BMW CCA GGC Autocross Season Update: Leading in Gonzo Class!
Ivan Pandev, June 2023
This year’s focus will be on competing in the Golden Gate Chapter of the BMW CCA’s autocross series, including the Top Driver Shootout at the season’s end. Cars are classified in this series based on a points-for-mods system, as opposed to the SCCA’s permitted-mods-per-class system; I reckoned the Battle Bimmer would fit in much better to the former style system. As of this writing, the Battle Bimmer currently leads the Gonzo class, with 46pts, an 11pt lead.
Round 5
The topic of the day was new Hoosier A7s. The below G-plot quantifies the grip advantage of brand new slicks. Lateral grip clustered around 1.1G on the old A7s, while the new A7s cluster around 1.3-1.4G lateral. Note that peak G hardly improved at all and isn’t a reliable indicator of grip potential. Longitudinal grip increased a surprising amount as well, especially on acceleration. The new rubber shot the Battle Bimmer back into contention for raw FTD this round, eventually beating out a GT3RS for the honor. Indexed FTD however, remains elusive for us; this club is home to some seriously quick drivers. Is there a path to grabbing indexed FTD for the Fechner Cup and Top Driver Shootout in a month’s time? Dropping approximately 150lbs of passenger, seat, and harnesses will help, but there is lots of detail work left to do as well. That brings us neatly to the driver notes from this round.
While the Ground Control rear sway bar has put the mid-corner balance in a much better place, the increased lateral load transfer on the rear axle means the torsen differential doesn’t lock up and induce rotation on corner exit quite as well as I like. Additionally, the wear pattern on fresh Hoosiers indicated the front axle can use still more negative camber. Time to dream up some solutions before the Top Driver Shootout!
Round 4
This round was the first outing on the Ground Control SpecE46 rear sway bar, and immediately the mid-corner balance was greatly improved. (More detail on the motivation and results of that install here). A quick spinoff on the first run highlighted just how much less understeer there is. Final results of the day had the Battle Bimmer back on top of Gonzo class, however not the contender for raw FTD that is was in Round 2. These Hoosier A7s are not only old, but also deep into their heat cycling life, and started to show chords the day after the race. Still, you can’t ask for much more than a class win and reliable running all day.
Round 3
This round was a farce, a long way down from winning official raw FTD in the previous round. There was no time to sharpen up with sim racing in the days leading up to the event, and we arrived so late the morning of the event itself they practically closed the gates behind us. Arriving so late, I didn’t have time to walk the course and was stuck unloading the trailer. We didn’t even have mental bandwidth to check tire pressures before our first run, which could’ve been done later in the day had we not violated Laguna Seca’s noise limits, which meant we only ran 4 out of our 10 runs. Of the runs we did complete, the tires couldn’t find any grip or temperature, owing to incorrect pressures and a cold-ish, dusty track surface. Finally, the depowered 13.8:1 rack works great at Crow’s Landing, but wasn’t effective at the multiple tight hairpins of this Laguna Seca course. Overall, this was a pretty comical performance and a great reminder of how much lap time is hidden in preparation. We placed last in class this round, around 4 seconds off the leader, and are very happy the remaining rounds are at Crow’s Landing Airfield.
Round 2
We arrived at Round 2 with small-but-steady improvements to aerodynamics (beyond the obvious of the re-installed front clip), and about 200lbs extra weight in the form of passengers and associated seats and harnesses. The track was dry all day with long straights to favor the Battle Bimmer’s high-end power curve, so we maintained our lead in-class. The fastest time for 59.022 was also the fastest officially classified time of the day, although still 9th in PAX terms. Going forward in the season, the goal will be to reach the top of the PAX times as well, to stand a chance in the Top Driver Shootout at the end of the year. We eagerly await Round 3 to drop those 200lbs and hopefully push further up the PAX standings.
Round 1
We arrived at a wet and cold Crow’s Landing airfield for Round 1 with a barely broken-in clutch and slicks – no grooved tires. The goal was to survive mechanically, run the car to its limits, and collect telemetry for future ChassisDyne development. Both morning and afternoon sessions featured a rapidly drying track, so class leaders changed multiple times towards the end of the session. In the end, we pulled the Battle Bimmer into the top spot of Gonzo class by around 1.5s, and got the second-fastest officially classed absolute time that day, although the PAX time was lacking. It was a joy to adapt to the changing track conditions run-by-run in a chassis which has had tons of work but very little run time over the past few years. Telemetry showed a peak lateral acceleration of 1.68G, and maximum sustained being around 1.4G.
Suspension Refactor II Shakedown
Ivan Pandev, Aug 2021
After this year’s suspension refactor, as detailed in the development blog, we had the fortune of a private test day at Thunderhill Raceway’s Big Skid Pad. The goal was to run the Battle Bimmer hard for as many runs as possible, validate the performance and installation of the many new suspension mods, get comfortable driving the new equipment at the limit, and end the day with a well-used oil sample to send to the lab and check engine health.
In a word, the suspension refactor is a marked improvement in every way. The 12% quicker steering ratio makes the entire chassis easier to place through fast-paced slaloms and kinks, but the step-change in steering perfromance is the welded pinion shaft; I can’t understate how much more precision and better response comes from eliminating the compliance from the torsion bar. It transforms the feel of the car from a stripped-out passenger coupe and a 2300lb kart.
Meanwhile, the new brakes’ response bite is a clear step above the previous version. The larger 298mm rear rotors and relocated rear calipers, estimated to increase rear braking torque 16% for the same pedal pressure, assisted rotation nicely in trail braking.
Finally, the new control arms with re-clocked roll center correction (RCC) bearings made much better use of the front Hoosiers, which now wear evenly and seem to ride slightly negative-cambered mid-corner, instead of graining the outer bands and underworking the inner ones. We’ll shoot for an outer-to-inner tire temperature difference of around 10-15F next time, a rule-of-thumb to balance maximizing contact patch and camber thrust.
Driving without the front fascia highlights another benefit of the RCC bearings; the dropped control arm pivots have kept the arms roughly level in roll. The snapshots below estimate that the roll center probably stays within several inches of the ground plane, probably a few inches above, which agrees with our Racing Aspirations model. This is just a sanity check at best, but at least we can quantify the effects of changed roll geometry going forward.
Like most weekends, the engine showed perfect reliability. Intake temperature was difficult to keep below 160F on that ~80F day, but the VF Engineering tune should be safe at that temperature at 93 octane and above.
2019 Solo Nationals Recap
Written by Ivan Pandev
We had big goals coming into the 2019 SCCA Solo Nationals. Although we fell short, the Battle Bimmer showed total reliability, a step-change improvement in overall speed, and a clear path forward for better results in the future.
New Eqipment, New Challenges
Project Battle Bimmer arrived on Wednesday with two step-changes to its performance; a VF Engineering superchager kit and the widest (315mm) Hoosier A7s its run yet. The supercharger was barely shaken down two weekends ago at Crow’s Landing, our home track, and the new tires were an unknown. Short on wrench and track time, Project Battle Bimmer promised a step-change in overall speed, but wasn’t fine-tuned around the new parts or the new track conditions at Lincoln airport.
The courses at Lincoln airport are generally tighter and have lower maximum speeds than those at Crow’s Landing, and the surface affects tire marbling much differently as well. These new effects would expose two weaknesses. Firstly, the lack of negative camber adjustment range had been a known fault, but at Lincoln airport’s new surface, it resulted in the underworked inner tire circumeference accumulating marbles like never before. On the second day of competition especially, this caused overwhelming understeer and squandered the potential of the new hardware. Secondly, some hairpin corners left both drivers wanting for more steering angle. This results from the de-powered factory steering rack, which has a 15.4:1 gear ratio, along with a few degrees of compliance coming from the torsion bar in the pinion shaft.
Overall Results
At the end of two days of racing, Savion and I were sitting 12th and 11th, respectively. Whereas in 2017 my time was 109.5% of the lowest trophy time, and 113.8% of the championship time, this year my time was 106.0% of the lowest trophy, and 109.1% of the championship.
The pace-killing marbling only came into effect the second day, however. At the end of the first day, when we were able to keep our tires relatively clean, I was in a battle for 9th with my top time being 105.5% of the lowest trophy. If I wave my bench-racing wand and forget that I coned my fastest time, that figure falls to 104.2%. None of this counts of course, but it does demonstrate how much our performance fell off on the second day; what our final results might’ve been had we learned these hard lessons the year before.
A Clear Path Forward
After these underwhelming results, it was clear that there wasn’t much sense returning the Battle Bimmer to competition without some chassis and driveability changes that can capitalize on the potential of the 315 A7s and VF Engineering supercharger system. These would include increasing the negative camber adjustment range and decreasing the steering rack ratio, and many other opportunities for improvement throughout the car.
AAS Practice Course, Aug 2018
It’s good to look back on how far you’ve come. Wrong differential ratio, sloppy steering, senseless alignment, and only 190hp to push the Battle Bimmer along. Here’s to 3 years of progress!