This time attack event would be my last of the season competing in the OnGrid PNW time attack series. I was already on track to win the chamionship pointswise, but I had one second place finish by .1 seconds that I need to beat. At my last event I was able to beat that time by .3 seconds... and with some refinements to the platform and some extra aggressive tires I am hoping I can finish strong!
Leading Up To The event
Going into the event I knew the chassis needed some serious attention. I'd really narrowed down all the other stuff and it was time to get serious.
FIRST: Fixing my Bump Steer
I now had some concerns that bumpsteer was the root cause of a lot of my handling problems. I'd chased my problems in all sorts of outher areas...some of which sort of worked, but it all ultimately started to add up to bad bumpsteer.
Three things recently made bumpsteer obviously the candidate to look at:
The first was describing my offset rear toe arms and the guy said "Wouldn't that majorly change the factory geometry?!" and yes... yes it would I admitted.
The second The other slap in the face obvious example was encountered when I checked toe on my car at full droop a few weeks after an alignment. At that time noticed I was toed out 6mm total, which really surprised me. I attributed it to the alignment folks making a mistake and inverting my request for toe in with toe out, or maybe I was measuring poorly, or maybe it was how I transport my car with the straps on the tires. Hindsight being 20/20 though I should have known something was off at that time, but of course I was in denial.
So I borrowed a bumpsteer setup from a friend in the community and took to measuring the bumpsteer on the the rear of my Corvette. That confirmed it for certain: BUMPSTEER WAS DEFINITELY A PROBLEM!
The blue line above was my baseline at time of measuring. As you can see... I was bump steering nearly 3.25mm from full droop to full compression. That's nearly as much as my total toe in... and that's just ONE SIDE! In theory that's 6.5m of total toe, and since I only run 4mm of total toe... I could quite possible be toed out under heavy braking! Explains the sketch factor on my entry into turns!
Now more than likely my car is not going from full compression to full droop. In the range of travel my car likely operated at, the difference was 2.2mm per side or so. I generally have targeted just shy of 3/16" total toe in, which is an easy to measure 4mm total toe. Thus, I was quite possibly bumping near all the way from toe in to toe out under heavy braking since 4.4mm of toe change would exceed my 4mm of toe in! Add in that if I am trail braking my turn entry, weight also transitions laterally as well as longitudinally, meaning that unweighted inside rear corner is now toed out which really messes with my cars handling.
So now knowing this number was off, I decided to try adding more spacers to get the results on my curve. The results of additional spacers were the even worse red line above. That confirmed I needed to drop down in size, resulting in my next measurement of the blue line, which was good but not quite there yet. Then, went all the way down to purple, which was clearly too aggresive as the car would start to toe out as it squats. I finally settled on the green line for my curve on that first corner, as that curve is pretty much dead flat across the usable range of my suspension travel and I fancy it'll yield some significant improvements!
Next up was the front. Not wanting to go through the whole rigamorale of removing the calipers, I made some adapters that bolt onto the hub or rotor face and use M12 bolts to hold on the measuring plate. In theory this might exacerbate my measurements, but it was also only 45mm offset and my wheel is ET-54 so it all seems well within the realm of reasonable. If you're interested in this hub spacer, let me know I'll make it available for printing!
The initial measured front bumpsteer curve wasn't nearly as problematic, but was still not ideal. My front left changed 1.5mm over the entire travel, and the front left changed 1.7mm, giving me a total toe change of about 3mm total toe, which wasn't great, but literally half as bad as the rear of the corvette!. Taking a few measurements and already knowing the direction to adjust in, I settled on 2.3mm of spacer on the drivers side and 2.0mm of spacer on the passenger side. This gave me 0.3mm of toe change over the full range per side and 0.2mm over the likely range of travel, giving me a total change of 0.4mm. That amount of toe is well beyond what I can measure when doing a string alignment for myself, and so I feel happy settling on a 300% improvement.
Final Bumpsteer Chart:
There is the final chart, which might look like there's a lot of variation there, but keep in mind the Y axis is pretty small resolution, with the upper and lower end of the chart being 1mm of toe in each direction, with most of my variation sitting under a half mm, which I'll be damned if you can measure doing a string alignment! Anyways, all of my total bump steer accross the entire range doesn't exceed 0.6mm total bump, with most being at 0.3mm or so. In the actual usable range of travel I am going to see about 0.5mm total of bump steer, which is a substantial improvement from 4.4 in the front and 1.7mm in the front.
Data for those interested:
I'm really really hopeful that all of these measureents, changes and assumptions are correct. I was reading about one of the most problematic oversteer cars out there, the AP1 S2000, and it's measured bumpsteer. It's numbers aligned fairly close to mine... which also explains a lot of how the car felt and handled.
Next, Leveling Up My Alignment Game
So having my bumpsteer dialed is great, but since I've been chasing my handling with excessive toe in and questionable string alignment techniques... it's now time to properly align my car. One of the problems I've had though is getting everything setup to align the car properly and in a way I can trust my measureents. I can't measure on the car on ground as I can't reach the tie rods, and I can't measure in the air, but the suspension is unloaded and possibly inaccurate there as well.
The solution was clear to me: I needed to build a platform, and then on that platform eliminate drag of the tire on the ground so I need a slide plate... and then I might as well have my scales up front which lets me both corner balance the car AND it lets me roll the car suspension out.
Building out this platform was a fun excuse to bust out my fabrication skills that I don't get to use all that often. So out came the welder, the plasma cutter and chopsaw... and off to the steel yard I went. I was fortunately able to source most of what I needed from the remnant pile and pay the screep steel price, though I paid full price for a few specific cuts which was fine as it helped ensure my platforms are as level as reasonably possible.
With the car now able to be up on platforms I could access the adjusters on the car with it's full weight on the tires,. That meant it is time to string align!
Something I was doing wrong before was assuming the front and rear track width were the same. They are not. The front is approximately 3mm wider per both GM numbers and several articles measuring. So I'll be dialign me string in to be slightly ahead on the front axle at 2mm, which eagle eyed math wizards will note is not half of 3mm... but since I can't easily eyeball 1.5mm, I'm going with 2mm. This does skew the line slightly towards adding toe out, but numerically this works out well! I zero'd the front which means the car is ever so barely toed out up front. In the rear I'll run 2mm of rear toe in on each side, which gives me 4mm total,. The rear toe in even with a slight subtraction for the line possibly introducing slight toe out lands me right between spec corvettes suggested starting point of total toe in of 1/8" (3.175mm) and my best luck toe setting thus far of 3/16" (4.7mm). This ultimately seems like a safe place to land, and I can always dial back the toe a bit better in the future!
Also... probably worth mentioning that YES I did indeed set the car up at race weight. 7/8 tank of gas, my weight in the drivers seat, and the cool suit cooler 3/4 full of water.
With the alignent in place and bump steer sorted... I went to corner balance the car, but didn't really need to do anything. I'd previously corner balanced the car and it was pretty dialed still. Technically the corss weight was 50.13% or something like that, but no sense chasing such small amounts when my setup isn't guaranteed perfect anyways. The platforms could be slightly off, the drivers weight in the seat may be a bit imperfect, I could go on a donut binge before the event, etc etc.
All this to say... my setup is as dialed in as I am capable of measuring!
Fresh STICKY Tires! ... again.
After my first foray with the Vitour Sonics was fast yet short lived I wanted to try them yet again. I was pleased with the grip the sonics delivered... but let down by the durability and the steep alignment requirements one supposedly needs to not wear them out preemptively. Reflecting on a few details with my setup and the tires very early demise, I think bumpsteer was contributing to some of my wear issues and limiting the potential of the tire. That issue, followed by my alignment being imperfect and excessively toed in to compensate for my bump steer issues all possibly contributed to their early demise. Past that, all of that can then multiplied by a recent conversation with other folks about how ORP was cording other folks tires there as well!
Fortunately for me Vitour was willing to comp me a new set of tires, which I greatly appreciated. I got the tires mounted on my second set of Podium FF6 wheels (thank you guys for being amazing sponsors) and the car was dialed in to shred!
How The Event Went
The drive out to the event was fortunately uneventful. My wife decided to join and bring the dogs so she hauled our camper with my truck while I hauled the race car with the van. This time the van hauled without issue, especially since we could put more of the tools and equipment in the back of the truck.
We arrived, ran into some friends, had a good evening of hanging out, and passed out for the night.
Day 1 - Chaos And One Okay Session
Day 1 began WET, and not only was it wet but it was the first wet day in quite a while which meant there was a lot of oil and other contaminates coming up to the surace. I went out and found myself ice skating around the track and rolled off hard. Just as grip was maybe starting to show up though... a friend rolled his very expensive car into the woods. They were okay, but the car was ruined, and it was a VERY nice car. We all got off the track a bit shaken and passed on the news to the next sessions: there's no wins out there, only lossses.
The next session unfortunately wasn't any better. While conditions had improved, another driver went hard early and found themselves high centered on the outside of turn 8. That location being unsafe, the track went all black and the session was yet again a total loss with most folks maybe getting 2-3 laps.
Then to top all of that carnage... the timing was broken and only one driver got a time at all... so we had no times, our grid was all mucked up from people fibbing about their lap times... So it would all come down to session 3 on day 1 and people wre chomping at the bit.
Fortunately for me on my final session I set my space right on the outlap and found a flying lap of 1:48.34. I knew it wasn't enough to win... or was it? Others were struggling to find grip on the freshly dried track as well, leaving me fourth overall and first in my class. That was nice to discover, plus our grid order was now sorted. This was a good place to be going into day 2, but the gap between me and my competition wasn't close enough that I could ease off.
I went out in the last two HPDE sessions and tried some minor adjustments. I added a full rotation of height to my rear coilover perches in the hopes of improving front grip and increasing rear throttle off oversteer. While throttle off oversteer sounds undesireable, I often use it on transitions to increase the pace at which I rotate with just a brief liftoff. That full rotation of additional perch height was a bit excessive, and thus backing off one third rotation yielded a far more balanced car.
So day 1 wrapped up with me feeling good about where I was at and the potential to squeeze more out of it all and secure both the podium and championship.
Day 2 - Strained Start Then Strong Finish
Day 2 was looking like PB potential in the AM, which meant it was game on. Partly cloudy weather, cool temps, dry track... there was nothing in the way and so the competition was going to be FIERCE!
So we all go out and... somebody instantly breaks down before even exiting the track entry fully. We all just can't catch a friggen break and get a clean session! Now we're all strained and time is burning as we go around several laps under full track yellow with a white flag for the tow truck. I also for some reason was still gridded behind the slow ass audi that is only fast on the straights and definitely turned a slower lap time. What a disaster...
We go around a few times under full yellow, I'm trying to give the gap I need for the friggen pokey audi, everyone is stacked up... and some dude jumps the whole line from the back under standing yellow!
I was livid... so I chased him down because he wasn't at that pace for that position, repeatedly gave him the universal "get out of my way" hand signal, watched him nearly wreck 5 times, got the pass... and went about the rest of my session on adrenaline and anger... which was not the best version of me nor the reflection of how I like to conduct myself.
...but to be honest that session was a bit of fun. I don't drive in anger, and rarely hopped up adrenaline. My shifts were fast, everything was hyper dialed in... but risk aversion had left the chat so perhaps not the best. I did run a 1:47.69, a time I felt confident would keep me in first.
This was proceeded by more conflict. I flip the dude off getting off the track, dude follows me back to my garage... and uh... I wasn't my best and that wasn't the best reflection on me as I was game on ready.
Fortunately, cooler heads prevailed, it all got sorted out, driver was found at fault for passing under standing yellow, and we all later apologized. I'm sure we won't be sending each other christmas cards after that but man... to jump the whole grid from the back of the pack without the times to back it up. PLUS, you're the ONE guy to do that? Take a hint man... but I get it, everyone was stacked up, time was burning, tensions were high.
After a cool down, I sort myself out and go out for the second session and decide not to drive in anger and deliberately try the cool and controlled approach. While I certainly took some lessons from the angry run, I took a cooler approach, connected more of the right dots, and ran a 1:47.45. This felt especially good to connect together and I was feeling confident my times were standing. My competitors did find some gains over their previous laps, but did not drop into the 1:47s.
The final session isn't much worth mentioning. Temperatures increased, track conditions eroded, and I can't recall if we went out late due to a break down in the previous session or if we finished under all black... but it just didn't come together better for anybody and I was able to secure the win.
Exhausted and sweaty and honestly still not fully feeling recouped from the big conflict earlier... I was happy to call it a wrap on the win.
Final Results:
In the end I took first place in GT class. Hell yeah.
I was the only person in GT to drop in 1:47 territory. It was not actually my PB though as I had thought, but previous conditions were in may and perhaps had been a bit more optimal. Or maybe I drove better and used that throttle off oversteer with my bad bumpsteer better. I can definitely tell you turning this 1:47.457 was nowhere near as harrowing as turning it before!
Long term I should be working to find more time. My two competitors had nowhere near maxed out their platforms, though I think 20+ years of engineering development and triptonic shifting, motorsports level ABS, and traction/stability control likely give them more edge than they'd like to acknowledge. My car was all me... and I built it up myself as well. To me, that feels good, but I can't sit on that.
My overall results were great. 4th overall out of 27 entrees is pretty good. I can take that to the bank as well.
Takeaways / Future Plans
More front camber. The car was still understeering and running wider than I'd like at the limit. I'd also get turn in understeer if I carried deep braking into the turn. I know that the Vitour Sonics are suggested to be at -3.5 degrees or more of negative camber and I am only -29. I need to get more negative camber out of this Corvette and I think this will yield significant improvements up front. I believe I can get more if I let caster be damned, and... as I understand it Caster matters more for dynamic camber under tight cornering, not so much at speed on track? I'll investigate all of this first, but I think I can get closer to that -3.5 target and find some handling gains there as well.
What do I give up without "full aero"? This is a fun data analysis exercise between my most recent SCCA PB and this OnGrid PB. Now, this isn't a totally fair comparison since between the two events I resolved a clear as day obvious bumpsteer issue on my C5 Corvette and I switched tire compounds... but for fun I ran it by AI and graphed the above.
Entering the lap, full aero (SCCA configuration) was faster... and you can even see inthe entry to turn 2 (second dip) where full aero and rotation seemed to yield a faster time. Through turn 5 the red SCCA line remaind fairly ahed... but it all changed come about halfway through the track where increased confidence braking and being able to get on the throttle, likely due to improved bump steer, yielded the most improvements.
My theoretical takeaway: Aero did help produce downforce with less drag clearly based on top speeds. For most of the track though it was neck and neck, and only on the front straight above 100mph was it fully realized. That extra speed slowly eroded over the lap though as improved braking zones (extra confidence from dialed in bump steer I wager) and improved exit speeds started to eat away at the full aero lead.
Can I bring back some controlled throttle off oversteer? Being able to lift slightly in a section to induce slight rotation was fast in several sections, or... was it actually fast because that's time I wasn't on the gas. Do I need to re-evaluate my lines or... what should I do?
Long term, I'd love to investigate going LS3. Watching that annoying audi time travel away from me on the straights only to park it in front of me drove me crazy all weekend. The jump from 5.7L to 6.2L would let me pick up 60 or so horsepower supposedly, and I could move into some of the LS3 based tech for some improvements there as well perhaps.
Oh and this one is a critical one... Don't get that heated again. Conduct myself better. Be better. It all worked out in the end but it was not a good reflection on me. It's possible to be right and lose.