Phat Trakka 

Part 6: Swingarm

Time to commit

Compared to a stock GT750 swingarm, the donor GSX-R 600K1 swinger is longer, wider, much lighter and much stiffer.  The stocker is incredibly heavy and yet has little in the way of rigidity.

Of course the GSXR swingarm was slightly too wide and the pivot bolt was a massive 25mm diameter compared with the silly little 14mm one on a GT750. To solve the first problem, Monty at KTM machining in Elk Grove Village machined the requisite amount off the width of the pivot section and he also made the center spacer between the needle roller bearings smaller to match.

All swingarms are not created equal.

The next job was to design a way to mount the swingarm and its pivot bolt. 

Alternatives:

 - Use the stock GT750 14mm spindle plus spacers or new end caps to allow use of 25mm GSXR axle.

 - Machine a new spacer tube to slide inside the GSXR bearings and to fit the 14mm GT750 spindle

 - machine new pivot points to be welded into the frame to take the GSXR spindle.

I liked the last one, but concluded it was not necessary and in reality would add little extra stiffness because at this stage teh frame was the weakest link. Option 2 was more expensive that option 1 and offered no advantage, so I used option 1 which Suzukidave has also used.

First order was to cut down the GSXR spindle so that it was marginally wider than the swingarm, so it could be dressed down accurately later.  That was achieved quite quickly with a cut off wheel in an air tool.

Then pivot tube as machined out to clean it up.  Two sleeves were made to clear a 14mm GT750 spindle and were pressed in to the cut-down GSXR spindle. The ends of the tube/spacer were skimmed to the appropriate dimension to give just the right amount of end float when the pivot bolt is tightened.

From an engineering perspective, this isn't an elegant solution with so many pieces, but it is quite adequate for the task.

The swingarm was loosely assembled with the new pivot arrangement and the next task is to work out the exact dimensions of the shock mounting brackets. 

To be continued..................