making a racing motorcycle chassis - cdn.instructables.com

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Making a Racing Motorcycle Chassis

by tonyfoale

In the early 1970s I built a frame for my racingmotorcycle to house a 350 cc Aermacchi engine (HDsprint in the US).

In 2006 after being away from the track for 30 years Istarted racing again in classic races in the US and Ibuilt a close replica of my 1970s bike in 2008/2009.

This Instructable, which is an entrant in the “Metalscontest”, describes how I built it mainly throughphotos which generally show more than words can.The above photos show the orinal 1970s bike and theother shows the new one being exercised at Loudonin NH, USA. in 2010.

Making a Racing Motorcycle Chassis:

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Step 1: Tools and Materials

I will not give a detailed list of the materials usedbecause that will be obvious from the descriptions ineach step. As always tools used depend so much onthe facilties available to an individual builder but whatI used will become apparent as the build proceeds. Iwill describe one tool though before we start becausemany readers may not have see one. That is what iscalled an optical table. I managed to buy it from alaser optics company that was downsizing. From thephoto it can be seen that it has a lot of depth whichkeeps it rigid. The top is stainless steel to a flatnesstolerance well within that needed for the task. It also

has the top drilled on a 1” (25.4 mm) spacing andtapped ¼ “ unc. These holes are very handy forholding parts and fixtures/jigs securely to the table.

I used this table as an accurate and solid base foraligning the frame during construction. I have neverliked the common type of frame jigs used by manypeople who have made frames. This common typemounts the frame in the orientation that the frame willassume when on the road as a complete motorcycle.This type of jig usually has a horizontal base with atall vertical piece to hold the steering head. I find this

type of jig to be lacking rigidity and accuracy. On theother hand I find these problems a non-issue whenusing a solid flat base like the optical table or a largeenough surface plate and I have used the bed of anold flat lathe as well in the past.

The second photo shows the simplicity of some of thefixtures which are used in conjunction with the table.

Step 2: First Steps

The construction of this frame was largely composedof steel tubes welded together, but at the rear of theengine there are two 12 mm thick 6061 T6 aluminiumalloy plates which give support to the rear swingingarm and footrests. If I had a bandsaw at the time Iwould have cut the plates out with that, I did not but I

was able to use a CNC milling machine. I wrote asimple Gcode programme and cut them out that way.The mill also made it very easy to drill and reamvarious holes equally located on each plate.


Step 3: Making the Head Stock

The head stock is a part which holds the two steeringbearing to which the front forks are mounted. Usuallyit is made from some relatively thick walled steeltube. It order to save weight I wanted to waist thecentre section down. To do this I machined it from asolid bar of 4130 Cr.Mo alloy steel. The photos showthe main steps and the finished part. The large

diameter end sections are bored to take the steeringbearings. Pieces always distort to some extent whenwelding and so I always leave the bores undersizeuntil the frame is finished and then do the finishmachining which is described later.


Step 4: Preparing the Tube Fitting.

When round tubes are welded together it is usuallynecessary to prepare the tube ends to get a nice fit.This process goes by many names such as fishmouthing or tube profiling which is what I call it.

There are many ways to do this, for example you canmake or buy holding fixtures to permit the use of ahole saw in a manual drill, drill press or millingmachine. You can also saw close to the shape andfinish with a file or grinder. You can find freewareprogrammes on the net which enable you to print outtemplates to wrap around the tube as a cutting guide.

However for tubes of the size that I used, 22 and 19mm I prefer to use a milling cutter of the same size asthe tube to which the tube being cut will fit. To make itquick, simple and secure I made clamping fixturesfrom some scrap aluminium blocks. These blockswere relieved such that the vice pressure acteddirectly across the tube to ensure secure clamping.The blocks could be mounted at any angle in the vice,so I only had to set them with a digital angle level toget a perfectly fitting joint.


Step 5: Profiling the Tubes.

The photos say it all. Note how little “stick out” there is when machining. This ensures the greatest possible rigidityand accuracy.


Step 6: The Fun Part Begins

I get to do some welding.

The tubing is 4130 Cr.Mo. Both 1 mm and 1.5 mmwall thickness, depending on the loading of eachtube. I used what I and most British old timers callbronze welding but which is more commonly knownas brazing in the US. The heat is supplied by an oxy-acetalene torch and the filler rods are manganesebronze which melt at a slightly lower temperaturethan the more usual nickel-bronze. There are threemain reasons for preferring bronze welding overfusion welding for this type of structure;

The steel tube is heated to a lower temperature thanfusion welding which helps retain its strength.

The fillets are usually made with a larger radius andthat reduces stress concentration.

The bronze is more flexible and that softens the loadsa little further reducing the tendency to fatique.

I do not have a photo of the welding of this frame butshown is a picture of an earlier frame that I preparedfor a friend.

Step 7: Starting the Structure.

Aluminium plates attach to the tubular structurethrough one cross tube. If that was done with a capscrew through a parallel drilled hole then over timeand use I can guarantee that the holes will loosenand accurate location will suffer. To avoid that inthese cases I countersink the plate and make steelbosses with a matching angle which get welded to

the cross tube as shown in the photos. After weldingthe bosses, they get finish machined in a lathe andthe ends are drilled and tapped M10 for clampingbolts. When clamped up the taper fitting ensuresaccurate and rigid connection.


Step 8: The Base of the Frame.

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


Step 9: Adding Some Shape.

The next stage was done using the optical table andsome very simple fixtures to hold it square and level.The fixtures were machined and drilled in the mill toensure accuracy. Although I have the pictures aboveshowing the sub assembly tacked together I amunable to find any pictures of the jigging for this part.

It simply consisted of two matching plates on eachside to hold the upper tube in place. The secondpicture above just shows the sub-assembly on thecrankcases purely as a progress photo.


Step 10: Somewhere to Sit.

Now it is time to add the seat tubes and rear sub-frame. For this operation the sub-assembly is orientated as itwould be on the road. This makes it very easy to align the seat tubes by using some spacing blocks to hold theends of the tube at the required height. This was a one off construction and so where simple fixturing wasappropriate I was happy to go that route.

Step 11: Fitting and Checking.

The first photo shows the close fitting that the profiling gave, note that the top tube was profiled in two directionsone to mate with the top cross tube and the other to match the diagonal. The second photo shows the assemblybeing checked with a vernier height gauge.


Step 12: Re-orientation.

Up until now the frame was orientated as it will endup on its wheels but I like to bolt the head stock atright angles to the table to ensure rigid alignment, thismeans that the rest of the frame must be tiltedforward by an angle equal to the angle of the forks(called the rake angle) when on its wheels. To do this

I raised the rear by an appropriate amount using tallerside plates as shown in the photos. One photo showsthe headstock clamped in position. The grid of holesin the table makes alignment very easy.

Step 13: Joining the Dots.

The next stage entailed fitting the tubes connectingthe finished rear sub-frame to the head stock. A frameis pretty useless for racing unless the headstock axisis accurately aligned at right angles to the pivot in theplates for the swingarm. As I have already mentioned,welding always causes distortion. If I had welded allthe tubes at this stage then it would be most likelythat it would be out of alignment as soon as it wasreleased from the jig. I know that many frame builderscannot fit finished frames back into a jig with any

accuracy. To avoid such problems I always weld inthe head stock after all other welding is done and hasbeen allowed to cool. That way any distortion is donebefore the head stock is welded and that rarelycauses much movement. The photo shows all thedots connected although the head stock has not beenwelded the other tubes have been tacked and nowthe frame will be removed to get good access to finishweld all the joints.


Step 14: Welding the Tubes.

The previous step described the reasons for welding the head stock last. The next step describes in detail howdistortion is minimised. This step shows the pre-headstock welding.

The upper tube has no bracing until welded to the head stock so I clamped a plate between upper and lower tubesduring the welding, off the jig, to keep them aligned. That worked well.


Step 15: The Finish in Sight.

The frame was placed back in the jig and I checked tosee that distortion was not forcing the frame againstthe headstock. There was a little bit of interferenceand I removed the frame from the jig to file the areasapplying the pressure. After two or three fittings thehead stock was free without any binding. I tacked thehead stock top and bottom which completed the

structure and prevented and significant misalignmentfrom the final completion welding. Once cool andremoved from the table it would fit back onto thefixtures perfectly. In the 1970s and 80s I madeseveral frames of different types and always used thesame guiding principles to ensure alignment.

Step 16: Milestone Achieved.

The structure completed.


Step 17: It Is Not Over Yet.

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

Step 18: Do Not Forget the Bottom Bore.

I had no fixtures to hold the frame true for the bottombore but when doing the top one I machined the topsurface so that it was square to the bore. That gave atrue surface which held the head stock in alignmentwhen it was clamped down onto the table. To relievethe force on the head stock from the overhung frame I

positioned it so that the seat tubes just touched thetable, hence taking the overhung weight of the frame.

Little things make a difference.


Step 19: What Next.

We need some way of holding the forks, these are clamps know as fork yokes. I made these from some pieces of6061 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 millingmachine.

Step 20: Post Yoke Machining

After machining the yokes a steel stub for the bearing was pressed into each yoke. A spacer was made such thata little preload was placed on the bearings when everything was clamped up. that required careful measuring. A10mm clamping rod connected top and bottom yokes.


Step 21: Now for the Rear End.

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


Step 22: Swingarm Design.

The swingarm consists of three main pieces, A crosstube at the front which is bored to take the pivotbearings, and two side tubes. The front is alsostrengthened with a sheet metal gusset and bracketsare added near the rear for mounting the shocks.

Note that the two side tubes are a composite of round

and rectangular tube. This is a result of the rules forthe class of racing which dictate a round tube. Therectangular wheel mounting ends were deemedacceptable. Without such a rule each arm would havebeen totally made from rectangular tubing.

Step 23: Shock Mounts.

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


Step 24: Finishing the Job.

As with the headstock I finish machine the bearing bores after all the welding is done. To hold the arm in the mill Iclamped two large V-blocks to the table and then clamped the bearing tube to the V-blocks to keep the tube inlinefor boring. It was a simple matter to turn the swingarm over to do the second side.

Step 25: All Done.

The rough appearance to the edges of the welds isflux residue. Many people use what is called a gasfluxer for supplying flux to the weld via the gas flow tothe welding torch. This requires very little cleaningafter welding. I am a bit old school and I do not likethe gas fluxer very much instead I use a heated

welding rod dipped into a tin of powdered flux. Thistechnique leaves a hard residue of flux, as seen inthe photos, which must be cleaned off. Sand blastingthe finished parts is the easiest way to remove thisflux residue.


Step 26: Fun Time.

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

Step 27: The Spoils of War.

2009 AMA grand national champion in the 500 cc class.

If you liked this instructable please vote for it in the metals contest. Thanks.


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