mburchard

Will be building up a new tandem next week, during my search for other than 105 Octalink BBs had someone suggest running a triple BB in the back (118.5mm) and a double BB (109.5mm) in the front, suggesting that a few mm doesn't make a difference with the long timing chain, and the narrower spindle puts the pedals where they would be on my single.

I was planning to go with 118.5s front and back, but now curious if any of the experts on this board also think it's a better idea to use a narrower BB in the front?

TandemGeek

The difference in sync chainline offset is less than the width of a chain... so no big deal there.

As for putting your pedals where they are on your single bike (matching the tread or Q-Factor), does this imply that you are using the same made/model cranks and same width BB on the tandem that you have on your personal bike?

Or, if you're using different cranks on the tandem, have you already calculated the resultant tread / Q-factor of the candidate tandem cranks & 109.5mm BB spindle to establish that it's the same as what you have on your personal bike?

mburchard

Single bikes are DA 7700 and 7800 (doubles), new tandem will have Santana carbon cranks (made by FSA, I think). Haven't measured everything, so don't know if DA and Santana cranks are the same/similar. They are at the LBS, I guess I can get them to make some measurements.

TandemGeek

You'll definitely want to measure everything...

Santana uses VERY wide spaced rear cranks to correct the chainline on their 160mm rear spaced wheels in combination with a special front derailleur clamp called the "far-out" that also moves the front derailleur cage further outboard. I could be mistaken, but I suspect you'll be hard pressed keeping the same Q factor on your front cranks given how wide the rear Q will be on the Santana.

Are you very sensitive to Q-factor or are you just trying to eliminate variability? If you're not positive that Q-factor is a major factor in your mechanics, I'd be inclined to compromise a bit there. Also, have you confirmed that crank length will be the same? Most tandems, including Santana, use 175mm front / 170mm rear as their default crank lengths. Not a big deal if you already ride 175mm on your solo bike, but if Q-factor has you concerned I'd check on that too.

ricardo kuhn

My Ibis tandems (I have three of them over the years... cousin-it/uncle fester, MTB type 26") came with 126MM bottom brackets with XT type cranks (175MM front/170MM rear) and a 56T big chainring, to me the clearance with the chainring/chainstays (135MM rear end) is the most critical issue, but with the 126MM BB I was even able to run a 62 Paragon machine works downhill chainring (Yes.. shifting is horrible even with a none microdrive Suntour XC pro front derreileur but the bike goes like hell)

Anyway I install 121MM Philwood BB with adjustable cups for the back cranks, moved about 1.5mm to the right to gain sufficient clearance for a 54 T ring (not much road riding on this machines), on the left install the Timing chain ring on the "Inside" of the cranks were the middle chainring will go and then install a 116mm BB for the front and install the chain ring on the "Outside" were the big chainring will go, this represents a gain of 10 milimiters in Q factor terms, but more than that much better ground and side clearance when on the trails specially since I run shimano SPD downhil pedals that are super wide (the big red ones).

I take pictures of the setup soon, including the bash rings and everything (camara broken at the moment)

Here is how it looks before just for reference.


on the new set up the frontal bash plate seats on the inside of the cranks, next to the frame and actually makes the bike more stable (closer to the center line of the frame) than before when sliding down rocks and woods.

In short I will say the narrower you can run your cranks (actually pedals, but you need narrower cranks) the better (frame design permit, specially wide chainstays like yours), but I don't see any reason why the front cranks need to be as wide as the rear ones (other than stetic, after the chain is align perfectly) to optimize Q factors as much as possible and also gain valuable ground clearance.

ricardo kuhn

Yeah a extreme example, but you get the idea.




Just look at the way the rear Bash guard is on the outside (like is soppose too) and in the front is on the inside really close to the frame.

mburchard

Good stuff folks, thanks.

LBS informs that Calfee frame has 118 BB shells front and back, while Santana has 130, so thinking Q factor more influenced by BB than crank design. But will check!

ricardo kuhn

Older style MTB cranks like the Deore's you see in so many tandems from the 90' used to build with really wide Q-factor, I don't know if was the casting/forging technology or just a trend of having really "Paralel" arms that came all the way from the center of the crank to the pedal mount.

Over the years the Q-factor became narrower and narrower, when the cranks became more "Diagonal" and the difference in between the crank bolt area/spider and the pedal hole became more notorious.

What you really need to look for on a crank (maybe even before you purchase the cranks) when selecting a Bottom bracket are.

* Optimal chain line, usually in basic terms the middle chainring is soposse to be aling with the middle of the cog set, in the Santana's case the chainring need to seat farder away to cope with the wider hub and location of the cogset..

* Chain ring clearance Depending on the shape of the chainstay and the location in space of the spider, you can only go so close to the center line of the bike before the chainring touches with the chainstays, specially the big (56T,<, etc) and middle ring, since the chainstays are usually taper in that area coming from the bottom bracket shell..

Again certain cranks have the chainring spider, closer and/or farder away from the crank/spindle junction, so that distance is relative to the cranks you are using.

* Crank touching the chainstays...
Many cranks end up touching or at least being really close to the chainstay at the pedal end, even if you have about 2mm clearance the cranks and the frame can flex such a amount..

Personally I have grind many of my cranks in a shallow angle at the pedal end (about 10/15 degrees) to conform to the shape of the chainstays so I can run narrower bottom brackets for many years (since the 80') with out any issues at all. (except cosmetic ones on the colored anodized cranks)

Anyway Still puzzles me why the two bottom brackets need to be the same size on a tandem if the frontal chainring is not bound by any of the rules mention above, except for the obvious requirement alignment of the timing chain, but then again that can be solve by using a "staggered" chainring like I do on my tandem.

TandemGeek

Your new thread on the Calfee tandem finally broke the code. When I read your earlier post I wrongly assumed you had picked up a Santana.

bikeriderdave

FWIW, I'm sure that it's 68 mm for the Calfee vs. 73 mm on the Santana. Doesn't necessarily matter. Some TruVativ ISIS BBs are designed for use with either. They fit as-is in a 73 mm (MTB pseudo-standard) shell and require a 5mm spacer/washer when used in a 68 mm (traditional road) shell.

mburchard

Yeah. Calfee frame, but LBS planning to put on a set of carbon cranks he got from Santana, which are Octalink (and maybe made by FSA, I am not sure). Managed to find a couple of new Dura-Ace BBs in the correct size, was not easy, I guess they have not made them for awhile.