November 2003

I had made some enquiries, but no one seemed interested in the remains of the second TZR, so in desperation I put an ad on the BEMSEE website Ė asking for just £120. I got four phone calls in the first two days! Fantastic! A few days later and the bike went to a teacher from Kent who is taking it back to the racing track. Ainít the internet great?

Fan frame

The fan frame carries the drive from the lower pulley via the belt, to the top pulley which is fixed to the fan hub. At the other end of the fan hub is the fan blades which rotate within the duct.

The Constructorís Guide recommends that it should be built from 1" square-section steel tubing, in a simple H shape. I bought the fan shaft pre-welded into a 50mm x 25mm rectangular section and this section forms the cross piece of the H.

Itís important that the fan shaft sticks out of the frame at exactly 90 degrees, so that the top and bottom pulleys end up being parallel. It would also be good if the fan frame uprights are parallel- although this isnít so important. I welded two pieces of angle to the ends of the fan frame legs, top and bottom. The frame was then braced firmly and so couldnít warp. Then I tack-welded the fan shaft cross member into position and checked that it was nice and square. The easiest way to keep the fan shaft perpendicular to the cross-piece while itís being welded is to drill a hole in the bench to allow the fan shaft to poke downwards. I welded up the cross member in, a run at a time on opposite ends, so as not to let too much heat build up. I deliberately made the uprights too long so that I could trim the excess lengths off after it was fitted into the duct.

Having fixed the fan-shaft into the fan frame, this seemed like a good time to try out the fan inside the duct, so I assembled the fan, hub, and bearings. I put the duct on its back and dropped the fan in. While the bellmouth end (front) of the duct was very circular, at the rear of the duct the fan-tip clearance varied considerably as it rotated. Experts on the HCGB website bulletin board were talking about a tip to duct clearance of well under 10mm. The duct had been kicked around my garage for about a year, and it clearly needed some bracing to keep it circular. I cut out a former from MDF and screwed it into the back of the duct Ė much better.

The idea is to mount the engine and the lower pulley so that they are both the same height above the cockpit floor Ė that way the flexible couplings wonít have to flex much Ė meaning they should last longer. I measured the height of where the engine crankshaft would eventually be. For that engine height / lower pulley height, I will need a belt circumference of 66¼". The nearest belt size is a 660, which means Iíll need to raise the pulley (and the engine) by about 3mm. So I ordered a belt and some bigger anti-vibration mounts, and 5 flow straighteners a front cone and a half rear-cone.

Air holes
There was a discussion on the HCGB bulletin board about the size of air-feed holes. The opinion seemed to be that the ones at the back should be smaller. This seems sensible as thatís where the fan is. So I cut out the air holes using hole-saws Ė 68mm holes on the sides and the front and 45mm holes at the rear. At either side, one of the holes happens to land exactly where one of the T-section triangles already is. Thereís no way Iím moving that, so I cut two smaller holes either side of the T-section.

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