Monday, 17 December 2012

Saddle Mounts Part 2

Last shedweek I made the saddle mount, the near vertical post that clamps to the backbone. This shedweek, I've made the little doodad that clamps to it and carries the saddle springs. I'm not sure what the correct name is for this part but I'll call it the spring carrier for want of a better term. It's a lovely little thing, very pleasing to look at.

Spring Carrier Doodad.

Calculations on the original facile example and a few others I have handy showed me that I could machine the shape from a 1" bar.

The first step is to mill an identical flat on either side of the bar so that the final thickness is 0.75". This was the first proper use of my new milling machine, the finish is OK but not that great at present, I'll try a new belt to see if I can even out the finish. The finish is not important in this case since so much of it is going to be removed through subsequent operations.

The next step is to drill three holes in one of the flat faces, the two end holes are tapped at 1/4" cycle thread as a temporary means of mounting to a face plate, these end holes are where the springs will be mounted. The middle hole is used to centre the bar on the face plate.

Then bore the central hole to be a nice fit onto the saddle post.

Next machine the side on profile using a profile tool. This type of interrupted cut is very hard on tools, even using small feeds. The finish is unimportant since it will be hand filed later.

Next remount at 90 degrees using an old section of angle iron. Mark and bore the cross hole for the 3/8" bolt to clamp the carrier to the saddle post.

Then machine the profile of the side.

Flip around and machine the back of the carrier in the same way.

Finish with hand files and make the 2 bolts to hold the springs and the larger one to secure the carrier to the post.

In other news, I'm being tossed on the horns of a dilemma. In our house, we try hard not to go mad on Christmas presents. Sure, the kids get a little spoilt and they get things that I would have loved as a child but I'm talking about presents to each other, to my lovely wife. Officially we have a complete spousal gift moratorium. We both agree in advance not to be silly and to treat the festive season as a time to spend together as a family with the necessary reduction in shed based activities. Unfortunately, I happen to know for a fact that my lovely wife has bought me something this year. My daughter told me. What do I do? If I buy her something, I'll have let on that I know that she bought me something and if I don't buy her something I may as well go and live in the shed until next year. I can't win. I think the solution is to buy her a present and wrap it in a generic Christmas/birthday style wrapping paper, then hide it down the back of the sofa rather than under the tree. If I'm right and she has got me a present I can whip it out and still be in favour and if I'm wrong and she hasn't broken the gift embargo, then I can just keep it until her birthday in August. Sorted.

Whatever, happy Christmas to both of my readers.

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.