So I’ve had this question enough times to warrant a blog post on the subject: “Why do you build in sub-assemblies?” vs just welding the entire frame up at once. The short answer is: Because it breaks building a bicycle down into bite sized pieces. The LONG answer would be this:
So the first step I take is I cut the chainstays to a rough length (generally 20-22″ long pieces). If they are chainstays I’m making from raw tubing, I put the tube into the tubing bender and mark where the die’s outer edge comes in contact with the tube. This is the very first reference point. I make the bend and repeat this with the second chainstay, loading it into the bender, mark it and repeat the same bend. I now take these out, stack them, and mark the second bend. The first bend sets up the bend for chainring clearance and tire clearance. The second bend is for crank arm and heal clearance. Once that is marked on both, I attach a block which I now can use as a reference to keep things in phase. I know that bends in my bender start about 1.25″ from where the tube first contacts the die. So I mark a reference point 1.25″ behind that second set of marks I made as the start point. These get loaded into the bender, bent to the proper degree, and repeated for the opposite one.
So you see I’m already starting to create A. reference points, and B. breaking these procedures down into repeatable steps to create repeatability and maintain accuracy. Now it’s time for the dimple for added tire clearance. But hold on there cowboy! Editors Note: It’s very important to make the point that the very first bend I made was actually “under-bent”. Meaning I know from doing this long enough, that when i make an impression on the chainstay that creates the dimple, I’m moving material and that increases the bend. So I’ve taken that into account with the first bend. So these get loaded into my new and improved dimple die:
Fresh out of the die:
Now that the chainstays are finished and in their “long john silver” phase, the first cut I make is at the dropouts. The distance from the center of the axle to where the tires outer most tread pattern is located is a constant. This differs obviously depending on wheel size and tire choice. These two things are discussed with the client – yes I even ask you what tires you run, what size they are and what make… etc. This is important. I actually load up the dropouts on a dummy wheel, with tire on and inflated, and hold the chainstay right up to where I want it to sit. I mark where the chainstay and dropout meet. I know from my drawing what the angle needs to be cut at. I do it this way JUST in case there is any slight variation between the matched pair of chainstays and the actual drawing. I do this so I’m not splitting hairs nor trying to pull my own out. The stays get loaded one at a time into the vice on the bridgeport and get mitered:
Once both are done, I mark and vent the dropouts:
Now here is where we get to that first sub-assembly and why: The dropouts need to be in phase with the chainstays and at the correct distance from the dimple. If I miter the bottom bracket side first, I need to keep that in phase with the dropout miter. It’s much easier if I first miter the dropout end first as i can easily keep it in phase with the dropout by using a small tool I made which is simply the exact same size and shape die that I use to create the dimple. We need to maintain perpendicularity with certain parts and center lines. Once everything is cleaned up, I load them separately into the chainstay mitering jig like so, tacking them one at a time on the top and bottom:
Once that step is done, I now have two matched chainstays, that have the dropout faces in phase with the dimples. The dimples are also in phase with the bends. The chainstays were matched. We’re breaking down each step into easy to digest steps to maintain accuracy. If any adjustments need to be made, I can make them now. If all looks good, I now fully weld the dropouts to the chainstays. Whamo:
This step is important. I have two parts that are matched and a true pair. These get loaded into the chainstay mitering jig. The reason I tacked and welded the dropouts is so that I can register them in the tool and the chainstays now have another surface that can resist the cutters ability to want to “twist” the tips when cutting. This is the next important part of how I work: Creating rock solid foundations so I can carry accuracy throughout the job. Each step of the build a mistake can happen. A tube can slip. A hole saw can grab the tube. By taking these extra steps, I help to minimize the “work of risk”. Here’s the pair of chainstays loaded into that chainstay mitering setup:
You’ll take note that I’ve shortened the bottom bracket side, AND I’ve cut the two pieces at an angle so they’ll fit. Some even cut them dead on so the two tubes but up against each other and they then tack the tubes together. This also helps to keep the tubes from twisting. Rigidity in a mitering setup is paramount. Having the cutting tool as far up into the headstock is paramount because you’re utilizing the mass and diameter of the head of the mill. The more that cutting tool sticks out, the more of a chance for you to create what’s called “runout” as well as chatter. Chatter is bad. That can lead to the cutter skipping and pulling on the tubes walls which can end up in inaccurate cuts or even worse, tearing the tube.
I measure the chainstay length (in this case 16.5″ on a 29er) mark where the center of the cutter should be, make the cut and check them in my bottom bracket sub assembly jig:
This has a movable bottom bracket holder with a ruler for chainstay length:
This enables me to check the miters for fit, but also to double check the miters for the correct chainstay length. Since the distance from the dropout to the dimple is constant, I now can fine tune and determine chainstay length by cutting both chainstays at the same time. Also note that since the chainstays are tacked AND welded to the dropouts, I’m maintaining parallelism with the axle and bottom bracket but also perpendicularity with the rear axles center line with the dimple and the dropout face as well as with the bottom brackets center line. That is key. Both miters for the chainstays at the bottom bracket are done at the same time so they are in phase with the dropouts which will then be in phase with the bottom bracket. Procedure is very important.
Skipping ahead, I set up the jig from the drawing, load all of these parts into the frame jig, miter all the tubes check fits and such and clean inside and out and de-bur everything:
You will notice that I have NOT tacked or welded the chainstays to the dropouts. This is something I recently stopped doing until the front triangle was all mitered and ready to be tacked. I’ve been increasing the size of the vent holes on the bottom bracket to reduce weight and allow much/water to drain properly if it collects to avoid large surface areas at the end of the tubes to collect moisture and rust. It’s much easier to have everything prepped and mitered, trace the outlines and THEN do all the vent holes last:
Once the entire front triangle is finished and ready to be tacked, I now weld the bottom bracket and chainstay sub-assembly. Why? Because if you’ve ever tried to weld chainstays to a bottom bracket while the seat tube is in place, you’ll know that it can get tight as you come across. Not having to deal with the seat tube in place makes this a lot easier to weld IMO. In this shot, my head and entire body is pretty much standing where the seat tube and front triangle would be:
This just allows for a lot of freedom of movement and ease of welding in a normally tight area. There’s enough tight spots to get to. So I try and eliminate a few if I can. I also do this when I’m tacking the frame. Certain parts are tougher to get at with other tubes in place. So I fully wrap certain welds after the tube is tacked and in place before I move forward with tacking another tube in place. So that’s the long and short of why I build with sub-assemblies. I hope that helps to shed some light on my methodology. For the full story, check out this link here: HOW IT’S DONE.</a> This is a step by step set on Flickr that breaks down each and every step of building a 44 from start to finish. Enjoy!