Mine is a minimalist design, the object being to maximize the power to weight ratio whilst avoiding wheel spin. The chassis was reduced to two 5-hole narrow strips with one front wheel for good directionality. Minimum mass was also necessary to ensure maximum acceleration and minimum inertia.
I found that the tri-flat clutch/coupling components supplied with the set slipped before full wind-up was achieved, so I fitted plastic 57-tooth gears into the geared hubs, enabling grub screw security of wheels to drive shaft. The plastic pins through the gears and hubs were for extra security, but probably not necessary.