embankmentlb

How often do you adjust your timing chain? Our tandem has 800+ miles & i just adjusted the chain for the 2nd time. It seems i just adjusted it about a month ago but the chain jumped off unloading the bike from the car. Is 400 miles about normal? It's possible the concentric slipped over time. Thanks.

TandemGeek

As often as necessary to make sure there's not too much slack. Too tight isn't a big deal, but too loose is.


Not all that unusual as the bedding-in process will always make a chain 'stretch' at a far more dramatic rate when it's new as compared to the subsequent miles.


Yes and no. If you torqued it up to the manufacturers spec (yeah, I know... what the heck is that spec?) it should hold. However, given your recent visit to Tennessee and Metcalf Bottoms, I don't find it all that unusual that you had to make a post-rally adjustment. 20% grades will test the holding strength of just about every part of your tandem's drive train so a post-ride check on events like that are a pretty good thing to add to your mental tandem maintenance checklist.

embankmentlb

TG, thanks for the advice. With each event, I realize i have so much to learn about these machines.

merlinextraligh

Timing chain? Why would you use a chain for that?

zonatandem

When needed.
Usually about every 5,000 to 10,000 miles for us.

TandemGeek

Because a Burley is too short for a Gates Carbon Drive sync belt & pulleys.

uspspro

Good point TG.

I find our eccentric may slip a little after a ride with the really really steep stuff.

embankmentlb

I have now marked the concentric with paint so i will know if it slips. Putting a carbon drive belt on our Burley would be like putting Z rated tires on an old air cooled VW bug.

Monoborracho

After returning home from the TTR, I somehow "pushed" the timing chain off of our Burley as I was putting the bike into the workstand to clean it up. Of course it "pushed" right back on when I put it in place and turned the cranks. Then I "pushed" it off again to get the cranks back in phase and then rolled it back on.

I haven't tightened the eccentric.

We've done a couple of 30+ mile rides since we got back home last Tuesday, and now I've got an irregular clicking /creaking noise that is either the eccentric or the headset. The headset doesn't intimidate me, but I've never really worked on bottom brackets, let alone eccentrics, and I suspect the eccentric is where the noise is located.

Did I mention the nearest bike shop is 70 miles away?

Any advice where to start? I've a torque wrench and don't mind buying tools.

embankmentlb

It may depend on the type BB you have but our Burley recently had a clicking/popping sound in the BB. It was simply a BB lock ring that had loosened. You may wish to check that also.
Monoborracho, are you the team we met from TX?

Monoborracho

Yep, we were the other half of the Burley Bunch.

embankmentlb

Apparently TTR was unusually tuff on orange Burleys Adjusting the eccentric is very easy (if it's the same setup as ours). Loosen the two lock nuts on the shell & rotate the eccentric with a pin wrench. I bet you could do the job without the pin wrench because the eccentric is loose enough to just side out of the shell once loosened. I hope this is of some help. Nice to meet you at TTR!

DKMcK 

Here is a link to the technical specs for the Cannondale eccentric. It may not be the same as the Burley, but it should give you an idea on what you're working with. It's really a fairly simple concept.
https://www.cannondale.com/asset/iu_f...plement_en.pdf

Monoborracho

Thanks DK. Just got ordered a couple of pin spanners. I think I have the tools to pull the crank arms if I need to change the BB.

The Burley eccentric differs somewhat as it has two set screw on the top of the bottom bracket that hold it in place. The chain tightening procedure as shown in the article will be helpful.

Thanks again.

Aeroplane

New tandemer here, but I've had an eccentric on my SS MTB for quite a few years. A technique I like to use for making sure my chain doesn't get loose is to make sure that if the eccentric does loosen up, it gets tighter. If you are running the BB on the front of the eccentric, make sure it's on the higher part. If you are running it on the rear of the eccentric, make sure it's on the lower part.

Another way to think about it is this: If you divide the eccentric into quandrants, and look at it from the drive side, the BB should only be in the first or third quadrants. This way, if the eccentric slips, the bb will move further away from the stoker cranks, tightening the chain. I hope I didn't explain that too obfuscatedly.

WebsterBikeMan

From the Gear to Go website:Now, I'm neither endorsing nor contradicting Bill, but trying to understand just what it might mean to be "designed to have the front spindle rotated through the lower half". If there really is something to it, I shouldn't be putting the eccentric into the first quadrant, as you suggest. On the other hand, I'm not sure slippage will necessarily be in the direction of gravity rather than in the direction of loosening the chain - depends on which force is greater. As I am looking at an timing chain in need of tightening with an eccentric right about at 6 o'clock, I'm right at the point of deciding to go with "first quadrant" or trust Bill. As it is a Santana, I'll go with Bill at least for now, although your argument does make me think...

merlinextraligh

Another factor is pedal strike. Putting the eccentric in the upper quadrant raises the cornering clearence.

Aeroplane

If your tandem is so precision fitted that moving your center of gravity up 1/2" throws off all your stability, by all means, avoid the first quadrant. Personally, I wouldn't be able to tell the difference (but mine is a clunker anyways).

andr0id

Yeah, there's a lot of folklore around setting both fixed gear and timing chains with slack so that you can spin them freely with the wheel off the ground or the drive chain removed.

This doesn't make much sense when you think about it. As soon as you press your foot down on the pedal with 100 or so lbs of weight on it, the chain tension goes way higher than I could ever get it with a pin wrench. I just make it pretty tight with no binding and leave it at that.

Carbonfiberboy 

I adjust captain's saddle height whenever I adjust the timing chain. 1/8" difference in saddle to BB height makes a noticeable difference in my pedal stroke.

I had a clicking sound that I thought was BB or headset. BB and headset seemed tight, so I removed and lubed the captain's seatpost. Very annoying problem fixed, no money spent.

TandemGeek

Hmmm. I'm not sure that passes the sanity check for tandems.

At least on tandems, eccentrics tend to 'slip' when the amount of tension generated along the top run of the sync chain by the captain exceeds the holding power of whatever type of retention system is used to secure the eccentric in a fixed position. In other words, under load the front crank axle is always attempting to move towards the rear crank axle and if the eccentric doesn't hold -- and the BB spindle is not sitting well back or well forward in the eccentric shell close to the centerline -- the eccentric will rotate towards the rear of the tandem (period); having the spindle high or low in the eccentric really doesn't seem to make a difference.

When the BB spindle is closest to the back center line of the eccentric shell (~8 - 10 O'Clock) it may still rotate but the effect on chain length isn't all that great and when it's very well forward (~2 - 5 O'Clock) there's just not as much leverage on the spindle to make eccentric slippage all that common.

As a final note, you can use 1/2 links to mess around with the eccentric position if you're trying to bias the bottom bracket spindle to a position that just isn't supported by the addition or removal of a full link but as noted below and elsewhere in tandem fitting threads, most average tandem captains can easily adapter to slightly sub-optimal crank positions so the need to use a 1/2 link should be reserved for situations where it just seems like the sync chain is always too short or too long on a custom-sized tandem.

Meh....

I'm with Aeroplane & Merlin on this one: having the bottom bracket positioned high or low in the eccentric shell this is a personal preference issue where all of the various trade-offs must be considered, e.g., cornering clearance and fine-tuning of fore/aft, set-back and saddle height in relation to bar height seating positions. Having flipped eccentrics around a few times over the years to alter the up / down position of the BB spindle I won't say that you can't notice the difference -- I could -- but only for a few miles, after which I adjusted to the subtle change which always involves all kinds of adjustments to the saddle to compensate for related shifts in set-back, saddle height, bar reach and bar-saddle height. However, the gradual and very small rotations of the eccentric to adjust for wear don't mess me up all that badly until it gets to a point where the chain has worn so much that it's time to change the sync chain vs. putting any serious wear & tear on the timing rings.

conspiratemus

Raising the centre of gravity of a single-track vehicle (like a bicycle) doesn't make it tippier anyway. A bicycle (unlike a two-tracked vehicle that doesn't have to balance) is always inherently tippy. The higher the centre of gravity, the slower it tips and the more time you have to do whatever it takes to stay balanced, but it will also take you longer to make it lean over for a turn. Think of a metronome: for slow (Largo) you raise the weight to the top of the ticker, for fast (Prestissimo) you slide it down as far as it will go.

The best basis for adjusting your eccentric is to maintain pedal clearance. Striking a pedal as you try to power out of a turn scares the crap out of your stoker. Not nice to scare your stoker.

WebsterBikeMan

The higher the centre of gravity, the less stable. What happens as you raise the centre of gravity is that it gets further from above the base for a given amount of tilt. While you may have more time to react, you have to react more. Think of balancing a broomstick on your hand. Most of us have done this. If the broomstick were half the length, you wouldn't have to move so far to compensate when it wants to tip. If that doesn't work for you put a heavily loaded handle bar bag on your bike, ride it, and then take the bar bag off and put heavily loaded front panniers on. You might not want to try this experiment where you'll experience anything technical.

But that said, we're talking a cm or two here. If all McCready is talking about when he talks about them being "designed" to have the bb in the bottom half of the eccentric swing is keepig the centre of gravity low, I'm with TG, Aero and Merlin. I was puzzled as to what, if anything, about the design could depend on having the bb in the bottom half of the eccentric.

Having experienced pedal strike (once, on a single), I'm not interested in going there again. However, at my size (and my son's), being able to lower the stem by keeping the BB in the bottom may well be an advantage. Esp. on the used Santana I just picked up for him to captain.

conspiratemus

Come now. Do you think I would have posted what I said if I had not actually done the broomstick experiment for real instead of just "thinking" of it? If you actually do the experiment, you will find that a long (117 cm) length of 1/2" dowel stock is much easier to balance on your fingertip than a short (14 cm) piece of broomstick. The short stick just falls off your fingertip before you can correct. If you clamp a C-clamp to the top end of the long dowel, (thus raising the centre of gravity even further, à la the metronome) it becomes even easier to balance. Even if you clamp three C-clamps to the dowel in tandem, one clamped to the other, so you have a chunk of heavy metal sticking out to one side of the top end of the dowel "off balance", that makes it easier, not harder, to balance it as you walk around the room with it. The secret is that you keep moving your hand to get the base of the dowel back under the centre of gravity to arrest its tip before it falls. And the higher the centre of gravity, the slower it tips, which gives you longer to judge how far in what direction to move your hand to get it supported again. On a bicycle or motorcyle, you stay up by making the same type of correction: steering in the direction of the tip, which allows the steering geometry to push the CofG back over the tires again.

It's true that it's easier to make your stubby length of broomstick stand vertically on the table top (provided it's sawn off square) than it is to get a long dowel to do that, and it's true that Old West stagecoaches (and big boxy SUVs) with their high CofG's are "tippier" than Formula 1 race cars. But with a two-tracked vehicle that needs to keep its CofG between its wheels, or a broomstick standing on a table, there is no way to arrest the tip once the CofG becomes unsupported and it just falls the rest of the way over. Bikes (and dowels on fingertips -- even dowels sharpened to a point) don't behave that way because they are dynamically balanced, by steering.

I don't like heavy handlebar bags because they jiggle and bounce, which makes the steering feel funny. That's not a CofG problem. But I happily carry heavier loads in my backpack on my commuting bike and don't notice any steering or balancing trouble. But I do like all loads snug so they and me and the bike are all moving in the same direction at the same time....again, not a CofG problem.

I cheerfully concede that the few centimetres we are talking about in eccentric adjustment make no practical difference to CofG either way. I also agree that the choice of "high" or "low" flows from how you strike the balance between pedal clearance (which favours "high") and being able to reach the pavement with the left toe while still on the saddle (which favours "low".)