Sliding outside the window

» Posted by on May 7, 2012 in Current projects and Tech Notes | Comments Off on Sliding outside the window

Sliding outside the window

My second event in Freddie the Formula Ford was an autocross test day in a parking lot inside Auto Club Speedway in Fontana, CA. This was an ideal opportunity to get more familiar with the car in a safe, non-competitive environment with lots of quality seat time available. As you will see, I did a lot with the day and learned several important things, but ultimately found the car to be too far outside the tuning window to be competitive for autocross. The short story is that skinny rear tires aren’t viable for autocrossing a Formula Ford.

This was my first event with Speed Ventures, and I’m extremely impressed. The usual process hassles were completely absent, it started and ended on time, they did an extremely efficient job of cycling cars through the course, and it was an enjoyable, low stress environment. I’ll be back for more of that!

The lot was fairly small, so the course was short and tight. Here’s a map of the autocross course from the GPS logger that is part of my AIM EVO4 data system:

The lower right corner shows a 200 foot long scale reference. The blue cross is at the starting line and the color coding is speed, calculated from GPS data. My top speed was only 54 mph. The surface was slick and a bit gravelly, so the car pulled 1.3 g lateral compared to 1.5 g at Buttonwillow on the same tires. Here’s the friction circle from my best run:

That shows that I did a decent job of using all of the combined lateral/longitudinal grip available, which is surprising since I spent so much of the run sideways. Seriously, big-time-drifting sideways. The setup, even at the end of the day, was crazy loose except in the low speed twisties.

I’ll overload you with data some other time, so let’s get to the play by play account of the day. What follows are my notes from each outing and the changes I made for the next outing. You may find the pace and magnitude of the changes I made to be far larger than most people would do. Since it’s early days for setup development, and since the environment was low risk, I went for it. My policy at the track is to never run exactly the same setup twice. Every time the car comes to a stop, I change something. That makes every outing a development opportunity. My high school band program taught me a lot about the value of cumulative progress, and I apply that lesson to car development whenever I can.

2 runs: Low grip, lively balance, braking understeer, 2nd gear course. Changes: FARB full stiff after 1st run, bias 1 turn to the rear, pressure from 17/17 to 15/15 (front/rear). Didn’t look at lap times.

2 runs: 38.9, 38.7 sec. Less grip, off course in 1st turn because I drove through gravel. FARB full stiff for 2nd run, still loose. Need a cooling fan for grid: water 232°F. Steering lock and steering ratio are problems for autox. Changes: 17/17 psi, rear springs from 250 to 170 lb/in, disconnect RARB, R spring seats up 3 turns.

3 runs: 38.9, 38.9, 38.6 sec. Very strange: big oversteer 1st run, push 2nd run, either depending on driving technique 3rd run. Car or driver? Probably increasing tire pressure. Still huge power-induced oversteer. Changes: 18/18 psi, rear pushrods 3 turns longer for bottoming.

2 runs: 39.4, 38.6 sec. Better grip, worse driving. Tried to keep it stuck down, just coasted a lot. Still steps out to oversteer easily. Seems to work better when being over-driven. Changes: R pushrods +4 turns, press 19/19 psi.

2 runs: 38.5, 38.6 sec. Better driving 1st run, loose but driveable now. Over-drove 2nd run, got behind the course. Changes: front rebound + 10 sweeps, press 20/20 psi.

2 runs: 38.5, 38.2 sec. Better! Smoother reaction to braking to power transition, better grip. Still loose but driveable. Changes: F rebound +5 sweeps, 21/21 psi.

1 run: 38.9 sec. Worse driving, too loose early and pushing late in the run. Grip didn’t seem as good, especially forward traction. Reaction to brake-power transition seemed harsher, less controllable. Changes: F rebound -3 sweeps, 20/20 psi.

1 run: 38.3 sec. Better driving. Brake to power transition better. Still very easy to get into huge oversteer. Grip feels better. Press 20/20 is good. Changes: front springs 350 to 450 lb/in, front spring seats down 2 turns.

2 runs: 38.6, 38.4 sec. Behavior was the same – stiffer front springs didn’t change the behavior. Fronts still lock first. Changes: Rear toe in + 2 turns, bias 1 turn rear.

1 run: 38.2 sec. Worse driving, better car.

This was my first autocross in 16 years. Considering that, my run times were stunningly consistent, but slower than they should have been because of the very loose balance. I was surprised that I had no problems acclimating myself to autocross driving or car control, given the fact that I spent almost the whole time sideways. The lower speed range of autocross calls for much shorter gear ratios than road courses, so the car felt like it had infinite power. The data showed significant wheelspin with any significant throttle application.

The car was reasonably balanced with a bit too much low speed understeer at Buttonwillow with 350 lb/in front springs and 250 rears. Changing them to 450 front and 170 rear with the rear bar disconnected still wasn’t enough to tame the driftitude for autox. So, I have to conclude that running the front tire size on the rear as well just doesn’t work for autocross. I put all of the ballast as far forward as I could arrange, so the car currently sits at 46.6% front with driver but no fuel. I located the ballast forward to accommodate the same size tires all around, and I can’t go any farther. Given that limitation and the short gearing for autocross, my primary conclusion from this test day is that the skinny rear tires are not viable for autocross.