phate's autocross adventures

With the season approaching quickly, I thought I'd make a dedicated thread to talk about how I'm not driving my car effectively and lots of other fun things. Car setup and analysis are two hugely important things to me, so that's probably going to be the majority of this thread. In the coming week, I've got some work to do to finish prepping the car for this year. Big changes this year, so I'll recap the current setup.

I'll assume you've read my suspension thread, but feel free to ask questions.

• Coil overs built by me from Bilstein shocks, shocks revalved by FCM
  • 850/700 lb/in front/rear springs
• Steeda front bar (new for this year) [Was running stock front]
  
• Secondary rear sway bar that I designed
• XXR 527 17x9.75 +25 with Hoosier A6 275/40/17 (new for this year)
• Enkei NT03+M 17x9.5 with Hankook RS3V2 255/40/17 for colder/wetter events
• Polyurethane suspension bushings all around (front susp bushings new this year)

My 'general car setup' thread is posted behind the scenes and is waiting for a home in public space on the forum, so I'll link to that once it's up. This section is going to take those principles and apply them to my car. We're changing a LOT of things, so rather than heading out to autocross and figuring things out by trial and error, I'll take a 'by the numbers' approach to get things close.

We'll start with last year's setup I had at the end of the year. I was pretty happy with it, but it was still understeering a bit in steady state. It was 850F/700R springs, stock front bar with poly bushings, secondary rear bar on the softest setting, no bump stops, RS3V2's around 35psi. Front weight % was around 58.5%. Using those assumptions found in the 'general setup' thread, we can check the stiffness bias:

Old Front Wheel Rate
Springs: 850 x .73^2 = 453 lb/in
Bar: 222 x .73^2 = 118 lb/in, but roughly doubled in roll
RS3V2 @ 35psi = 1600 lb/in

Front total rate: ((453 + 118*2)*1600) / ((453+118*2) + 1600) = 482 lb/in

Old Rear Wheel Rate
Springs: 700 x .57^2 = 227 lb/in
Bar: 73 x 1.00^2 = 71.5 lb/in, but roughly doubled in roll
RS3V2 @ 35psi = 1600 lb/in

Rear total rate: ((227 + 73*2)*1600) / ((227+73*2) + 1600) = 301 lb/in

Old Overall Stiffness Bias: 481/(481 + 302) = 61.6% <--Higher than front weight %, so it's understeer prone





Picture of last year's setup in action (before the secondary rear bar):

Secondary Rear Bar Setup:






The new setup is going to change the front bar that doesn't have adjustability (makes things simple when you don't have adjustment!), the tire spring rate is going up significantly (tire pressure assumed), and I'll use the rear bar to tweak the stiffness bias.


New Front Wheel Rate
Springs: 850 x .73^2 = 453 lb/in
Bar: 387 x .73^2 = 206 lb/in
A6 @ 35psi = 2400 lb/in

Front total rate: ((453 + 163*2)*2400) / ((453+163*2) + 2400) = 634 lb/in

Since we're shooting to mimic last year's setup with the numbers, I'll have a target front stiffness of 61.4% and work backwards:

62.4% = 588/(588 + <rear wheel rate>)
Target Rear Wheel Rate = 396 lb/in

New Rear Wheel Rate (with bar as the variable)
Springs: 700 x .57^2 = 227 lb/in
Bar: B x 1.00^2 = B lb/in
A6 @ 35psi = 2400 lb/in

354 = ((227 + 2B)*2400) / ((227+2B) + 2400) --> B = 124 lb/in (my bar is adjustable up to 174 lb/in, so easily done)

Yes, it should still be understeer prone since it's the same stiffness bias, but I want to start with something that is familiar. I have plenty of adjustment left in the rear bar to dial in more rear bias when the time comes.



I suppose all that's left is suspension height choice and alignment. I could have gone lower last year but never did. Since we're chopping and flaring for the new tires, I think I'll be able to go a little lower. More about that in the next post.

 

Awesome thread, Clint.

I should really break the setup of the Miata down similarly to how you've done and start measuring things sooner rather than later, while the car is still at "baseline" and before I start fucking with it...