Old School Chopper Frame Fabrication - Knucklebuster

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Using an indelible marker outline the miter path around the inside edge of the paper template and slip if off the tube before starting the trimming operation. If you need to remove a lot of material you can trim the tube back to within an eighth or so of the marked line using a reciprocal saw and then finish up the fine cutting with a grinder. The objective is to create a nice tight joint at the intersection and also to move the wishbone tubes in towards, and parallel with, the centerline of the frame keeping the 8-3/8” inside clearance. Just keep grinding, filing and fitting a little at a time until the connections are perfect on both sides of the backbone and the distance between the wishbones is 8-3/8” right about where the fender mount cross member will eventually be welded. You’ve probably been in a bike shop somewhere or over at a friends house while someone is trying to install a new rear wheel and the swing-arms or the wishbones on a rigid frame are too narrow for the hub and spacers to be installed without using a small bottle jack to pry the rear end of the frame out a little bit. Well this isn’t supposed to happen but every time you make a weld a little bit of material gets displaced and shrinks upon cooling and on motorcycle in general the tendency is for the back end of the frame to get sucked in slightly during the final welding process so it’s a little tighter than the specs call for. To account for this phenomenon I strongly suggest that you fit the wishbone to the backbone in such a manner so that the measurement between the tubes near the axle plate location measures from three-sixteenths to a quarter of an inch more than at the fender mount cross member. If you have a choice here err on the side of too much space rather than to little. To get this clearance you'll have to trim just a very little bit more off the fish-mouths at the backbone connection so the wishbones flare out just ever so slightly on the rear end. Do not trim the fender mount cross member down below the 8-3/8" throat to throat dimension under any circumstances. Man, I love building motorcycles, hand me that hammer and where's the beer! Part 4 This new section was prepared after we made our move into the new digs so it may appear more disorganized than our normal state of disorganization so cut us some slack as to presentation quality but we've had dozens of folks asking us to get on with this series and to stop making excuses. At this point we’re ready to start building the down-tubes and bottom rails. Again the material is 1.25”x.120” ERW tubing. We’re going to make these pieces in two stages. Each down-tube/bottom rail piece needs to be 84 inches long. In the first stage we need to make the two bends where the down-tubes make their transition into the bottom rails as shown in Figure 19 below.
Figure 19 I call this area of the frame the front ramps. I’m sure there is a factory-legitimate terminology for this area of the frame but to be honest I have no idea what anybody else calls it. This is the section of the frame that will eventually hold the front motor mount bar. Many frame manufacturers gave up this particular factory configuration years ago in favor of a simple, and cheaper to fabricate, single long radius bend. I personally don’t think the big bends look very good or are as strong as the old factory method so this is way I continue to build them even though it takes a little more work. Figure 20 provides the dimensions and angles for the bends on the bottom rails. Click on the picture for a large image. Note that the down tubes, when they are rough cut, extend up past the location of the top tube and steering neck and that the rear end of the bottom rails, in their rough cut condition, extend past the area of the axle adjusters. We’ll trim the tubes to their final dimensions later. These two simple bends between the down-tubes and lower rails are fairly critical and should be made with care to insure that the angles are accurate and that the dimension between the two bends is within an eighth of an inch, preferably better. Figure Error! Bookmark not defined. What we want to do initially is to create a set of rails that have a 27 inch piece of straight down-tube, a forty degree bend, another section of straight tube four inches long, and another forty degree bend ending up in a long straight section that runs rearwards to the end of the seven foot piece of tubing we started with for each side of the bike as seen in Figure 21. Both of these bends in each piece of tubing are made in the same plane. They are not compound bends. If you take the pieces of bent tubing and place them sideways on a flat surface, like your driveway, they should lay perfectly flat. Do not attempt to make the final upward bend in the rear of the bottom rails at this stage of construction. We'll get to that point later in the construction. Some builders start at the rear end of the bottom rails and work their way forward and upwards towards the backbone connection but for me at least I think it's much easier to work from the top down and head towards the rear with the last
Page 1 and 2: Old School Chopper Frame Fabricatio
Page 3 and 4: 1. Steering head. Also called the s
Page 5 and 6: Figure Error! Bookmark not defined.
Page 11 and 12: Figure 11 This cross member sets th
Page 15 and 16: Figure 15 Figure 15 shows the end r
Page 17: Figure 17 There is a corresponding
Page 21 and 22: Figure 23 The slope angle of the 'r
Page 23 and 24: In Figure 3 we can see the two tube
Page 25 and 26: Figure 1 shows the basic components
Page 27 and 28: an eighth of an inch clearance betw
Page 29 and 30: Figure 6 In Figure 6 we've deepened
Page 31 and 32: Figure 2 if you look closely you ca
Page 33 and 34: angle finder. It explains is easy t
Page 35 and 36: Figure 6 In this shot I've propped-
Page 37 and 38: In case you're interested I messed
Page 39 and 40: Figure 3 Figure 3 depicts a plate t
Page 41 and 42: Figure 7 Figure 7 is another 'boxed
Page 43 and 44: Regardless of the style or type of
Page 45 and 46: Figure 13 At this point all you nee
Page 47 and 48: Figure 1 You'll no doubt realize th
Page 49 and 50: Figure 4 Actually the plate will be
Page 51 and 52: If you're building a frame without
Page 53 and 54: Figure 5 Now that the plates are in
Page 55 and 56: Figure 2 Clean the notch up slightl
Page 57 and 58: Figure 5 The frame in Figure 5 is o
Page 59 and 60: Figure 7 The tube in Figure 7 above
Page 62 and 63: We'll now begin doing the detail wo
Page 64 and 65: The charge has been removed from th
Page 66 and 67: Some people have reported problems
Page 68 and 69:
Steering Necks For some unexplained
Page 70:
Figure Error! Bookmark not defined.
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Old School Chopper Frame Fabrication - Knucklebuster

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