Tuesday, July 11, 2006

What's new?

Now that Sarah's bike is off for painting - I can turn my attention to some other projects.

I'm building a carbon frame won by one of the top 150 fundraisers at the 2006 MS150 Tour de Farms. This process has its own blog - so if your curious, look at: www.LindasNewBike.blogspot.com.

Meanwhile, I'm getting serious about Aram's new track bike. The frame will be of silver filet construction except for a lugged bottom bracket. At first, it looked like the BB angles wouldn't work out with the limited (60mm) drop on this frame. Closer examination revealed that it has brass clearances - giving me a number of degrees of wiggle room for both the down-tube and chain-stays. None of the tubes are super light, so I'm not to concerned about using brass in the BB.

The tubing is large diameter steel. The down-tube is 35mm, the head-tube is 36.5mm with a 1.2mm wall thickness, the top-tube is conical being 31.7mm diam in front and 28.6mm in back, and the seat-tube is 28.6 with an external buldge at the top (to help prevent heat distortion when connecting the top-tube and seat-stays). Oh yeah, the chainstays are ovalized for good clearances - so they'll probably get a bridge to stiffen things up. And for the seat stay bridge, I'm considering a polished stainless 3/8" tube. Do I really want to go through that polishing thing again? Hmm...

The jig is up and I've been working on its fit. The overall design was refined on Martin Manning's design spreadsheet - good stuff, especially if one wants to be precise. This is a fast handling track bike. It's not a single speeder for the road. Aram is a serious and competitive racer, so we want to be very precise to avoid handling anomolies.

Related to this, I'm going to deviate from my usual building sequence. The goal here is to create a strong and well aligned keel connecting the head-tube to the rear wheels. The first build will be chain-stays and then connecting them to the bottom bracket. This will be done with very careful attention to alignment. Then the head-tube/seat-tube will be put together, again with a critical eye on alignment. Finally, these two assemblies will be connected - with much fitting and testing to make sure that the head-tube is in a vertical plain aligned with the track of the bike, and that the track forks (the proper name for "drop outs" on a track bike) are perfectly parallel (on two axes), and equal distances from the ground and from the frame's center line. Once all this comes together, we're over the hard part.

After fitting the jig, it was time to start working on tube prep. So far, I've washed out the insides of the head, top, and down tubes. This is necessary pre-brazing, but also makes it easier to read the butts inside the tubes. By looking through the tube at a light, and moving it around, one can see where the butts transition. By so doing, it possible to make sure that the ends of the tube were marked correctly (one end usually has a longer butt and this is the end from which to trim excess length).

Then I set up some v-blocks on my reference surface along with a surface guage. If you're not familiar, a surface guage is a precision base with a post rising from it. A spindle is atached to the post and can be adjusted up/down & in/out. The post has a hinge at the bottom and a screw that delicately angles the top out or back. By adjusting the spindle close to an object, and then fine tuning with the post's screw, its possible to index a precise height from an object sitting on the same reference surface as the surface guage. By moving the guage around on the surface, one can compare heights from spot to spot.

We're using this a little differently here. A tube is set in the blocks, and the surface guage is adjusted to the highest point in the tube at a location midway between the ends. Then the tube is rolled in the v-blocks to detect if and how much bend there may be in its length.

The head-tube measured out fine, but its short so that's to be expected. Both the top-tube and down-tube have very slight bows. Not enough to be a problem, but enough that its important to be concious of the bows in order to insure the alignment of the finished product. For both of these tubes, the concave side of the bow will face down.

Then it was time to refine the position of the head-tube. With most jigging systems, this is he most complicated part. Most jigs don't provide a way to fit fork into place, as a means of locating the head-tube. Often, the head tube is difficult to remove and replace to the same location. It will be on the same axis, but may have moved up or down along the axis.

I go about positioning the head-tube from several different basis points so as to find any errors in my methods. The first method involves locating the top of the seat-tube and with a long level determining where the top of the head tube should be. The raw head-tube is too long, so there should be some extra sticking up above this point; this can be removed later.

A second basis is to measure up from the datum line on the jig to it's front horizontal frame member. Then, it' possible to determine where the bottom of the head-tube should be relative to this new datum line, based on the design produced in Martin's spreadsheet. Using a vertical measure and a small level horizontally, it's possible to put a guage mark on the head-tube. From there, it's possible to measure up from the mark, parallel to the head-tube axis, to a fixed point on the frame holding the head-tube assembly. This point becomes a new datum point, and the measure is retained as a basis for where the lower head-tube should end. Again, there should be a little extra tube below this point.

After adjusting the height of the tube, and double checking the measures for each end, it's possible to hold up the fork, parallel with the tube to the appropriate point. With a little help, a long straight edge, level, and a tape measure - its possible to compare the axle height to the rear axle height and then confirm that the drop to the center of the BB is correct.

Once all of this is done, we're part way there. From this point, we need to establish where the head and down tubes intersect. In some degree, this can be done visually, but we're trying to be precise here. So we employ a protractor to check the angle between the down and seat tubes. when this is correct, we check the angle between the head and down tubes. If these comply with our design, and the spacing between the down-tube and planned bottom of the head-tube look good, we should be a go.

So far, I've reached this point - but not marked anything. Next time around, I'll begin by confirming the basic measures as above. Then I'll add one more check. Holding the fork in place, I'll check the front center (that's the distance from the front axle center to the bottom bracket center. If all is good, I'll mark the vertical center of where the head and down tubes meet. I'll then do the same for the top-tube and head-tube. These will be aligned on a vertical line on the head-tube creating the point at which the breather holes will be drilled.

As I said, locating the head-tube is one of the more complicated bits of preparing to braze a frame.

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