Friday, June 17, 2011

When the mill is busy....

Switch to the lathe.  Got a bit of time in to make some parts for the front end, specifically the steering stem.  Although my bike does not have forks, it does have a steering stem, one that moves with suspension travel but does not pivot with steering.  The design is a throwback to my original Mk1 chassis that used a combination of tapered roller and deep groove ball bearings instead of spherical rod ends at the upright pivot.  Sorry for the vintage shot but those were the early days of digital.....

Those were the early days (2001) of nearly zero rake and an air cooled Rotax single.  In case anyone was wondering, do not use Daytona as a maiden voyage for a race bike, frame still warm from welding.  We both survived but it was a lesson learned!

I've almost come full circle, having experimented with various bearing configurations in the quest to maximize feel and minimize friction and hysteresis in a compact package. This is an area in front end design where just equaling the feel and feedback quality of telescopic forks will be a very good result.  With a linkage and multiple pivots there are many more areas where the vibrational energy that is feedback can be lost.  Careful detail design can minimize this loss and is a necessity for high performance race applications but the solutions are not always obvious.  For a while I was using high quality Aurora spherical rod ends that gave me easy adjustability but lacked in extended lifespan and were relatively expensive.  The bearings in the current version are different styles from what was originally used and should be an improvement in feel and packaging.

Back to today's part, the steering stem.  It serves to connect both suspension a-arms and provides a pivot axis for the wheel/legs/upright assembly.  It has 2 bearing journals that define the steering axis, a jam nut thread, and 2 cross holes once for each a-arm.

Herre's the bare part:

The first step is to turn the main part geometry from bar stock on the CNC lathe.

Close the door, press the button and...

Open the door and presto, a part:

To assist in getting the bike dialed in I had to make several different versions of the stem.  Think of them as different triple clamps and you won't be far off.  In combination with other interchangeable parts they allow near independent adjustment of several important chassis parameters.

 Here's the 3 different part geometries with the steering bearings that will be used.

These parts will now need to go into a 4th axis mill setup where I will add the flats and cross holes, finishing them.

That's all for now.