I am looking for some help with a tandem frame that I’m getting built. I’m wanting to know what BB shell width and chainline I should specify. I first tried going past my local bike shop to ask them for some help but they said that’s out of their knowledge base. I then started searching this forum and got even more confused.
My ideal setup is to have SRAM red 2x with 50/34 chainrings and a 12 speed 10-36t cassette. The wheels would be Rolf Prima and they come in either 142mm or 148mm boost. The crankset is Appleman 2xr and comes in various spindle widths. I plan on a standard English threaded BB. I would have 2 sets of wheels, one setup for road with 28 or 30mm tires and a second set for gravel with ideally 50mm tires (45mm would also be ok).
In reading some threads here, it seems like I would want 148mm boost for the rear hub since I plan to use wide gravel tires so I can get proper clearance? The rear BB and the front Eccentric BB come in either 68mm or 73mm, does this choice change things with the chainline? And, how do I decide on what chainline to tell the builder? I’m not looking to have a narrow q-factor.
(My first start with BikeCad, modeled mostly from my current Cannondale tandem)
Ff using the appleman cranks then just go with the 73 shell and use a chainline that allows the stays the frame builder chooses to achieve the tyre size and chain ring clearance. For the most part I would let the frame builder figure it out. That is what you are paying them for.
Here is my advice as a custom bike designer: One-size-fits-all bikes don’t work out well. Some builders will say yes to everything without trying it themselves.
If you plan on using 700x30 tires, then a 50x34 gearing makes sense.
If you plan on using gravel tires, you need much easier gearing to accommodate the larger tire diameter, higher rolling resistance, and steeper offroad grades.
What is the primary riding style and purpose of this tandem?
Bike will be split between both road and gravel. Current tandem is a Cannondale, running 700c 28mm road tires and swap to 650b wheels with 38mm gravel tires. 38’s are ok but I find them limiting on tricky roads. I run 45s on my single gravel bike. My thought on going up to 50s is that seems to be the trend for gravel, but if having up to a 45mm gravel tire is best for having a bike work good for road as well, then so be it.
If I had unlimited garage space and money I would have both a road and gravel tandem.
Curious why you wouldn’t use 85.5 width. Internal bearings handle road widths while external bearings work with MTB width cranks. Loads of room for big down and boom tubes.
I have completely standardized on the T47-85.5 BB’s.
I had never considered that, probably because the front eccentric would use an English threaded BB. I’ll have to check with the builder to see if they offer this BB shell.
Tandems need eccentric bottom brackets. That is why I think the original poster mentioned those widths. My guess is the BB’s will be PF30 to use these styles of eccentrics:
I agree 85.5mm T47 is the standard if you need a big downtube (>38mm)
Yeah, the eccentric limits one BB but not the other. I suppose there’s some value in having same BB’s. With boom tubes generally being pretty big tubes, the extra room on the stoker BB would make sense.
I was planning to use either a Bushnell Eccentric or Chinook Eccentric. Both fit eccentric shells 53.8 to 55.0mm diameter. I’ve contacted the frame builder to see if he can use a T47 for the stoker’s BB.
I don’t have solid answers for you. But I just built my first frame earlier this summer and ended up with chainline woes.
My mistake was to go by the listed chainline of my rear hub (in this case a Rohloff). That’s where the trouble started. Now I’m having to make my own chainring spider to clear the chainstays and, since my rings are “way out there” (~3mm beyond what RaceFace calls “Boost”), I need to make my own derailleur clamp to push the derailleur out. (Note: “make my own” is my choice, I may have been able to find solutions that exist, but I find searching for obscure parts more tedious than doing CAD and having things made.)
It seems strange to me to have a customer have to specify chainline. It affects so many things (including Q factor, which thankfully you don’t care too much about). Maybe an actual framebuilder can chime in as to why they would ask for that (instead of figuring it out themselves based on the other requirements).
I’ve been going back and forth with the frame builder and have almost finalized all details. I contacted SRAM support and they said 47.5mm chainline would work with 148mm rear. Also using a T47 external bearing BB for stoker. Thanks for that suggestion.
