Fathers Day Cake Lorry
Way back in February, Dawn had the idea of making a cake based lorry in the same style as the Cake cars we're been doing. It was a great idea but I knew to shift all of the additional weight of a trailer we were going to need four driving wheels and motors. The majority of the weight would be spread over the rear of the trailer so those wheels and axles were put in place with bearings to reduce the rolling resistance. The pivot point was again made with a bearing to ease turning friction. Other minor changes included a reversing beeper and a second set of brake lights on the trailer itself (with curly cable between cab and trailer). The whole thing worked well for this quick demonstration.
The problems came when I loaded the trailer down with any weight. I ran my tests using 10kg of weight which the cab was able to pull but the steering was totally ineffective. All the weight was pushed into the drive wheels making the steering light, the sheer power of the four motors just ploughed the cab on in a straight line. At this point I was lucky with some previous design decisions. Using different motors for left and right wheels allowed me to implement a software differential between the wheels. By making the left turn faster than the right, the whole cab will steer right regardless of what the steering is doing. A little bit of tweaking got the two to line up so that the whole thing turns the right amount based on the user input.
This major problem with the lorry has been back propagated through the cars. It was only a minor issue on the cars but now it's completely gone and the cars drive much better because of it.
Dawn eventually turned the lorry into a Carlsberg van capable of dispensing beer (from a 5kg keg) hidden in the back and it's probably the best fathers day cake in the world.