Aero advice

[petawawarace]

Two questions for the “experts” out there.  
 

1.  What is the lowest you’d recommend for front splitter height? I know lower is better, but not really practical if you have an off track excursion etc. I’m used to open wheel cars, so looking for a general idea here.  
 

2. I’ve added a splitter and air dam. Splitter is only 4”.
My car was very well balance before.  I’m mainly doing this to make the car more slippery.   I did not have a rear wing before.  The car was fine in high speed corners.  I can run the splitter level or just very shallow.  Will this affect the balance enough to need a rear wing? 
 

Thanks.  

[mender]

Nobody?

 

You'll find more speed from downforce than drag reduction so focus on that, add a rear wing to balance the aero and go fast. 

[shanehutton]

The last thing you want is the splitter touching the ground and cutting airflow under the car.   Between that and off track excursions, a bit higher is better than too low. 

[mender]

[TiredBirds]

I'd say our is/was about 4"

[wvumtnbkr]

As low as possible without getting ripped off.  It shouldn't flex down.  Touching on the outside edge of a corner is not ideal, but probably about the correct height. 

 

Mine is 3.25" off the ground and is probably a bit high compared to some of the full aero cars.

[Bandit]

3 hours ago, mender said:

Nascar splitters don't go back under the car and there are no undertrays so keeping them very close to the track, with the occasional touch, works as long as it doesn't hit enough to unload the suspension. With no undercar aero allowed they have a different strategy from the typical race car with splitters.

 

The pic of the 3 car is also pretty much fully compressed, not ride height. They'd never get the jack under it if it sat that way.

Edited March 8, 2021 by Bandit

[mender]

Mine is about 3 1/4" as well. No contact with the track but no curb hopping. 1/2" birch plywood, 4" beyond the air dam, no external supports.

 

I followed the DYI recommendations and used small splitter diffusers plus I also built in about 2 degrees of rake into the splitter. My car's weight distribution is 67/33, and according to most sources I should only need half as much downforce in the rear as I have in the front to maintain the mechanical balance, which was quite good. From that, the wing that I installed on the rear was oversized and I was expecting to run it at a shallow angle to balance the front aero. 

 

At the one race that we've run the full aero so far, the aero package worked better than expected, and the wing angle that I used (7 degrees) as a safe starting point proved to be very popular with the drivers. I suspect the car would have been a major handful and several seconds slower without the wing.

 

My suggestion: use a rear wing.

Edited March 8, 2021 by mender

[Ian]

On 3/5/2021 at 9:20 PM, petawawarace said:

Two questions for the “experts” out there.  
 

1.  What is the lowest you’d recommend for front splitter height? I know lower is better, but not really practical if you have an off track excursion etc. I’m used to open wheel cars, so looking for a general idea here.  
 

2. I’ve added a splitter and air dam. Splitter is only 4”.
My car was very well balance before.  I’m mainly doing this to make the car more slippery.   I did not have a rear wing before.  The car was fine in high speed corners.  I can run the splitter level or just very shallow.  Will this affect the balance enough to need a rear wing? 
 

Thanks.  

 

Splitter height depends a lot on suspension compliance.

 

4" could be too low if your car has a lot of roll or dive.  Our old car was set at 4" with suspension compressed after a drive.  Even that high, we'd still drag the corner in certain turns.

[mender]

5 minutes ago, Bandit said:

1. Nascar splitters don't go back under the car and there are no undertrays so keeping them very close to the track, with the occasional touch, works as long as it doesn't hit enough to unload the suspension. With no undercar aero allowed they have a different strategy from the typical race car with splitters.

2. The pic of the 3 car is also pretty much fully compressed, not ride height. They'd never get the jack under it if it sat that way.

1. Most Champcars won't have undertrays either. I suspect most of the problems with "being on the splitter" are from the splitter supporting the car and taking weight off the front tires, not from a large change in the aero under the car.

2. Yes, fully compressed so the splitter is skimming the track and that's how they work best. 

[mender]

Here's a picture of the Civic as it left the pits:

 

And in a moderate speed corner with a fair bit of banking:

 

I'm building a bigger splitter this year and will lower it until I'm getting scrape marks on it. I may also put rub blocks on the outer edges so I can sneak up on the minimum height without grounding the splitter. I may have to increase the spring rates.

Edited March 8, 2021 by mender

[Bandit]

36 minutes ago, mender said:

1. Most Champcars won't have undertrays either. I suspect most of the problems with "being on the splitter" are from the splitter supporting the car and taking weight off the front tires, not from a large change in the aero under the car.

2. Yes, fully compressed so the splitter is skimming the track and that's how they work best. 

True. I was just pointing out why a Cup car's splitter is different from a car with a full undertray where they do want some air going under the car.

 

 

On the typical CC splitter going back to the axle centerline I agree unloading the suspension with a track hit has more effect than any loss of downforce as few, if any, corner flat enough to close off the splitter.

[wvumtnbkr]

48 minutes ago, Ian said:

 

Splitter height depends a lot on suspension compliance.

 

4" could be too low if your car has a lot of roll or dive.  Our old car was set at 4" with suspension compressed after a drive.  Even that high, we'd still drag the corner in certain turns.

I would say that if your car rolls enough to have the splitter contact the track at even 3.5 inches, I would be looking at stiffening the front end up!

[LuckyKid]

Mine was around 3" off the ground.  Stock sways, but 1000lb springs in the front of a light miata to keep it off the ground enough.   We have splitter blocks that scrap in a couple corners/brake zones.

 

If your car isn't that stiff, I'd recommend higher.

[erich]

Our splitter is about 4" off the ground as well, but we need the airflow for a large rear undertray/diffuser.

 

For you guys I recommend a parachute. It'll keep the back end from coming around.

[Originalsterm]

22 minutes ago, erich said:

Our splitter is about 4" off the ground as well, but we need the airflow for a large rear undertray/diffuser.

 

For you guys I recommend a parachute. It'll keep the back end from coming around.

 

The parachute is great advise, I wish more teams would run these.  Our 318 isn't fast enough to keep the chute deployed, but it would certainly help with the faster teams.

[petawawarace]

Thanks guys.  Most of that was helpful.  Adding parachute brackets tonight.   

[LuckyKid]

Make sure to duct to the radiator and seal it then vent the hood.  This will reduce drag and increase df.    Radiator opening should be about 1/3rd the size of the radiator and should have smooth edges inward so the air stays attached.  

In looking at the hood, you probably have low pressure as the hood radius's up righty at the start.   Vents would likely be most effective further back or and on the sides if stuff is in the way.

[petawawarace]

6 minutes ago, LuckyKid said:

Make sure to duct to the radiator and seal it then vent the hood.  This will reduce drag and increase df.    Radiator opening should be about 1/3rd the size of the radiator and should have smooth edges inward so the air stays attached.  

In looking at the hood, you probably have low pressure as the hood radius's up righty at the start.   Vents would likely be most effective further back or and on the sides if stuff is in the way.

Basically all of that is in progress. Rad opening is exactly 1/3 of rad opening size.  Plastic is ordered for the ducting to the rad.  
The green tape you see on the hood is getting cut and folded down around the front of the motor, and will remove a good majority of the air coming out of the rad.  

[LuckyKid]

1 minute ago, petawawarace said:

Basically all of that is in progress. Rad opening is exactly 1/3 of rad opening size.  Plastic is ordered for the ducting to the rad.  
The green tape you see on the hood is getting cut and folded down around the front of the motor, and will remove a good majority of the air coming out of the rad.  

You will want to add a gurney at the front to help direct the airflow up, and the hood exit air can join with that flow easier.  

These guys do a bunch of actual wind tunnel testing and I try to use their info on our zero point versions:  https://racelouvers.com/

[Gkuhn41]

We havnt measured our splitter height but we can just fit our toes under it when we wear shoes.....yep thats how i measure it lol. probably around 3".

 

If we run just a splitter and no rear wing its a handful to drive. Super fun if you like a loose car but its terrifying when going through the kink at Road America counter steering at 100+.

[55mini]

6 hours ago, Gkuhn41 said:

We havnt measured our splitter height but we can just fit our toes under it when we wear shoes

Same!

[turbogrill]

12 hours ago, LuckyKid said:

Make sure to duct to the radiator and seal it then vent the hood.  This will reduce drag and increase df.    Radiator opening should be about 1/3rd the size of the radiator and should have smooth edges inward so the air stays attached.  

In looking at the hood, you probably have low pressure as the hood radius's up righty at the start.   Vents would likely be most effective further back or and on the sides if stuff is in the way.

 

Why should the rad opening be 1/3? 

[mender]

Because that's what works...

 

[turbogrill]

6 hours ago, mender said:

Because that's what works...

 

 

Aha thanks.

 

But if you don't have a radiator. Would a completely close airdam be best or would you want to have a path for the air to go airdam and then up thru hood?