Scratch Building a Metric Frame IIII aim for a flat surface across the seat rail/top tube junction. Which makes the fish mouths a complete headache to do. once again the "drawing" on the board comes in handy as it gives a surface to align everything. The general method is to make a vertical cut along the seat rail and then fettle each rail until they fit on the centerline of the top tube, then the upper edge of each rail is relieved until they fit whilst sat on the board.
Once that's done, the cross tube is cut and mitred and then the top tube is slid along and notched to meet the cross tube. As the top tube is 1.75" and the cross tube is 1.25" I weld a small section on the end to tidy the appearance up and then tack the whole sub assembly together.
This one needed a bit of a bend in the top tube to get clearance for something or the other, or it might have been to get more of a slope on the seat area. I really can't remember why I did it to be honest! As a rule I like to see a straight line form the headstock to the axle plates, but that isn't always possible. To figure out the angle of the bend I needed I positioned the tacked up sub assembly on the axle plates and measured how much the front end of the top tube needed to drop vertically. The entire assembly was then set up in the bender and bent until the necessary drop had been achieved. Notice the magnetic spirit level in Fig.10 to ensure the bend is in the right plane.
At this point it's time to fish mouth the top tube on to the headstock. Part of the point of cutting the seat rails over-long is that if you make a slight cock up out of this you can usually slide the assembly forward to compensate. This joint is one of the most critical on the frame so its worth taking the time to get it spot on. As you can see in Fig.11 it was just as well that there was a little slack in the thing!!!!
Once the top tube is sitting nicely on the headstock the whole top tube/seat rail assembly is tacked to the headstock and the axle plates. At this point there is clearly a motorcycle frame in the making and you can start to see how the thing is going to look when it's finished. The bend in the top tube will be under the gas tank when its all done which gives a cleaner line than kinking it at the junction with the seat rails. My opinion is that a "kick" where the top tube and seat rails join makes the frame look like a camel. But, that's just personal opinion and nothing else, if you want a camel go right ahead and build one.
Moving swiftly on, the next step is the down tubes/bottom rails. Out comes the angle finder again and it gets positioned so as to let me visualise the position of the rail, whilst at the same time establishing the angle for the front bend which is the next one to do. the angle finder is made out of 1 1/4" by 3/16" strap steel by the way. This is because I use 1 1/4" tube for the vast majority of frames and the angle finder gives a very good indication of what any given rail will look like and how much room it will occupy.
The same test bend is used again by placing against the angle finder to establish where the bend line will be in relation to the headstock. The angle finder is also marked about 1" past the headstock, which represents the cut end of the down tube. The separation of the two marks gives the position of the bend line from the cut end of the tube and a little judicious tape measuring gives the overall length to cut.
Using pretty much the same technique as the seat rails, two pieces of tube are cut, marked with a bend line, and bent to form the down tubes and bottom rails. Notice the hand just appearing to the right of Fig.16. this is because at this stage these things will always try and make a break for freedom. You can also see the 1" box the provides a location for the rails as well. One VERY important point at this stage is to check that the exhaust is going to clear the down tubes. You can add another bend rotated 90 degrees to the first one to get extra room at this stage without too much hassle. But it is very important that the pipes will clear.
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