Thursday, 6 December 2012

Saddle Mount

I am a slacker. There, I've said it in public. I've had so little time in the last three weeks that progress has been slow. Also we've had a few weekends away and consequently my shedweeks have been minimalist to say the least. My focus at present is to get as many parts as possible ready for brazing. I don't own any oxy-acetylene equipment, I just hire it for half a day when I need it. This works out cheaper and safer. About a hundred years ago, I used to be a fire-fighter in the UK and I've seen a few cylinders go pop with fatal results. Acetylene is truly horrible stuff, totally safe when used correctly but lethal when abused. I get nervous just transporting it in my car. But I digress. To get value for money out of a half day hire, I like to get as much ready as possible before hand, hence the work on the saddle mount. In addition, once I've got the handlebars brazed on, I'll be wanting to make and fit the T grips. Before I can do that I need to be able to sit on the machine to position the grips at the best ergonomic angle for my wrists.

The original saddle mount.

As you already know, the backbone is elliptical which means that the seat mount clamp has to be elliptical to match the profile. It is easy to find a suitably sized round thing to use as a form when bending a round shape, elliptical things are not so common. I had a go at free bending a few samples but as I've previously noted, I'm not great at bending things and I wasn't happy with the results. The solution was to make a former so that I can repeat the bend as many times as I like. I have future projects that require elliptical backbones and I'm sure I'll be re using it.

Making the form tool.

First find some strip steel that is the correct gauge and then mill it to the correct width. I found a piece of an old galvanised dustbin that was almost the right size, I don't want the zinc plating but that's easy to resolve later on.

Place the strips into the form tool and using a series of suitable sized round things, squeeze the strip into shape.

The stages of bending the clamp to shape.

Next mark and drill the 5/16" holes for the bolts. Then mark and file the ends to shape. The bolts are 5/16" cycle thread with a slightly domed head, these will be brazed into the upper half of the clamp. Of course these need to be made from scratch, not being an off the shelf item. I used 10 mm coach bolts as the starting point, the dome being machined with my new ball turning attachment. For some reason, I forgot to take any photos of all of this so you'll just have to take my word for it.

At some point I removed the zinc galvanised finish by dunking in some acid for a few minutes. This is a violent reaction that gives off large amounts of hydrogen. I will not say if we collected the hydrogen for purposes of amusing children.

Zinc meets hydrochloric acid - violence + hydrogen.

The upright post that the saddle itself clamps to isn't vertical as you would ordinarily find on an ordinary Ordinary. Instead, it is inclined at the same angle as the front forks. You can see this on this woodcut from 1891, this is right at the end of production and the bike has more inclined forks than the earlier more upright examples.

Note that the saddle clamp is parallel with the front forks.

I've carefully measured the original I am copying and also measured directly from the assembled frame of my copy. The angle was the same in both cases which was pleasing. The saddle post has this angle milled onto one end to locate it at the correct angle on the clamp, to add extra support an additional piece is fitted around it, this piece has been filed to accurately fit onto the curved clamp at the correct angle. To hold this all in place a small bolt is screwed from underneath, the head will be filed off after it is all brazed up.

The saddle post has a flat milled on the front facing surface, I'll mill this after brazing. This is in contrast to the usual rear facing clamp found on most Brooks B70 saddles from the period.

Rear facing clamp, note the flat milled on the rear of the upright member.

Initially I thought that the clamp I measured was a 'Friday afternoon special' until I started looking at contemporary images and sure enough, the clamp is forward facing on the facile. It does makes sense when you think about it though, The geometry of the facile means that there is ample space to get in with a spanner, unlike a penny farthing, and it's out of the way when mounting the bike.

Front facing clamp, lots of room for a spanner.

I'm not completely happy with the clamp, but I'll braze it up and see how it performs when bolted onto the bike. If I need to, I'll simply make another one.

The clamp seems to be very flexible, I may need to use slightly thicker gauge strip.

In other news, I've been press ganged into being part of a works team for a mountain bike race next February. This means I have three months to get lean and fit. Over recent months I've been developing an excellent abdominal 'one pack'. Clearly things will have to change. We visited friends in Ash Vegas at the weekend and I rode the last 50 km from just the other side of Methven. The route is across the Canterbury plains and is slightly downhill all the way, I calculated the gradient at 0.6% which is nothing but boy does that 0.6% make a difference! Admittedly, I had a strong Nor'Wester blowing straight up my bottom bracket in the early part of the ride but this had died to nothing by the end. I've even ridden to work twice this week, once in 30 degrees Celsigrade no less. Last year I rode 15,000 km, this year will be much much less mainly due to injury, museum leave and badly inflamed wimp glands.

No comments:

Post a Comment