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CMC Meeting, November 08

The next meeting of the Christchurch Meccano Club is the Christmas Break-up on Friday 5th December.

The challenge

Last month's challenge was to build a model powered by a single rubber band, and a race was run. This was a great success, with a large number of entries showing surprising variation.

The first entry is from Neil, and he's not read the instructions carefully enough. The rules carefully included the term "self-contained", in order to prevent the entry of catapult-type vehicles (indeed, since the model didn't actually have to have wheels, once you have a catapult you may as well just fire a nut and bolt across the room). One down – make sure you read the rules next time Neil!

Hugh's first entry relied on the weight of a medium flanged plate staying vertical, and the band pulling around the gearing and driving the entire thing along. Sadly, the wheels (circular strips) were so large the gearing just didn't allow enough speed off the line and it was knocked out early on in the race.

His second attempt was more successful, but still slightly over-engineered. That old hub disc with the non-Meccano 'tyre' reared its ugly head again, too. But the speed was not enough to bring it into contention.

Another technique is shown here, where the rubber band pulls down a long strip attached to a gear, and that rotation is geared up to the final drive. Again, insufficient acceleration meant that this didn't last long.

Robert's entry used the band cut to form a single long piece, with a knot in each end. Much lighter, looking like a dragster and even with a driver, this was a much quicker device. Your author did try using a band split like this, but found the results to be inferior to having the band as one piece doubled up. It's likely that the wasted elastic in the knots at each end is too big a cost.

Here's a most unusual model, with yet another different form of drivetrain. The elastic is attached to a rack strip, guided between orange rollers, which drives the pinion on the main axle. Interesting that the model is also using the flex gained from the yellow flexible plate at the front. This might be considered to be using a driving force other than the band itself, which is specifically excluded by the rules.

Craig's entry slipped through the scrutineering process, and it was only during the semi-final stages that Craig complained that his spring was stuck. Of course, the spring attached to the rubber band is a complete breach of the rules stating that the rubber band must be the sole provider of power. Thankfully, his entry lost at this point and thus there was no need to disqualify it. We'll have to watch him in the future! :-) This model was, however, the only one with an in-built winding device!

And if you thought you'd seen all the drive methods, here comes another one. The 'wheel' of wheel flanges is driven very unusually by the untwisting of the band inside it. The driver sits behind holding the 'chassis' which is just a single strip. The main design problem seemed to be the friction between the strip and the socket coupling, which is of course highest at the start with the band under tension. The sharp-eyed will also spot that this entry disqualifies itself by the use of the short socket coupling, a part Meccano never made.

Cameron's entry was small and light. The powering mechanism was very simple, and the modern plastic wheels with big rubber tyres meant that it got off the line quickly. The use of a single bolt head to 'catch' the band meant that at the end of the band's run it falls off and the model continues with its own momentum. No need for complex pawl arrangements here.

After just losing in the first round to Charles' entry, it came back into contention through the repechage and semi-finals to be in the grand final, where it just lost out again to the same model. A clear second place for Cameron.

The physicists among you will have long worked out that we only really have one consideration to make – given a small and fixed amount of power available in the rubber band, the quickest accelerating model is going to be the lightest one. Cameron's was close, but Charles' model was even lighter, using just narrow strips and a single double-angle strip at the back. The brass bush wheel at the front gives just enough weight to stop the front lifting too far, and the light but big rubber tyres from the 1980s give enough traction to avoid slippage. The band is held by an anchoring spring for cord, just looped over the 'wrong' side of it, such that it falls off at the end of the run. The 27f gear gives two further gears, by initially winding the band over the axle, then the boss of the gear, then finally a part of the 27f you get a very low starting gear and a gradually increasing one through the model's run.

We can see that in this challenge, light and simple is the only way to go.

The CMC Challenge

No challenge this month, as the next meeting is the Christmas break-up. This is normally very well attended, and this year will take the form of a mini "exhibition" for the other users of the Church. In return, the Church is laying on a barbeque, starting around 6pm, so get here early!! Remember to bring along a range of interesting models to show everyone.

The December meeting

The next meeting is Friday 5th December, 2008, at St Johns Church, Ferry Road, Woolston.  Bring a friend! And, as usual, bring whatever Meccano model or curiosity you're currently working on, whether it's finished or not!

Remember that it starts earlier than usual – around 6pm for this month only.