believe this was for a 1/4" or 3/8" fender gurnery which started at the forward bottom of the wheel arch and went to axle centerline height. The car was the original Riley DP.

The wheel arch gurney added 19# to the front and took 17# from the rear. Drag was under the noise floor to measure. So, on this car, it was really more of a tool to shift balance rather than something which actually added to total downforce. We ran some version of them almost all the time, but, at some place like Daytona, they were either very small or removed altogether.
Pretty cool to see those numbers. Kinda backs up my post above. Since DP's had bodywork coving the tire already, sealed radiators front and rear and (mostly) flat floors, there was probably zero extra air through the cooling system so no drag gain from that, and the small front Df gain was just about equaled by leverage loss off the rear. The minimal amount of extra air under the splitter wasn't as detrimental with the flat floor either. And they were only 1/4"-3/8" tall, which is much smaller than people in this thread normally reference. So its kinda cool to see those, but totally apples to oranges.
 
194
146
Exp. Type
Autocross
Exp. Level
20+ Years
So Cal
Yep, the gurneys did nothing on the cooling side and the tires were flush to the fender opening, but not protruding. It's a different ballgame for sure. It's just kind of a sanity check on the overall magnitude of change.

Those numbers were at 150 mph, btw.
 

Grant 302

basic and well known psychic
View attachment 75785
I've played with versions of these and always found a gain, because it helped to evacuate the wheelwell, which makes the underside of the nose work better (at least, that was the logic). I think the biggest ones I've played with were maybe 1/2", but it was definitely noticeable. I think that lower dive plane might be along for the ride, but you only know by testing. I had one car that we'd run double dive-planes which had no real effect, but if you knocked the top one off, then the bottom one would be better than nothing.
The upper one *could* exit at a much steeper angle and/or more separated from the spat at the trailing edge.
 
5,122
5,605
Exp. Type
W2W Racing
Exp. Level
20+ Years
Pleasanton: 1/2 way between Sonoma and Laguna Seca
The upper one *could* exit at a much steeper angle and/or more separated from the spat at the trailing edge.
What we have decided to do is reduce the height and width of the spat to fee up the Canards some but leave enough to create some outwash....then hopefully find the test time to try removing one pair of canards at a time to see if that helps. Comments and opinions are welcomed.
 

TMO Supporting Vendors

Latest posts

Buy TMO Apparel

Buy TMO Apparel
Top