Tuning APR GT-250 wing?

[MFKN3]

Purchased a 67" GT250 as well as their front air dam.

Does anyone have any pointers on what bolt settings to use for the rear angle?

Should I start at 0 or 5 degrees and can I literally measure with a clinometer/spirit level?

[roastbeef]

I started an aero thread a while back. I found the general consensus is to add aero evenly, then it is mostly trial and error from there.

[1MOREMOD]

I run apr gt 250 on my e46 and have the rear set at about 4 degrees aoa if I remember correctly

[dogbone]

Originally Posted by MFKN3

Purchased a 67" GT250 as well as their front air dam.

Does anyone have any pointers on what bolt settings to use for the rear angle?

Should I start at 0 or 5 degrees and can I literally measure with a clinometer/spirit level?

I have a 67" GT250, along with the APR front splitter that extends 5" from the bumper. The 5" length was chosen for time attack rules. (I also have a generic lip filling in the gaps between the bumper and splitter.)

May I ask why you chose the air dam over the splitter? The 67" wing provides significant downforce. Since you went with such a large wing, I'm assuming you wanted solid downforce in the rear, but I'm concerned that you won't use all of the wing's capabilities because you can't balance out the front.

For awhile, I drove on track with just the rear wing without any front aero at all and while driving on rounders at speeds over 80mph, the understeer was incredible. Literally turning the steering wheel had no affect. The splitter did a nice job to balance it out overall, but I still find myself wishing for even more front grip in fast rounders. So, I'm concerned that the air dam just won't balance out with such a large wing.

It is possible to dial the wing back to some extent by reducing the angle of attack (AOA), but once you get a taste for that rear being planted, you just don't want to give that up.

Measuring the AOA has to take into account that there's a Gurney Flap sticking up on the rear of the wing that will affect all measurements. APR said to lay some kind of straight edge from the rear of the wing to the front and measure that angle and then subtract 4º to account for the Gurney.

Here's how I measure it:

-try to have the car sitting on as flat ground as possible
-I use the top part of my floor jack handle as the straight edge, and place that from front to back on the wing as perpendicular as possible.
-The iPhone can measure angle with the built in Compass app. If you go to the Compass app and swipe to page 2, there's an inclinometer. I place the phone on the jack handle and measure the degree of angle of attack. Whatever the number is, I subtract by 4 and that's the angle of attack.

It's been a long time since I adjusted mine, but I believe on my car, the iPhone shows somewhere between 8-9º. Subtracting 4 for the Gurney means I'm running around a net of 4-5º AOA.

But again, I have a sizable front splitter. If you end up running only an air dam, you may have to run less AOA to get it to balance out.

Ultimately I love the APR aero setup. The car sticks like crazy compared to non-aero. My times instantly dropped by 2-3 seconds at every track once the front aero went on. To me, that is an amazing result. I was very worried that with the full front splitter that I would rip it off the first time I slid off the track into the desert. I am happy to say that two years later, I have never damaged the splitter even with at least 2 dozen offs into Southern California deserts. I even slid into the rocks at Laguna Seca at the Turn 2 hairpin and the splitter lived to tell the tale. Of course, splitters that run longer and sit lower are more likely to rip off during landscaping excursions.

If you don't have a relationship with a shop that can give you specific aero advice, Racewerkz Engineering gave me very solid advice. Casey @ Racewerkz designed the cutout shape for the APR splitter that sits under the E9x M3.

[Datka]

Dogbone, your feedback is always informative with details, always learn something new from your posts
Just ordered APR 250 wing 61 inch. Was about to start searching how to measure the AOA, and here it is.
I also have cosmetic front lip and planning to add splitter, like you did.
One question. How many inches should it extend from the bumper with 61 inch wing. I spoke to Casey and he told me to run 5 degrees AOA, but I forgot to ask about the splitter length

[dogbone]

Originally Posted by Datka

Dogbone, your feedback is always informative with details, always learn something new from your posts
Just ordered APR 250 wing 61 inch. Was about to start searching how to measure the AOA, and here it is.
I also have cosmetic front lip and planning to add splitter, like you did.
One question. How many inches should it extend from the bumper with 61 inch wing. I spoke to Casey and he told me to run 5 degrees AOA, but I forgot to ask about the splitter length

Glad my ramblings are useful to someone!!

As far as front splitter length, for me, there was a priority list of things to consider:

-If you engage in time attack or some kind of racing, usually there are regulations on how far you can go from the tip of the OEM bumper with aero. I do time attack and try to compete in the Limited Class whenever possible. With Global Time Attack, I get bumped to Unlimited because of my tires.....With Redline, I can get into Limited with Hankook TD. So, the limit for me seemed to be 5" based on the rules for Limited. I guess if I was willing to throw the car in Unlimited all the time, I could pretty much do whatever I want, but I haven't gone that route. So, first you need to take into account regulations of any competition you want to do.

-Next on the priority list was practicality. My car is a track-only car now, so street driving was no longer a worry. That's good. But as I mentioned before, ripping the damn thing off while pretending to be a weed-whacker was a main concern for me. At Buttonwillow, there are times when I test the limits and end up in the desert. It's generally safe to go off track there, so it happens occasionally. I've seen many ripped splitters and it takes at least $2500+ to recover from that little adventure (bumper, paint, lip, splitter, install). It should be fairly obvious that the longer the splitter is, the more susceptible it is to getting ruined. So, I decided to keep the splitter a "reasonable" length---whatever that means---to try to keep it intact if I slid off the track. I can happily say that it has survived so far.

I have a friend who ran a longer splitter on the same car as me. At one point, I think his splitter was around 8" from the bumper. We learned some interesting things from his setup:
(1) - The longer splitter which was the same material as mine didn't automatically translate into magical grip.
(2) - The longer splitter in the thin carbon material was difficult to keep rigid under high wind loads with the support rods. It simply gets more floppy/flexible as it gets longer. I run two support rods and it's fine. At one point he was up to 4 rods trying to get it to be stiffer. I don't think it really worked. There are stiffer materials and from what I saw with him, I'd say if you're going to go longer than 6" you should use a stiffer material than the APR carbon. APR makes a honeycomb that is very very stiff. But it's really expensive and quite a bit thicker, so it will be closer to the ground.
(3) Not surprisingly, the 8" length really was susceptible to damage and he ripped the entire thing off the car during an off-track excursion at Buttonwillow. The spot he went off was challenging because it was at the bottom of Phil Hill and it was a bit muddy around the track, and that thing dug into the ground and literally disappeared. There was no sign of the splitter when I was looking for it later in the day. I joked that a carbon tree is going to grow from that spot now that he planted the seed!! (I went off on the far side of Phil Hill too at Buttonwillow (when it was drier) and my splitter lived. I have also gone off in the mud in flatter spots of the track and still been fine. hehe I know it sounds like I go off a lot, but I really don't.....) (Racewerkz built him a new splitter from some other metallic material that is light, thin, stiff and not so expensive.)

-Last on the list was the technical aspect of how it would balance with the rear wing. The truth is that there is virtually no empirical data available on the amount of downforce these things make that is directly applicable to the E9x M3. It's actually quite frustrating. I called APR and asked. They simply pointed to the generic data on their website....And I can't really blame them.....I mean, we're mounting this thing on the trunk for goodness sake.....hehe....how do you factor in the trunk flex into the equation? And then the stiffness of the springs affects things and the rake of the car affects it, and it goes on and on.....So, in the end, for us hobby guys, I would say the hardcore technical aspect of this with respect to data is actually the least thing to worry about---unless you're willing to spend the money putting the car into a wind tunnel.......yeah, good luck with that.... I actually looked into that for the heck of it. Holy cow, wind tunnel time is expensive---many thousands of dollars.

I would say starting at 5" from the OEM bumper tip is a good place to start, even with a 61" wing, versus the 67" I have. The E9x M3 tends to understeer by nature. Adding a wing of any kind is going to exaggerate that tendency. And as I said before, I still wish I had more front grip, so the 61" wing with the 5" splitter could be very nice!

Since you've already spoken with Casey once, it might be worth one more phone call to him as well.

[MFKN3]

Originally Posted by dogbone

I have a 67" GT250, along with the APR front splitter that extends 5" from the bumper. The 5" length was chosen for time attack rules. (I also have a generic lip filling in the gaps between the bumper and splitter.)

May I ask why you chose the air dam over the splitter? The 67" wing provides significant downforce. Since you went with such a large wing, I'm assuming you wanted solid downforce in the rear, but I'm concerned that you won't use all of the wing's capabilities because you can't balance out the front.

For awhile, I drove on track with just the rear wing without any front aero at all and while driving on rounders at speeds over 80mph, the understeer was incredible. Literally turning the steering wheel had no affect. The splitter did a nice job to balance it out overall, but I still find myself wishing for even more front grip in fast rounders. So, I'm concerned that the air dam just won't balance out with such a large wing.

It is possible to dial the wing back to some extent by reducing the angle of attack (AOA), but once you get a taste for that rear being planted, you just don't want to give that up.

Measuring the AOA has to take into account that there's a Gurney Flap sticking up on the rear of the wing that will affect all measurements. APR said to lay some kind of straight edge from the rear of the wing to the front and measure that angle and then subtract 4º to account for the Gurney.

Here's how I measure it:

-try to have the car sitting on as flat ground as possible
-I use the top part of my floor jack handle as the straight edge, and place that from front to back on the wing as perpendicular as possible.
-The iPhone can measure angle with the built in Compass app. If you go to the Compass app and swipe to page 2, there's an inclinometer. I place the phone on the jack handle and measure the degree of angle of attack. Whatever the number is, I subtract by 4 and that's the angle of attack.

It's been a long time since I adjusted mine, but I believe on my car, the iPhone shows somewhere between 8-9º. Subtracting 4 for the Gurney means I'm running around a net of 4-5º AOA.

But again, I have a sizable front splitter. If you end up running only an air dam, you may have to run less AOA to get it to balance out.

Ultimately I love the APR aero setup. The car sticks like crazy compared to non-aero. My times instantly dropped by 2-3 seconds at every track once the front aero went on. To me, that is an amazing result. I was very worried that with the full front splitter that I would rip it off the first time I slid off the track into the desert. I am happy to say that two years later, I have never damaged the splitter even with at least 2 dozen offs into Southern California deserts. I even slid into the rocks at Laguna Seca at the Turn 2 hairpin and the splitter lived to tell the tale. Of course, splitters that run longer and sit lower are more likely to rip off during landscaping excursions.

If you don't have a relationship with a shop that can give you specific aero advice, Racewerkz Engineering gave me very solid advice. Casey @ Racewerkz designed the cutout shape for the APR splitter that sits under the E9x M3.

Brilliant, thanks. I will try starting with 3 degrees on the rear. Was originally told 0-5 degrees but saw another thread on here of a track build and his wing was definitely angled very high.

Sorry, I do have the splitter if that's what it's called. Time attack rules here dictate I am limited to 100mm front protrusion which is what I will install the front at.

Do people leave the gurney flap on?

[1MOREMOD]

100 mm is roughly what my apr splitter is set at maybe slightly less. Yes run the gurney on mine. Originally though about removing but it creates more downforce with a relative reduction in drag compared to running without at a steeper aoa to get equivalent downforce.

[dogbone]

Originally Posted by MFKN3

Brilliant, thanks. I will try starting with 3 degrees on the rear. Was originally told 0-5 degrees but saw another thread on here of a track build and his wing was definitely angled very high.

Sorry, I do have the splitter if that's what it's called. Time attack rules here dictate I am limited to 100mm front protrusion which is what I will install the front at.

Do people leave the gurney flap on?

Yes, leave the Gurney Flap on. The Gurney gives you more downforce at a lower Angle of Attack. Running a wing with less AOA results in less drag. So it's a win-win: more downforce, less drag.

[MFKN3]

Thanks for all the information, much appreciated. Those clever engineers (gurney flaps)!

[dogbone]

Originally Posted by MFKN3

Thanks for all the information, much appreciated. Those clever engineers (gurney flaps)!

The APR website talks a bit about the history of the Gurney Flap. Race car driver Dan Gurney came up with the idea in the early 1970's. But I bet he didn't have an awesome carbon one on there!

[TheDirtyThirty]

Originally Posted by dogbone

I have a 67" GT250, along with the APR front splitter that extends 5" from the bumper. The 5" length was chosen for time attack rules. (I also have a generic lip filling in the gaps between the bumper and splitter.)

May I ask why you chose the air dam over the splitter? The 67" wing provides significant downforce. Since you went with such a large wing, I'm assuming you wanted solid downforce in the rear, but I'm concerned that you won't use all of the wing's capabilities because you can't balance out the front.

For awhile, I drove on track with just the rear wing without any front aero at all and while driving on rounders at speeds over 80mph, the understeer was incredible. Literally turning the steering wheel had no affect. The splitter did a nice job to balance it out overall, but I still find myself wishing for even more front grip in fast rounders. So, I'm concerned that the air dam just won't balance out with such a large wing.

It is possible to dial the wing back to some extent by reducing the angle of attack (AOA), but once you get a taste for that rear being planted, you just don't want to give that up.

Measuring the AOA has to take into account that there's a Gurney Flap sticking up on the rear of the wing that will affect all measurements. APR said to lay some kind of straight edge from the rear of the wing to the front and measure that angle and then subtract 4º to account for the Gurney.

Here's how I measure it:

-try to have the car sitting on as flat ground as possible
-I use the top part of my floor jack handle as the straight edge, and place that from front to back on the wing as perpendicular as possible.
-The iPhone can measure angle with the built in Compass app. If you go to the Compass app and swipe to page 2, there's an inclinometer. I place the phone on the jack handle and measure the degree of angle of attack. Whatever the number is, I subtract by 4 and that's the angle of attack.

It's been a long time since I adjusted mine, but I believe on my car, the iPhone shows somewhere between 8-9º. Subtracting 4 for the Gurney means I'm running around a net of 4-5º AOA.

But again, I have a sizable front splitter. If you end up running only an air dam, you may have to run less AOA to get it to balance out.

Ultimately I love the APR aero setup. The car sticks like crazy compared to non-aero. My times instantly dropped by 2-3 seconds at every track once the front aero went on. To me, that is an amazing result. I was very worried that with the full front splitter that I would rip it off the first time I slid off the track into the desert. I am happy to say that two years later, I have never damaged the splitter even with at least 2 dozen offs into Southern California deserts. I even slid into the rocks at Laguna Seca at the Turn 2 hairpin and the splitter lived to tell the tale. Of course, splitters that run longer and sit lower are more likely to rip off during landscaping excursions.

If you don't have a relationship with a shop that can give you specific aero advice, Racewerkz Engineering gave me very solid advice. Casey @ Racewerkz designed the cutout shape for the APR splitter that sits under the E9x M3.

This explanation is amazing, this should help me out a ton as well for this weekend at SOW. I just installed a 61" GT-250 and had APR build be a custom splitter that extends past my dtm style air dam. Only difference is this is on my e30 but hopefully the rules still somewhat apply

[Richbot]

I'm just a poor country racecar driver but it seems to me if you could find a way to flatten and blank or mostly blank the front side intakes you could pick up a good amount of front grip. You'd need to get air to the intake and trans cooler somehow, but it's a thought I had. Make the front bumper cover one big airdam with a splitter at the bottom, similar to what all the GTLM cars do

I also think we run big enough front tires and they stick out far enough into the air that louvers would make some sense

[dogbone]

Originally Posted by TheDirtyThirty

This explanation is amazing, this should help me out a ton as well for this weekend at SOW. I just installed a 61" GT-250 and had APR build be a custom splitter that extends past my dtm style air dam. Only difference is this is on my e30 but hopefully the rules still somewhat apply

Cool.

Just a side note: Since Streets of Willow is a fairly slow and tight track in most spots, I would say that SOW is not the best place to really make any final judgements about the aero's performance or even the settings. With an E30, I think you'll struggle to get to speeds where the wing is really allowed to do it's magic. A higher speed track is necessary. Big Willow is good for sorting out the effectiveness of aero. The high speed rounders of Turn 2 and Turn 8/9 are ideal for putting aero through it's paces. Plus the front straight at Big Willow will clearly demonstrate how much drag the wing is creating on straights. If the wing is going to slow you down, you'll see it very easily on Big Willow's front straight.

In my experience on my car that weighs 3250lbs, I find aero does very little-to-nothing below 60mph. At 60-70, there's some decent benefit. 80mph it kicks in nicely and begins to hold the car down. 90mph+ the car--specifically the rear--is just glued to the ground. In rounders above 100mph--like at Big Willow Turn 8 or Buttonwillow's Riverside, the rear is getting so much downforce that understeer in the front becomes the big issue.

Since an E30 is lighter, perhaps the speeds can be a bit lower for aero to kick in. But I would argue that the lower horsepower will make it harder to achieve high speeds at SOW. Anyway, all I'm saying is, don't expect night and day difference at Streets of Willow since the track itself is not a high speed track and the aero may not be kicking in much.

PS. I think the best track to tune aero in SoCal is Chuckwalla. It's just one rounder after another at 70-90mph most of the way around the track. You can REALLY get a good feeling for aero at Chuckwalla.

[Richbot]

Autocross people have demonstrated the benefits of aero at sub-50mph speeds, gaining tenths or more on 60 second laps that never exceed 75mph and where cornering speeds are usually in the 35-55 range. Agree finding a good aero balance for fast corners is tough at a rinkydink track though

Helps to have ride height sensors/frickin' lasers for tuning obviously

[dogbone]

Originally Posted by Richbot

Autocross people have demonstrated the benefits of aero at sub-50mph speeds, gaining tenths or more on 60 second laps that never exceed 75mph and where cornering speeds are usually in the 35-55 range. Agree finding a good aero balance for fast corners is tough at a rinkydink track though

Helps to have ride height sensors/frickin' lasers for tuning obviously

Of course, I understand what you're saying. I was just trying to dampen his aero expectations at Streets of Willow since it's not a fast track. My average speed there is in the 60's. It's 80+ at every other track I've ever visited.

[TheDirtyThirty]

Originally Posted by dogbone

Of course, I understand what you're saying. I was just trying to dampen his aero expectations at Streets of Willow since it's not a fast track. My average speed there is in the 60's. It's 80+ at every other track I've ever visited.

Once again great advice and yes I wasn't expecting too much of a difference at SOW because of the track characteristics. But getting up to those speeds is pretty easy considering the car has an S54 swap

[dogbone]

Originally Posted by TheDirtyThirty

Once again great advice and yes I wasn't expecting too much of a difference at SOW because of the track characteristics. But getting up to those speeds is pretty easy considering the car has an S54 swap

Ah--some tricks up the sleeve! Very nice!

How much does it weigh and what tires do you run?

[TheDirtyThirty]

Originally Posted by dogbone

Ah--some tricks up the sleeve! Very nice!

How much does it weigh and what tires do you run?

I believe it weighs around 2500 lbs. Had it corner balanced maybe a year ago but can't remember the exact numbers, currently running 225/45 NT01s on 15x8s.

[dogbone]

Originally Posted by TheDirtyThirty

I believe it weighs around 2500 lbs. Had it corner balanced maybe a year ago but can't remember the exact numbers, currently running 225/45 NT01s on 15x8s.

So 2500 lbs with 300-350hp....and now with aero. Sounds like you could cause some good trouble with that car! If you're chasing lap times, you'll need better tires. You slap some 225/45/15 BF Goodrich R1S on that baby and I promise with aero and R1S, you will DESTROY any lap time you ever did pre-aero on NT01.

[TheDirtyThirty]

Originally Posted by dogbone

So 2500 lbs with 300-350hp....and now with aero. Sounds like you could cause some good trouble with that car! If you're chasing lap times, you'll need better tires. You slap some 225/45/15 BF Goodrich R1S on that baby and I promise with aero and R1S, you will DESTROY any lap time you ever did pre-aero on NT01.

haha this is what I like to hear. One last question do you think its better to take a tire like the 225/45/15 R1S and mount it to a 8" wide wheel or go to a 9" wheel? I keep reading that a moderate stretch on 225/45 tire is faster on a 9" wide wheel. (at least this what the Miatas guys are saying)

[dogbone]

Originally Posted by TheDirtyThirty

haha this is what I like to hear. One last question do you think its better to take a tire like the 225/45/15 R1S and mount it to a 8" wide wheel or go to a 9" wheel? I keep reading that a moderate stretch on 225/45 tire is faster on a 9" wide wheel. (at least this what the Miatas guys are saying)

Personally, I cannot speak about whether a stretched tire is faster or not. To test that properly would take more resources and time than I am willing to throw at it---two fresh sets of tires on two sets of rims that are different widths on the same day with tons of open track to be able to guarantee pushing the limits. Plus, the reality for me is that I'm already committed to 3 sets of rims all at 10.5" which is filling the wheel wells pretty full as it is. Trying to put a wider rim in the front is impossible for me. I might possibly be able to go wider in the rear, but I prefer to run square to prolong tire life. So for me the stretch issue is not something I consider. I just try to get the fattest thing I can under there.

It is worth noting that R1S tires run very wide for the listed size. And they have a very boxy shoulder. Since a wider rim will cause a tire to run a bit wider than when it sits on a narrower rim, if you're close to the limit in terms of available room under your car---ie. rubbing the damper or fender, I might hesitate moving to a wider rim with a wide, boxy tire like the R1S. If you have the room---when I look on tirerack.com at the tread width of the R1S 225/45/15, it's 8.4". I don't think that a 9" rim is really much of a stretch in this case.

I have always sought to run my sidewalls as perpendicular to the rim as possible. Along the way, people seemed to generically say that the tire deals with load best that way. Perhaps there's more detailed info on that somewhere.....

I've had a set of 27/65/18 Michelin slicks on a 10.5" rim. They had a 9.8" tread width. They ran fine. The edges held up well. There was a bit of a stretch there. But I didn't detect any issues.