Now for the engineering behind this work of art. The staff at NZ Classic Motorcycles were kind enough to remove the tank and seat assemblies, so we could see it all. First I should explain that, while I have owned and enjoyed motorbikes, I have never raced one. I’m a chemical engineer, not a mechanical engineer, so I don’t really know what I’m talking about here. But I do remember clearly some of that 1994 lecture by John Britten and have listened carefully to other people since. So here goes.
Just to remind you of the whole machine, here’s that poster again. One of the first things that you notice is the absence of any fairing around the lower half of the machine. There were apparently a couple of reasons for this. John Britten wanted the engineering and innovation to be visible, and a fairing wasn’t necessary, it just added weight. That’s because the lower part of the bike is so skinny that the rear tyre is its widest part! (Check out the rear view in my earlier post).
Next: the engine is water cooled, but where is the radiator? Not in front of the engine, where it adds frontal area and blows hot air onto the engine being cooled. That would be silly. No, it’s tucked up under the petrol tank and seat.
High pressure air is ducted from intakes at the front of the bike, through the tiny radiator into the low pressure zone above the rear wheel. The pressure drop across the heat exchanger creates high air velocities which greatly increase heat transfer rates. I recall John saying the velocity was 2-3 times that of the bike.
Actually, heat transfer is stuff chemical engineers do actually know a lot about. I remember struggling to convince a pulp mill superintendent that we could drastically improve the performance of a shell and tube heat exchanger by welding caps over ¾ of the tubes. It didn’t seem logical that reducing the heat transfer area by 75% would improve the heat flow. But the unit had been totally over designed and the fluid velocity (and thus the Reynolds Number) through the tubes was much too low. Increasing the velocity by 4x far outweighed the 4x reduction in area: the heat flow more than doubled when we modified it!
So the result of this innovation on the Britten bike was not only better performance but also a weight saving (the radiator holds only 1.1 litres!)
Notice that the bike doesn’t have a frame, everything hangs off the engine. That’s not particularly new, the Vincent motorbikes for example used that design 50+ years ago. The Vincent engine was much fatter than John’s though ( look at these Vincent Black Shadow studio shots which illustrate this point). John handled this design beautifully though, using lightweight materials not available to Vincent Motorcycles back then. That rear wishbone is a good example. In his 1994 lecture I remember John saying the carbon fibre wishbone weighed less than 2 kg, compared with a 12kg Ducati model at the time.
The front suspension also shows some similarity with the old Vincent. This next shot shows the setup quite well:
Check out the Guy Martin video listed in my earlier post for his comments on how well that works. Another shot from a different angle:
One small detail: the location of the rear shock absorber. See how it’s positioned well forward, improving weight distribution and providing much better cooling (of the shock).
Finally, a few more images:
and with the tank back on: