The motors


These are bespoke units and are very heavy. They are lightened a little by removing the mounting feet. The curvature of the body, clamps within the shell hold the motors (and further strengthen the body) and the original bolt holes are used to mount the motors even more securely. Conductive paste now improves heat conduction to the hull and matt black paint (on surfaces which get hot or which face hot surfaces) improves radiation.

To convert electrical energy to mechanical energy efficiently, large magnetic fields must be used. Our motors are large. These motors are so efficient that they will spin with considerable torque even on an AA battery. We expect that the improved efficiency would allow us to save on battery mass.

Note that the so called 'permanant' magnets will lose their strength if subject to trauma or if the magnetic field is not concentrated through the comutator. To minimise loss of magnetism, the motors are never dismantled so the commutator acts as a magnet "keeper". Thus, the motors were specified with tapers and keys to suit our pinion gears and will not need to be taken apart after manufacture. Over time, and with heavy (ab)use the magnets will start to weaken which will cause the motors to run faster.

For the technical minded folks - loss of magnetism will cause a corresponding loss of back-EMF when electric motors are running. To produce the same back-EMF, motors with weakened magnets will have to run faster. Our 'Joker' will have a higher maximum speed as she ages.

To reduce inertia the motors are quite long compared to their diameter.
Pugno had the motors mounted in line at one end of the machine. This resulted in a wide configeration. 'Joker' has the motors mounted at either end in a symetrical configeration. The hull is much narrower.


Motor Specifications

• Nominal operating voltage 36V
• Weight 2x 11kg
• Will run equally well both directions (zero timed)
• Rare earth permanant magnets, the mounting improved by filling the voids between the magnets with epoxy resin.
• Fully enclosed design to prevent the ingress of swarf
• Maximum usable torque - 5Nm at 6V across the terminals.
• Maximum efficiency - 80% at 2350 rpm
• Maximum estimated current - 160A (limited by the magnets being demagnetised)
• Estimated maximum power 2kW for 2s (4kW total)

The Axles

Pugno had axles which went through the width of the machine. This added strength but reduced the usable space considerably. This was ditched in favour of stub axles bolted to the monocoque and also onto pillars within the machine which further strengthened the structure.


Axle Specification

• Axle lengths - 200mm
• Diameter - 25mm
• Material - aluminium rod, bored out to reduce mass

The Wheels

Kart components are retained. Weight has been saved by welding the pairs of wheel and sprocket carriers together and then lathing off surplus metal. The carriers are modified to carry two sprockets. The weight of nuts has been saved by Helicoiling the mounting holes and using short bolts. As a bonus, the carriers now do not need not be tightened up to prevent them slipping on the axles due to motor torque since they are now welded together. The transmission chains are now located by the wheels rather than in a central chest as previously. The tyres can be foam filled to prevent them being disabled due to punctures.


Wheel Specification

• 10 inch front kart wheels and very sticky tyres
• Grip improved by wiping the tyres with grip improver
• Motor Sprocket teeth 9
• Wheel Sprocket teeth 66
• Gold kart chain
• Primary chains from motor and secondary chains joining the wheels on each side together.
• Speed at 2350 RPM = 9.5 mph

Next Chapter:    Power