Buell Motorcycle Swing Arm Modifications

Buell Motorcycle Swing Arm Modifications.

Wicked change over and modification this job.

My customer has asked me to modify and extend a cast aluminium swing arm to give him a wider rear wheel section.

Here is the Non standard swing arm to modify.


Now the Buell motorcycle that this aluminium swing arm is to mate up to has a simple steel swing arm affair (the difference shown below). And there are also pictures showing the aluminium swing arm offered up to the original steel frame and back end gearbox pivot mounting. Note that we are extending the swing arm by 100mm to ensure the next size belt drive fits and extend the wheelbase marginally which helps reduce front end lifting so easily under accelleration.


If you have ever considered a similar modification for your motorcycle you will know that there are many things to consider.

Points to consider

  1. Mountings
  2. Bearings
  3. Swing angle
  4. Coil over / linkages if used?
  5. Offsets of wheel
  6. Offset of Mounting in Frame
  7. Offset of drive sprocket to wheel centre and frame centre
  8. Relocation of pipework, cables
  9. Chain or belt clearance
  10. Revised mountings for chain guard, rear sets, exhaust, rear brake
  11. Any areas of interference e.g. is the assembly going to hit any part upon compression of suspension.
  12. Maintaining rear / front wheel alignment / centres
  13. Accurate and equal measurements between wheel centre and new pivot point centres,otherwise rear wheel will be out of alignment.
  14. Strength, rigidity, flexibility
  15. Welding and machining skills, design and manufacture skills
  16. and so on !

So all in all not a job for the feint hearted. As you can see there are many things that need consideration before any work takes place.


How did I complete this job?

A complete breakdown of this modification can be seen below.

Measure, measure and check again! 

I use any number of methods to work out all the above. First and foremost an image in my head of how I think I can tackle the job, what the end result may look like, will I be able to make it “work”. Its no surprise what people throw at me as far doing custom made jobs. Often upon initial conversation / email I may not know if I can make / modify / fit / manufacture what the customer requires. Often its a case of getting more information / pictures / sketches / notes / dimensions etc to fathom out if indeed it is even possible and will work.

Once I have pictured the overall job and agreed that I can do it, there may be several steps involved in being sure I can make what the customer wants as I may have a different picture in my head to what the customer thinks.

Anyway this modification was quite logical in planning and implementing.

A marked out layout was completed on my work bench using the wheel centre as a datum point. I then overlayed the original and also the new swing arms. Instantly this gives me a layout that I can pattern from. By doing this I can compare what dimensions I have to work from to make the new swing arm fit to the back end mounting in the frame for the original pivot point. At this stage a flat template was made from aluminium as a check and compare guide.

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Once the template had been made and offered up to the frame and original pivot mounting points I could clearly see that the pivot mounting point and frame centre line was some 10mm out of alignment. This meant that the template and layout had to be adjusted to ensure that the centre line of the rear wheel was moved over to correct the offset and ensure the frame and rear wheel centre line is on the same plane.

Designing the new Swing Arm Extension

Once the correct layout, centre lines and overall plan is checked and double checked again, I know what we have to work with. At this point I have designed a 3d model of the infill piece which will extend  the swing arm by 100mm. The item will be machined from one block of material and be pinned and welded to the aluminium swing arm.

This positive positioning will be via 2 aluminium “top hat” pins that will be a press fit into the adapter / extension piece and also the bearing bore on  the actual aluminium swing arm. Once happy with the fit these will be welded in position and the slab side of the extension will also be fully welded all round to the swing arm. Creating a fully fitted extremely strong extension piece which is now one part with the swing arm.

Initially several sketches were made to clear my mind dimension were correct and also to put my ideas on paper to be able to produce a model. Careful thought needed putting in to design of the item to meet all the parameters and miss vital components such as the back edge of the main framework.

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Sketches always help me with considering a design for a one off job and clarifies points in my head and what I want it to look like. From the sketches I can easily make a 3d model as below
swing arm mods (2)

The above image shows a 1 piece extension unit complete with front pivot bearings and rear top hat pins in place. From this an engineering drawing was produced to enable manufacture. This included all dimension required to ensure dimensions and fits for bearings and pins are all to tolerance and “work” accordingly as designed.

swing arm dwgpin bosses dwg

Points to consider when designing this part !

Strength, weight, size, machine-ability, weld-ability, fit, finish, conflict with other parts (as its a moving part).

So all above considered this was the final machined extension piece manufactured from T6 Aluminium.

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Once checked dimensionaly and any sharp edges removed the next step was to offer up to the cleaned up swing arm, removing all the original paint to ensure no weld contamination in the welded areas we could tack up and check against the frame before final welding.

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2 images above show the main extension welded to the original swing arm. Location was critical and so top hat pins were machined up to a light push fit into the new extension and aligning with the original bearing bores in the swing arm. This ensures a “true” aligned extension piece. Once all tacked and aligned the whole unit was fully welded using the TIG welding process. As the swing arm incorporated an oil tank within the pivot end of the swing arm, some difficulties occurred in welding as due to the nature of alloy being porous oil soaks into the material somewhat. This is fine but during the welding process this contamination “burns out” and floats on top of the weld pool.

Once welded the surface of the weld was cleaned with a skotch pad to remove any surface contamination. Weld integrity is critical on a job like this where so many forces are travelling through and acting on the pivot point. In this instance there is in excess of 600mm of weld all around the added extension piece to original swing arm so its not going anywhere.

Infill panels were then cut, positioned and welded in position to finish the “boxing in” of the extension piece.

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To add more strength and infill the area above the new extension I added a fabricated triangulated section which ties in the extension fully to the original swing arm – see image below.

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Final shots of the new swing arm fitted up to the frame – below

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Thanks for looking, hope this blog was useful to you and remember if I can help you with your project please don’t hesitate to give me a call, or use the contact form above.