Monday, 17 August 2015

New Bike #1

I've been trying to clear a few projects lately. You know how it is when you have just too many things you have to do to make progress on the stuff you want to do. I also dislike having bikes in pieces. I'm increasingly finding that I'm forgetting details and mislaying or worse losing components. For these reasons, I've either been giving away or selling off some of the herd over the last year or so. I'm still not done but I'm making progress. The remainder is getting reassembled into complete bikes that I want to keep. The first example is this "BSA" from the late 1920s.

A BSA bicycle, yesterday.

The BSA is in quotes because although the frame is entirely made from BSA components, I have no idea who actually made it. The date is also a best estimate based upon components and a trawl of BSA catalogs. This approach is difficult here in NZ where local manufacturers had boxes of stock that was used until it ran out. In many cases long after the parts were manufactured. NZ was slow to accept new trends in bicycle fashion during the first few decades of the last century and lagged the rest of the developed world by a some way. When the first NZ & Australian riders took part in the Tour de France in 1928, the Europeans found it amusing that they were going to compete on technology that was 20 years out of date. The Australasian team upgraded as soon as possible to try and level the playing field. Even so the Kiwi in the team, Harry Watson, wrote home to complain about the reliability of the European bikes not being as good as locally made NZ ones. The bikes they took over to France would have been similar to this one.

I'm not sure why there was this lag. I've mentioned the likelihood of using up hoarded stocks of lugs etc. There is also the fact that NZ was very isolated from the rest of the world which would have meant an ignorance about emerging technology. But I suspect a large reason was shear bloody mindedness. "That bike was good enough for your granddad son, so it's good enough for you. Harden up". This continued right into the middle part of the century. The concept of a racing bike here was very different from in Europe. Steel rims were the norm although wood and rarely aluminium were used. Wide, heavy tyres were used as racing routinely took place on gravel, most roads being unpaved. Single fixed gear was normal and braking rudimentary or non existent. None of this detracts from the achievements of the riders though. These were genuine hard men, the equal to any in the world on two wheels. And of course since everybody was on the same equipment it didn't matter anyway. Genuine old racing bikes are not that common here. It seems that many were converted to roadster duties after retiring from racing. It's fun to spot these lurking beneath the modifications of time. Many were worn out and discarded, but even so very few seem to survive.

This one is something of a bitsa. Most components are BSA and are from the same period although the whole thing has been built up from spare parts. The frame is made entirely from BSA fittings. It has BSA lugs, brake and chainstay bridges, fork crown, steerer and dropouts, bottom bracket, rear forkends for the BSA cam system and a steering lock. I have no idea what the tubing is but is of the older 'D' profile chainstay, seatstay and fork profile. The steerer and crown are nickel plated and have numbers stamped which may or may not indicate 1928. The frame and forks are lightweight.

The wheels are 28 x 1 3/8" (ETRTO 642, 700A), a size that was obsolete in England by 1910-1915 but was still popular in the colonies right up to the sixties. Tyres are not interchangeable with the common 28 x 1 1/2" (ETRTO 635, 700B). New tyres are still available in a few specialist shops although I have a small stash of original Dunlops. These are the same tyres that were raced on back in the day, heavy block tread and all. The rims are wooden and I think French. The rear rim is the one that badly split a couple of years ago. I repaired the rim with modern adhesives and rebuilt the wheel, however the rim was unable to hold riding pressure in the tyre and it split again when sitting out in the sun.

The pressure was only about 40 psi at the time this happened. 

The new split was in a different place from the first one. I think that this rim will need to be for show only. The hubs are both BSA, the rear with the cams to adjust the tension. The rear hub also has my new cones made from through hardened 4130. I was going to run these as a trial to see how well they handle the real world before I start remaking all the worn parts on the facile. I'm going to have to learn how to make wooden rims in the near future if I want this bike to be ride-able on the road. More on that later.

The handlebars are early nickel plated racing bars. They are too narrow for me but I can't nip down the shop and exchange them so they will have to do. The saddle is a modern Brooks Pro that I bought new in 1995. I will change this when I find a suitable replacement. I'll put the original  gallows style seatpost back on at the same time. It doesn't work with this saddle, pitching it too far forward.

There are no brakes other than the fixed gear and no provision for them either. The frame is scruffy but has original paint and pinstripes so I've simply cleaned it up and oiled it. The chainset is an early nickel plated Durax, I've never seen a nickel one before and I didn't think the company was that old. Does anybody know when they started making these hollow octagonal fluted cranks? The bottom bracket cups and axle are Bayliss Wiley, the axle being a very elegant hollow example.

Tightly bound while the glue dries.

In other news, we've been tuning the trebuchet. The original design brief for these devices was to hurl the heaviest possible projectile a set distance. That distance being how far you had positioned yourself away from the target and out of range of troublesome archers etc. Our goal is subtly different, we want to throw a projectile as far as possible. Over the last few months the trebuchet has undergone a transformation. I've been studying the theory regarding the efficiency of these things (it's quite low by the way) and there are several things to pay attention to.

The weight needs to drop as vertically as possible to put as much energy as possible into the projectile. There are several ways to achieve this. The counterweight can be hung on a connecting rod as in our original design or the trebuchet can roll on wheels to allow the weight to rock the machine back and forth as it falls. It turns out that the length of the connecting rod is important, a longer rod allowing more correcting movement. In order to achieve this Kung Fu Pete came round and we moved the weight to hang horizontally.

Thomas, helping out.

At the same time I machined some better bushes for either end of the con rod. The whole machine was still being bodily moved with each throw, so it got some wheels added.

The wheels run on short tracks and the weight now drops more or less straight down.

As the weight reaches the bottom of the fall it has no option but to turn and swing horizontally. To get maximum energy into the projectile, you need to tune the release point to coincide with this direction change which will usually be when the throwing arm is vertical. There are two ways to do this, modify the angle of the sling hook and alter the length of the sling. These two variables affect each other so they need to be tuned together. A longer sling will release later as will a bigger angle to the release hook. It's a bit of a juggling act. Using a video camera and replaying in slow motion is an extremely useful tool to tune these variables. I imagine it's also what the medieval builders did.

The design of the throwing arm is worth some careful consideration. Ideally, it should be neutrally balanced so that the centre of gravity is coincident with the pivot point. This is hard to do and means that the weight side has to be bulky and the throwing side light and slender. I've consistently used a 1:4 ratio between the lengths of these with the current arm being 0.75m and 3m resp. The overall weight of the arm should be as light as possible to reduce the moment of inertia, again care needs to be taken that it doesn't get too weak. Since the tip of the arm on the throwing side is travelling extremely quickly, it makes sense to give it some useful aerodynamic qualities by making it wider in the vertical plane and narrower in the horizontal plane. I've also given it a rough aerofoil shape. I think I'm probably pushing my luck with the latest arm but it seems to happily throw 1 kg rocks without breaking or flexing too badly.

Swish it goes, very loudly.

The net result of all this work means that we've extended the range to just over 175 metres (~575 feet). There is a lot of variation between throws which is caused by having variable projectiles. Heavier stones describe a higher trajectory and  lighter stones a flatter one. I'm going to find a set of river boulders of equal size and weight and as round as possible. I can then fine tune the sling length and hook for maximum range using the video technique. Maximum range doesn't happen at a release angle of 45 degrees as you may think, it's more like 40 degrees. Air resistance having a large effect at the higher speeds means that releasing too high makes the projectile fall short. Mr. Middleton has badgered me into doing some accurate measurements in terms of range, height and duration. Watch this space.

We're pretty much at the end of the road in terms of more development of this design. That's probably not a bad thing though as there's now enough range to punch a hole through my nearest neighbour's roof if I get a foul shot. I really don't want that to happen. Loading and shooting this thing is also reasonably frightening, the forces and speed involved being potentially lethal. It has been an interesting exercise and it has certainly got people talking...


  1. Bob,

    Very nice bike. My great granddad worked at BSA, not that we were ever alive at the same time to meet each other though. Maybe he worked on that frame!?

    I am reminded my of something I read - that a lot of Mansfield saddles were imported into NZ.


    1. Hi Stephen,

      your memory is quite correct. We do see a few Mansfield saddles here. I've got my eye on a suitable candidate for this BSA, it's the right age too. I'll update the blog if I manage to buy it.