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Old 09-03-2018, 12:59 PM
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AlanG AlanG is offline
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A small thread for a small Yamaha YA-1

The YA-1 was the first motorcycle manufactured by Yamaha, from 1955 to 1958, though its design was heavily influenced by the pre-war German DKW RT125, and its numerous post-war derivatives. It had a tubular bicycle-style frame carrying a 125cc two-stroke engine like the RT125, but differed in having springing on both front and rear forks.

The model is the latest, and apparently the last, model published by Yamaha Paper Craft. It was brought out in December 2017 amid much fanfare, to celebrate the 20th anniversary of their paper model web site. Given this enthusiastic promotion it was a major surprise to see them announce only six months later that the site would be closed at the end of September 2018. The model is billed as being in their "Ultra-Realistic" series, though it is to an even larger scale than the others in the series. No scale is explicitly given, but the overall length of the model is cited as 49.5cm and Wikipedia lists the prototype and being 1980mm long, so we get a scale of 1:4.

This is far too big for me, so my version will be at my usual 1:14. This is a significant reduction, so I can expect to have to rework some of the components, possibly with some simplification to make things buildable. I hope the result will be reasonably faithful to the original model, though. We have already seen excellent builds by spiral and (sadly unfinished) huubvc, so I shall not go into too much detail, but will show a few photographs and touch on some of the modifications I have had to make.

Alan
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Old 09-03-2018, 01:01 PM
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I have always found it rather odd that the order of construction of the various subassemblies in the instructions bears very little relationship to the final assembly sequence. I like to be able to check the fit as I go, so prefer to follow the final sequence. Accordingly, I start with the frame.

This is a fairly complex tubular structure, with several oblique tube-to-tube "T" junctions. Several of these are implemented as tabbed joints, with a large hole in the cross piece meeting edge-to-edge with the shaped end of the "upright". In some cases the shaped end appeared to be somewhat approximate. I decided that this would be hopeless in my scale, so redrew the tubes to butt accurately with each other. At the same time I reduced the number of segments in the curve of the bottom tube beneath the engine from 7 to 3 - this curve is mostly covered by the front engine mount and the engine itself, so there seemed little point in making life too hard for myself.

The frame assembly leaves the ends of the rear forks dangling and very vulnerable, so I went straight on to the rear suspension units and fitted them to give some protection against clumsy handling.

A small thread for a small Yamaha YA-1-01-frame1.jpg A small thread for a small Yamaha YA-1-01-frame2.jpg
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Old 09-04-2018, 01:07 PM
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The engine consists of a cylinder block with prominent horizontal cooling fins, a cylinder head with vertical fins, and a teardrop shaped crank case with bulbous sides.

The first observation was that the cylinder block consisted of two shaped boxes making up the central core, and a lot of fins cut out to fit over the core. The core boxes had several slots to accommodate tabs on the inside of the fin cutouts. It was not hard to see that I was never going to get this to work, so I changed it to a sandwich construction, with alternate paper fins and card-thickened slices of the central core.

On the cylinder head I simply edge-glued single thickness fins instead of the complex built-up ones, and ignored the spacers. Finally most of the slices forming the crankcase are filled with cardboard, so that all the tabs can be discarded.

A small thread for a small Yamaha YA-1-02-engine1.jpg A small thread for a small Yamaha YA-1-02-engine2.jpg
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Old 09-04-2018, 09:27 PM
Saybur Saybur is offline
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Love how this is coming together. Thanks for sharing.
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Old 09-04-2018, 11:18 PM
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As usual I shall enjoy following this thread. Your way with tubes is always impressive.
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Old 09-06-2018, 04:15 AM
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Saybur and Maurice, thank you for your interest - I hope I can continue to make the thread worthwhile. At least it may remind someone to download the Yamaha models before they disappear at the end of the month.


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Just for a change I made the carburettor much as supplied in the kit, though with many of the tabs removed to allow rather neater edge gluing.

On the other hand, I rejected both the "standard" and "special" versions of built-up chain, and drew a conventional flat chainwheel-and-chain combination, though I did laminate extra copies of the chain on both sides to give some extra thickness.

A small thread for a small Yamaha YA-1-03-carbchain1.jpg
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Old 09-07-2018, 01:22 PM
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Next is the tank. My first attempt was a disaster, largely because there is a recessed box beneath the top cover, and the model tries to reproduce this. As a result the upper strakes of the bulbous sides have a big cutout in them, making them very weak, and prone to bad twisting. There seemed to me to be little point in having this recess only to hide it with its lid, so I made a new design with a solid top. Each of the sides could be made independently and (largely!) without distortion, and then joined by a strip across the top. Then I could just glue the box lid and filler cap on the top, and no-one would know.

While struggling with the redesign of the tank, I took some light relief doing the saddle. Not that light, though, as on the first attempt I had enormous difficulty fitting the completed underside to the completed top, as was required by the tabbing scheme. For my second attempt I moved a good number of tabs from the central piece of the underside to the adjacent side pieces. This was so that I could fit the side pieces to the top first, and then tie everything together with the central section. This turned out considerably more successful.

Finally for this time I made up the exhaust pipe and muffler. These came together reasonably well, though I did simplify the top flange where it meets the engine, and the "fins" at the back.

A small thread for a small Yamaha YA-1-04-tankseat1.jpg A small thread for a small Yamaha YA-1-04-tankseat2.jpg
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Old 09-08-2018, 01:37 PM
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At last something which could be put together largely as published! I did not try to line the fender with the printed pieces, but just coloured the inside by hand. Very little of the inside will be visible, so as long as it is not glaring white it will be fine. The stays are very tiny - I ended up rolling them around a 0.6mm twist drill - so the end fixings are a good deal simpler than the original.

A small thread for a small Yamaha YA-1-05-rearfender1.jpg A small thread for a small Yamaha YA-1-05-rearfender2.jpg
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Old 09-09-2018, 08:42 AM
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We come to the front end, starting with the headlight. Like the tank, this is a fairly straightforward piece, except for the large hole in the top for the recessed speedometer. In this case, however, this has to remain, as the speedo is visible. The first try was pretty unacceptable, so I moved a lot of tabs to an internal joining strip to get a better fit between the front three rings, and made a former to keep everything firm and round, so the second go was reasonable. The intended mounting is by a rod which passes right through both brackets and the body of the headlight. I ignored this - the body will just be glued to the inside of the brackets, and the ends of the rod will go to tidy up the outside.

In a similar way, the middle section of the handlebars passes through the brackets which hold it, but the holes in the brackets make them unmanageably fragile. Instead I left the brackets unpierced and sliced up the central handlebar section.

A small thread for a small Yamaha YA-1-06-steering2.jpg

The front forks have bellows-like gaiters to protect the telescopic springing, which in the model are simulated by numerous conical rings placed alternately up and down. These were pretty hard to assemble, especially as adjacent rings had slightly mismatching outer diameters. In the end I managed a fairly acceptable appearance.

A small thread for a small Yamaha YA-1-06-steering1.jpg

Finally, the front fender, like the rear one, was quite easy. Again I just hand-coloured the inside instead of layering in the printed strips.
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Old 09-09-2018, 12:26 PM
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Cool bike Alan, thanks also for the info about Yamaha site, there's a couple there I've looked at and better grab em.. Tricky bit on the front shocks, not sure I have a better idea though. They look good.
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