Nacho.

hi all,
Iīm building my first tandem, and my plan is to build it with 73mm BSA shells front and rear. So, no excentric BB, as I donīt find them particularly appealing. Somehow I need to tension the front chain.
Has anyone tried this with excentric BSA Shells? (which are also not nice) Or any other solution to tension the front chain? Somehow I hope to find a perfect match of chainrings and boob tube length but itīs probably unrealistic.
Thanks!

Nacho.

unterhausen

I'm sure someone has done it, but probably most of them wish they had just done an eccentric bb

Andrew R Stewart 

"A perfect match" will wear and chain length growth will result in a loose chain in time.

I don't know what is meant by "Has anyone tried this with eccentric BSA Shells?" In my understanding this is what many tandems (and some single speed one person bikes) use already. Every tandem I have serviced (a few dozens over the years) use an eccentric front shell. Now how the eccentric is secured in place does vary. Binder slot and bolts is the most common but set screws, expanding wedge and expanding "drum brake like shoes" are also used.

What about the eccentric method do you find unacceptable?

There are two other methods to tension the timing chain that I know of. One is found on cheap single speed (usually with a coaster brake rear hub) tandems. A pulley is mounted on a bracket that has a slot. The pulley is mover up or down within the slot to provide the adjustment of tension. These don't do well with strong riders as the bracket is often bent from the forces the chain is seeing. The other is to place a chainring in the chain's loop about halfway along it. The ring will stay in place and the chain will run over/under the ring. This is also a poor method with strong riders as the twisting of the frame can allow the ring to "derail" and fall out of the chain's loop. Andy

JohnDThompson 

Another option is a "floating" chainring for the timing chain:



I suspect it could fall out if/when you hit a large enough bump.

Personally, I'd go with the standard tandem eccentric bottom bracket. There's a reason these were invented.

Cynikal 

You could use a PF30 BB shell and one of these https://www.squidbikes.com/collectio...bottom-bracket
But I would assume that a standard eccentric BB shell is better for the needs of a tandem.

bulgie 

I've never heard of anyone who stuck with that "ghost ring" method long-term. Everyone I know who tried it gave up after the ring fell out. No bump needed; just hard pedaling can do it.

In addition to the Columbia, Huffy, Schwinn Twinn method of a single pulley on a brazed-on bracket, I've seen one French lightweight that used a derailer cage with 2 pulleys, and the normal derailer cage spring to put some tension on the chain. I think that would be reliable, but slightly more friction from the pulleys. The eccentric BB method is super efficient (no added friction) but weighs more and you have to remember to tighten it as the chain wears and gets longer. Riding with the chain loose has caused crashes. The chain generally derails when you're sprinting, so having your foot fall out from under you at maximum sprint often causes a big swerve. I watched an experienced team swerve all the way across the oncoming lane and almost off the road on the other side, luckily no cars were coming and no other bikes were right next to them at the time. The sprung derailer cage method would mean never having to adjust for chain wear, maybe a safety feature.

Fun fact, the bike I saw that on was a front drive, i.e. chainrings on the cap'n crank with an extra-long chain going all the way to the rear wheel. So the two-pulley cage thingamajig was on the upper run of the timing chain, which looks weird but it's correct. With the typical rear-drive, the slack side of the timing chain is the bottom, and that's where you'd put a tension pulley or a two-pulley cage.

But of the couple hundred tandems I've made, 100% had eccentric F BB. It's the only way I'd consider.

But I'm old and retro and out of touch, so maybe there's some newer way worth looking at, I wouldn't know.

Mark B

unterhausen

I have to remind myself there are advantages to people trying things old retrogrouches have seen a couple of times before which didn't work. Sometimes the new guy gets something to work because things have changed! But I feel like if someone is going to repeat history they should know that's what they are doing.

My favorite example is the Sachs wheels that had the cassette that stayed with the frame. Which Cinelli reinvented. Then 10 years ago they were reinvented again. Then 8 years ago, someone reinvented it again. It makes me wonder how many times between Sachs and now it has been reinvented. It always worked, but it never lasted. Well, Cinelli might have invented the through axle, which seems like it's going to stick now.

Darth Lefty 

I think the cheap style tensioner is rigid, not sprung. (Check me if I'm wrong.) If it's sprung then it's going to be a problem if there's ever tension on the lower run. Timing chains can derail. I've had my kid manage it with his stoker crank. The chain doesn't have to be tight, but tight enough that single speed teeth aren't going to let it off. It has to be better than a single speed bike just because there's enough more length to sag. The PF30 adapter might just not have enough travel.

I do think 73 on both ends is smart, or 68 front / 73 rear, with right side timing. That lets you use a Boost rear and e-bike wheels and the whole ecosystem of Race Face Cinch to get whatever chain lines you need. You could even give the captain a road crank. Left side timing made way more sense with road bike stuff. MTBTandems uses Middleburn cranks

bulgie 

I don't know what boost or Race Face Cinch or Middleburn mean, but that sounds well thought out, you might be onto something. I'm just ignorant there.

There isn't normally any tension on the slack side, unless the stoker and cap'n try to fight with each other, one trying to make the other one pedal. I guess that could happen, and make the tight side of the chain go slack with a sprung pulley. So yeah, a fixed pulley like on a Huffy solves that problem. The sprung pulley definitely wouldn't work with a fixed gear and no brakes, like a track tandem! Anyway I've only seen the sprung pulleys idea used on one bike. That team liked it but they probably learned early on not to fight with each other about whether to pedal or not.

About how much eccentricity you need: The offset (let's call it "e") needs to be 1/4" (6.35 mm) or greater if you want to be able to use any size chainrings with any length of keel tube.
Back in the '70s some commonly-available eccentrics had e<6 mm, which meant that certain lengths of keel tube required you to use odd-numbered rings. Doesn't matter what size they are, only whether even or odd-numbered teeth, since chains have 1/2" pitch.

Santana found that out the hard way, made a tandem with a 25" keel and found that even-tooth rings wouldn't work (without a half-link), so they changed the rear TT to 25-1/8" for their production models (Keel and rear TT are equal if and only if the two ST angles are equal. I often designed with different STA, front vs rear)

My old tandem frame design spreadsheet had a field for e, and then it would tell you if you were in the danger zone, the "bad" keel tube length that doesn't take even rings. But later, all the available eccentrics wised up and had e>6.35, so the issue was moot. I mention it only as a historical oddity, or if you run into an old eccentric in some old builder's junk drawer.

Remember chain wear makes such a long chain get longer by more than you're used to on a single bike (or the rear of a tandem). It's a lot of delta to take up. The timing chain needs to be kept tight to avoid derailing, which is more than annoying, it can cause a crash. The chain only gets looser in use, never tightens (even from pedaling -- we're talking about the lower, slack side here) so there's no need to adjust it with some slack. Just tighten it, short of where it starts to bind or add friction. and keep it tight as it wears. That's why you shouldn't start with the eccentric adjusted forward of center with a new chain -- you want all that available adjustment range to be in the tightening direction.

Your binding mechanism, whatever locks the eccentric adjustment, needs to be easy to work with even on the road. The Bushnell is good, or the traditional two pinch bolts with a slit shell. Give yourself a large hole through the insert that'll take an allen wrench, or any shaft really, screwdriver or what-have-you. Then you can bring the crank arm to bear against that shaft for rotating the eccentric in the frame to tighten the chain. (Pad the shaft with a cycling glove or some such, to keep the shaft from making a dent in the soft alloy of the crank.) Since I use pinch bolts that take a 5 mm allen, I make a through-hole in the insert that the 5 mm allen fits through. Handy, since it's already in my hand.

There have been some truly bad eccentric fixing methods that gave eccentrics a bad name. Two to avoid:

1. Setscrews that bite into the insert. (Santana did this on some early bikes, to their eternal shame). The setscrews make divots in the alloy, and then when you try to make a slight adjustment, it wants to creep back into the existing divot. Inept and lame.
2. Wedge-lock, like on some Cannondales. Made people hate this otherwise pretty decent tandem. Like a quill stem, you had to knock the wedge loose to get the insert to move, and on many, after a little grit or corrosion bound it up, it was almost impossible to do, even in the shop let alone on the road.

Bushnell's insert uses a type of wedge-lock but I've never seen one get bound up, they seem pretty reliable. Genius design. Too complicated for my taste though, so I still favor the tried and true two pinchbolts on a slit shell.

I bet that's more than anyone ever wanted to know about eccentrics.

Mark B

duanedr 

"My old tandem frame design spreadsheet had a field for e, " How many people in the world, EVER, could start a sentence with that?!

Andrew R Stewart 

Nacho- Have you decided what to do? Andy

Nacho.

Hi John, yes Iīve seen this and it could potentially fall out.

Hi Andy, sorry for the not-so-accurate description. Iīm referring to the BSA eccentric shells, like the one from TrickStuff (here in europe) and there are a couple of others in the market. These normally have around 6mm of adjustment, some up to 7mm.
My biggest problem is that in order to have an eccentric BB shell Iīd need to machine an adapter for my jig. Easy if you have a lathe, but I donīt. Also about the looks I just donīt like the off proportion of a chunky bb, but thatīs just personal taste.


Yes, thatīs exactly what I was planning. Right side timing, and in this case Easton (same as RaceFace) I really like the Cinch system, and I think it would fit to the build very good.

Hi Mark, thanks for your input!
I didnīt know the Bushnell BBs, they seem pretty reliable. Iīll do some research on that, in case I go with classic BB, which is what everyone is recommending!

Thanks everyone for the info and the comments!

Darth Lefty 

People are now using NW rings for timing which seems like it should reduce the amount of perfection asked of chain tension, but require inch increments for the stoker compartment. Looking at a few, Ventana's are all 29" and Salsa Powderkeg was 29 or 30

bulgie 

I've never used NW timing rings (because the last tandem I built was ~25 years ago), but as soon as I heard about them, I immediately thought tandem timing chain would be a perfect application.

But I don't see why they add more restraint on keel tube length. If you have enough eccentricity in your eccentric, then any size ring should still work with any keel length. And all modern eccentrics have enough "e". (It needs to be 1/4" or more, for a 1/2" range of adjustment)

Or am I missing something? Seems to me the only thing you can't do with NW rings is a half-link. Or odd-numbered teeth.

Note, changing the subject a little, I didn't mention previously on the pros/cons of eccentric: It will change the cap'n seat tube angle a little. For those who are fanatical about their STA or setback, when you adjust the eccentric through its range you can get about a half-degree of change off the theoretical STA. More angular difference for short riders than tall ones. I never had a customer notice that, let alone complain about it, but I must admit I never pointed it out to them. Why poke the bear? A lot of the ideas people have about setback are based on lore and wanting to be like Eddy, and almost no one needs to keep their setback within such a tight range IMHO. A lot of people think they're The Princess And The Pea but couldn't actually feel the difference in a blind test. But if you truly are that sensitive, then use an alternate method of tightening the chain

Mark B

Darth Lefty 

They force the outer links to sit on the bigger teeth, in the same way on both rings. So between two rings in phase you need pairs of links.

Unless I'm missing something which also seems pretty possible.

If you have a 4 arm spider and 2n+2 teeth then you can mount a ring 90 degrees off.

I looked at a few more websites. This fat Calfee is set up pretty much like I'm describing although I imagined a larger outer drive ring and smaller inner timing rings so the chains wouldn't interfere in high gear. Calfee website says they have 28.5 in boom tubes. In this photo it looks to me like the stoker crank is leading. I've been clicking around pictures on MTBTandems' Facebook page and this lead seems really common




This one is lagging. Or am I imagining it?

Andrew R Stewart 

WRT in phase cranks and the slight off phase they can be- I spin faster than my wife (when on singles) and like to have the pedal slightly higher up off the ground when pushing off (we both dismount for stops) so her/stoker crank slightly leads the captain's.

My first tandem had little 36T timing rings (on the LH side), classy but flexy TA cranks. My current tandem runs 42R rings and this feels better to me. Maybe biased because I was told long ago larger timing rings are more efficient. Andy

unterhausen

I don't think anyone would notice the cranks being a half link out of phase. I tried 90 degrees out of phase and didn't like it at all. For all I know my tandem is set up sloppily enough that they are a half link out since I just eyeballed it.