The mode is press fit: t ighten the bolts to install. Then you install your 24mm integrated spindle crank, adjust the fit—you may need spacers depending on what crank you have—tighten all bolts to within the correct torque tolerance. Also check out our T47 Eccentric BB. It might be just up your alley! Getting an answer to that involved looking at what they are designed to do and then what they actually do.
Eccentric BBs perform two functions:. When you rotate an eccentric bb there is both horizontal and vertical movement since the unit moves in an arc. On a single speed we take advantage of this horizontal or forward and aft movement. You loosen the tension bolts built into the eccentric BB, easily rotate the unit until the chain or belt reaches the right tension.
Then tighten the bolts. The tension is maintained just as you like it. The added convenience factor is that you can do this anywhere with a minimum of fuss. No turning your bike upside down. On a mountain bike we take advantage of the vertical movement. With an eccentric bottom bracket you are able to quickly and easily adjust your bottom bracket height. What difference does it make? Any at all? On the subjective side of things, the answer is it depends on the rider.
Every rider also has a certain level of physical adaptation to the mountain biking discipline. Add to that again the trail conditions rocky, flat, or somewhere in between means there is no single definitive answer to the question. If you are interested in the nuances, a wide ranging discussion of all the issues is available on this MTBR Forums thread.
Check out the video for a detailed demonstration of how to remove your existing bottom bracket and install the model we examine in the rest of this post. For the most comprehensive coverage on removal and installation of a square taper bottom bracket check out this article. The eccentric effect is achieved by offsetting the sealed bearings within a pentagonal structure.
Unlike most other eccentric models, the bolts on this model are uniquely positioned. There are no bolts laterally installed across the unit. Three of these bolts are evenly distributed around the outside casing. Each is easily accessible on the casing in between the chainring and BB shell. Achieving very light finger-tight fit is all you need. First up you need to remove the existing bottom bracket. Our Square Taper Bottom Bracket article can help you remove that type if that is indeed what you have.
Loosening the axle nuts releases the chain tension. I should note that this will be the last time you have to touch the wheel nuts if chain tension is the main reason you normally have to fiddle with them. Having loosened the bolts, the chain will be slack enough to lift cleanly off and away from the crank and chainring.
A procedural reminder: the chain will likely scratch the chainstay clear-coat and even chip the paint. Protecting the chainstay is easy. Secure the chain around the chainstay whilst you install the eccentric BB.
With the chainstay protected, dislodge the chain from the chainring. Protecting the seat stay is also important. Remove the drive-side crank. An eccentric bottom bracket is actually an adapter that holds English threaded bottom brackets and allows users to adjust the fore-to-aft position of the crankset to add tension to their chain.
This is done by rotating the off-center or eccentric axle position within the round frame shell to move it forward and pull the chain taut. This type of mechanism has been commonly used by tandem and singlespeed riders for decades and are still popular thanks to their clean aesthetics, ease of use and reliability.
Neither option was really great, as the pressure was only applied to a small area and overtightening could easily strip the threads, bend the frame shell, or gouge the component. This avoided the problems of earlier versions, but still required some hammering to adjust, which could easily cause damage if not done carefully. It used a winged design that applied pressure more evenly around the entire circumference of the shell and could be easily tightened or loosened to adjust positions at any time.
The Patented pat. It does not rely on parts welded to the frame to hold it's adjustment. 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.
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 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 Mark set pin design ebb. Mark design leaves terrible detents in the ebb, and is really not easy to adjust. Benefits: Extremely inexpensive Drawbacks: Harder to use, and not very reliable. Modified Mark uses a 'pinch bolt system'. Let's step forward to or so I can't remember if it was the late 's or the early '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.
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