Building the mounting framework

The lidar and camera needs to go on the roof and the PX2, batteries etc need to go in the boot, so I need to build some sort of framework. Ideally this needs to attach to the subframe as it should be as rigid as possible. All the cameras and lidar are going to need calibrating and any movement will make that calibration useless. I ordered some extruded aluminium profile, bolts, corner brackets etc etc from a company called Motidis.

I think Motedis are based in Germany. The aluminium profile is surprisingly cheap, but I guess the money is made on the other items, as it quickly adds up. I could have got the stuff from RS or another UK site but I find the RS website a nightmare to navigate. I particularly liked the Motedis website as it listed all the accessories that matched each type of profile, so this made the choice a lot easier. The quantities were a conservative estimate of what I would need. This was based on a back of the envelope calculation that would provide some form of roof rack and a frame for the boot to mount the electronics etc. In the end I had enough alu profile but needed more corner brackets, short bolts, channel nuts, compression washers and nylon lock nuts. One thing to note, originally I ordered fixings with the brackets included but these turned out to be 4mm and I didn’t use them as there was just too much play in things. Pretty much all the aluminium frame was bolted with 5mm. Note the 5x10mm bolts are actually too long for the depth of the channel unless you use a split/compression washer. You actually need the 5x8mm. However, as I’d bought 10mm, I went with split washers but as these are notoriously useless at resisting vibration, every bolt had blue thredlocker applied. I had some rough sketches but what I wanted to achieve largely existed in my head and has evolved as the build has proceeded. However, once I took the panels off to get access to the subframe, it became difficult to see how the hell I would attach anything to the roof without serious modification to body panels. In the end, after a lot of scratching of heads and even trying to make some aluminium brackets which were going to make the car look more like an elf, I decided that maybe I could modify a roof rack without too much damage to the car. Very early on Id looked at roof racks and found nothing other than this video and a couple of blogs about dangerously clamping a Halfords roof rack to the plastic bodywork, but this would have been a bad idea as any flex in the plastic body panels would result in movement of the rood rack. However, despite having previously ruled out roof racks, late one night while desperately searching for a Twizy roof rack for the millionth time, I decided to buy a cheap universal roof rack from Amazon. As it clamped onto the roof via metal straps, I thought perhaps I could adapt it. Although the roof is semi rigid plastic and does flex under pressure, the edges where it would be clamped partially lie on the subframe, so this might work.

On Sat 9th Feb ’19 I went into work on a Sat morning, it was going to be the first of many Sat and Sundays in the lab. The first thing was to cut the roof rack to size. This involved drilling out the rivits, cutting the main bar to size then re-reviting it back together. 

T To mount to the subframe I needed to cut through the fibre glass cover that encases the subframe along the roof line. I did this with a dremmel, and it was dusty as hell. At the time I thought it was plastic, but I subsequently read somewhere its fibre glass, so I probably should have had a mask on !!! The cuts should be fine as they wont be visibile under the bodywork.

Although I found one place where a roof bar could be mounted, it wasn’t obvious where I could put a second due to the size and structure of the roof. So I decided the rear of the roof rack would be mounted on steel stud work that I could bring up through the roof and attach to the subframe internally in the boot. This would have the added advantage that I could use it to level and or change the angle of the roofrack.  As you can see, once the roof bar was moutned, the straps would still get in the way of the side panels over the doors. I spent a long time thinking how to get around this and finaly came to the conclusion that I shouldn’t worry about it,  we would probably never sell the car. Once we had finished with it, it would probably be scrapped and at some point I was going to have to start cutting holes in it. So I might as well get on with it. Let’s face it, who is going to want a used Twizy with no doors and full of hole so?? With the one roof bar and two 8mm studs protruding through waterproof glands in holes I cut in the back of the roof (see boot framework when I write it) this gave a pretty solid base on which to build some sort of custom roof rack. The basic frame was cut and bolted together from the extruded aluminium profile. Quite a few of the end connectors were tapped and bolted through to give extra strength. The remainder of the connections used 45′ angle brackets that Id got from Motedis but also galvanised steel corner brackets that I picked up at B&Q. The front bar was most of the width of the car, this is because I new we would need to mount the side cameras so they could see down over the edges to some extent. However, although we also needed a camera near the front, I didn’t extend the frame in that direction as I was concerned that doing so would result in a very unstable cantilevered structure which would bounce around making any calibration impossible. Id worry about it later, if the cameras were too far back on the structure, then they would need to be fixed directly to the roof above the windscreen. I did however, need to mount the lidar on the structure. The lidar has a range of +-15.5. The centre of the car where the lidar would be mounted was about 50cm from the back which means I would need to lift the Lidar up give it a clear view over the back of the car. Some simple trigonometry said the lidar needed raising so it had a clear view of the road and not the car/roof rack. I spent quite some time trying to mount underneath the roof rack bar to keep the whole thing as low as possible but there just wasn’t enough room space.

I got some 125x125x5mm alu offcuts cheap off eBay. Ideal size as slightly larger than the lidar, so I built the tower frame to fit which saved having to cut it. I bolted the lidar down with 8mm stud so not only does that hold the lidar down but compresses the whole tower to keep it rigid.