Test equipment (part 2)

This is pretty much a continuation of the previous post, in which the probe points of interest were identified for each of the three rake locations. Step one was to pull out the coordinates of the points and put them into CAD. Each point then had a line drawn, extending rearwards by 70mm (the length of the Kiel probes) at the angle of the local velocity vector. One probe was registering a velocity with positive Z component (i.e. moving forwards relative to the car); any reading close to or less than 0 would be considered an accurate correlation for this point. From here, the rearmost point of each probe was joined with a pair of splines to create a 20mm ribbon structure, forming the rake. 

The choice of connections between each point was a compromise between minimising frontal area (both overall and locally near each probe) and maximising structural rigidity. Structural strength came from avoiding parallelism to maximise resistance to skewing. Triangles would have been more effective but would have nearly doubled the weight and frontal area. 

I've been told by three separate people that this particular rake looks like how Picasso might have imagined a cat. 

After making rakes for all three cases (two straight and one corner), they were all printed in individual 150mm x 150mm sections before being glued and taped together. The mounting method had been finalised by this point (shown later in this post), and was such that no additional changes were required to any of the rakes to support it. It was at this point that I realised I really hadn't planned how to mount the Kiel probes. I should have incorporated mounting holes at the nodes of the structure, but instead the probes will have to all be mounted slightly offset from the intended position, and held with temporary glue. This wasn't ideal, but was resolved by adjusting the probe points in CFD accordingly. This meant the positions weren't exactly where we wanted them, but it didn't change things too much. After initial construction, the rakes were stiffened with a layer of carbon fibre. It's not the prettiest construction, but that can be solved with a bit of sanding and black paint once we're done testing to make a nice part we can show off to people who will likely have no idea what they're looking at. Other than a Picasso cat. 

To figure out how to mount the rakes, the CAD models were imported into the full-vehicle CAD model. The front mount would be suitable for use on both sides of the car. 

All mounts were designed from 3mm aluminium sheet. The front mount fastens to the underside of the floor, and the rear one shares the seat mount. The non-fastened end of each mount was designed to sit snugly over the chassis, held by a cable tie and a thin sheet of rubber bonded to the mount to avoid slipping on the chassis tubes. The chassis mounts were interfaced with the rakes by three 100mm m5 bolts. By clamping the rake between two nuts on each bolt, the angle and lateral position of the rake could be easily adjusted in-situ. 

The final mounting setup provided impressive in-plane rigidity, with longitudinal rigidity provided by applying forwards tension with a cable to counteract drag forces. The sensor PCBs were mounted rather awkwardly next to the driver, with the requirement of being close to a CAN breakout point. The PCB mounting will be much improved for future tests, with a single smaller board and the addition of short sections of soft/flexible tube attached to the sensor barbs.

And before you ask, no, there isn't normally a gaping hole in the bottom of the cockpit. There's a carbon cover that mounts over the shocks, rockers, and datalogger, keeping to the cockpit closeout regulations.