Saturday, 14 January 2012

The "Lethal Contraption"

The title of this post come from Rachel's pet-name for a bit of off-season engineering i indulged in. Whilst the name is tongue-in-cheek, if you decide to try making your own one of these, you do so entirely at your own risk, and i encourage you to take all reasonable safety precautions to protect yourself in case anything should go wrong. This includes wrapping the pressurised parts of the system in blankets, and wearing eye and ear protection, and only using this setup outdoors.

Not a lot of people know this, but the humble 2 litre fizzy drinks bottle is designed to withstand an internal pressure of 175psi before bursting. Your average person would find this (a) not very interesting and (b) not very useful, but it transpires that science-minded mtbers have been using this dry fact to great effect for some time. Well, mtbers and water-rocket enthusiasts...

Enter, stage right, the nightmare job that has bedevilled the days and nights of many an mtb rider and racer alike, seating tubeless tyres, or worse still non-tubeless specific tyres onto lightweight racing rims. Conventional wisdom suggests that it is "merely a case of getting a sufficiently snug fit between the tyre and whatever you're using to seal the spoke holes in the rim". Except most of us that have spent half an our furiously pumping only to be met by a soggy and uninflated mess of tyre, soap bubbles and latex solution know that this isn't quite the case. Those of us who are a little further from their student days than me and Rachel are faced with two choices: return to the humble innertube, which has served cyclists since 1891 when Edouard Michelin first set them loose upon the world, or indulge themselves with an expensive, and not very portable compressor. But thanks to our intrepid two-wheeled inventors, there is now an alternative possibility.

With a collection of bits and bobs you can buy from your local handy DIY emporium (the Mica home store in St. Andrews was particular good for this kind of thing) you can assemble for a couple of quid something that will do the same job as >£100 of compressor. To do the full Blue Peter thing, you will need:

2 Litre Pop bottle (note, it MUST be one that is used to contain fizzy drinks, if you use a bottle not designed to withstand pressure, it'll just burst before you have enough air in it, and you'll be picking bits of plastic out of your face for weeks)

Old track pump hose and nozzle (or do what i did, and treat your elderly track pump to a new hose etc, and use the old one for this contraption)

2 old innertubes with replaceable valve cores

2 jubilee clips

some rubber o-rings that will fit over the valve stems on the innertubes

a bulldog clip big enough to clamp over a kink the hose to prevent air escaping

eye protection, and a blanket/doormat to cover the bottle when full of air

Tools: drill bit which is the same diameter as your valve stems, vice/clamp, screwdriver for jubilee clips

Creating the "compressor" is pretty simple.
(1) Drill two holes in the top of the bottle (clamp it in the vice to do this), leaving a decent space between the two of them. Cut out the valve stems from the inner tubes, leaving a small amount of the rubber from the tube around the bottom of the stem to aid sealing.

(2) Push an o-ring over each valve, and down to the bottom of the stem, and then insert the valve through the holes in the lid so that the stems will stick out of the bottle top like a pair of antennae. Use the little metal rings that come with innertubes to secure the valve stems to the bottle lid, and to squash the o-rings to form an airtight seal.

(3) Remove the valve core from one of the valve stems, and push the free end of the track pump hose over the top. Secure the hose to the valve stem with a couple of jubilee clips to make sure you have another airtight seal.

(4) Finally, wrap several layers of duct tape around the curve of the bottle at several positions along its length to give it a bit more structural strength.

(5) Test the whole rig-up by filling the bottle with water (an effectively incompressible fluid, and therefore much less capable of storing lots of energy) and then kinking the hose and securing it with the bulldog clip. Attach a track pump to the open valve, and pump up to 120psi. Assuming the pressure doesn't drop, and water doesn't spray everywhere, you have a working tubeless compressor.

To use the compressor, kink the tube near the track pump head, and secure with the bulldog clip. Pump the chamber up to around 120psi, attach the pump head to the valve of the tubeless tyre you're trying to seat, undo the bulldog clip and hopefully the inrush of high pressure air will be enough to seat all but the most stubborn of tyres!

Et voila, a tubeless compressor for a few quid!

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