May 21-22, 2005

Homestead-Miami Speedway

Session 2

8 lap race -2:55 pm May 21

We do not have a data trace to calculate kw-hrs used per lap. We do know that after three laps, our average cell voltage was 3.86. We fall back to our 70 amp discharge study and estimate that we have used 30% of the 22.7 kW-hrs or 1.031 kW-hrs per mile. That means we should be able to go 10 laps at full power. The race is eight laps. We can race at full power.

Our Data Acquisition specialist has to use some tricks on the grid to make sure the computer and inverter are talking. This problem means there is no time to set up the video camera. The officials signal 'clear the grid' and we roll out on the pace lap.

I warm up the tires by slowing up and then going hard through the corners. To save power, I only warm the brakes a little.

I am out of the last chicane and pointed straight as the pack accelerates towards the start line. Full on the power and the green flag flies. My timing is good and the Imp's power pulls us past four cars. I brake early to compensate for cold brakes. The car starts to slow, then seems to accelerate again, then slows again. I am closing too quickly on the car braking ahead. I turn the car to avoid, and spin. I roll backwards as cars pass on both sides. One car does not turn fast enough and whacks the left front of the Imp.

As traffic clears, I respin the car and aim back onto the track. The car is running and I hear no odd noises and no smoke. I rejoin at the back of the pack and begin to work my way back up.

The car is running strong. I am reaching over 105 mph at the end of three longest straights. 600 amps and no cell is falling below 2.8 volts.

Lap 2, according to plan, I start using the regen pedal. This provides an additional 160 ft-lbs of slowing. Because the car is not stopping well, I brake early. The car slows way before the corner. I ease off the brake and then can't get the car slowed down enough to hit the apex. This happens again and again.

While most of Homestead's corners require good braking, corner 8 requires good grip and nerve. First lap around, I have traffic and must brake behind a slower car. Next lap, I just breathe the throttle. Then back to full throttle and turn in. It is bumpy and the car lifts and bobbles. It settles just in time to brake for the next corner. My second lap is 2 seconds faster than my qualifying time.

Lap 4, coming out of turn 6, smoke appears directly below my seat again. I pull into the pits. We guess it is another shorted wire but can not take the chance.

James and I confer. The drilled rotors show signs of cracking. The paint on the calipers have changed color from the heat. The infrared thermometer shows the front brakes came in much hotter than the rear brakes.

The car feels like the throttle is sticking. We check the linkage. There is no binding or problem. A look at our data shows a number of incidents of unexplained surges of motor torque including the surge I felt right before the accident in turn 1. At 1.5 seconds into the lap throttle is at 0%. Current suddenly jumps from 0 to 512 amps. Torque to 178 ft-lbs. This continues for 3/10's of a second. Then amps drop to 118 and torque to 34 ft-lbs for another 4/10th of a second. Then everything goes back to zero.

lap1 chart 052105 Homestead Speedway

Our first guess is that interference created voltage in the throttle cable is sending a throttle signal. The problem with this theory is that if stray voltage fooled the inverter into thinking it was being asked for power, the inverter would record that signal voltage as a throttle signal. Throttle signal is 0 all though the glitch.

We are mystified!

We get to work. The seat comes out and we find another burnt signal wire. Clearly, despite our best efforts, the pack is shifting back and forth minutely under the seat and chaffing the wires. We adjust the sensor wires we can reach but without disassembling the pack, we can not reach all.

We adjust the brake bias three turns to the rear to get more braking from the rear wheels and re-route the wires between the throttle pot and the inverters as a precaution. The crew then bangs out the fender. We will charge in the morning.



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May 21, 2005 Session #1
May 22, 2005 Session #3


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