Rotary Racer - Car Performance

Merryfield 2010-09-19

Results

A great race for us. We had lowered the CofG by building some new 16inch wheels and replacing the foam sides of the car to match. A new, even stronger, roll-bar was fitted as well with stronger subframe mountings. The steering gearing raito was changed to make the steering less sensitive. We had decided to go relatively slow this race to test the car after its changes, to allow the drivers to re-gain confidence and beacuse it is a relatively short track with tight corners. We used the old, half dead batteries and decided to try and race with an average current of about 15 Amps. We calculated that with new/good Yuassa batteries at 28 degreesC we could expect a single pair of them to be able to put out 15Amps on average over a whole 4 hour race. So the plan was to emulate this with our two sets of duff batteries.

Summary

Car Computer Log

This shows the main items measured by the Rotary Racers car computer. They are:

Item	Description
Throttle	The position of the twist grip throttle from 0 to 100%
MotorCtl	The motor power output setting. The Car computer sets the motor power based on the drivers throttle position, the speed of the car and the current being used. There is a software algorithm to implement this "drive by wire" system. The Car computer drives the PWM (Pulse Width Modulation) Motor Speed controller with a duty cycle proportional to this value. 0 - 100%
Speed	The speed of the car in Km/H calculated from the magnetic read switch pulse generator by the Car Computer.
Voltage	The batteries voltage under load.
Current	The current being taken from the batteries.
Charge0	The charge left in the battery pack 0. This is calculated by the car computer using an algorithm that has been honed to match the battery discharge curves for the Yuasa batteries. It currently bases its results on the current being used from the battery.
Charge1	The charge left in the battery pack 1. This is calculated by the car computer using an algorithm that has been honed to match the battery discharge curves for the Yuasa batteries. It currently bases its results on the current being used from the battery.
BatPack	The battery pack in use (0 or 1), the turbo button (2) and the Pitstop warning (4) . We generally change the battery pack with each driver change.
Temp	The Temperature of the motors casing in degrees C.
GPS	GPS position, speed and time together with lap position information
Misc	Other itmes describing the operation of the control system

The graphs show the overall data, the actual ASCII data log files are also available, from the links below, for detailed analysis.

Practice

Car Data Log

Car Data File

The full race

Car Data Log

Car Data File

Battery Pack	BatA	BatB
0 (RR7,RR8)
1 (RR9, RR10)	12.61	12.60
0	12.42	12.43
1	12.40	12.39
0	12.29	12.36
1	12.46	12.45

Notes

The Car was RR8f. This was the cars eighth race. We had built new 349mm (16 inch) wheels, remade the foam sides to match the new wheel sizes, replaced the roll-bar with a thicker stainless steel one, improved the subframe mounting strength and added screws to allow the roll-bar strut to take forward as well as rearword forces. The steering gearing was changed to make the steering less sensitive and new steering protection plates where made and installed.
The CofG was lowered by about 28mm with this work and the wheel track increased by 18mm to make the car more stable on tight corners.
The car was running with a 17:49 (2.88) ratio on the chain drive, but now with smaller wheels.
The Merryfield circuit is 1.094Km long and has some slight hills with three tightish corners. It is a bit bumpy in places. There was a reasonable wind on the day (30Km/H ?).
The software was car19 which included average speed computation and GPS data and lap timing measurements. The only change was the wheel circumference change for the speedo. The Car electronics worked well. The speed measurement was fixed and the GPS worked fine.
The drivers reported that the car felt more stable on cornering and appeared to be less buffeted by the wind. They also said the brakes felt a bit smoother and the steering better. The drivers we going steady as agreed. The drivers were: Ben, Tom, Liam, Dan, Gareth and James. Pit-stops were excellent.
We started off near the front using around 17Amps and on the limit for the speed limit for this race. We then reduced the power levels to try and average around 15 Amps.
Battery charge levels looked fine until Gareth's turn. It soon became obvious that the battery pack had serious problems as the voltage quickly fell below 20 Volts. We turned down the power substantially untill the car was running around on about 9Amps at about 34Km/H. We swapped batteries and driver early because of this and left the power on low to make sure we could finish the race. Later on these batteries were holding up well and we were in 3rd position not to far behind 2nd place. The compatitive spirit kicked in and we increased the power levels again to get us a second place result :). Obviously the battery packs are getting bad as to be expected although the quick collapse of this one was not expected.
When the batteries failed we were using about 9 Amps at 21 Volts (189Watts) and going at about 21MPH.
On testing batteries RR7 and RR8 after the race, off load RR7 12.33V, RR8 12.40. Under 20Amps load RR7 was at 11.90V and RR8 was at 7.70V. Obviously RR8 has one or more faulty cells. Looking at the logs the voltage level became spiky during Liams drive, so it will be worth looking for such spikes in the future as it will probably indicate a faulty cell. Also its terminal voltage fell while in the battery quarnteen area.
We managed to average 15.3Amps, a little higher than the 15 Amps we were aiming for due to the bad batteries and the competative spirit :). But pretty impresive, we effectively raced with the energy of just one pack of two good batteries and managed second place !
An excellent race for us and a good test for the changed car.