The Evolution of the Bushnell Eccentric Bottom Bracket


No matter where one comes down on the different theories, we know of one place where both exist. R+E Cycles, of course!

Featherweight Bushnell Eccentric

When we joined forces with Dennis Bushnell, one of the things that we inherited (along with Dennis, Jenny and Toby) was the Bushnell eccentric business. Now, you may think that we've always been a little bit eccentric and you would be correct, but this is a little different. In the tandem world the eccentric is one of those components that lives on a tandem, and when functioning properly goes unnoticed. It's the part of the tandem frame where the captain's bottom bracket (where the crank arms mount) is installed. As the timing chain (the chain that connects the captain and stoker cranks together) stretches, the eccentric allows the captain's bottom bracket to be moved forward to tighten the chain back up. It's a very necessary piece of equipment on a tandem, and now the single speed and fixed gear bike manufacturers have discovered this slick item for their purposes.

Up to our knees:

Dennis pioneered the developement of the eccentric that we use in our Rodriguez tandems years ago and I've always thought it was a fantastic product. When Dennis and Jenny were preparing to move their shop up to Seattle a few Novembers ago, Jenny called to tell me they had orders for 100 Bushnell ecentrics. She said that they could cut up the material for the 100 pieces and then we could machine them after the move. I suggested that they cut up the material for 400 eccentrics (100 is the biggest single run that they had done in the past). Well after the move, we spent some time designing and making some new tooling to speed up production. With that aspect conquered, Todd, Jenny and myself spent the entire month of January up to our knees in aluminum shavings making 400 Bushnell eccentrics. After that batch of 400 was done, I had a whole new respect for this quality component, and the design work that has gone into it. In 2005 alone, we sold nearly 900 Bushenll eccentrics.

The difference between eccentrics is hard to see from just looking at a tandem. This led me to write this article to describe the different evolutions eccentric bottom brackets have gone through over the years.

Evolution and intelligent design:

Your Great Grandfather's eccentric (prehistoric):

Before the eccentric, tandem manufacturers used an adjustable pully wheel to take up the chain slack as the chain stretched. Then someone (I don't know who) came up with the idea of moving the front crank forward in the frame to adjust for chain stretch. This idea looked a lot better than the old and caught on with most tandem manufacturers.

Your Grandfather's eccentric (he's really just fool Grandma):

The early eccentrics were made from a chunk of aluminum held in place by pinch bolts on the frame. The pinch bolts could be loosened, and the chunk of aluminum could be rotated to tighten the chain. It worked pretty well, and looked a lot better than the old way. Another type of eccentric bottom bracket used sharpened screws instead of pinch bolts to dig into the aluminum chunk when tightened. A lot of tandem manufacturers still use these types of eccentrics.

The draw back to these types of eccentrics is that sometimes the threads on the frame will strip out, or the threaded piece may even break off of the frame. This requires a costly frame repair and paint work. The sharpened bolt style also has a gouging effect on the aluminum chunk which we thought of as crude.

Your Father's eccentric:

In the late 1980's a company (we'll call them Large-Gun dale) came up with a design that didn't use threads on the frame, but in the eccentric itself. We're not snobs, and we recognize good design when we see it, so we switched. It used a wedge that could be tightened by a nut/bolt system. If the threads got stripped, it was just a small repair instead of a costly frame repair.

The drawback to this system was in trying to loosen it. To loosen this system properly required a long punch and a hammer: when someone is coming at your expensive tandem with a hammer, it's kind of scary. A mechanic had to loosen the bolt and then hit the large piece with the hammer and punch until it broke loose from the frame.

The other problem that we ran into was non-tandem shops loosening the bolt and then hitting the bolt head with a hammer, driving the threads right out of the special nut. Then we would have to rush a new special nut to whatever distant shop needed it.

The modern eccentric:

Well, a few years after that, the Bushnell design was born. Dennis came up with an ingenious design that used the bolt in the eccentric to both tighten and loosen the aluminum chunk in the frame. The need for the hammer and punch was gone. The Bushnell eccentric looked great in the bike, and was the easiest design to adjust as well. Like I said, we recognize good design when we see it, and we switched immediately.

Dennis then went to work improving and lightening up the design to make not only the most evolved eccentric on the market, but also the lightest and best looking.

It has now become a sought after component for manufacturers of some of the highest quality tandems and single speed bikes on the market. You didn't think we sold 900 Rodriguez tandems the first year we manufactured eccentrics, did you?

Function over flash:

Well, the Bushnell eccentric has gone through several evolutions to get where it is today, but is by far the best designed little piece on your Rodriguez tandem that you'll never notice.

Our choices of eccentrics over the years illustrates just one of the ways we focus on putting the best performing product on our bicycles, not just the most 'magazine-hyped' products. The Bushnell eccentric is more expensive than an aluminum chunk, but it serves the bicycle owner much better.

After my experiences in building Bushnell eccentrics that January, I gained a new found respect for the development and manufacturing processes of this incredible component, and I hope you will too.