Past due for an update on this one- I was pretty bummed that the 2.8 tires wouldn't fit the rear on my -057, and decided to try and find the biggest tires that would fit. On the MTBR plus bike forum there is a thread on 27.5+ tires that has a spreadsheet of estimated tire widths on different rim sizes by Craigsj (
http://forums.mtbr.com/26-27-5-29-plus-bikes/27-5-tires-931412-post12516812.html#post12516812) that gives an idea of how different tires will compare based on the bead-to-bead measurement, or how wide the tire casing is. My measurements for the b2b width and mounted tire width are all wider than his measurements/calculated values, but it does give a good idea for ranking the tires.
His formula predicts that the 2.8 Trailblazer should be 64 mm wide on a 34 mm internal rim, whereas I measure closer to 68 mm wide on mine. Pretty close, but since the chainstays on the -057 are 70 mm it makes the difference between "enough clearance" and "rubbing through the paint". The next smaller tires by b2b measurement are the 2.4" Geax Goma, WTB Trail Boss, and Continental Trail King. I decided to go with the Trail King, and so far it works well. I got a wider b2b measurement, but the actual width of the mounted tire came in just under 64 mm, just 2 mm above his calculated width. This gives me just over 3 mm (1/8") clearance on each side. I'll still need to keep an eye on it (and not ride in thick mud, but I wouldn't do that on trails here anyway), but this is probably the fattest tire that will fit. My 29" rims are 25 mm internal, and Continental X-King 2.4's on them measure 58 mm wide (3 mm wider than Craigsj's prediction).
The bottom bracket clearance is about 1/2" less with the 27.5 wheels than with the 29" wheels- dropped from about 10.75" (27.3 cm) to 10.25" (26 cm). I do get more pedal strikes, but that will probably change as I get used to the lower clearance. The feel is almost the same as with the 2.8 tire on the rear- it certainly is smoother than my 29" wheels, due to the lower pressure. My 29" rims are 25 mm internal, and Continental X-King 2.4's on them measure 58 mm wide.
Tough to see, but there is clearance
No problem going up this if you pedal at the right time
Summary of my numbers-
29" wheels-
25 mm internal, X-king 2.4- 58 mm wide
27.5" wheels-
34 mm internal, Nobby Nic 2.8- 69 mm wide
34 mm internal, Trailblazer 2.8- 68 mm wide
34 mm internal, TrailKing 2.4- 64 mm wide
