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Evolution and Intelligent Design

The Most Popular Eccentric Bottom Bracket on the Market - The Bushnell Eccentric

No matter where one comes down on evolution or intelligent design, we know of one place where both exist. The Bushnell Eccentric....of course.

Now, you may know some of us here, and you might even think that we've always been a little bit eccentric, and you would be correct, but this kind of eccentric is a little different. In the tandem world, the eccentric is one of those components that lives on a tandem, and when functioning properly, pretty much goes unnoticed. It's the part of the tandem frame that 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 or fixed-gear bike manufacturers have discovered this slick item for their purposes.

Up to our knees:
Dennis Bushnell pioneered the development of the eccentrics that we currently use in our Rodriguez tandems. I've always thought it was a fantastic product. When Dennis and Jenny were preparing to move their shop up to Seattle in November, 2004, Jenny called me and told me that they had orders in for 100 of the Bushnell eccentrics. She said that they could cut up the material for the 100 units 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).

After the move, we spent some time designing and making some new tooling to speed up production, and then 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. Since then, it's even gotten busier, and our production methods have gotten even more efficient than ever.

Because of its extreme lightweight (just 140 grams) and the high quality of this part, the Bushnell eccentric has become a sought after product for several manufacturers of the new single speed mountain and bmx bikes, as well as 'fixies'. Because of these new bike types, demand for this product went through the roof!

The difference between eccentrics is hard to see from just looking at a bike, so I thought that I would write this article and describe the different evolutions that they have gone through.

Evolution and intelligent design:

Your Great Grandfather's eccentric (prehistoric):

Before the eccentric, tandem manufacturers used an adjustable pulley 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 was accomplished by mounting the captain's bottom bracket off center in an aluminum chunk that could be rotated. This idea looked a lot better than the old and caught on with most tandem manufacturers.

Your Grandfather's eccentric: - 300~400 grams

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. Photo 1.

Another type uses sharpened screws to dig into the aluminum chunk when tightened (instead of the pinch bolts).

A lot of tandem manufacturers still use these types of eccentrics today, as they are much less expensive to make.

The drawback to these types of eccentrics is that the threads on the frame can strip out, or the brazed-on threaded piece may even break off of the frame entirely. This requires a costly frame repair and paint work. The sharpened bolt style also has a gouging effect on the aluminum chunk (kind of crude we always thought).

Your Father's eccentric: - 305 grams

In the late 1980's a company came up with a design that didn't use threads on the frame, but in the eccentric itself. It used a split/wedge design (just like an old fashioned bicycle stem) 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, it required a long punch and a hammer (when someone is coming at your expensive tandem with a hammer, it's kind of scary). One had to loosen the bolt and then hit the large piece with the hammer and punch until it broke loose in the frame.

One other problem that existed was that non-tandem shops would loosen the bolt, and then hit the bolt head with a hammer. This would drive the threads right out of the special nut. Then they would have to have one red labeled to their shop and appease a mad customer.

The modern eccentric (the missing link):
What was missing was an intelligent design that looked good, was lightweight, and easy to adjust (even by a Neanderthal). Well, a few years after split/wedge design, Dennis Bushnell set about re-inventing the eccentric all together. Dennis came up with an ingenious design that used the bolt in the eccentric to both tighten and loosen aluminum wings inside the 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. Gone were the wedges, and in were the 'wings' as we call them here. 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. In its current design, it is less than 1/2 the weight of the competition (just140 grams).

It has now become the natural selection for manufacturers of some of the highest quality tandems and single speeds on the planet. (You didn't think that we sold 2,000 tandems since last year 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 choice of eccentrics over the years reflects our long held philosophy on putting the best performing products on our bicycles, not just the most 'magazine-hyped' products. The Bushnell eccentric is much more expensive to manufacture than an aluminum chunk, but it serves the bicycle owner much better in the long run.

After my experience in January, I have a new found respect for the development and manufacturing process of this incredible component, and I hope that you will too.

If you've made it this far, you're as eccentric as I am! Thanks for reading - Dan