- Notch, bend and weld 7/8” unistrut into a 32” x 10” rectangle. Be sure the short sides center on a hole, not a space. When done, it won’t be square and flat; hammer and bounce it around until it is. Widen the center holes in the short sides to 1 1/8”. Make two 10 ¼” brackets from unistrut or ½”x2” channel to hold the differential in the center.
- Build up a transaxle, 48” long: cut the ends of the differential’s original axles back to 6” each from the center of the differential, then weld a 22” piece of ¾” i.d. tube to each. Fit this assembly, with its 4 bearings hand tightened, into the rectangle, with the two 10 ¼” struts clamped in place so that the differential sprocket is 1 5/8” to the right of center. This will line it up vertically with the drive sprocket on the bike. Align the 4 bearings. Mark the bearing flanges, disassemble, and weld half of the flange of each bearing into place. Reassemble, checking that the differential sprocket is attached, then weld the center struts in place. The transaxle is now permanently assembled, and any gearing change will have to be done from the cranks.
- Drill a 3/16 hole to fit a large hitch pin as tight as possible to the each outside bearing. These will hold the assembly together.
- Cut the 5” x 10” x 3/16” plate into two 5”x5” squares. Drill 4 holes in each to match the hole spacing of your rear wheels. Measure from the center of the transaxle 24” each side, mark and cut the axles, then weld a 5” x 5” hub to each end.
- Flip a bike frame upside down, and insert the rear assembly through the triangle of the seatpost (left), seatstays (bottom) and chainstays (right). Mark the seatpost so that when it is cut and welded to the front of the rectangle, there will be 9” from the top of the axle to the center of the cranks.
- Cut the chainstays just before the dropouts, so they can be bent and welded,
or cut off the chainstays and replace with 3/8” round stock.
- Cut the down tube(s) to 8” from seat tube. Fit a #40 sprocket to an ashtabula crank so that the gear ratio is about 22:30.
That’s all for the drive unit.
- For the front end, cut a 33” piece of 2” telespar, centering it on a hole, not a space. Cut 2 more pieces 6” each, and place these 10” to each side of center so that the space between them is 20”. Weld in place.
- Cut two 24” pieces of 1 ¾” telespar. Notch a half-cylinder groove 2” in from each end to receive a bicycle head tube. Don’t weld this yet – it will have to be clamped and measured when the frame is together to determine the right height for the front end.
- For the chassis, cut two 68” pieces of 1 ¾” telespar. At 30” in, make a 30° notch, bend and weld.
- Place the rear end on a table. Prop the front end up 16” higher than the rear axle, about 56” forward of the cranks. Place the long ends of the chassis pieces into the 6” sockets welded to the front end. The short ends of the chassis should rest on the rear of the unistrut rectangle, so that the top of the chassis is level.
Pin the front ends of the chassis to the sockets. Cut two 12” pieces of 2” telespar, slide them up the sloping back ends of the chassis so that they rest on the rear of the unistrut rectangle, and pin these to the chassis also. Measure and adjust in every direction so that the front and rear ends are equally spaced, level, and diagonally square: take plenty of time, because this alignment determines both the ease of assembly and the centering of all the belly tools. The two 12” pieces will be welded to the unistrut and the alignment will be permanent.
- Weld the rear chassis sockets to the rear of the unistrut, then weld diagonal braces of 3/8” round steel from the top of the socket to the front corner of the unistrut.
- Put the rear end, chassis and front end together on the floor, with the front end propped up so that there is a clearance of 28” from the floor to the bottom of the chassis. Clamp the head tubes to the half-cylinder cutouts in the front end, with the forks and tires assembled. Since head tube length varies, you’ll have to find the correct height by moving them up and down. Floor to bottom of telespar should be 28”. When that’s done, weld the head tubes in place.
- Cut two 9” pieces of ¼” x 1” bar steel, and drill a 3/8” hole in each end, 8” apart on center. Clamp one to the underside of each fork for steering arms. To determine a reasonable Ackerman layout, put the forks at a 51” spacing – halfway through the front end’s range of 42” to 60” – and point the clamped steering arms at the center of the rear axle. Tack weld, then take off clamps and weld.
- For a steering knuckle, bend the ends of a unistrut elbow so that the outer holes line up to receive the steering column at a 30° angle. Slide a bicycle kickstand through and weld a washer to the top and bottom so it stays put. Cut an 8” piece of ¾” square tube and weld it to the top end. The steering column will slide over this. Weld a ½” hex nut to the bottom to pin the inside ends of the tie rods.
For tie rods , if you’ll be staying with one front end spacing, just weld tie rod ends onto any tube or solid piece of steel that fits, such as this 3/16” x ¾” galvanized bar from the scrap pile:
Or for telescoping tie rods, use ¾” square tube inside 1” square tube.
- Cut a 48” piece of ¾” emt, and find the type of handlebar that fits tightly into this. Drive the handlebar as far in as possible, drill a ¼” hole through both, and secure with a bolt. On the other end, weld a 3” piece of 1” square tube to the emt, taking care to line it up square with the steering knuckle and the handlebar.
- Drill a 1/8” hole in the down tube, 6” to 8” in front of and below the cranks, to mount a pivot pin (a 16d common nail) for the base of the inverted pendulum seatpost.
Cut the seatpost from the bike frame about 9” from the top. Cut a 28” piece of 1 5/8” unistrut and fit the cut seatpost into this so that the total length is 31”. Clamp the seatpost to the open side of the channel and weld.
On the bottom end of this assembly, lengthen and widen the 2nd and 3rd stamped holes into one hole to fit around the down tube(s). Do the same to the 4th and 5th holes if the frame you chose has 2 down tubes like this one.
The seat is now set up like an ordinary bike - in this case the middle of the seat is 28” above and 6” behind the center of the cranks. But since it pivots on the nail which is in front of and below the cranks, and since it will have a spring always trying to pull it forward, it will exert a forward and upward force on the operator depending on how tight or loose he or she chooses to set the spring. This action is good for shock absorption and varying the body’s position. Also, since the seat height and fore-and-aft position are adjustable, and the handlebar position as well, the machine can accommodate most people between 5 ½ and 6 ½ feet tall.