Gravel MTB Build Advice

Thinking about doing a build next winter/year. 

Currently on a 15 Specialized Diverge Expert Carbon with plenty of mods (carbon wheels, larger cassette etc.). 

Love the bike and with small block 8 tires I can go 90% of places I want to go BUT I am thinking I want to do a ultra light no suspension carbon MTB with drop bars so I have the option to go with proper MTB tires if necessary. 

My questions...

What do people recommend for a drive train?

I want to be able to ride it on the "road" as well or at least the mountain passes near my house so I would like a decent 2x system up front but as is the case with riding mixed surfaces I think I will need at least a 42 in the rear when things get steep off road. 

I may end up going with one of the Chinese Carbon frames for the build because I have the Specialized already and with a project build why not try something funky. Also don't have thousands to sink into a project. I have Light Bicycle wheels on the Spec and they are really nice so I am not all that worried about "cheaper" carbon. 

What bottom bracket standard will give me the best shot at getting where I need to be with gearing?

My Spec weighs in just over 18lbs. Want to get the MTB gravel build right at 20lbs or so (will depend on the size tires I use). Seems like the frames and fork for the MTB stuff I have been looking at are almost the same weight as my Specialized...so with similar drivetrain and seat, bars, brakes, etc. I should only be adding weight with the larger tires. 

owly wrote:Seems doable. You're thinking of a 29er wheelsize frame, right? (as this would negate chasing down one of the wider tyre-clearance 27.5 china gravel/dropbar framesets). 

To save money on some lighter weight components I'd be looking at used, like a carbon crank. Sram Force 22 spiderless crankarms, for example, to suit 2x. 

If you'd also like somewhat of a road-gearing mix, then you could look into a drivetrain with a smaller cassette. The small ring on the 2x being low enough to suit, before you start considering larger cassettes with bigger jumps. Big ring, large enough to suit. 

My recent 2x build only has a 40t large ring, so its not built for pedalling on screaming downhill runs. Rather, the small cassette's tight cog jumps suit road riding cadence across flatter areas. The small ring up front was chosen to get me up the steepest hills where I ride. 

17.5 lbs ti frame with ThunderBurts 2x. Starting with a carbon frame and careful part selection you could easily do sub-20 with 2"+ tyres.

Ya looking for a pretty good mix for road/trail. 

My Diverge has a 50-34 FSA carbon crank and 11-40t cassette. My biggest issue has always been finding a wide enough range for road and trail riding. I suppose getting a larger MTB crankset upfront and going as big as possible in the rear will suffice. 

The only issue I have with my Diverge is that when at super high altitudes (I do most of my riding over 10,500 feet where I live) with steep dirt, rocky trails I am at the very limit of my drive train when I am on the 34t front and 40t rear. I imagine having larger tires on the new bike will help with the traction aspect but the entire purpose for building the bike is to access some very high out of the way roads and trails and connect them via the local roads etc.