Tracklotrail, tracklocross, fixed off road, trail fixie. Is there any difference compared to a single speed?
There are a few characteristics with fixed trail riding to think of, when designing a frame and when building a bike from it.
And no experience or riding condition is universal. So we write this from a Nordic, often wet, always rooty, rocky and rolling terrain point of view.
Our key idea is agility. When riding fixed, you will not need the usual stable high speed properties or leverage force in the bars. You will need ultra light and fast response, agile body, your firmest deathgrip of the bars and a riding presence sharp as a razor.
Those fast moves call for a fast handlebar. No bus driver 80 cm thing. Narrow bars, in the size of your shoulder width, and comfort for your hands.
Skidding? Strongly discouraged on the trails. That should be avoided, is not cool and is not necessary but it messes up these trails if you do it, so don't. Keep your pace planned, meditative and consistent instead. But you need to keep the wheels light just in case. Rotating weight is a powerful force. And a low, short and steep bike turns and moves ultra fast just like you need when you have no suspension or brakes to save you.
So, the way to build up the Heroux Tao fixie we have found to be a narrow bar (680?), a mid-long stem (75?), the widest grips you can find (145+), light strong rims (20-25 mm inner) and fat, light, low-treaded tires (e.g. XR2 3.0) for summer or rugged grippy like Vigilante 2.8 in the offseason.
This build makes it flickable, light, fast to respond.
You should not have to buy a bunch of fancy stuff when you get a new frame - so rear spacing on our fixed frame is 135 mm, BB is 73 mm threaded, and ZS HS with 1 ⅛” straight steerer (or a tapered, should you find a short fork with it).
Just get an old MTB 29" wheelset, pull the discs off, QR, cogs and tires off. Flip the rear over and bolt a cog on the disc mounts, get a pair of massive tires, get the strongest bolt-in axle and a tensioner, a nice old steel 26” fork, a stripped classic 3-speed crankset which gives a perfect 54 mm chainline. Cheap yet good stuff all of it, get it from your friend’s bin of “not fancy enough” parts and build something awesome!