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Each week, I get dozens of emails asking if they can upgrade their existing bicycle to our patented (pat. 8,070,633) Bushnell Eccentric Bottom Bracket. These are from people who've bought expensive single speed bikes, Rohloff equipped bikes, or tandems that use eccentric bottom brackets (referred to as EBB throughout this article). As they have examined their bike, they've discovered the fact that their bike came stock with an EBB that was designed in the the Fred Flinstone era. While this may have saved their frame builder a hundred dollars or so, it has no benefit for them, and actually makes their bike heavier and harder to use. They discovered our design on-line, and want to know if they can retro-fit one into their custom bicycle. Most of the time it is possible, but sometimes the bottom bracket shell that is used is too small to fit any EBB into the bike except for the one that came with it:-( For this reason, it's important for you to find this out before you order your new frame.
In this article, I aim to help customers understand the different designs so they can steer their frame builder to the eccentric that works best, not just the cheapest option. As a customer, it's up to you to educate yourself on the different styles, and then ask your builder to use the one you want before the frame is built. Otherwise, they will usually choose the cheapest option. It's an easy place to skimp if your customer doesn't know any better, right? Customer education has always been our friend here at Rodriguez Bicycles, and here's a new area to focus on.
Note: All of the designs mentioned in this article, including the Bushnell, are available to every frame builder through their standard frame building supply companies. Any reputable frame builder is able to build your frame to accept any design, but you have to express your preference.
The first benefit is the fact that the design does not use parts welded to the frame to hold its adjustment. That means if you get a little heavy handed and 'reef' too hard on the bolt and strip the threads, you've just stripped a nut, but have not damaged your frame. Every year we upgrade several people to Bushnell EBBs after they've stripped the threads in their frame. Sometimes we even get a call from other bike manufacturers trying to help one of their own customers who've stripped their frame.
Ease of adjustment:
As you read through this article, you see that there are many eccentrics on the market but only one Bushnell. The Bushnell was designed by Master Tandem Builder, Dennis Bushnell, to address all of the design drawbacks of the other designs. Ease of adjustment was key to the design. A 4mm allen wrench is all that is required to adjust your chain tension when you are using a Bushnell. No hammers required (yes, some ebb styles require a hammer).
Although we're not complete weight freaks, a lot of customers want the best performance and the lightest weight option. The Featherweight Bushnell EBB is that answer at just 140 grams. This is why it is used as the standard on light-weight tandems and other bikes in the industry.
The eccentric itself is just a solid block of aluminum that the user rotates in the bottom bracket shell welded into the frame. The frame builder welds nuts onto the outside of the frame on the bottom bracket shell. You will use a wrench to drive the set pins through the nuts and into the aluminum EBB. Once the hardened steel pins have created a deep divot into the softer aluminum EBB, it's very hard to make a fine adjustment because the pins always try to turn the EBB right back to the divot.
Aside from being very difficult to use, the Mark design has very little surface contact so is the most likely to slip out of adjustment. This is because the amount of contact bewteen the frame and the aluminum EBB is very little. The set pins drive the unit against the top of the frame's bottom bracket, so the points of contact are about 25% at the top, and the set pins themselves.
As if this is not enough, the Mark design is often rendered useless when a rider over-tightens a set pin and breaks the welded nut right off of the frame. The good new about the Mark design is that a Bushnell EBB usually drops right in and works beautifully.
Here's an example of a custom titanium frame rescued with a Bushnell EBB. Every month we sell several Bushnell EBBs to customers who have the Mark design in their frame. I'm very surprised at how many manufacturers still use the Mark design in their $4,000+ bicycles.
The Good News:
The pinch bolt design holds its adjustment better than Mark because of the fact that there is full contact all the way around the aluminum EBB. It also doesn't gouge 'memory' marks into the EBB so you can more easily make micro-adjusments.
The Bad News:
Other than its relatively heavy weight, there are a few other drawbacks to this design. The biggest drawback is the fact that, like the Mark design before it, the bicycle frame is used as the method to actually hold the adjustment and not the EBB component itself. This means that if the pinch bolt breaks off of the frame or the threads are stripped out, the fix is not an easy one. It requires frame work and re-painting. The next drawback is the fact that since the frame is split at the bottom bracket, an expanding design like the Bushnell will not work in the frame unless frame modifications are made.
I can't remember if it was the late 1980's or the early 1990's, but around that time Cannondale came up with a good design that we started to use instead of modified Mark.
We liked the design because it didn't use threaded parts welded awkwardly to the frame to hold the adjustment, but was a self-contained unit. This meant that the frame was safe from the gorilla type torque that bike mechanics often applied to eccentric bottom brackets.
The design was similar to a handlebar stem with a wedge and a bolt that pulled the wedge into position tight against the bottom bracket shell. There was very good friction between the parts and the adjustment held extremely well. The design was heavy like the others, but all in all was nice looking in the frame, and kept the frames safe.
It was not without its problems though. The Cannondale wedge design required a very specific method to loosen it when you needed to adjust the chain. That technique? Hammering! "Woooh, wait a minute! Are you going to use that hammer on my bike?" was a phrase anxiously hollered by customers watching me preparing to make an adjustment to their expensive tandem.
The design was not intuitive for most bike mechanics either. When you break a stem loose, you hit the stem bolt with a hammer, not the actual stem. For this reason, most bike mechanics who hadn't seen one of these before usually took a good 'whack' at the bolt after loosening it up like they would on a stem. Well, this would not loosen the eccentric, but rather drive the threads right out of the special nut required for the eccentric. I spent many hours on the phone (and still do) explaining to mechanics how to get one of these out of a bike after you've stripped the nut.
With the rise of popularity in single speed bikes and Rohloff equipped bikes (we're the biggest Rohloff builder in the U.S.A.), the eccentric bottom bracket has new life. The Bushnell was originally designed for tandems, and that's why we have a big head start on every other manufacturer of EBBs. The fact that ours was in development for years means that you're getting a product that's tried and true. This goes for any bike that needs an EBB.
We stand behind the Bushnell EBB just like we stand behind everything we make. When a Bushnell EBB customer emails us for help, they're talking directly to the folks that have designed, manufactured and assembled that part. Remember, demand the best for your custom bike. We think you'll agree that the Bushnell EBB is that choice.
If you'd like to read an evolutionary history of the eccentric bottom bracket, click here.