Saturday 5 May 2012

Forks are done


We planned from the outset to give the 1925 Invincible a complete once over to inspect and repair any small area that could or might cause problems during our ride.
This blog outlines what has been done with the front forks.
The forks were pulled off, disassembled and completely stripped to look for cracks, fatigue marks or just poor workmanship and bad design.

Well this task found much wrong with some poorly engineered items that could benefit from modification.
The headstock shaft itself was worn and was loose inside the front fork stem, and the headstock bearings were just plain worn out and deemed just usable at a pinch.
While this would have been fine for a local run, or around the car park of a shopping store it was not up to the task of a sidecar trans-continental.
The loose headstock shaft had caused the headstock bearing cup to fret in the frame and the wear was excessive providing another area that could be a problem and we decided to rectify this.
The headstock  cups being loose within the frame was more of an issue that the bearings them self.
A metric bearing conversion (Timkin Tapers again) were sourced and new cones machined to accept these. These are slightly oversize on the O/D and under size on the I/D which allowed modification with the original outside dimensions being maintained
We could have machined the existing cups to take these taper bearings however as the cups were loose within the frame it was decided to machine new ones.
The head stem was removed from the fork blades and re-machined to accept the diameter of Timkin bearing prior to being located and brazed back into position.

New lock nuts and top headstock nuts were machined and the main fork leg re-united with the frame.

Not a major task, but one that was well worth undertaking and hopefully will result in some reliability throughout our tour.

The front fork (which is the same as Excelsior) was pulled apart , cleaned and inspected.
To our surprise this appeared okay however it quickly revealed a broken spring.
The reason was the the inner spring on the cartridge had fretted on the outer and caused this to fatigue.
This was rectified by machining two central collars to hold the springs concentric minimising the fretting by not allowing the springs to rub.

As fate would have it  i had a spare spring in a misc box of springs that had the same pressure but was 3/8" shorter, allowing the collars to be fitted.
It actually made the assembly quite neat and this is know finished.

It would be interesting to see how the Excelsior forks differ to the Australian copy on the Invincible Jap.
The Invincible forks are quite well made, robust and strong.
Considering these forks are close to 85 years old and have had minimal maintenance i am surprised at the overall condition.

We spent time discussing the options of securing the bolts on the lower fork legs on both the inner and outer legs.
It was decided to fit Castle nuts and split pins to ensure these are held in place and install jam nut (1/2 nuts) on the outer side of the fork rockers.

The front cartridge was painted, and the assembly was re-assembled.
Other discussions were on what grease to fit to the shocker, and we are still undecided on this one.

This is the first component that is finished for the 1925 Invincible Jap and with much more to do we at least have a tidy set of forks.

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