Now, due to
-How tall do I have to be to ride a 36er?
There are 3 answers:
A: If you aren't concerned about great fit or handling and just want something fun and different to toodle around on, 5'8" or so is probably the lower limit.
B: If you're moderately concerned about fit and performance, but are willing to give up a few seconds to your buddies, 6' is about the limit.
C: If you're over about 6'10" or so, a 36" wheel is actually probably your best option, assuming you're ok with a rigid bike.
-What's the minimum effective toptube length?
As I've said before, I'm not a huge fan of ETT for fitting bikes, but for the sake of comparison, someone with a size 48 foot, 175mm cranks, and otherwise normal frame design ("normal" for a 36er is pretty weird, though - 69 HTA and 90+mm rake for front center in the 73cm range!) a 25" effective toptube is about the minimum.
-Can I get a suspension fork?
No. In theory I could build you a custom headshock fork for a 36" wheel, but nobody has ever asked me to do that - because the resulting handlebar height would be truly insane (think bars 6+ inches above your saddle if you're anywhere under about 6'6") and the travel is pretty limited (70mm) on those old headshoks.
-How much will my 36er weigh?
Somewhere in the low to mid 30s (that's pounds) depending on the parts you use to build it up and whether it's running full gears, 1x or singlespeed. You might be able to geek out and get under 30 but it would cost an arm and a leg.
-Can you build wheels for me? What rims/spokes/hubs?
The best combination of parts I've found is Shimano low flange (meaning centerlock) hubs paired with 14g straight spokes and Nimbus aluminum rims. 36 spokes, 3 cross. And yes, I can have these built for you. Cost ranges from $700-900 depending on hub choice. High torque generated by the big wheels will mean that the 29er-specific hubs (ie M529) are your best bet for durability.
-How about an internal gear hub (IGH)?
In general I advise against these due to the enormous torque loads and low gears used on the 36" wheels. Remember that Rohloff (for example) has a minimum ratio of 2.4x1. On a 26" wheel your lowest gear is equivalent to about a 22x34. Scale that up by 40% and your lowest gear is going to be about 30x34. If that's low enough, great. If you're willing to ignore the Rohloff ratio limits and take your chances, you can get lower, of course.
-Ok, so what's the best drivetrain?
I like an 83mm BB shell and 50mm chainline GRANNY GEAR. Why? Because a 22t chainring on the front is like a 30t ring on a 26" wheel mountain bike, so you want to put the granny gear where the middle ring would be - that's the gear you'll be in almost all the time. In many cases I just recommend running granny gear only and a wide range cassette (11-36 10sp or 10-42 XX1 if you've got the budget) in the rear. Front derailleurs are doable too if you want one, of course but you'll *never* need a big ring so granny/middle is plenty.
-How short can you make the chainstays?
51cm is about the limit, really. Yes, that's super long. No, you won't be manualing very easily unless you're very tall. Tough luck. It's a 36er.
-What dropouts do you like to use?
Paragon low mounts for geared bikes, Paragon rockers for singlespeeds. Why? They both have super long tabs that allow the use of some commercially available oval chainstays. The new Polydrops would work well too, I imagine.
-Can I get my bars to the level I like relative to my saddle?
Assuming your saddle is 77cm above your bottom bracket, the lowest the bars can easily go (assuming no spacers, an inset headset, and a -17 degree stem/flat bar) is about 1" above your saddle. So to run perfectly level (this calculation assumes a ~12" BB height) you'll need your saddle up at about 80cm.
-What's the standover like?
Lowest standover without doing any weird curving of tubes is going to be in the 34" (measuring to the midpoint of the TT) range. Some amount of shenanigans (curved down toptube, lowered/braced toptube, etc) can squeeze out another inch or two if needed but if standover is a big concern of yours, a 36er is probably not the bike for you.
-Are 36ers fast? Are they fun?
36ers are, for most normal sized humans(meaning under about 7 feet), slower than other mountain bikes. They have heavy wheels and tires (much more so than you would expect just scaling up from smaller parts, since the components are mostly made for unicycles) and no suspension options. There is really only 1 decent offroad tire.
Fortunately 36ers are super fun. Traction is excellent, the feeling of sitting "in" the bike is amazing, and lean-steering them is almost mandatory. For XC riders who want a totally different experience on the trail, plus lots of compliments and questions at the trailhead, they are a blast. For folks who are concerned about every last second/gram/watt, or cleaning that tough section, or hitting all the drops and jumps off the side of the main trail, they are less attractive.