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Discussion Starter #1 (Edited)
This thread started with me discussing design details as I designed our custom ti fat mtb tandem. Fast forward 4 months and here she is.

Goals were to run the largest possible tires (5.05"), good fit for us as smaller riders, retain the ability to run dropper posts, give her a little more room to see in the back, and have the largest/most powerful brakes possible.

Here's the finished product.
Specs:
Head angle 68 degrees with a 483 A-C fork like the Surly ICT
Seat tube angle 73 degrees
Wheelbase 1900 mm
BB drop 40mm
Front top tube actual 575.8mm effective 590mm
Rear top tube 760 c-c on the two seat tubes and 580 effective for stoker's handlebar attachment seat tube
3rd "seat tube" for stoker so that her bars are free from mine and so we can both run droppers
Chainstays 465mm
Bent seat tube (stolen shamelessly from our Surly fat bikes, which we love)
31.8 seat tubes (all 3) and solid posts (for now)

Components:
780mm bars, stoker will likely cut hers to 740ish
60mm stem for captain, stoker probably going to go shorter, 35mm is on order
Raceface Ride cranksets, 2x spider, 22t timing chains, 32t main drivetrain ring, bash guard on front, all 12 speed Eagle chains on both.
54mm bottom bracket eccentric, 100mm wide for fat
Manitou Mastodon Pro Ext 100mm fork
Vee Snowshoe 2XL tires. Rear didn't quite make it on the clearance and I snipped the lugs way up the sidewall off on one side to clear.
Onxy Classic hubs laced to Surly MOBD 80mm rims, tubeless, SRAM XD hub, built by Mike @ lacemine29
SRAM GX 1x drivetrain, obviously a custom length shifter cable. 52-10 cassette
Magura MT7 Pro brakes with 220 mm rotors

Frame weight was 8.3 pounds before all the stuff I put on it. I haven't weighed it complete. Normally I don't care about weight but given that the tires are over 8 pounds by themselves I'm surprised how light it is.

We've had it out twice already and it is incredible so far. I can't wait to have more time to ride it.
133839079_238349701058443_3001016260413480031_o.jpg
 

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A guy on a bike
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I'm far from an expert in frame geometry, especially for tandems. But I think that 29" are awesome, especially on a tandem. Since you really can't lift the front or back wheels as easily as on a regular bike, you'll end up rolling over more stuff--making big wheels really shine. That said, they might be too big, since you say that you are "tiny"--so definitely stick with what fits you and your stoker.
 

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Curious if anyone has anything to say about this tandem. Captain and stoker are tiny people as you can tell from the design. We are 5'5" and 5'4" tall.

Specs:
69 degree head angle (should slacken a bit with 100mm suspension fork)
73 degree seat angle
590mm effective top tube for captain
Stoker top tube is 730 c-c, this should be long enough but curious?
Standover 734mm for both of us which is great with the flatter top tube.
Clearance for the widest tires (26x5.05")
Chainstays are at 485mm
44mm headtube

31.6mm seatposts with what looks like enough room for droppers if we want to run them.

I'm excited about the design so far but obviously I don't want to get this wrong so I'm open to any advice.
We are just tandem lovers not designers, engineers or anything to qualify us to give advice but we just designed with the help of our frame builder for mainly the rear end and built a tandem that I wanted to switch between 27.5x4.0 and 29x3.25. It is built and maiden voyage this morning. Awesome, custom is the way to go! I did a ton of research and between that and our past 7 tandems, the two most recently a Full Suspension Symbiosis and Salsa Powderkeg, and picked the best from each to make what we wanted. The frame specifically with what you are asking we went with 70 degree headtube 73 degree seat tubes a trade off between to slack and sloppy when going slow on switchbacks to enough at speed for stability. Any steeper we felt like we were crashing into obstacles verse riding over them. Slacker was a handful when going slow and running technical single track. Most single tracks are designed for singles so there will be times where you need to really slow down and get through it instead of flying through it. I would also look at your top tube or tubes and look at tightening up the whole frame with a lateral tube running from the headtube to the stoker seat tube. If you are a strong rider you will find without it that the tandem will sway not just while pedaling but on sweepers making it a handful. Also on a tandem stand over height is huge so with the above mentioned design it lowers stand over to a minimum for safely mounting and dismounting where you wouldn’t on a single, water crossings, log clearance, sudden riser that the front wheel is up already. We also opted for a longer the usual heattube though make sure you consider which headset you put in a the stack hieght but feel this adds to the whole solid feel of our build. Enjoy the ride!
 

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Discussion Starter #4
So this thread went nowhere, but I finished the design and built the bike. See the (edited) first post for details.
 

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It's amazing. I've just arrived here thinking of building two tandems for my family. I'm 6 foot 4 and my wife and two children are all 5 foot 4 (photo is old!). I live in NZ and intending to have two offroad touring tandems that can also be playful. Where on earth did you start to design the geometry? I'm hoping to get frames made in China and then build the rest of the items up here.
 

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Discussion Starter #6
It's amazing. I've just arrived here thinking of building two tandems for my family. I'm 6 foot 4 and my wife and two children are all 5 foot 4 (photo is old!). I live in NZ and intending to have two offroad touring tandems that can also be playful. Where on earth did you start to design the geometry? I'm hoping to get frames made in China and then build the rest of the items up here.
We both already owned fat bikes, so I incorporated a lot of that into the design. I can definitely help you in any way you need! As for steering, don't worry about it being heavy, make it as slack as you'd want for what you're using the bike for. Make your rear cockpit longer so your stoker can see something besides your back. Make sure you have a short enough seatpost so you can use a dropper if you want.

If I had it to do all over again, I would make my chainstays even longer for more rear tire clearance (5.05" don't quite fit), I'd go 28.6 on the "stoker stem seatpost" for easy no-shim install, and I'd probably have made her reach a little shorter but her top tube even longer, not worried about the wheelbase. I have zero regrets though, the bike rides amazing and we absolutely love it.
 
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