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Tandem Cycling A bicycle built for two. Want to find out more about this wonderful world of tandems? Check out this forum to talk with other tandem enthusiasts. Captains and stokers welcome!

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Old 03-15-11, 08:35 PM   #1
tiggermaxcocoa
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Da Vinci crankset questions

Hello,

As many of you probably know, I'm building up my first tandem. We've decided to go with da Vinci cranks (I think) and have been in conversations with our builder (R&E Cycles) and da Vinci about what size bottom bracket to pair with the cranks. Da Vinci is saying 103mm in front, and then probably 107-113mm in the back, but R&E is saying that we should use the same for each, and that 116mm works the best in their experience.

I would guess that I should trust the builder here, since they install these cranks fairly often on their tandems. But I'm really confused about this issue as a whole. So I have the following questions:

1) For anyone running da Vinci cranks, what bottom bracket sizes are you using?

2) For a tandem, do the bottom brackets need to be same?

3) Will going to a wider spindle size increase the flex in the bottom bracket?

Thanks!
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Old 03-16-11, 06:25 AM   #2
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I would guess that I should trust the builder here, since they install these cranks fairly often on their tandems.
But then again, I would have to assume DaVinci installs their own cranks on their own tandems all the time. And DaVinci only makes tandems, so far as I know.
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1) For anyone running da Vinci cranks, what bottom bracket sizes are you using?
I don't know, as I didn't assemble my bike, or spec the bottom bracket size. But I can say that where the crank arm attaches to the spindle it is about 2.1-2.3 cm from the seat tube, whereas where the pedal enters the crank arm, it is 5.0 cm from the seat tube. This gives you an indication of the shape of the crank arm, and more importantly the Q-factor that will result from various choices of bottom bracket size.
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2) For a tandem, do the bottom brackets need to be same?
They should be aligned with each other on the left. On the right rear, you need enough space for the chainrings to clear, and the chain, when it's moving between rings, should not strike the chainstays. If you want the pedals symmetric about the centre of the bike that means that pushes the left rear out some, which then pushes the left front out and, again, if you want perfect symmetry, pushes the right front out to match the rear. So yes, if you need perfect symmetry. I don't think we have perfect symmetry, but without taking the bike apart, it's hard to get in there to measure.
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3) Will going to a wider spindle size increase the flex in the bottom bracket?

Thanks!
Yes, but whether you'll notice it or not is a different matter.
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Old 03-16-11, 07:43 AM   #3
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1) For anyone running da Vinci cranks, what bottom bracket sizes are you using?

I believe these are the front / rear combinations we've used on five different tandems that have been fitted with daVinci's cranks. Some of the numbers might be off by one or two digits, as I'm going from memory. All of these are on tandems with 68mm rear bottom brackets and 145mm rear spacing running triple cranks and cross-over cranksets.

113/113
103/108
115/115
115/118
111/118 +5R
108/111

2) For a tandem, do the bottom brackets need to be same?

Uhh. No. And, if you're using a sync chain drive the alignment of the left side timing rings doesn't have to be spot-on. The typical length of a tandem's sync chain will accommodate quite a few millimeters of mis-alignment without any detrimental effect. If you're having a Rodriguez tandem built, they typically use fairly long stoker compartments which means an even longer sync chain and increased margin for less than perfect timing ring alignment. The newer Gates Carbon Drive sync belts are a different story... those do need to be aligned.

That said, I've always tried to use BBs that have two adjusting cups, (e.g., Phil Wood and discontinued Race Face models) which allow for up to 5mm of left-to-right adjustment without resorting to the use of spacers on BB's with right-hand fixed-cups. However, even then I would only worry about getting the stoker's drive-side chain line adjusted for optimal performance and the left-side would typically fall where it falls.


3) Will going to a wider spindle size increase the flex in the bottom bracket?

In theory, yes. Will it be enough for you to detect, not likely. Bear in mind, Santana spec's 129mm spindles on all of it's tandems, and has for nearly 20 years. Those were originally square tapers and are now Octalink. I'm pretty sure Co-Motion uses something like 115mm - 118mm BB spindles and always has. The trick is making sure your spindles are along enough to accommodate your chain rings, timing rings and providing enough heel clearance relative to your chain stays. With Debbie's little feet, we've gone as narrow as 108mm in back without any trouble on tandems with straight stays.
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Old 03-16-11, 07:44 AM   #4
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Whatever you do if you make the decision you are also assuming all of the risk.

If it was me I'd go with the builder's recommendations but I'd hold onto the last payment until I had the opportunity to test ride the bike and determine that it met my satisfaction.
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Old 03-16-11, 08:14 AM   #5
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We have a DaVinci crankset on our Rodriguez tandem because we wanted 170mm captain and 165mm stoker cranks and that was the best way to get them. What is your reason for considering a DaVinci crankset? It didn't occur to me to ask about spindle length; I just let them build it. Unless you have some specific fit issues and know exactly what you need, how are you going to know until you try it?

Last edited by swc7916; 03-16-11 at 08:28 AM.
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Old 03-16-11, 09:10 PM   #6
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We have a DaVinci crankset on our Rodriguez tandem because we wanted 170mm captain and 165mm stoker cranks and that was the best way to get them. What is your reason for considering a DaVinci crankset? It didn't occur to me to ask about spindle length; I just let them build it. Unless you have some specific fit issues and know exactly what you need, how are you going to know until you try it?
We're probably going with the da Vinci for the same reasons as you are. The stoker uses a 165mm crank and I use 172.5, and although there are other cranks that you can get in that size (FSA models), they really aren't any less expensive than the da Vinci after taking into account how terrible the mega exo bottom bracket is. I also like how you can change out the spider on the da Vinci, and the one-piece spider/chainring for then timing chain looks nice as well.
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Old 03-16-11, 09:27 PM   #7
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TandemGeek,

Are there any other factors that could require a wider spindle for the rear? I'm still somewhat confused why R&E is insisting that a 116mm is optimal to achieve clearance for the triple chainring, while you and many others have used as narrow as 108mm without issues. The rear dropout spacing on our bike will be 145mm, and the only other factor I can think of is whether you have straight or curved chainstays. All the photos of R&E tandems I can find show straight stays, just like yours. Any ideas?

Thanks to everyone for your thoughtful input!
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Old 03-17-11, 07:13 AM   #8
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Any ideas?
Idea #1: Like a lot of semi-production builders, standardization can keep you out of trouble and 116mm is a 'good' standard for most tandem buyers who aren't sensitive to crank tread width (aka, Q-factor). Again 116mm is actually narrow compared to what you'll find on other stock tandems. So, I believe this is where the folks at R&E are coming from, particularly if your original quote from them was accurate, i.e., they find that 116mm works 'best'. Two of our tandems -- the Erickson's -- were fabricated by R&E's current master builder, Dennis. Erickson and Rodriquez tandems share the same pedigree, hence the R&E name on the shop. Therefore, Dennis would be about the only one who would be able to know with any certainty if there are any significant differences in the chainstay diameter or perhaps the length of the stays that would limit the size of the spindle that could be used with the daVinci cranks. daVinci's cranks have the benefit of using 34t timing rings which also factors into making them more compatible with a narrow spindle vs. the more common 39t and 42t timing rings used on other tandem cranksets.

Idea #2: Not knowing how you've made your request for a BB spindle configuration that diverges from what your builder recommends, if you haven't done so you may want to approach your desire's by asking these questions:

1. What's the most narrow width spindle that I can use for the rear bottom bracket without having any granny ring or daVinci 34t timing ring interference?
2. If the answer remains 116mm or isn't as narrow as you'd like, follow-up with, "What is the source of the interference: insufficient drive-side granny ring clearance to the chain stay, insufficient timing ring clearance to the chain stay or possible heel clearance issues?
Note: When I was running the 108mm rear spindle on our Ericksons I had about 2mm of clearance between the granny ring and the stay, which is 'tight' by builders standards, particularly with square taper since over-torqing the fixing bolts when installing your rear crank could easily consume that clearance.

3. Not knowing what type of BB's they are using, "Would using something like a Phil Wood BB that can easily be off-set to adjust chain line solve any of the potential intereference issues?"

Bottom Line: A good builder will try to steer customers towards "good" configurations using reliable components that work well for the vast majority of average consumers. Unique requirements provide unique opportunities for future problems and uphappy customers that the builder must address as well as the potential bad rap they may get for delivering a product that didn't "work right". When you start to move out of that box, you have to recognize that you will encounter some resistance and further from the center of the box you move, the more resistance you get until you enter the realm of a true-custom buyer... and that'll cost extra and you get what you ask for. It's also at that point that -- as someone else already mentioned -- you assume the risk for stuff that doesn't work. If you're OK with that, and are willing to take the "hit" for the stuff that doesn't work (that is, not blaming your builder or dealer), life is good. But it clearly is a quid pro quo. You get what you want, but you give up the right to assign blame to your builder or dealer. Yes, I've paid to have things I got wrong fixed and ate crow on a couple "I told you so" things over the years. I'm OK with that.
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Old 03-17-11, 02:58 PM   #9
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I used a 108 front and a 113 rear on a Cannondale road with 145 spacing. I have purchased 2 sets of daVinci tandem cranks, including a 3 hole rear, and all pedals threads were too tight. I have thread chasers and had to do all threads on both sets. Other than that they are fine.
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