Building a flat bar road bike

I built a flat bar pub bike from my Btwin 500se with a set of Claris shifters from Rose in Germany, a Claris front derailleur and a cheapo bar from Chain Reaction. It was very easy to fit, the gears hardly had to be adjusted at all and the Btwin brakes work even better than with the stock shifters.

Go for it!

Regarding counting teeth - three things to be aware of.

The range. This is the maximum difference in number of teeth the cage of the rear mech has to cope with. As you are going 1x, this is just the difference between the big sprocket and the little sprocket at the back. 34-11 = 23, so you need a cage that will cover at least 23 teeth. Any MTB mech will probably do that.

next is the max sprocket size. Most MTB will go 32, but check the spec if you want a bigger sprocket for hills. 

Third ( probably most important) is your overall gearing, discussed above. There are 2.5 x as many opinions on this as there are cyclists. They are of course all wrong. The correct ratio for bottom gear is 3xthigh/waist

You know your own level of cack-handedness.

Question I'd ask myself, is, how will I feel at the top of a hill where my speed is going to hit 40mph, sitting on a bike I've screwed together myself.

My own answer, is I'd get off and walk.  So, for me, not worth doing.  Your call!

http://www.bikecalc.com/speed_at_cadence

Alter the numbers to get a very good idea of what gearing does what.  I used it to build a touring/mountain climb bike.  I use 44/28 chainrings and either 11-32 or 11-40. 

 I can climb up 25% inclines at 70+rpm and also speed long the flats/ slight downhills upto 32mph.

  This gives most riders all the gearing they will probably ever need.

My main ride is a flat bar and has been since I 'converted' in 2001 (to a Ridgeback Genesis Day 02, race bike geo with flat bar) after 18 years on drop bar bikes only. Don't get me wrong I love my drop bar bikes but for around town and urban riding flat bar all day long.

My current steed is a Specialized Globe Pro with a 3x10 (48/36/24) and 12-28 but tbh I'm mostly on the 36, depending on what gearing you like you might end up spinning out a lot by going with a 1x system. I try to hold onto 36x12 for as long as possible simply so that I can at least put some wear into the sprocket but even on the shortish 2% slope leaving town to the next but a short few metres away boundary wise I'll be at 110rpm @26mph and I'd have to be closer to 125rpm to hit the 30mph I'm usually travelling at.

36x32 with 28mm tyres gives you an approx 30" bottom gear, a 36x11 about an 87" (26mph at 100rpm). Might give you an idea where you need to be for your front ring.

Enjoy the build.

If you're willing to buy a bike instead of building one then it looks like a Triban 100 from Decathalon would tick your boxes and be fairly affordable:

https://www.decathlon.co.uk/triban-100-flat-bar-road-bike-id_8500949.html

www.sheldonbrown.com is a great resource for finding out which mixes'n'matches of components will work together (and sometimes how to actually put them together, or take them apart). Part of the fun is figuring out what works with what, particularly when the manufacturers didn't intend it!

If using a square taper bottom bracket, pay attention to your chainline - without a front derailleur to catch the chain, it can jump if the chainline is too extreme (usually if your BB axle is too long and the chainring too far out from the frame).

32t or 34t on the back sounds fine - remember to use a long cage derailleur, such as MTBs do (only some road bikes do) - but it also depends as much on your chainring. There are some pretty steep braes in the 'Burgh, so you might (depends on your) be looking at something fairly modest on the front, especially if you're carrying loads. But you don't want always be riding (and thus rapidly wearing out) the smallest couple of sprockets on the flat and downhill either, so you mightn't want too small a chainring...

I'd hazard a guess that the chainring sweetspot is somewhere in the 34t to 44t range - but think about what kind of bike it'll be, what you'll use it for, and what sort of rider you are.

I have an 8 speed hub geared flat bar bike for getting to and from work most days and for general errands. I live in Bristol which is fairly hilly and it's fine, although noticeably slower than the road bike. To be honest, I only ever really use gears 3, 4, 5, 6 and sometimes 7, so an 11-32 with an appropriately sized chainring would be fine I'm sure. If you don't have all the bits already, then think about getting a hub gear. For me, for a town bike, what they lose in weight and spread of gear ratios (they always seem a little gappy), they more than make up for in ease of cleaning and general maintenace – basically all you need to do is oil the chain once in a blue moon and adjust the brake pads as they wear, then give the magic gubbins inside the hub an oil bath every few thousand km (I have no intention of doing that part myself!).

Certainly not ludicrous at all! In fact, it's a very good idea - I'd quite lack a flat bar road bike for around town.

My advice would be stick with 9 speed - that way, firstly everything is WAY cheaper, but also, the MTB and road pull ratios are the same i.e. you can interchange road and mtb parts no worries.

It's also very doable, even if you only know the basics. GCN do lots of great videos on youtube for how to setup a bike, follow them and you'll be fine (that's how I learnt).

I've also had a 9 speed DIY 1x bike, worked pretty well.