Tire Selection for NASA TT

Vorshlag-Fair

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I missed some of these updates but did notice your car at Nationals. Congrats on the times, but Dennis is fast (and a customer of ours). I've run against him off and on for years, and had a perfect record against him in our S197, but he has continued to develop the car in the years since we sold that car. He is quicker than he used to be!

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I still think you are giving up some time by not using the GOD TIRE: the 345/35/18 Hoosier. And 48 psi is waaaay higher than we ever ran. I used to go on track at 26-27 psi cold and come in at 33-35 psi hot with these and the 335s. We ran at 3802 pounds in TT3, which was the then limit for the highest bonus on minimum weight brackets.

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The TruFiber flares look good, but since my last 2018 post here I have since found a number of options that are less costly but also less "bolt-on". I made a post about this last week on the Corner Ponies FB page...

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We took a chance late last year and ordered some of the cheapest flares I had ever run across for my wife's 86. I had used Duraflex/Carbon Creations parts before and the quality was really hit-or-miss. In this case it was so cheap that I didn't have a lot to lose.

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It turned out better than I had hoped, but there was a little mis-match on a few pieces that my paint and body guru will have to fix. Lots of hours of doing the "underneath" work, of course...

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But this allowed us to fit 18x11" and 315mm Rival-S tires at all 4 corners. Made for a 2.5 second drop on the 90 sec MSR Cresson 1.7 CCW course, our biggest drop over the course of 6 track tests on the same car/track/driver over a 2 year period. You can read more about the flare install at this link - its a lot of the same technique needed for an S197 flare using Duraflex flares...

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I don't think these flares existed last year for the 2010-14 Mustang, but they do now. And ignore the prices shown - their Minimum Advertised Pricing policy is a joke. I don't even bother selling this brand (and I'm a direct dealer) because eBay and other sites just sell these at cost.

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They look pretty big - and might be a copy of the TruFiber flares for all I know. They say "75mm" on both, but who knows? Next time we get an S197 in here that needs big tires we'll try this new kit.

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https://www.clinchedflares.com/universal-fender-flares/

There is a company called Clinched that makes a bunch of styles and widths that are listed in their "universal flares" section. I've put my hands on these recently and they are kind of a game changer for cars that don't have a lot of flare options.

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Her is a set of their "Euro" style flares on a CTSV road race car. These are made from vacuum formed thermoplastic and are very easy to cut and modify.

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We just mocked these up on his 2 door 96 Subaru Impreza body with F14 18x12's and 335/30/18 Hoosier A7s. The ends need to be trimmed to fit, but like I said, these are easy to cut. It might be a solution for those not wanting the styling or look of the TruFiber or Duraflex bits.

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Anyway, I have rambled enough... still wish the OP would try the 345/35/18 A7... its worth some time, you gotta trust me. We use this big boy as the drive tires on lots of RWD cars...

Cheers
 

ddd4114

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Yeah, Denis said he didn't really get serious into TT until ~5 years ago (can't remember exactly what he said). Since then, he's been putting a lot of focus into coaching and data review, and it shows. It's a bit embarrassing that he out-drove me at my home track with a car that has basically the same performance envelope. He shared some data with me, and our cars are VERY close. Our speed traces were almost identical except for 2 turns - where we took different approaches, and he was (obviously) faster. Otherwise, our grip seems to be the same, our top speeds were the same, etc. Hindsight is 20/20, but I think if I had also invested a little bit more into coaching, I could have beat him. Oh well, maybe in 2021.

I'm actually considering the 345mm Hoosier in the rear, but not so much for the (possible) additional grip. I'm running out of gear in a few spots at several tracks where it's not worth shifting, and the taller tire will probably help as long as I keep my apex speeds high enough. I am struggling with rear traction issues, but I'm wondering how much of that is because I'm still running stock control arms without relocation brackets, and I'm lowered ~1.75". I'm going to try addressing that issue first to see how much a bit of anti-squat and stiffer bushings will help.

Regarding tire pressure, my settings came from multiple iterations of taking tire temperatures (the correct way, not using a silly infrared gun in my paddock spot), and the trends I saw are pretty repeatable. Maybe we were running different enough setups, but until I was in the upper 40's (front), my center temperatures were colder than my outside temperatures. I know you guys have a lot of track experience, but I just don't see how you can run 35psi hot in a 3800lb car (with aero) using the exact same tire and a smaller wheel size than me. If anything, I'd expect you'd need higher pressures with the smaller rim. How did you establish that pressure target? Did you take tire temp data? If so, what was the spread if you don't mind sharing? I've noticed the tires wear a lot better with the higher pressures, and I don't seem to be losing any grip. Hoosier recommended 50psi, but the tire temp data doesn't support going much higher than I already have. Another benefit of the higher pressures is the reduced aligning moment and higher cornering stiffness makes the car easier to drive. This was really the biggest difference I noticed.
 

Norm Peterson

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Seems to me if you're running out of gear while having traction problems, solving or at least mitigating the corner exit traction problem is only going to make you run out of gear sooner and/or more often (and be in and of itself a push to go 345-wide).


Norm
 

ddd4114

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That's a good point, but the 345/35 is a lot taller than the 335/30 (27.5" vs 25.9"). That would give me another ~6mph in 3rd gear (the one that usually causes issues), and it's very unlikely that I'll be exiting corners 6mph faster with a tire size change. I'd be thrilled to exit 2mph faster.
 

Vorshlag-Fair

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Regarding tire pressure, my settings came from multiple iterations of taking tire temperatures (the correct way, not using a silly infrared gun in my paddock spot), and the trends I saw are pretty repeatable. Maybe we were running different enough setups, but until I was in the upper 40's (front), my center temperatures were colder than my outside temperatures. I know you guys have a lot of track experience, but I just don't see how you can run 35psi hot in a 3800lb car (with aero) using the exact same tire and a smaller wheel size than me. If anything, I'd expect you'd need higher pressures with the smaller rim. How did you establish that pressure target? Did you take tire temp data? If so, what was the spread if you don't mind sharing? I've noticed the tires wear a lot better with the higher pressures, and I don't seem to be losing any grip. Hoosier recommended 50psi, but the tire temp data doesn't support going much higher than I already have. Another benefit of the higher pressures is the reduced aligning moment and higher cornering stiffness makes the car easier to drive. This was really the biggest difference I noticed.
Yea I have just never heard of tire pressures being used on the wider Hoosiers anywhere near that 50 psi range. Have run 100s of Hoosiers (with probed pyrometer testing) on .... 7 of my own cars (had to go count), and the R6/R7 and A6/A7 generations always seemed to find their happy place in the 30-38 psi range hot. I tried lower pressures, but not much higher...

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Never even thought to go that high - I guess I need to test with 50 psi hot and see if I am missing something? There might be time available that I never knew of. :) #500psi

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They always wore well, with fairly even temps across the tire, etc. And yes, wider wheels tend to help in many ways. We ran the 315s on an 11" wide wheel for 2 seasons then moved to 12" wide wheel, and they wore better on the 18x12s. We always ran the 335 fronts on an 18x12" and even ran the 345/35/18 rears on an 18x12", unfortunately (it should do much better on a 13" or 14" wide wheel). It was the widest wheel we could get affordably at the time (Forgestar makes an 18x13" now) but as you can see above... it didn't look super squeezed on the 12... but of course we would have run it on a 13" wheel if we could have. I was worried the 314s would wear funny, but they wore evenly, even with this squeeze.

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The only "big tires" that we noticed very weird wear patterns on were these Conti slicks (not the re-branded Hoosiers) from the Daytona Prototype series. These wore the outside shoulders off very rapidly, in one day on track, using our "normal Hoosier" tire pressures. I would have spent time dialing in these tires for better wear we had pallets of new ones) but being 5-6 seconds off the pace of an A6, I wasn't going to waste any more track time or tires on such a slow, poorly wearing and slow tire. ;)

Your results may vary! :)
 

ddd4114

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Never even thought to go that high - I guess I need to test with 50 psi hot and see if I am missing something? There might be time available that I never knew of. :) #500psi
Hey man, you never know until you try! I might drag a nitrogen bottle to track next time!

For years, I ran in the 36-38 psi range with everything from a 245mm to a 315mm Hoosier R7 on my S197, and they always felt "fine". Actually, similar to you, when people with lighter cars told me they were running 40+ psi, I advised them to try lower pressures. You would think (or at least I thought) that for TT, you'd want to run on the lower end of the useful tire pressure range because it's more compliant, should heat up a little faster, and should offer a little more mechanical grip. However, I think what I overlooked is how stiff Hoosier sidewalls are - especially with a 30 aspect ratio (i.e. 315/30 and 335/30) - and its effect on tire pressure settings.

When I was brainstorming setup ideas last year, I called Hoosier and told them my configuration (which was ~37psi hot at the time), and they told me I was running WAY too little pressure. I was skeptical but tried 42 psi at the next event, and... it felt better but wasn't magically faster. It wasn't until I started taking tire temps and using that data to adjust alignment and pressure that the car really started responding, and at that point, I was at the 48psi setup I mentioned. Of course, we're talking tenths and not seconds assuming your car is remotely dialed in.

Of course, as you said, "your result may vary". The more I think about it, if the 345/35R18 Hoosier is that much better, I can see needing less front tire pressure because you can shift load transfer further rearward to achieve the same balance. Furthermore, you probably won't need any more tire pressure in the back because the sidewall is much taller and should be a bit more tolerant.

Anyway, I'll get off my high-pressure soap box. I'm definitely interested in playing around more with setup next year to see what happens. In fact, I'll have to give you or Jason a call in a few weeks to order another set of rims to prepare myself... ;)
 

ddd4114

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UPDATE

I tried the 335/30R18 front and 345/35R18 rear setup this past weekend at Mid-Ohio, and... I don't think it really works for my car. I think it can be a little faster on certain cars on certain tracks, but I'm pretty sure I'll be switching back to a 335/30R18 on the rear.

I had a friend help me take tire pressures and temperatures during the Friday test-and-tune day, and I found the larger rear tires took an extra 1-2 laps to get to operating temperature compared to the "smaller" front tires. Perhaps this was my fault because I was still a bit rusty from not driving since October, but the car didn't "feel right" during this transition period. Also, since I can catch the back of the TT field on hot-lap 3, I need the tires to be at operating temperature ASAP. I ended up band-aid fixing the problem by increasing the rear spring rate from 450 lb/in to 550 lb/in. Initially, I liked that the car was more responsive and had more neutral handling, but I later changed my mind. This change reduced front tire pressure by ~1 psi and added ~1 psi to the rear tires along with helping the rear tires come up to temperature faster, so that was good. Based on tire temperatures, the tire pressures didn't need to change much from last year. I settled on adding 1 psi to the rear tires, which makes sense from what I've read considering the 335's are more "pinched" on the 13" rim whereas the 345's sit more evenly.

Unfortunately, while I hoped to figure out how to make the car handle well with the new 550 lb/in springs and the 345mm rear tires, it never happened. The higher spring rate made the car a lot less predictable and (as expected) terrible over bumps and hill crests. I tried softening the rear sway bar and messing with the rear shocks, but I could only make small improvements that I can't confidently say were not actually me just getting used to the car. The end result was that I didn't really trust the car, and my driving suffered. I also managed to cord the brand-new 345's in three days (~45 hot laps), which was infuriating but not that surprising. I was also frustrated because I missed beating my old track record on the Club Course layout by only 20ms. After looking at data and video, I had a few opportunities to beat it by several tenths, but I couldn't seem to put together a consistently good lap. For sure my driving is to blame for that and not necessarily the car setup. Oh well, it was a good learning experience.

Here's a video from the end of the weekend after I was done messing around with the car:

Hopefully the stupid Youtube bots don't spam the comments again. If I'm honest, while the car is still pretty fast, my driving wasn't very good. Taking turn 1 at less than 80mph at the beginning of the video is pretty embarrassing with a car as capable as mine, although it was clearly a driving error and not a car limitation.

However, I'm not completely dismissing the 345's for a few reasons:
  • Even though it was the end of May, the track was pretty cold this weekend. It was ~55deg in the mornings and only ~65deg in the afternoons and partly cloudy the whole weekend. The 345's might have been up to operating temperature by lap 2 on a hotter day.
  • I swapped rear springs after only 2 sessions, and this might not have been enough time to dust off the cobwebs. If I gave it more time and pushed the car harder, I might have been able to get the original setup to work with small tweaks.
  • I made a lot of setup changes but never went back to try different combinations. Against my better judgment, I think my desire to find a "perfect setup" with the 345mm rear tire blinded me from retracing my steps a bit.
I'll have to look through data a little more and decide what to do next, but I'm not very excited about purchasing another new set of 345's. They're rather expensive, and I didn't have enough competitors to win anything this weekend. I'll continue adding to this thread to share what I learn if I figure out anything interesting. Hopefully it can serve as a guide for anybody else looking to compete with an S197 with a fairly open rule book.
 

ddd4114

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UPDATE

I experimented a little more with the 345/35R18 vs 335/30R18 and I think I'll stick with the 335/30R18 unless I really need the taller gearing. Maybe I just can't get used to it, but after adjusting the suspension for the two tires, the car just feels more nimble and easier to drive with the 335/30R18 to me. For sure this could be affected by driving style and car setup, so of course your results may vary.

After my last post following a TT event at Mid-Ohio, I ran a TT event at Pitt Race. I registered for the Friday test-and-tune and did some back-to-back testing of the two tire sizes with no other changes. I was actually faster on the 335's by a few tenths, but again this could be because I wasn't used to them. Before doing this, I sorted out the handling issues I induced at the last Mid-Ohio event by softening the rear spring rate from 550 lb/in to 500 lb/in and reducing rear compression a bit. I think this is the best compromise because the car is now very predicable, has slight understeer, and doesn't spin the tires over bumps and crests as easily. However, the car is (obviously) still stiffer than my previous setup using 450 lb/in rear springs and as expected the car is a bit more twitchy and skips over bumps more easily. I need to play with shock tuning more at the next event to remedy that.

I did another comparison with the two tire sizes at Mid-Ohio last weekend, and I noticed another issue with the 345's - they're definitely rubbing on my fender flares a little. Interestingly, the right-rear tire rubbed the worst, but this isn't too surprising because turns 1 and 11 are left-hand turns and the fastest on the track, so the rear is being compressed another ~1/4" from the downforce. Another possibility is the stupid bump at the end of the carousel. Unless I perfectly aimed the car to straddle the bump (which you can't easily see while driving), it was a pretty wild ride coming out of that turn. Shock tuning will probably help, but if anything I need to reduce rear compression and that will make the tires rub worse. The car becomes undriveable if I add more rear compression with the current setup. It might also help to stiffen both sway bars, but I'd hate to reduce suspension compliance through the rest of the track just to improve one corner. Raising the rear ride height is yet another option, and the extra rake could add a bit more downforce. However, I don't like the idea of raising the CG more than necessary.

Another thing I learned at Mid-Ohio this weekend is that the 335mm A7's actually are up to temperature after the warm-up lap even on a cooler day (not including the first session of the day). I didn't think this was the case previously, but it turns out that it was my brain that wasn't fully warm instead of the tires. Once I figured this out, I destroyed my previous Club-Course track record by ~0.9s. There's definitely more power available, and the tires are probably overheating less by the end of the lap. At Pitt Race, I ran 0.3 seconds faster than my previous track record, but unfortunately it didn't count because it was during the warm-up session. After that, I couldn't repeat it because (insert typical racer excuse here).

Here is a video of my best lap from Mid-Ohio yesterday:

You can definitely see how much the car struggles over the bump at the end of the carousel, but otherwise I really like how it handles. Taller gearing would be nice at the entrance to Thunder Valley, but based on the data it's not costing me too much time.

After this, I'm interested to try running 315mm R7's again on 12" rims. They might not be quite as fast as 335mm A7's at cooler temperatures, but I wonder if they'll actually be faster when it's hot outside. They should also last a lot longer, so they'll be nice for practice sessions. Previously I was able to get ~25 heat cycles from a set of R7s, and I'm lucky to get more than 15 heat cycles from a set of A7s.
 

Vorshlag-Fair

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I see that Dan is still killing it in NASA TT3 in his region - congrats, and keep it up!

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https://nasaspeed.news/race-coverag...-great-lakes-time-trial-action-from-mid-ohio/

I hope he is still reading this forum... Dan, you wanna update the thread with your current aero changes? Car is lookin good.

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We bought the same set of TruFiber S197 flares and modified the crap outta them to fit over a set of Jongbloed 18x13" / 335mm front and 18x14" / 345mm Hoosier rear.

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Making the front air deflectors / flare extensions was tricky, and took a lot of fabrication time.

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Wasn't easy and I don't know if we would do this work again. Really gobbled up a ton of time. That's kinda why I want Dan to update this thread with his current dive planes, and even the last iteration!

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We got a little greedy and went with the 13" wide front and 14" wide rears. These do support the tires better but it required even more bodywork to make the flares... wider. Lots and lots of bodywork.

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In the end I'm happy with how it turned out, but I would be cautious about recommending these flares for even 12" wide fronts and 13" wide rears. Did you modify the flares for your 335mm tire setup, D?

Also note: NASA has penalized TT2-TT3 and anything over 315mm now gets an extra weight penalty. So my "run 335/345 tires at all costs" no longer applies. There IS a cost, and our blue Mustang customer got stuck with this penalty, too. :/
 

ddd4114

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Thank a lot! I still lurk around here, but unfortunately the content (at least in this section of the forum) has really dried up over the last few years. I also took a break last year due other stuff that needed my money/attention (houses are f**king expensive), so I've been a little disconnected from the racing world. However, I'm back this year and going for Nationals at Pitt Race in September!

The new aero bits are honestly just an excuse to do a project on my mind for a while, which is creating homemade carbon fiber parts. It was definitely a little more work than I anticipated (like most things), but I think they turned out pretty well:
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The design is basically a more streamlined version of what I had been using since 2020:
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Obviously with carbon, the parts are a bit lighter, should have better aero performance, should be less sensitive to changes, and most importantly... look pretty cool. Realistically though, it's not a noticeable difference on lap times. Aero development for mortals like us is certainly a challenge with diminishing returns. I'm already way into that territory. A proper flat floor and rear diffuser would probably be a measurable improvement, but those are deceivingly tricky to get right. They're invaluable for pro race cars, but I think amateurs over-value them. It's pretty easy to install them and basically just end up with more weight and an overheating transmission.

The wing-element dive planes are 3D printed and skinned in carbon, and the large end plates are created using a 3D printed mold (using several parts glued together):
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I was close to remaking my entire splitter in carbon, but... one of my friends (who does stuff like this professionally) talked me off that ledge. Even doing all the work myself, it would be >$1000 in materials, and I don't really have the time right now to design an optimized 3D shape for it. It's not totally off the table, but it probably won't happen this year.

The only other parts I've made recently are new 3D printed brake ducts for 4" hoses, also skinned in carbon where my old ones normally break. Sadly you can't seem them after they're installed, but they also turned out pretty decent:
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The frustrating part after all this work is... I really haven't made the car much faster in years. I'm MAYBE a few tenths faster than I was at the end of 2019. I hate to say it, but without totally gutting the car and optimizing it, I might be near the limit of its performance. It's certainly a fast car, but when well-sorted Corvettes show up, it's a real challenge. On that note, a Corvette guy who move from Texas has been grabbing a bunch of the TT3/ST3 records recently (fortunately not mine yet!). You guys are fast down there!

Your modifications to the TruFiber flares certainly look way better than mine, but as you said yours look like you spent a lot more time on them. I was on a time crunch so I threw some sheet metal parts together in ~2 days. They served their purpose (and provide MASSIVE aero improvements compared to that area being open), but they're certainly not the prettiest. I think the only real modifications I did to the flares themselves was drilling holes for mounting (I used Rivnuts in the body) and cutting out the large air reliefs in the front flares.

I agree on being hesitant to recommend flares like these, at least for TT purposes. I honestly don't think the 335's are THAT much faster than 315's, and you can make 315's fit in the stock fenders. I saw improvement with 335's, but it was maybe a few tenths. For people who aren't super serious, that's a ton of money and fab work for small gains. If you're actually sprint racing though, it's a different story. 335mm A7's overheat in a few laps with my car, and even R7's would get pretty toasty after a long session. I think the extra 20mm would be worth the effort for a 30-min race, especially in Texas heat. Up here, we can typically get away with a lot less tire aside from 2 months in the summer.

Are you guys going to NASA Nationals this year? Pitt Race is quite a hike for you guys, so I assumed not. If you are, definitely stop by my trailer for a beer! ;)
 

modernbeat

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Dan, we (Terry and I) are just getting the LS550 on the track. First are some tests with zero aero. Then the aero is added. First and second stages won't be as complicated as your aero is. Third stage -might- be complicated, including a belly in the front splitter.
 

Vorshlag-Fair

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Jason is right - we haven't even touched anything relate to aero in our latest Mustang build, this LS550.

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Our first event (above) was on some 4yo 315mm R7s and it was glorious.

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Last week I took it out with a new front end (GT500 nose & fenders from Anderson Composites) with a wider 18x12" wheel and 315mm Yokohama 200TW tires. It was a hot mess (throttle mapping is junk) but it is already 2 sec faster than my red 2018 GT ever was on this same track with these tires, and it is almost as quick as our S197 was here on 335/345mm Hoosier A7s.

Having 200 horsepower more ...doesn't suck. Can't wait to tackle aero, and we're looking at your updated front aero elements closely!!
 

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