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Cannondale eccentric frozen?

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Cannondale eccentric frozen?

Old 07-24-20, 12:44 PM
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Freerojo
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Bikes: 1997 Specialized Hardrock GX, 2008 Specialized Roubaix Triple 105, 1996 Cannondale RT2000 tandem

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Cannondale eccentric frozen?

Hi, I went to adjust the eccentric on my 1996 Cannondale RT2000 and it would not budge. Bolt backed off easily. I tapped on the bolt to try and loosen with a rubber mallet. The spanner pin holes on the exccentric look a little rough. I have a Park Tool Green spanner that will fit but, because of the previous wear mentioned earlier, I notice the spanner can slip out when i put some force into it. Would it be wise to spray some PB Blaster into the eccentric and wait a week or too for it to do its stuff? I suspect the eccentric hasnt been touched in years. Thanks Ron
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Old 07-24-20, 05:04 PM
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Leisesturm
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WD-40 and 40 minutes (or less) should do it. In any case I can't see it taking a week!
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Old 07-24-20, 05:19 PM
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WheelsNT
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The answer is here: https://hobbes.ucsd.edu/tandem/maintenance.faq

I finally broke the code, it’s an http, not https link!

For my single-wedge Cannondale eccentric, this was the complete and total answer. I now carry the bolt and washers on the bike, just as the author encouraged.

Here’s what it says:

****************************

----------------------------

Cannondale eccentric adjustment and overhaul.

1) Before starting

Inspect the timing chain and the chainwheels for excessive wear; clean and replace as necessary.

2) Loosening the wedge.

(If you are going to be overhauling the bottom bracket, remove the crank arms and the cartridge first.)

The Cannondale uses an expending wedge eccentric, similar in design to a handle bar stem. The bottom bracket shell is a featureless aluminum cylinder. The bottom bracket axle is mounted in a aluminum rod whose outside diameter is slightly less then the inside diameter of the shell. The axle is mounted off-center ("eccentricly"), so that rotating the holder (the "eccentric") moves the axle fore-and-aft (and slightly up and down). A diagonal cut is made through the cylinder, producing a wedge shaped piece. A nut is mounted in the wedge, and a countersunk hole is drilled through the eccentric from one end through to the wedge. A bolt is inserted into the hole and threaded into the wedge nut. When the bolt is tightened, the wedge is pulled into the bottom bracket shell, causing the effective diameter of the eccentric to increase, wedging it into the shell.

The common and wrong way to loosen the wedge is to back the bolt out so that the head is exposed, and pound on it with a hammer.

To loosen the wedge the right way, you need a short bolt with the same threading as the wedge bolt and a stack of a couple of washers, of increasing sizes. The bolt, with the stack of washers, is inserted into the eccentric from the wrong side, i.e., into the big end of the wedge (on the right side of tandem), and threaded into the wedge nut. The largest washer should be large enough to bridge across the large end of the wedge from the bottom bracket shell to the eccentric. Now, when this bolt is tightened, it gently pulls the wedge free. When the wedge is free, remove the bolt and washers, and put them in your on-bike tool bag.

The next step is to show this to your local bike shop mechanic, so that he stops pounding on frames with a hammer.

3) Overhaul

If you're just adjusting the tension, skip to "4) Adjustment".

If you're overhauling the bottom bracket, remove the bolt and wedge, and slide the eccentric out. The wedge nut can be shaken out of the wedge. Clean the inside of the shell, and all surfaces of the wedge and eccentric. A rag should be sufficient to wipe them down unless them have some especially icky stuff on them. Inspect the inside of the shell and all surfaces; they should be smooth; any illregularities may make adjustment difficult.

*** The following advice about lubrication is based on my personal experience and observations. I do not know what the official recommendations of Cannondale are. ***

Put plenty of lubrication on when reassembling. The forces trying to change the eccentric alignment during normal use are small compared to the gripping power and leverage of the wedge. I use any available "anti-seize" compound designed for normal temperatures; a high quality grease will work well also. Insert the eccentric (the right away around; the wedge will go in on the left). Reinsert the wedge nut into the wedge, and skootch it around so it's in the right position (you should be able to see the threads of the nut centered in the hole that the wedge bolt passes through). If it won't stay, a dab of grease may help hold it in place. Insert the wedge. Insert the wedge bolt (from the right side of the tandem) and get it threaded into the wedge bolt.

4) Adjustment

Tighten the wedge bolt until it's just finger tight. To turn the eccentric, you need a tool, a "pin spanner," available from you local bike shop, or good auto parts store. The tool is a U-shaped rod. The ends of the rod are bent 90 degrees, forming two parallel pins about 2 cm apart. The pins fit into two holes on on right side of the eccentric, one on each side of the head of the wedge bolt. You should be able to turn the eccentric with the pin spanner easily and smoothly. If it's not smooth, overhaul the eccentric, and figure out why it's not smooth. If it doesn't turn easily, it may be underlubricated, or you may need to loosen the wedge up some.

(When doing a field adjustment, you can use a small allen key to turn the eccentric. The short end of the key goes in the pin hole, and long end is braced against the allen key in the wedge bolt.)

If the timing chain is off, thread it on, turning the eccentric if necessary to make it easy.

I have marks scratched on two timing chain links, and on the timing chainwheels so that getting the phase correct is a snap.

Turn the eccentric to tight the timing chain, tight the wedge bolt (Cannondale has a recommend torque, but I don't know what it is; I would describe it as snug when using a standard small allen key. I would be cautious about overtightening, I suspect that the wedge can generate a lot of force).

When adjusting the chain, remember the chainwheels are neither particularly round or centered. The chain tension will vary considerably as the chainwheels turn. Find the "high spot" and adjust the chain tension there; and check it when you're done by looking again for the high spot.

How tight? I've heard two schools of thought. First the "tight" school. Grab a link midway between the chainwheels. It should not have any for-aft play, and about two cms. total vertical play. The other school is the loose school; it should be just loose enough so that it can be remounted without tools if it comes off. For racers and offroaders, I'd probably recommend the tight school, for tourists, the loose school.

After the eccentric has been adjusted, you may need to check the seat height, and the adjustment may cause the pedals to have moved positions.

Charles A. Clinton

**********************************
And whoever Charles Clinton is, we all owe him a big vote of thanks.

WheelsNT

Last edited by WheelsNT; 07-24-20 at 05:46 PM.
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Old 07-24-20, 06:35 PM
  #4  
Freerojo
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Thank you for this. I will spray some PB Blaster to get things going.
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