Bushnell Tandem Eccentric

 

New, lighter weight and hard-anodized eccentric (144 grams):

       

The photos above are linked to larger images; just click on each photo to see the larger image.

The redesigned, lightweight and hard-anodized 3rd generation of the original Bushnell eccentric is the result of a collaboration between Dennis Bushnell of Bushnell Cycles and Todd Shusterman of daVinci Designs. The 3rd generation Bushnell eccentrics were produced specifically for use by daVinci and are not at present available for re-sale. According to Bushnell Cycles, a decision to produce an additional lot of the lightweight eccentrics for re-sale beyond those produced for daVinci will not be made until the existing inventory of 2nd generation Bushnell eccentrics are reduced to the re-order point. It is noted that builders are still using both existing inventories of 1st and 2nd generation Bushnell eccentrics on new tandems and 2nd generation models are available for purchase through Nova Cycles Supply of Rocklin, California (www.novacycle.com).

As to how the 3rd generation design came about, daVinci obtained one of the 2nd generation eccentrics from Bushnell Cycles to evaluate for possible use on its daVinci branded tandems. The 2nd generation eccentric was differentiated from the 1st by a redesigned and larger wedge-bolt assembly, but was otherwise still very much like the original, 190 gram eccentric. daVinci's initial assessment of the eccentric was that while it was an excellent design, there was room to improve it by reducing excess material. daVinci used the original Bushnell design as a point of departure for a redesign effort with the goal of eliminating excess material to reduce weight while still maintaining the eccentric's strength and holding power. Bushnell Cycles subsequently provided daVinci with raw stock for the main body and Todd Shusterman then hand-machined several prototype bodies before having daVinci's machinist cut a master on an NC lathe. Todd then created new masters for the "wings" that, along with the new main body master, were sent to Bushnell Cycles for use in developing a production model. Dennis Bushnell made some final refinements to simplify the design for produceability and the initial production lot was fabricated, hard-coated, and shipped to daVinci for use as the OEM eccentric for daVinci tandems.

The following are a summary of the changes:
- Weight reduced by 25% (Was 190 grams, is now 144 grams)
- Components hard-anodized for increased strength
- Wedge/bolt design simplified & made more robust with larger Wedge bolt
-- Original wedge bolt assembly = 16 grams; new design = 29 grams
- Excess material machined away from main body and wings
- Two retaining spring clips used instead of one

I first learned about the development of a lighter-weight Bushnell eccentric while riding with Dennis and his brother Andy at the 2003 Midwest Tandem Rally. Dennis was kind enough to send one to me in early May 2004. The photos don't do it justice as it is a work of art.

 

Original aluminum eccentric (190 grams):

Click on photo for larger image

Exploded view of eccentric: main body, clam shells, wedge bolt & spring clips

Click on photo for larger image

Assembled eccentric

Ready for installation with wedge bolt fully extracted

 

As wedge bolt is tightened, the clam shells are spread

Wedge bolt at max. closed position

 

NOTE: Bicycle was placed in workstand upside down for these photos.

 

Solid bottom bracket shell ready to receive eccentric

 

Eccentric placed into BB Shell. Note, we install ours with the BB opening in the "up" position to maximize cornering clearance / BB height.

Eccentric is tightened with 4mm allen head wrench. Caution: Do not over tighten -- getting a stripped wedge bolt out is tough!!!!

Bottom bracket installation

These bottom brackets came as standard equipment on our Erickson road tandems & our Ventana El Conquistador de Montanas off-road tandems



      

Mark's Take on the Bushnell Eccentric Design

The words that go with the pictures; from a dialog on the  
Tandem@Hobbes newsgroup which challenged why I thought the 
Bushnell eccentric design was "The Best" I've come across...


      

Someone asked:

What makes an eccentric bracket "better?" To me, "better" than the rathergeneric ones I have would mean a wider range of fore/aft adjustment of the spindle position. Does the Bushnell offer a wider range ofadjustment?

The only other thing I can guess about is the increased ridigity of a solid bottom bracket shell - is this is a real world issue?

 

And I replied:

Again I'll preface with "In My Humble Opinion", it is the design, quality and performance of the component.

In this regard, Dennis' internally expanding DESIGN provides a bottom bracket that:

a). in the case of the pinch bolt bottom bracket designs, will not cause reduced lateral strength of the frame in that highly stressed region.

b). in the case of the dual set screw designs, does not create a progressive series of gnarled screw marks in the eccentric's body, i.e., damage by design as a method of suspension/retention.

c). in the case of wedge design used by Cannondale, well just check the archives to see how many people have had problems with that particular design.

These are the KEY discriminators that come immediately to mind as the things I don't find ideal with the other designs. There are also what I perceive to be drawbacks in these designs with regard to how they hold the eccentric in place and how they distribute the loads between the eccentric and the shell. They all work to achieve the purpose of the component; however, each of these designs has its draw backs (trade-offs if you will). One could also argue that the wedge designs are generally more aesthetically pleasing than the pinch or set screw designs.

If you (or others following this thread) are interested, Sheldon Brown provides more detailed descriptions of these various eccentrics on his site at: http://www.sheldonbrown.com/synchain.html#eccentrics

Cannondale has a picture of theirs along with instructions that suggest you can "tap" the wedge bolt out with a hammer should it be installed too tightly (Bwaaa haaa haaa) on page 27 of their on-line Acrobat manual at: http://www.cannondale.com/tech/pdfs/99tandsupp.pdf

Returning to Dennis' eccentric, it is an elegant design that incorporates solutions to the problems which plague the others. It allows frame designers and builders to use the eccentric with all frame materials and it allows them to use a solid, symmetrical bottom bracket shell without voids. From the standpoint of weight, you'll notice the clam shells have had non-essential materials removed so that, from the standpoint of total mass, this component is lighter than its peers. With regard to how it holds the eccentric in place and distributes loads (aka Performance), by expanding the eccentric through the entire axis of the bottom bracket shell, the loads are equally distributed around and across the shell. The latter allows you to use less torque on the wedge bolt assembly to achieve a firm installation of the eccentric. The design similarly makes removal of the eccentric easier than the 'other' wedge designs since the wedge bolt is loosened independently from the portions of the eccentric that are in contact with the shell. In other words, it is always easy to remove -- you loosen the bolt and then, if you even need to, rotate the eccentric with a spanner to free it. Finally, as to quality, it is a beautiful, well machined component made from high quality alloys and metals. More importantly, it is a product that is backed up and fully supported by Bushnell Cycle Designs. Trust me on this, if you need help from Dennis he will astound you with his responsiveness.... and he's one heck of a really nice guy to boot!

Frankly, I enjoy knowing and speaking with the folks who design and build the things I use... it DOES make a difference when you can gain an appreciation for their philosophies, love of their work and their sincere interest in providing their customers with the best products that they know how to make.

Oh, in regard to fore/aft adjustment, Bob Brown of Bob Brown Cycles (http://www.bobbrowncycles.com) provided me with a good explanation:

 

"When speaking of fore-aft adjustment what is probably meant in the overall throw of the eccentric. It's different amoung different manufacturers. Most 54mm diameter eccentrics have a little under 1/2" of adjustment or "throw". Since the minimum amount of chain you can take out is 1", that's not quite enough for every possible gear/boomtube length combo. You get some gears/chain lengths that cannot be properly adjusted for regardless of boom-tube length.

This is why Co-motion started making their own eccentric, it has slightly over 1/2" of travel, thus it covers all possible conditions. Supposedly the Bushnell eccentric is of similar travel, but I have not measured one to be sure, and the design of the eccentric would make it very difficult to achieve (but probably possible, and I bet Dennis did it!"

Hope this answers your questions.

Contact Info:
Bushnell Cycle Designs
3108 C St. SE
Auburn, WA 98002
Phone: 253-939-9778
Fax: 253-939-8651
E-mail: bushcycles@aol.com