When I sat down to think about making a tarp, my train of thought went something like this:
Keep it as light weight as possible >> Using as little fabric as possible would help achieve this >> Using less fabric would keep the cost down >> If I use less fabric the packed volume will be less for any given fabric >> I should utilise the bike as a frame to pitch the tarp over to allow use in open areas if no tree cover available.
I came across some fabric that was silicon coated on both sides, was cheap and dark green. Having looked at numerous tarp set ups including some that used a bike as a frame, I worked out that I could use the wheels at either end of the tarp would use least fabric. I lay down to work out how much horizontal room I needed if I lay full length between each wheel. Being of average height, 2m was enough. I then figured that a 50cm flap over the wheel to a guy point would probably be sufficient.
I ordered 3m of fabric, 1.5m wide. When I arrived, it measured 3.10m by 1.57m, which was a bit of a bonus in both directions I thought. It was soft, handled quite well and wasn’t particularly noisy when you flapped it about, like some spinnakers are. The fabric weighed 398g, which converted to a weight of 2.4 ounces per square yard. This gave me a comparison to other fabrics like ultrasil/ silnylon, which are typically 1.1-1.3 oz/sq.yd.
The next step was to determine the shape the fabric needed to be when stretched over a wheel. With more time and a lot of thought, we could have gone to the trouble of stitching in tapered sections to allow a better fit over the wheel, but we decided against this in order to keep it simple. The obvious areas of excess fabric were going to the back corners, away from the front edge where the wheel was. We cut off two triangular pieces about 50cm by 75cm which gave us some extra fabric to reinforce the guy points. All the edge seams were folded over twice and double stitched. Some sections that were going to be under greatest tension were also reinforced with some gross-grain tape into the hem, with small loops at the ends to tie the guys to. We also stitched in a line of gross-grain between the front and back guy points, in line with the wheels for extra strength and added durability over an obvious contact point (the wheel).
The finished product seems quite good. Though it’s not been out of the back garden yet, it was easy to erect and easy to get a good even tension. There are eight guy points which go out to six (titanium) peg locations. I’ve got 2.1m between each of the wheels, taking the extra 10cm of material supplied to give me more internal space.
The rear tapered section where we took a triangle of fabric off didn’t work as envisaged, but instead allowed a sort of drop down at the back which would be more effective at keeping weather from coming under the edges. The fabric does go over the wheel quite well and does appear to have equalised the tension in the fabric very well without a lot of very tight or very slack areas of fabric.
The guy lines were 1.5mm dyneema (15m, total) which is nice and light, easy to handle and knot. For further ease of tensioning, I’ve used mini line-loks which are hugely effective and weigh hardly anything. The whole lot stuffed easily into a Size 1 Granite Gear air bag to tip the scales at just over 480g, with six titanium pegs. Now I just need to find an occasion to test it out for real 😉
Middle Ring All The Way 
this looks ace matey, looks like you could do with something to hold the wheels a bit steady though, the way the corners are pitched forwards looks like the tarp may lose some stability? Wonder if you could fashion something to either hold it to the tarp, or pin it to the floor, like 2 pegs and some bungy line or something. Awesome effort though.
whats the giant overshoe thingy on your bike?
Two loops under the main seam would be easy to add in so you cold tie it to the top of the wheel. It was quite stable though, and provided it was pitched the right way to the wind, I think it’d be fine.
The “overshoe” things – do you mean on the handlebars? Those a Bar Mitts – super warm for riding in freezing temps.