Mach 2 Instructions

The principle change of the Mach 2 bike is to flip the head tube in the frame assembly. Depending on the donor bike, this results in a more acute head tube angle for the recumbent bike, which reduces the ‘tiller’ effect in steering. This improves the handling at the top end. But there are consequences. That sharp a head tube angle requires you to add considerable rake to the front forks. The added rake corrects the ‘flop’ in the steering caused by making the head angle so sharp. This added rake actually lowers the bike closer to the road, which means we have to make the frame design narrower (reduce the overall height of the bike frame) to keep the crank far enough off the ground. No heel scraping allowed, even in the tight turns. The new bike is better handling, by a dramatic little bit. It feels better because you don’t have to swing the steering handlebars out so far to accomplish a tight turn. The Mach 2 looks distinctly different too – you can see the narrower height of the frame, and the head tube is sharper in angle than the old seat tube right behind it.

The differences between the two designs are subtle, and unlikely to matter much to a beginning rider just learning to ride recumbent. After 1000 miles, it may matter to you. The differences in building the two designs are acute. I am telling you – build the Mach 1 first, learn from it. Build this bike later, with the lessons learned from the first bike. The Mach 1 is very forgiving to first time layout and alignment mistakes. The Mach 2 is a harsher mistress.

I put 6000 miles on my yellowbike1 before I was ready to move to the Mach 2 design. There's an old story about the sage, the student, and the donkey. No, not the one about the cart before the horse. The punchline counsels "Patience!"

SHEET 11 - This is a different cut than the work you did on sheet #1. Instead of using all of the top tube from bike 1, we use just enough of it (we will trim it more a little later) to be our downtube in the flipped assembly. Cutting off the rear triangle is the same as for the first bike. I cut the downtube about 5" up from the bottom bracket, leaving the rest with the head tube. The more length you can 'leave' with the head tube, the longer the resulting bike you can make.

SHEET 12 – there is no Sheet #12, the cuts are the same as on Sheet 2. Use as tall a donor bike for the back as you can find.

SHEET 13A – Fork bending. In general, about 4 1/2” of fork rake is needed on this bike. The fork from a donor bike has (usually) 2” or 2 1/8” of rake. A 3/4” conduit bender works very nicely to bend fork blades. AGAIN, put the bender in a bench vice and move the fork, not the bender. The pictures show the wrong way to do this (on the floor). Use a salvaged piece of 1 1/8" bike tubing as an extension on the head tube for leverage, and bend away! Notch the fork leg into the bender right at the drop out and BEND a good 2” – roughly. Take pains to keep the fork blade centered, and the fork crown level. Try to match your bend with the second fork blade. You can measure the change by laying the fork on any flat table. Check the evenness of your bends by the reverse test shown in the drawing – both drop outs should touch at the same time – no rocking allowed. Expect to adjust on one blade or the other. Take some time with this – if you have a spare fork or two (from other donor bikes), do a practice run first. There are formulas for deciding exactly how much trail is right for a given head tube angle. Unless you are set up for some real precision measuring, this rough alignment will do just fine. Check the pictures on Picture Page 4.

SHEET 13A2 – Fork bending. I added this sheet as a second test for checking your newly raked fork. Draw the indicated parallel lines – a center line with lines ½” away on either side, a pair of key lines 100mm (3 15/16”) apart, with some parallel lines either side of that. 100mm is the typical spacing for a front wheel axle. After you are happy with the rake and the table top test above, set your fork down on this pattern. Put the curve arc up – the fork is resting on the end of it’s stem and on the two drop outs. Look down from directly over the fork, visually align the top of the head tube on that center line, place the two drop outs on the 100mm lines down the way. NOW sight through the old brake mounting hole in the fork crown. Is it on the center line? You are lucky if so. Are the dropouts the proper distance apart? Time to adjust. You need this finiky alignment, or your front end will want to stray right or left when you ride. Centered and balanced. It’s hard to do, and just as important as the rear wheel alignment. I make adjustments here by clamping the fork blade in my vice just below the crown. Then I (with an extension tube over the stem) and push and pull the fork into alignment for this test. Get happy with it. Then, go BACK to the table top test and make sure the rake is even again. And check this again too!

SHEET 13B – Layout. Here you get to decide how long to make the final downtube dimension. Use the real parts. Take your time with this - use that fork you just bent, with the 20” wheel in it. Use the Frame #1’s parts – Put the fork into the headset, put the wheel into the drop outs on the fork. Take the piece with the bottom bracket and position this over the assembled parts on the table. Have a donor crank in hand, Handle this along with that bottom bracket piece - test your layout now. There is a picture of this on Picture Page 1. Here is the question – how short can you make that finished downtube without the crank interfering with either the front tire or the fork? Is there still enough room on the uptube (the one we will cut off later to fit under the top tube) to hang a front derailleur? I have been making my finished down tubes about 8” now where the drawing shows 9”. Make it as narrow as you can while maintaining clearances – your crank height will thank you later.

SHEET 13C – Shows the same assembly drawing as we saw on the original Sheet #3. Except that the head tube in this arrangement is flipped upside down. What was the donor bike’s top tube is now the new bike’s down tube, sleeving into the stub of the bottom bracket. What was the donor bike’s down tube is now the new bike's top tube.

OK? Trim the NEW downtube to sleeve into the stub at the bottom bracket at the ordained height from the layout in 13B. Trim and fishmouth the up tube from the bottom bracket to snug up under the new top tube. Braze/weld joints as in the earlier bike. There is a picture of this too in the sequence

Second assembly is just like the Mach 1, except that our ‘splice’ joint is here on the top tube, instead of on the down tube. Set the length of the bike to your preference. Make sure everything is in plane and braze/weld away! Mark Stonich suggested to me that, on a splice joint like this, one should braze one end of it into the outer tube first. Then clean that up, insert onto the second tube (and do the alignment), and braze that in place second. Mark is wise.

SHEET 14 – The principal difference between this and Sheet #4 is that you bend the rear triangle a bit farther – a new dimension here. Otherwise, the offset for the top stays, and the layout and alignment of the rear triangle to the bike frame, is exactly the same as on Sheet #4. The Clones are a forgiving geometry, but the more time and effort you put into this alignment, the better your bike will be. See alignment notes and methods in Mach 1 notes section!

SHEET 15 – The final assembly with ¾” EMT bottom rails is the same as on Sheet #5 – this sheet shows slightly different proportions needed for the finished Mach 2 bike frame. A narrower frame means a higher bottom bracket. At the rear, one is tempted to eliminate the 5 degree bend and just 'do' the frame straight back to the dropouts. NFG. Leave that angle in the frame - it's about chain clearance to the rear cluster. Trust me on this.

SHEET #16 doesn’t exist. Use Sheet #6 – it just explains the bell formed joint, which is the same here as in the Mach 1.

SHEET #17 – The finished bike. Use Sheets 8, 9, and 10 from the original Mach 1 set to complete the bike. Heck, use the parts from the earlier bike!

Always, I suggest a complete assembly and test of the bike before your finish and paint it. You may want to adjust that fork, depending on how it handles. Fork flop is that feeling you notice when a bike seems to ‘fall’ off center. It seems to actually lower itself into a turn (actually, it really does lower into the turn). This is caused by too much trail from that sharp head angle. Add rake in ¼” or ½” increments. Test it, see how it feels. Hold off putting a bridge on your fork until you are happy with this.

If your ride is too wobbly and feels unstable, you may have over-raked your fork. Start a new one, find a new donor bike if necessary (just for the right fork, not for starting over). Bend to 4” of rake, and try that.

I like to try a fork on a bike until I reach a point of satisfactory handling. THEN I bother to strip it down and add the brake bridge. There is a formula and a modest program for predicting geometry of preferred rake and trail - if you know the head tube angle and the trail, you can calculate the rake, etc. I hope to add the reference to this site with the author's permission. Eventually. But I find the guidelines mentioned above, plus the careful testing on the road for your bike, is the best teacher.