2026 Rd 8: Buttonwillow CCW1A

Event details

• Date: Sunday, September 13, 2026
• Track: Buttonwillow 1A CCW
• Official result: 86 Challenge Stock results

Stock top 5

Fastest Lap Onboard

Summary

Setup saga

We're back after a long, long summer break. This season, the car has just been too snappy on throttle for me to drive it confidently. I've tried:

• Reducing camber to -1.5 to improve longitudinal traction
• Replacing the rear sway bar bushings
• Reducing the rear tire pressure by 3psi
• Getting a pedal commander to make the 0-30% throttle range easier to modulate
• BRZ front spring (3.1kg/mm instead of the stock 2.9)
• Bilstein B6 dampers
• 75w90NS diff fluid instead of gear 300 (which has friction modifiers)
• Reverting to stock brake bias instead of using more aggressive rear pads (mostly a remnant from when my car was understeery)

But nothing was enough for me to really drive clean and confidently.Obviously it's probably a skill issue, but I've found myself repeating the same mistakes and erring towards underdriving instead of breaking past my ceiling. Something has to change if I want to win.

So I increased rear camber (1.5*->1.7*, kept toe at 0.1* each side), downrated my rear bar to 14mm and tacked on 11mm rear spacers. The idea behind the bar is simple - softer, more independent rear means a more forgiving rear end at the cost of a duller front. The spacers are a bit more convoluted. There are 3 complementary effects at play:

• Wider rear track means less lateral load transfer - more inside grip means the torsen works better. This is obviously marginal.
• Longer effective suspension arm means lower motion ratio, which means a softer rear. Equivalent to a ~3-5% reduction in spring rate.
• Larger rear scrub radius (yes that's a thing) increases compliance toe-in under power.

The last one is what I suspect made the biggest difference. I tested the 14mm bar and alignment change on their own and the car still felt like it was rotating too much on throttle, despite understeering more everywhere else. But once I put the spacers on, the car just felt like it was digging much harder on power, and I could dump a lot more energy into the rear axle without it snapping.

If you just watch my best lap above (P1 by 0.3s by the way), you'll probably feel that it looks like a sunday cruise. And it is - I'm losing time in many braking zones, but the idea is to build a car that makes a decent lap easy. If I can get within 0.5s of a top tier lap without risking a spin or off, that makes it easier to find that last 0.5s on my own time.

It's a bit humbling to "neuter" the car because I'm incapable of handling it. But I've spent a long time punishing myself for not being good enough, and honestly, it's about time I started being kinder to myself. I'm clearly faster with a more tame car, and I definitely had more fun.

Lap analysis vs Ivan

• Ss: I didn't go flat out. Skill issue. Didn't lose much time though - 4th gear limiter reduces how much you gain from being flat out.
• Mazda: I carried more speed in, but braked and turned in a bit too early - a habit I developed to cope with the oversteery car.
• Phil Hill: I braked earlier so I could use 3rd instead of Ivan's 4th. Mostly a wash
• Riverside: I braked earlier than Ivan, and Ivan's vmin was higher and yet I gained ~0.3s on exit. Ivan had a better riverside on another lap but mine was clearly not bad despite being conservative.
• Bus stop: I have some PTSD from when I bent my wheel here 2 years ago. Apparently a lot guys were carrying enough speed for 4th to make sense, including Ivan. I still think 3rd is faster but I was clearly overslowing to make it work. I lost my entire advantage from riverside and then some.
• Grapevine: Ivan brakes way too late, and V601s don't forgive that. He loses ~1s vs my pretty solid grapevine. Delta is now -1s
• Cotton corner: I thought I could stay flat over the final apex curb with this setup, but nope - I lose ~0.4s on exit. Delta is -0.6s.
• A-section: I brake and turn way too early (again), which meant I spent more time in the turn. I lose ~0.1s. Delta is -0.5s
• Sunrise: I brake, turn and get on the gas too early, losing ~0.1 and my transmission loses me another 0.1 in the drag race to the finish. Total delta: -0.3s

Overall: Brake later, pivot harder.

Downloads

↓AIM XRK Data - Session 2Download
↓AIM CSV Data - Session 2Download