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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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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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Nice bike! Sorry I haven't posted earlier, but I just discovered this thread today. My wife and I have been riding road tandems for 35 years, and even though we both have fat bikes, I highly doubt that I'll ever be able to get her on an off-road tandem. Still, it's cool to see that people are riding them! Great job on the design!
 

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I'm new to the forums and just saw this thread - and the tandem looked vaguely familiar... And then I remembered I saw it on youtube when searching for fat tyre tandems. Never rode a fat tandem, but, man, that looks enjoyable! I wish there were some shops here that would lend such bikes. Unfortunately, electric MTBs (not tandems!) are getting the most attention.
 

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Discussion Starter · #9 ·
I'm new to the forums and just saw this thread - and the tandem looked vaguely familiar... And then I remembered I saw it on youtube when searching for fat tyre tandems. Never rode a fat tandem, but, man, that looks enjoyable! I wish there were some shops here that would lend such bikes. Unfortunately, electric MTBs (not tandems!) are getting the most attention.
yes sir! We are on youtube as Bad Bully MTB

 

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Nice bike and very individual.
Your tandem with such a slack angle and tyres like that must have a giant fork trail. Is the steering in tight corners not too slow? What's the rake of the fork?
Our rake is 42 for both forks we're using, with the 10 cm fork the tandem steers sharp, with the other and certainly combined with the 29 wheel steering is working.
Is the comfort for the stoker not hindered by the rebound of the tyres?
We ride 2.6 to 2.8 tyres in 27.5 on a full suspension. Up front I'm testing right now a 29 wheel with a 2.6 tyre(room enough in the 27+ fork).
My first impression is that combined with the 15cm fork with a lot of sag it rolls over obstacles better than the 27.5. The27.5 rear wheel follows very well (I think the rear suspension is helpful)
How much sag u have on your fork with what pressure?
I also wanted more room for the stoker.
I cut her bars first down to 62 cm than 60 and now 56. Ideal : before we hit a lot of trees and other stuff with her bars when turning and even
with the very stiff tandem we have now her moving around is far less influencing the power I need to steer.
With a 15 cm fork bracket height was 38 but to my surprise this was not to high. When I ride with a 10 cm fork her bracket and pedals have too much ground contact, in downhills
the rear bracket is too low and keeps hanging in technical terrain.
Every tandem has awesome traction(mine has a 192cm wheelbase with 48cm chainstays) but with those tyres It must be possible to climb the sky.
Uphil and mud no problem with 2.6 rear tyre, when flat he starts spinning.
Keep posting about the handling of your bike.
 

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Discussion Starter · #11 ·
Nice bike and very individual.
Your tandem with such a slack angle and tyres like that must have a giant fork trail. Is the steering in tight corners not too slow? What's the rake of the fork?
Our rake is 42 for both forks we're using, with the 10 cm fork the tandem steers sharp, with the other and certainly combined with the 29 wheel steering is working.
Is the comfort for the stoker not hindered by the rebound of the tyres?
We ride 2.6 to 2.8 tyres in 27.5 on a full suspension. Up front I'm testing right now a 29 wheel with a 2.6 tyre(room enough in the 27+ fork).
My first impression is that combined with the 15cm fork with a lot of sag it rolls over obstacles better than the 27.5. The27.5 rear wheel follows very well (I think the rear suspension is helpful)
How much sag u have on your fork with what pressure?
I also wanted more room for the stoker.
I cut her bars first down to 62 cm than 60 and now 56. Ideal : before we hit a lot of trees and other stuff with her bars when turning and even
with the very stiff tandem we have now her moving around is far less influencing the power I need to steer.
With a 15 cm fork bracket height was 38 but to my surprise this was not to high. When I ride with a 10 cm fork her bracket and pedals have too much ground contact, in downhills
the rear bracket is too low and keeps hanging in technical terrain.
Every tandem has awesome traction(mine has a 192cm wheelbase with 48cm chainstays) but with those tyres It must be possible to climb the sky.
Uphil and mud no problem with 2.6 rear tyre, when flat he starts spinning.
Keep posting about the handling of your bike.
68 degrees is far from a slack angle. In hindsight, I probably would make it even slacker if I could do it again. The Mastodon fork has a 51mm offset if that's what you're asking. The steering geometry was copied from my Surly Ice Cream Truck. I was reluctant to make it slacker because I didn't want the steering to be too heavy. It turns out that doesn't matter. Obviously I do want to keep enough weight on the front to have good traction though, and this is a good balance. It's unlike a half bike in that it's not really all that practical to deliberately weight the front.

We run a thudbuster seatpost in the back for the stoker. Keeping the stoker comfortable is a big priority. The fat tires don't really rebound more than the plus tires we run, all of that is a matter of finding the right pressure.

Fork is set at 120-130 psi with about 25% sag.

We are running 3" 29 plus tires for the warmer weather now. The 4" tires are good but the 3" is probably the sweet spot.

The handling of the bike is excellent and I can flick it around pretty good considering we have 300+ pounds on board.
 
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