Making a motorcycle frame.

Introduction.

In the early 1970s I built a frame for my racing motorcycle to house a 350 cc Aermacchiengine (HD sprint in the US).In 2006 after being away from the track for 30 years I started racing again in classic racesin the US and I built a close replica of my 1970s bike in 2008/2009. This article describeshow I built it mainly through photos which generally show more than words can.

Tools and materials

I will not give a detailed list of the materials used because that will be obvious from thedescriptions in each step. As always tools used depend so much on the facilties availableto an individual builder but what I used will become apparent as the build proceeds. I willdescribe one tool though before we start because many readers may not have see one.That is what is called an optical table. I managed to buy it from a laser optics companythat was downsizing. From the photo it can be seen that it has a lot of depth which keepsit rigid. The top is stainless steel to a flatness tolerance well within that needed for the task.It also has the top drilled on a 1” (25.4 mm) spacing and tapped ¼ “ unc. These holes arevery handy for holding parts and fixtures/jigs securely to the table.

I used this table as an accurate and solid base for aligning the frame during construction. Ihave never liked the common type of frame jigs used by many people who have madeframes. This common type mounts the frame in the orientation that the frame will assumewhen on the road as a complete motorcycle. This type of jig usually has a horizontal basewith a tall vertical piece to hold the steering head. I find this type of jig to be lacking rigidityand accuracy. On the other hand I find these problems a non-issue when using a solid flatbase like the optical table or a large enough surface plate and I have used the bed of anold flat lathe as well.

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First steps.

The construction of this frame was largely composed of steel tubes bronze weldedtogether, but at the rear of the engine there are two 6061 T6 aluminium alloy plates whichgive support to the rear swinging and footrests. If I had a bandsaw at the time I wouldhave cut the plates out with that, I did not but I was able to use a CNC milling machine. Iwrote a simple Gcode programme and cut them out that way. The mill also made it veryeasy to drill and ream various holes equally located on each plate.

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Making the head stock.

The head stock is a part which holds the two steering bearing to which the front forks aremounted. Usually it is made from some relatively thick walled steel tube. It order to saveweight I wanted to waist the centre section down. To do this I machined it from a solid barof 4130 Cr.Mo alloy steel. The photos show the main steps and the finished part. Thelarge diameter end sections are bored to take the steering bearing. Pieces always distortto some extent when welding and so I always leave the bores undersize until the frame isfinished and then do the finish machining which is described later.

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Preparing the tube fitting

When round tubes are welded together it is usually necessary to prepare the tube ends toget a nice fit. This process goes by many names such as fish mouthing or tube profilingwhich is what I call it.There are many way to do this, for example you can make or buy holding fixtures to permitthe use of a hole saw in a manual drill, drill press or milling machine. You can also sawclose to the shape and finish with a file or grinder. You can find freeware programmes onthe net which enable you to print out templates to wrap around the tube as a cutting guide.

However for tubes of the size that I used, 22 and 19 mm I prefer to use a milling cutter ofthe same size as the tube to which the tube being cut will fit. To make it quick, simple andsecure I made clamping fixtures from some scrap aluminium blocks. These blocks wererelieved such that the vice pressure acted directly across the tube to ensure secureclamping. The blocks could be mounted at any angle in the vice, so I only have to setthem with a digital angle level to get a perfectly fitting joint.

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Profiling the tubes.

The photos say it all. Note how little “stick out” there is when machining. This ensures thegreatest possible rigidity and accuracy.

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The fun part begins.

I get to do some welding. The tubing is 4130 Cr.Mo. Both 1 mm and 1.5 mm wallthickness, depending on the loading of each tube. I used what I and most British oldtimers call bronze welding but which is more commonly known as brazing in the US. Theheat is supplied by an oxy-acetalene torch and the filler rods are manganese bronze whichmelt at a slightly lower temperature than the more usual nickel-bronze. There are threemain reasons for preferring bronze welding over fusion welding for this type of structure;

The steel tube is heated to a lower temperature than fusion welding which helps retain itsstrength.The fillets are usually made with a larger radius and that reduces stress concentration.The bronze is more flexible and that softens the loads a little further reducing the tendencyto fatique.

The aluminium plates attach to the tubular structure through one cross tube. If that wasdone with a cap screw through a parallel drilled hole then over time and use I canguarantee that the holes will loosen and accurate location will suffer. To avoid that in thesecases I countersink the plate and make steel bosses with a matching angle which getwelded to the cross tube as shown in the photos. After welding the bosses they get finishmachined in a lathe and the ends are drilled and tapped M10 for clamping bolts.

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The base of the frame.

The easiest way to make this piece and to ensure that it will fit the engine properly is touse a pair of crankcases as the jig. This is the only part that does not use the optical tablefor alignment.

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Let’s get some shape to it.

The next stage was done using the table and some very simple fixtures to hold it squareand level. The fixtures were machined and drilled in the mill to ensure accuracy. AlthoughI have the pictures above showing the sub assembly tacked together I am unable to findany pictures of the jigging for this part. It simple consisted of two matching plates on eachside to hold the upper tube in place. The second picture above just shows thesubassembly on the crankcases purely as a progress photo.

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Somewhere to sit.

The next stage is to add the seat tubes and read subframe. For this operation thesubassembly is orientated as it would be on the road. This makes it very easy to align theseat tubes by using some blocks to hold the ends of the tube at the required height. Thiswas a one off construction and so where simple fixturing was appropriate I was happy togo that route.

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Starting and ending.

The first photo shows the close fitting that the profiling gave, not that the top tube wasprofiled in two directions one to mate with the top cross tube and the other to match thediagonal. The second photo shows the assembly being checked with a vernier heightgauge.

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Reorientation.

Up until now the frame was orientated as it will end up on its wheels but I like to bolt thehead stock at right angles to the table to ensure rigid alignment, this means that the rest ofthe frame must be tilted forward by an angle equal to the angle of the forks (called the rackangle) when on its wheels. To do this I raised the rear by an appropriate amount usingtaller side plates as shown in the photos. One photo shows the headstock clamped inposition. The grid of holes in the table make alignment very easy.

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Joining the dots.

The next stage entailed fitting the tubes connecting the finished rear subframe to the headstock. A frame is pretty useless for racing unless the headstock axis is accurately alignedat right angles to the pivot in the plates for the swingarm. As I have already mentioned,welding always causes distortion. If I had welded all the tubes then it would be most likelythat it would out of alignment as soon as it was released from the jig. I know that manyframe builders cannot fit finished frames back into a jig with any accuracy. To avoid suchproblems I always weld in the head stock after all other welding is done and has beenallowed to cool. That way any distortion is done before the head stock is welded and thatrarely causes much movement. The photo shows all the dots connected although thehead stock has not been welded the other tubes have been tacked and now the frame willbe removed to get good access to finish weld all the joints.

Welding the tubes.

The upper tube has no bracing until welded to the head stock so I clamped a platebetween upper and lower tubes during the welding to keep them aligned. That workedwell.

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The finish in sight.

The frame was placed back in the jig and I checked to see that distortion was not forcingthe frame against the headstock. There was a little bit of interference and I removed thejig to file the areas applying the pressure. After two or three fittings the head stock wasfree without any binding. I tacked the head stock top and bottom which completed thestructure and prevented and significant misalignment from the final completion welding.Once cool and removed from the table it would fit back onto the fixtures perfectly. In the1970s and 80s I made several frames of different types and always used the same guidingprincipals to ensure alignment.

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Mile stone achieved.

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It is not over yet

I mentioned that I always finish machine the bearing bores after the frame is welded. Toachieve this I mount the frame on a milling machine table with the same fixtures that I usedon the optical table. This make sure that the frame is aligned perfectly for boring. Anadjustable boring head makes it easy to bore accurately to size.

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Don’t forget the bottom bore.

I had no fixtures to hold the frame true for the bottom bore but when doing the top one Imachined the top surface so that it was square to the bore. That gave a true surfacewhich held the head stock in alignment when it was clamped down onto the table. Torelieve the force on the head stock from the overhung frame I positioned that so that theseat tubes just touched the table, hence taking the overhung weight of the frame.Little things make a difference.

What next?

We need some way of holding the forks, these are clamps know as fork yokes. I made these from some pieces of 6061 T6 that I had in the recycle bin.

I rough cut them out with a chop saw with a fine wood cutting blade. And then finished them in the CNC milling machine.

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Now for the back.

We are not going anywhere without something to hold the rear wheel and suspension.This is called the swingarm and is another welded structure. Like the main frame onlysimple but carefully aligned fixtures were needed to hold the parts in place for the welding.

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The design.

The swingarm consists of three main pieces, A cross tube at the front which is bored totake the pivot bearings, this welded to two side tubes. The front is also strengthened witha sheet metal gusset and brackets are added near the rear for mounting the shocks. Note that the two side tubes are a composite of round and rectangular tube. This is aresult of the rules for the class of racing which dictate a round tube. The rectangular wheelmounting ends were deemed acceptable. Without such a rule each arm would have beentotally made from rectangular tubing.

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Shock mounts

With the main part of the swing arm welded together it only remained to add brackets forthe shocks. The photos show the simple jigging to hold those in position. Firstly only theouter plates were held in place by a couple of uprights bolted to the table. Then thoseplates were used in conjunction with some threaded rod and spacers to hold the innerplates at the correct spacing for the shock fixing.

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Finishing the job.

As with the head stock I finish machine the bearing bores after all the welding is done. Tohold the arm in the mill I clamped two large V-blocks to the table and then clamped thebearing tube to the V-blocks to keep the tube inline for boring. It was a simple matter toturn the swingarm over to do the second side.

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The finished job.

The rough appearance to the edges of the welds is flux residue. Many people use what iscalled a gas fluxer for supplying flux to the weld via the gas flow to the welding torch. Thisrequires very little cleaning after welding. I am a bit old school and I do not like the gasfluxer very much and I used a heated welding rod dipped into a tin of powdered flux. Thistechnique leaves a hard residue of flux, as seen in the photos, which must be cleaned off.Sand blasting the finished parts is the easiest way to remove this flux residue.

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Fun time.

Here we see the initial test fitting of the engine and other parts. The other photo showswhat it is all about.

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Spoils of war.

2009 AMA national champion in the 500 cc class.

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