Cannondale Tandem BB Clunk ??

[MyPC8MyBrain]

We have a Cannondale MT800 26" tandem that seems to have a problem in the cranks or BB somewhere. When the Captian's RT side crank arm is at 1:00, a clunking noise is heard as pressure is applied to the arm. The more pressure on the crank arm, the louder the clunk noise is. It will do it on every revolution in the same position.

I took the whole eccentric BB apart, greased and reassembled. [It is the single wedge design] Noise was slightly less for maybe 20 miles, but is back in it's full glory now.

Has anybody, by chance, seen this before ?
What may be causing the issue?
BB is Octalink if that makes a difference.

Thanks !!
Don

[Posting in Bicycle Mechanics board as well]

[DocF]

We also have an MT800. The bottom bracket clunk is a different noise than any we have. Most of ours sound like they are coming from the head set/head tube area. They are more like a metallic version of the noises one hears from an old wooden sailboat than a clunk. I would examine the crank-bottom bracket interface very carefully. Also, can you rotate the chainring on the crankset to a different orientation?

I detest noise calls in my job and I tend to ignore noises on our bicycles. I guess I'm not the best person to answer. :D

[zonatandem]

The first year that C'dale tandems were built in the 80s, some had a very annoying creaking sound eminating from the BB area. Solution was to pour lindseed oil down the seattube! . . . and yes, somehow it worked!

[MyPC8MyBrain]

I'm thinking that I'll swap the pedals from the back to the front and see if it makes a change. They are my next guess. I can feel it in my right foot, so it's something close by there.

Linseed oil down the seat tube ? Interesting...you have to wonder what the guy that first tried that was thinking. I'ts not a creak, or I'd even consider that :)

[tandemonium]

Our MT800 will also make the clunking noise!
The noise is intermittent and may not occur on some rides. I did notice that when it does occur we are usually 2/9 (on our middle chainring and 9 on the cassette -the smallest gear) and pulling a mild grade . Dropping to 2/8 or moving to 3/7 stops the clunk. The noise does not happen at the same position of each pedal stoke. We are both clipped in so it is hard to tell exactly how much pedal pressure triggers the noise. There is quite a feel transmitted through the cranks when the clunk occurs. It feels like bearings that are moving out of position. I can't figure out if it is the front or rear BB or the the stock Shimano fh-hf07 rear hub.

[winbert]

Hello, all. First time bikeforums.net poster, hopefully with some helpful information. I also own an MT800 (2002, size L/S, Octalink, 48x36x26, 9-speed 11x34t) and experience the same clunking from the stoker crank area when in the middle chainring, in the smallest 2 cogs, and applying moderate/high torque. After much investigation, we found the root cause to be the upper chain just barely catching on the big chainring shifting pins. When the tensioned chain slips off the pin and snaps back, the clunking noise reverberates thru the stoker cranks/rear hub/rear frame. I know, hard to believe - I never would have guessed that sound was a chain pong, either.

My short-term, quickie fix was to adjust the rear derailleur outer stop (H-screw) to not allow shifting onto the smallest cog. I figured by the time we're going that fast we should be in the big ring anyway. I still get some clunking from the 2nd smallest cog under high torque, but I either shift to the 3rd cog or we just back off a bit.

A more permanent fix I'm considering is spacing out the driveside stoker bottom bracket cup (you can use cassette spacers) to move the chainrings outboard to try & avoid the chain hitting the shifting pins. A drawback to this is that a longer BB spindle might be required, depending on how far you would need to space it out to eliminate the clunking. Also, we use the big chainring quite a bit when racing off-road, and often use the full cassette while staying in the big ring (since shifting between the middle & big ring is sometimes sketchy under load). The chainline from the big ring to the biggest cog is already marginal, so I don't want to make it any worse. I also considered spacing the big ring out a little off the cranks or filing the shifting pins a bit, but figured that would just make shifting to the big ring even more problematic.

WARNING - rambling now officially begins :-). The other idea I had was to switch to a 1x9 set-up with a moderately sized single front chainring (42t?). This would allow ditching the front shifter/derailleur (and any associated shifting problems/mechanical complexity/weight). And more interestingly (to me, at least), would allow moving the timing chain to the driveside (enabling the use of non-tandem cranks front & rear). I am currently running non-tandem 180mm XT cranks up front, and love the extra leverage! Of course running the cranks on the wrong side of the bike means the pedal threads are backwards, so I had to swap spindles on a set of clipless pedals and locktite the pedals into the cranks to avoid loosening. Moving the timing chain to the driveside would avoid all that nonsense, and allow the use of non-tandem stoker cranks (I'd like to try 177.5 or 180mm in back as well, plus the extra leverage will become more important with the limited 1x9 gearing). I'm hesitant, however, to give up the lower gears. When racing we could just run up the really steep stuff, but when riding off-road with my wife or 5-year-old (child stoker kit) the lower gearing sure is nice...

Hope that helps!
Winbert

[MyPC8MyBrain]

Im gonna look at where the pins lie in relation to the chain approach when i get home tonight. If they pass the chain at 1:00 on the right we may have a winner.

[MyPC8MyBrain]

It was the pedals. I snagged the pedals off one of the otherbikes around the house and slapped them on the front. We rode 15 miles last night without a hint of a funky clunky.

Cheapo Wellgo pedals.... Guess I'll have to buy a couple sets of pedals now :)

Thanks for your input !

[tandemonium]

I've had different pedals on our mt800 and the clunk remains elusive. It feels like a bearing to me. But which one?

[MyPC8MyBrain]

I would check the crank arm bolts, and the eccentric bolts too. You can also see the post above by Winbert regarding the chain rings...might be there. I wouldn't think that a bearing would clunk. Seems like bearings make more of a grinding up or growling sound.

Good Luck! Seems like tandems have lots of places for these things to hide :)