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.


  1. Very interesting system, specially the way you change adjustments. Does it really trnasmit asphalt feeling? We´re also designing an alternative suspension MOTO2 and from your expirience I'd like to know your opinion.


  2. If you're doing your own FFE then I hope you think it can transmit feedback! ;) Done correctly, yes, I think it can perform at the top level with proper rider development. Not an easy task but possible.

    Good Luck and maybe see you on the grid sometime.

  3. You have 3 different steering stems. It looks like the difference lengths which would change the relative angle between the 2 A-arms? Is this changing the rake. Wear is the jam nut located in your section drawing? Cool stuff. Love the explanations.

  4. Thanks. Yes, different lengths but I don't want to go into the details of what parameters they change. That is hard won information and part of what I hope will keep me ahead of others starting similar projects.


  5. Very impressive to see all the work and technical knowledge involved to make this wonderful motorcycle. In the tru spirit of John Britten...

  6. John,

    Thanks, John Britten was a huge inspiration to me. Now that you mention him it may be time for another viewing of the Britten video. Now who did I lend it to last?


  7. Why were you experimenting with a near zero rake?

  8. Chris, after you post I looked on the internet, because I vaguely remembered that there was going to be a re-release of the Britten footage, including some unreleased material. It is released, so if you need an extra copy

  9. Trom,

    To see how the bike behaves. Rake is not the important variable to bike handling but because the large majority of bikes all use similarly configured telescopic forks then rake is something easy to visually compare.

    When you move away from telescopics I was wondering if that 'magic' rake number of 23-24 degrees was still relevant. It was not.



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