Twosday bottom bracket spec?

[Chuck Anderson]

I bought an ancient Twosday from someone here a few years ago and recently enjoyed the fourth 2 week summer tour with my daughter, now 11.

Hopping on my regular bike at home a few days later, I was struck by how magically easier it is to pedal, with seemingly *far* less friction, and no squeaking.  I guess that implies the tandem's bottom brackets (I think both are squeaky) are overdue for replacement.  I'd like to try and tackle this myself.

Question: is there a way to be reasonably sure of the replacement bracket size beforehand, or do I really need to remove the old ones and see what they are?  I assume it's a JIS square taper, but how do I know if it's 68 or 73 mm and the spindle length?  This is the 3x8 dual drive, so single chainring.

[Eric Daume]

68 or 73mm is the shell (frame part) width. You can measure that on the bike. It will be 68mm, I bet.  The spindle (axle) length of the bottom bracket can’t really be measured until you pull the cranks off. Calipers make this measurement easier. 

Eric

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[John Thurston]

It will be a 68mm wide shell with British/ISO threading. As Eric says, you'll need to pull the arms off to measure the lengths. The stoker and captain spindles may be different lengths, and may be asymmetrical (i.e. more length to the right than the left). This last bit is unlikely, but you should check by doing an approximate measurement from the centerline of the frame.

You will need a puller to remove the arms. The threads will be 22x1mm. Before you use the extractor, make darn sure you've removed all of the nuts, screws, AND washers from the cavity in the crank! Failure to perform this check will cause Bad Things to happen (ask me how I know).

With the arms off the spindle, give it a spin before doing anything else. The bike probably has Shimano UN-5xx bottom brackets in it. Those have good longevity. So before trying to extract them (for which you will need an additional tool), confirm they aren't still smooth and running without play.

All that said, I advise some additional checks before even going that far.

Drop the timing chain and drive chain (no front derailleur, so it's easy once the timing chain is off) and check the play and rotation of the cranks. I suspect you will find them in good condition. Then check the condition of the idler pulleys on the rear derailleur. I suspect you will find they are squeaky or gritty (small wheels puts thos pulleys in the road spray and grime). Also check the chain wear (by measuring the length with a dedicated tool or 14" ruler).

You say it is a 3x8 hybrid hub, but you don't say which model. Sachs with a pull-chain? maybe, but probably a SRAM 'Dual Drive' with push-rod activation. If it is the pull-chain style, it may want a little oil dribbled in through the chain port. I have no first hand experience with the rod-style, but I do not think they want to be lubricated in the same manner.

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John Thurston
Juneau, Alaska

[Chuck Anderson]

Thanks John and Eric, especially for that very helpful checklist!

It's the old Sachs pull chain rear hub.  Like the drum brake-equipped front hub, it rotates pretty freely even if it won't go quite as close to forever as my road bike's wheels.  Ditto the cranks with chains removed: they spin well and with little observable drag, albeit enough that I can put them in any position (e.g. 10 & 4 o'clock) and they'll hold rather than fall with gravity to 12 & 6.  Similar story with those jockey wheels - I can feel their friction is more than zero, but seems too little to consider significant.

And yet in actual riding, it feels like there's more friction in the system than the sum of the parts.  The inescapable conclusion seems to be that my stoker doesn't merely loaf back there, she actively sabotages her dad's efforts.  And she's not even a teenager yet!

Well, I suppose it's also possible some components may behave a little differently in real-world cycling conditions than when gently nudged with an index finger.  e.g. mt intuition says "the small amount of fiction in these cranks would be completely swamped by the much larger force of pedalling" - but maybe that same force (or at least the portion of which isn't applied in a perfect circular motion) mashes the bottom bracket's bearings up against the wall and increases the friction proportionally?

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[Eric Daume]

My experience with kids on tandems is that they are a net drag 98% of the time!

Eric

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[John Thurston]

The free running of your rear wheel you describe is coasting. It isn't considering the drag of the planetary gears. That drag is only present if you are actually driving the wheel by the sprocket. The planetary of the Sachs is oil lubricated, independent of the grease used to lubricate the running bearings. To lubricate the planetary, lay the wheel/bike over, remove the pull chain (unscrew the attached rod), and dribble a couple of cc of oil in. For the Sachs, I use Phil Wood Tenacious. You want something fairly heavy-weight. Lack of planetary lubrication WILL make the Sachs suck energy during pedaling (and not while coasting).

I'm not asserting that the planetary needs lubrication, but if you've never done it, it probably does.

The next thing to look at relates to your observation about the difference between theory and reality. In this case, look at your stoker. When the bike is in the stand, it seems to work ok. But when you add the stoker, you perceive more friction. Regardless of the size of your stoker, you are adding weight and the frame is going to flex. That flex may be causing your timing chain to bind (not likely, but possible). A relatively easy check for this is to ride it without the stoker. Remove the timing chain and the captain's seat. Now the bike is a single (or a 'half bike' in the tandem world) with a really long reach. Does it pedal the same, or differently, than it did with the timing chain and stoker on.

re: crank holding position

That's what we expect. The effect of gravity on the two arms should be identical. We expect the crank (unlike a bicycle wheel) to be completely neutral. The test you describe tells us little of value.

And because swapping bottom brackets is a hassle, requires several special tools, and is (relatively) expensive, I'm going to ask you to keep looking elsewhere.

What tires are you running on the tandem? Please consider the idea that the tires on it are sub-optimal, energy sucking, sausages. There is a HUGE difference in feel and efficiency between garbage and good tires, and the difference will not be indicated by the maximum tire pressure they can maintain. A good tire at 40psi will roll more efficiently than a garbage tire will at 120psi. I'm not going to name and shame. I'm just asking you to consider another mechanical source for your perceived drag.

John Thurston
Juneau, Alaska

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[Gregory Behie]

Chuck,

Did you check the tension on the timing chain? I have found that when reassembling my Tuesday after a trip, that if I don't tighten the frame bolts properly, the weight of two riders, plus gear and the occasional pothole or rough surface can cause the bottom portion of the frame to spread incrementally and cause undue pressure on the timing chain which will make it feel like you're pedaling up a hill.

Greg

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[Squire Black]

If you have ridden for any distance with a timing chain that’s too tight, your bottom bracket bearing are likely shot.  Easy to check once you have removed the chains. 

Andrew

[Chuck Anderson]

Thanks again, John.

The hub is overdue for attention even if the previous owner rebuilt and generously lubricated it hours before packing it up to ship my way, and I'm guessing that's unlikely.  I'll try that in the next few days.  Easy to imagine that could be the main culprit.

Timing chain seems to have plenty of slack, and is very easy to slip on and off with the bike at rest.  Though the half bike test sounds fun, if I find I can reach the brakes from the stoker's saddle.

I've had that experience with tires: bought a touring bike years ago which came with exceptionally slow-rolling tires and was shocked by how much difference swapping them out for faster rubber made.  Doesn't seem to be the main bottleneck here though: a sidewall gash on our recent tour meant replacing a Big Apple with the only 20" tire I found in Mont-Tremblant that morning, a knobby Specialized Roller which (despite the name) should have a fair bit more rolling resistance.  But I didn't notice a difference in speed/effort, only in noise.

I'd note in passing that a bottle of that Phil Woods goo costs about 50% more than a UN-300... but of course that has no bearing (1) on whether I should replace bottom brackets if the problem lies elsewhere.  🙂 

(1) pun unapologetically intended

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[Chuck Anderson]

Decided to replace the knobby rear tire with another Big Apple, and once that arrived and I had the wheel off to change it, I also fed the hub some oil.  Should've thought to do a before & after ride, but even without having ridden this tandem for a couple weeks, the improvement is pretty striking.  Gone is the sense that I really should be going faster if friction somewhere in the system weren't eating up a significant portion of my effort.

I'll have to wait for the kiddo to return from summer camp to see how well that holds up with both of us pedalling.  If that slows the bike back down, I'll be back to ask how to oil a stoker.

John, I'm so grateful you mentioned "unscrew the rod attached to the chain" in your instructions.  Mine seemed to pull right out - it had been shifting properly so I guess the threads must've been engaged, but by so little that I wasn't conscious of having uncrewed it on the way out - and it wasn't immediately clear how to reattach it until I circled back to your email.