I had a good idea what I wanted to end up with before I started the 5cc buggy. It had to be 1/8th scale, had to have the same dimensions as a commercial buggy and shouldn't look out of place racing alongside them. I also wanted to use wheels and tyres that I could buy off the shelf. Basically I didn't want to rely on expendable items that I would have to make myself as this would discourage me from driving the car hard.
I copied the wheel track and wheelbase from a Kyosho MP7.5, a car I never maintain and is bashed into the ground. I picked it up on Ebay for about £120 which wasn't bad for what I got. It was an MP7.5 sport which had been hopped up with several of the Kanai Edition parts.
Once the wheel locations were laid out in CAD I applied a few more constraints that I knew I wanted. It was going to be rear wheel drive like the real thing, and had to have a more scale appearance than other buggies. I wanted to work the scale appearance around a chassis that could fundamentally perform well under racing conditions, so just scaling down a full size class-1 buggy would not cut it. My experience with the 5th scale buggy I had made before reinforced my belief that the parallel twin-rail chassis design is the optimum for this style of car, so I knew I was going to use that. The engine had to go behind the cockpit like the real cars, and I wanted the longest possible suspension travel I could get (another major bonus of 2WD and twin-rail chassis design). Bearing in mind some class-1 cars have 36" plus of wheel travel, this would scale to 4.5" which is a tough target to match.
With my high performance targets in mind, I was aware that what I wanted was actually a nitro version of a typical electric 2WD buggy like the Losi XXX and AE RC10B4 with some more scale features and at a bigger size. Therefore I studied the design features that are unique to those cars, such as the high degree of steering ackerman and front kick-up angle. Getting the front to bite when steering is a bigger challenge for a 2WD car with a lot of weight over the rear axle and AE and Losi cars do a great job at solving this. With the engine in the 5cc buggy being such a large percentage of the total vehicle weight, the importance of getting the front and rear suspension geometry correct can't be underestimated. I used the same kick-up angle as a 2WD buggy because of this. I also studied the relative lengths of the upper and lower wishbones and calculated out the tragectory of the wheels as the suspension compresses. The more unequal the length of these vital components, the greater the wheel camber will change as the shocks compress. The reference plane changes as the chassis rolls though, so you have to have a reasonable idea of how much roll you expect beforehand. This was difficult, as I hadn't even come close to finalising the design at this point. I expected a higher CG than a 2WD electric buggy, with scale details inevitably raising the profile of the car and I expected a roll cage like structure would be added too. So I chose my suspension mounting locations with these thoughts in mind, but of course I couln't be sure how it would work. That's one of the exciting things about a scratch build project!
