Custom build to do (nearly) everything...

The key difference is that the road bike has 55cm ST & 56cm TT whereas this design is 58cm square...

I thought I'd best check the geo to ensure It's comfy/efficient by correlating empirical data (De Rosa) with theoretical data (Internet bike fit)...

If we subtract 17 cm crank length from the 95.6cm saddle-pedal distance (bike fit) the saddle sits 78.5 cm from the BB.

Assuming the same stem length/slammage on both, the measurement from the centre of saddle down to the top of the headset is nearly the same for the 58 square proposal as it is for the 55/56 road frame.

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Moving onto the detail:
I'd been umming and ahhing about OS tubing cos I always rode OS steel MTBs BITD but that Secan convinced me of 28.6mm for neat minimalism - Cromor DB probably (cost effective and not too porky).

Clearance for 2" tyres + 2 sets of bottle bosses + mudguard bridge
(I'll need to work out where the bridge sits to give a neat mudguard line without clashing on a 42mm knobbly)..

Thru routing for rear brake - I'm not 100% sure how to handle this but think it would be neat.
My thought was to go in through the butted bit the at the top of the downtube and out through the bottom just before the BB.
The cable/hose would pass round the BB and enter chainstay on the underside, emerging at the top shortly before the flat mount.
I'll only ever want to run 1x so I guess gear outer could take the same route but via the right chainstay (that means cable outer all the way which I'd guess is normal for internal routing.)

The fork I have looks pretty neat with an internal headset so I think I'll go that way...

68mm BSA BB will keep my options open (from past experience with jumpbikes etc spacers seem to work OK if a wider axle is required)

With skinny steel tubes it'll be a 27.2mm seatpost and I think I'll go for a separate clamp just for toughness and simplicity.

I'll flesh out my drawings with these features see if it all makes sense....

Had a play on CAD last night and created the geometry (for CAD pervs I did it as a weldment in Solidworks) - Aside from being a mildly amusing exercise it has a few benefits:
1) useful to finalise detail (e.g. bridge/eye locations for mudguards) & communicate design intent with builder
2) handy for future reference (e.g. making a frame bag or checking tyre/wheel combos)
3) basic FEA modelling - e.g. check whether proposed cable-routing holes are a really stupid idea

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I'm also starting to think about wheels. I've been convinced to go tubeless by my nephew and adventure-riding hero. Here he is on his half-term jaunt in the Rift Valley.

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I was trying to remember the name of Meerglas - could be the source of some more inspo?

https://meerglas.org/blog/category/commuter/

Yes indeed, very much so.... thanks

And the details are super clean too

Is this somewhat related to Fern? Backdrop in the photos looks... familiar...

https://www.fern-fahrraeder.de/

Yes I believe they shared a workshop/painters/photographer but seem to have less presence than Fern these days 🤷‍♂️

Currently in the process of getting a custom titanium Albannach frame. Speaking one to one with the founder Jim about what I want, need, tyre clearance, little extras, mounts, aesthetics etc.

http://www.albannach.cc

We have taken my very comfortable Giant TCR geo and worked back to create a Torragar frame more suited to gravel/adventure/multi week/month touring.

All this customness for £1500. Granted hardly cheap cheap but this is a "bike for life" in my eyes. I will qualify this by saying I haven't got the frame just very happy with how its going so far.

Im going 2 x 11 Shimano GRX 810 on this build having used the 600 version on my current gravel bike. Hydro with real nice wide levers FTW

My advice would be to let things get out of hand with this one, squire!

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That sounds like a super-exciting project and the offerings from albannach look really nice too - you must be buzzing about it!

Do they manufacture themselves or just do the design and farm it out to Triton / Waltly / Titan / Hi-Light et al.?

Yeah its the second bike I'll build up myself after a much more simple All City single speed. Love all the research and sourcing etc.

So, from my snooping, Albannach work through Rothair Ltd which is also owned by Jim. They ship from Scotland, I assume after being looked over. Its definitely not a mass production line stuff.

"I don’t build the frames or stitch the kits but I design and engineer their unique details and style."

Its all very Scottish feeling and I like that! :oD

Yes indeed, for me there's often a correlation between the effort that goes into creation and the subsequent emotional connection.

re: Titanium - I did have a quick poke around into the world of Ti-fabricators and there are a surprising number of brands farming out some, if not all their work to Triton, Waltly, Titan, Hi-Light, XACD, Ora etc. Also some actually welding here in the UK (travers, enigma, sturdy etc)
Personally I don't have a problem with separating design & manufacturing but I guess that the if the person building it knows who you are and what you're planning there may be some added benefit/serendipity there....

Subbed with an obligatory comment fawning over the bobrat. Rip.
Excited to see how this comes out, enjoying the tube choice and geo so far and echoing @veLLo it seems a departure from your other builds so should be interesting

Cheers, I was just thinking about the Bobrat last weekend - I rode up staple lane and down Combe Lane and had a flashback of the last time I rode that combo - a rainy day in 2012 following the olympic route brakeless.

As you say, this build's definitely a departure from what I'm used to so I'm intrigued by how it'll pan out too

Returned to the cad model over lunch today and discovered that my short roadie chainstays could cause trouble with a big tyre- model shows 45mm wtb resolute...

Also thinking that the 832mm standover height might be asking for trouble given my 880 mm inseam...

Looks like a small geometry re-think is in order...

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Internal routing sounds great - have you looked in to T47 BBs? I don't know the ins and outs, but I know the shell is larger than a BSA shell, so you may be able to run the cables through it?

Also thinking that the 832mm standover height might be asking for trouble given my 880 mm inseam...

If you think standover should be lower I wouldn't worry. I've couple frames with the same standover as my inseam and with cycling shoes on there has been no problems whatsoever (raced cx etc.). I'm so used to it I don't even think about it. I can imagine that someone with long legs would be afraid of going even near french fit in these forearms length of seatpost -days.

I'd not really considered anything other than BSA having assumed all the proprietary stuff would be non-threaded but from a routing perspective T47 looks like a good plan.

I thought this page at Seven Cycles gave a rather good summary https://sevencycles.com/options/bottom-brackets.php

Thanks, that's good to know.
After reading that I checked my standover heights on bikes I've been comfortable riding off-road. They all came out around the 810-820mm mark which was reassuring.

thats a good looking one!

Back to the back brake routing question, this looks like quite a simple/elegant solution to route past a BSA BB

Elegant for sure, but won't that quickly fill up your chainstay with all the shite coming off the rear wheel?

A very good point....
I'm convinced that downtube external routing is too ugly so that will have to go inside, but I'm fairly relaxed about external chainstay routing...
As far as I can see, the options are:

• Thru DT - thru T47 BB - thru CS
  
• Out of the DT - under the BB - back into CS
  
• Out of the DT - under the BB - outside the CS
  Given your good point on the weatherproofing of the DT - CS concept above I think I'm inclined toward internal DT/external CS.
  
  

what about an eccentric BB shell to give enough space to run the brake cable internal from DT to CS?
Also means you can use any BB standard in the future without too much hassle and easy to run SS if you fancy it as well?

The internal hose routings like in your photos will be a smaller stainless tube brazed in so there's no access for water or mud to actually get inside the frame tube, it would just get into the hose routing tube which as I say, will most likely be stainless.