Wednesday, 11 March 2020

QR50 - Fork Bushes

As you know, I have decided to make a new set of telescopic forks for the QR50 since I can't find a new pair locally, or anywhere else for that matter, for a reasonable price. You've seen how I recovered the sliders and stanchions with new tube, and replaced the missing dust caps with machined parts.

Now it's time to machine the bushes and assemble the bits. I bought a 30 cm length of 30 mm white acetal, and cut off two 80 mm lengths, planning to make a complete leg-set of bushes in one setting. I wanted to make sure the set-up for the 22 mm bore was done without removing the material from the chuck.

I made a drawing to set my bush lengths against the existing design and the springs I had bought:


I set one of the 80 mm lengths up in the 3-jaw and centre drilled it, opening it up through 6, 10, 13  and 19 mm for the boring bar.


Here's the boring bar in action, taking out a 40-45 mm length to 22 mm or so to give a sliding fit on the stanchion.


From this bored part, I finished the top bush OD to 29 mm and parted off.


The top bushes fit in a 29 mm ID register in the slider, which is made from 32 mm OD seamless tube of 2 mm WT. The bush has a 0.5 x 0.5 mm register in one end to let it sit just below a circlip groove in the slider.

The circlips are wire type - I bought the sheet metal type initially before I realised there wasn't sufficient space in the leg to house them. Wire types are harder to get out but occupy much less space. The nominal 30 mm circlips need the ends trimming:


The clips fit in the end of the slider nicely - there is a 0.5 mm deep groove about 8 mm in, to let the clip retain the top bush:


The slider has a ground channel to allow you to get a scriber under the circlip:


It's very hard to see, but the circlip ends are ground flat to allow you to lift the ends out of the grooves:


The bottom bush is essentially a blind tube - bored 22 mm for the stanchion and finished to 28 mm OD for the slider, it has a 5 mm thick closed end to seat the spring. It's loosely retained to the stanchion with a small roll pin.

This shows the complete assembly, with the dust cap, main compression spring and the rebound spring. The springs were bought from ebay, selected for wire diameter and OD.


Here's the stanchion in paint:


The frame is also almost finished:


By the way. This interesting effect is called fisheye, and is what happens if you don't degrease the parts prior to painting...


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