The size of the model was partly dictated by the 300mm wheel centre to centre requirement and the need to keep weight within an acceptable range. However, the overall shape was determined with respect to the final recipient of the product, ie. Quinn the two year old.
I decided on four wheels rather than three. Two front wheels is likely to track better than one front wheel. I could have had two wheels at the front and one at the rear to deal with that issue. However, this raised subsequent issues about harnessing power from the rubberbands. I also speculated that four wheels would provide better stability (although this comparison is not tested in this project).
I decided on a "blocky" design. I looked at numerous toys aimed at this age group. Not surprisingly, blocks tended to feature largely among them. This makes sense for two reasons. First, from a safety perspective, it is best to avoid small parts. Secondly, if you watch a two year old, they tend to grasp things with their whole hand. They generally have not developed the fine motor skills at this age. As such, a block shaped model seemed appropriate.
Consideration was given to whether the whole model should be enclosed, like a car, or whether it should be more truck shaped. (I discounted a simple flat board option as it would lack sufficient "interest".) The latter provides the customer with the option for carrying a load. I was also conscious of the stakeholder's requirement for carrying a load and so the latter option was adopted.
Whether the wheels should be independent of the axels or affixed to their axels was another design consideration. If the wheels ran independently of their axels (eg on bearings), then the rubberband would have to power the wheels directly. This would contravene one of the customer requirements concerning exposed rubberbands. Safety is paramount when designing for toddlers and so this approach was discarded. The wheels could be powered indirectly also, although this would result in additional weight due to additional parts being required which, in turn, would add to the overall complexity of the model. This seemed unnecessary given that the axels could convey the energy from the rubberband directly to the wheels if the wheels are affixed to them. Simplicity of design seemed more appropriate given the age of the client. As such, I decided to make the axels and wheels a single component.
It is a design constraint that the rubberband compartment should be completely enclosed and generally inaccessible to the toddler for safety reasons. However, subsequent conversations with my wife suggested that toddlers at this age love to pull things apart and so there is an element of compromise in this regard. The rubberband mechanism should not require resetting and when the rubberband has fully unwound, it will act like a brake preventing the car from further forward momentum. This also means that all the rubberbands used to power the car will need to be the same length.
Continue to Rubberbands, Axels and Wheels
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