Homestead December 19, 2004

on track at Slip and Slide autocross.

Slip and Slide Autocross  

Preparing for an Autocross is very different from preparing for a road race. The autocross course is created in a paved area with cones and only exists for that day. The driver will only have, at most, four runs to set a winning time. Learning the course quickly is crucial to setting a good time.

Before runs start, we are given time to walk the course. Here I must, based on my knowledge of the car's capabilities, try to estimate how I should drive the course. If I think the car will have reached 50 MPH on the 1st straight away, then where will I need to start braking in order to be slowed down enough to make it through the corner?

Since this is the first time I have auto-crossed the Electric Imp, my guesses are a bit rough.

Run 1

The first run brings home how rough. The car accelerates all the way to 60 mph before the first turn. I have to brake much earlier than I had planned and the cold brakes and tires fail to slow the car enough. I miss the apex and have to stay on the brakes for too long. I am out of position for the fast line through the next corner. This sets the tone for the entire run. Always one step behind!

As the run continues, my job is made harder by the performance of the car changing. By the next corner I have revised my braking point to make allowances for the poor performance of the brakes. Since they have warmed up, they slow the car more effectively. Now I am going too slowly. Time is 45.407 and I miss one cone.

Run 2

Next run is better. The tires and brakes have a little heat making them more consistent. I have driven the track once and have a better feel for its rhythm. I brake at the same point but more effectively for the first turn and carry more speed through the slalom. Next corner is fine but I make a small error in the second slalom. I brake when I could have carried the speed.

At the last corner, I over-rotate the car. I steer into the skid to catch the car and hammer the throttle. The car feels like it is bunching up but not accelerating. What has happened is I have put the four wheel drive in a position where it is fighting itself. The front wheels are at full lock. The front tires are using their full grip to try and turn the car while being pushed straight ahead by the rear wheels. At the same time, the front tires are also responding to the full throttle by spinning forward trying to accelerate the car. When you ask the tires to do more than they can, they actually lose grip. The net result is the front tires are perpendicular to the car's path and sliding sideways.

I ease up on the throttle for an instant and straighten the wheels. The front tires grip and the car leaps forward and across the finish line. The timer flashes 44.490- almost a second faster than the last run.

Run 3

The third run starts well. I brake later but still carry the same speed through the corner and the slalom. Later brake for the next corner but slightly less speed. A wash time wise but I am full throttle all the way through the second slalom. A big gain. I brake for the next corner, get on the throttle and "CLACK", the contactors drop out. I pull immediately off the track.

The car will not restart. I have no voltage. We are done for the weekend.

Back at the shop, we discover what went wrong. When I flung the car into the corner a battery stack shifted. This tore a battery tab off. The cell is under one of the inverters, in the hardest place to reach. Once again, the fault is traced to a failure to secure the batteries well enough to withstand the rigors of racing.

Sum up
click to view RPM chart click to view Torque chart click to view Current chart click to view Voltage chart
For a first autocross the car did well. Being the only car in our class, B Mod, we won our class. More importantly, we did a faster time in 2 runs than more than half the 120 cars did with 4 runs. Electronically the car was perfect. The Kokam cells churned out 600 amps on demand. The data says 422 lbf-ft of torque at launch. The batteries and motors are giving us the power. It is up to us, ProEV, to make the system mechanically reliable.

in line for the next run


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