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REALLY! I've been throwing the idea around for a while and finally got around to pulling the
trigger on it. I tried the 29'er thing, and just didn't like it for several reasons. Now that there is a
good selection of product out there, why not give it a shot.
The biggest misconception about 650b is that it's new. It's been around forever, it was just overlooked when the "bigger is better" 29'er made it into production (Gary Fisher) first.
The 2nd biggest misconception is that it's 27.5". It's called 27.5" but it's actually 27.0". So it's not half way between 26" and 29", it's only a 1/3 of the way there. This is a big deal when I break out the equations below.
The cool thing is that most 26" frames have enough clearance for 27" wheels. 1" bigger diameter is 1/2" radius. It you have a bit more than 1/2" clearance everywhere, then you can do it. Most 26" suspension forks can also handle this. In order to accommodate 29" wheels, frame dimension and geometry's get really tweaked. My Niner hardtail was over 6" longer (front of front tire to back of back tire) than my 26" full suspension bike.
(This is were my Advanced Mathmatics and Physics background gets to come out and play)
The worst thing I found with 29'ers was the rotational inertia of the wheels. Marketing Departments at the 29'er companies are great at praising the "terrain-interaction" property of the bigger wheels. The one and ONLY property they are talking about is the geometry of the larger tire as it meets the ground. YES, they are correct in that the larger tire will "interact" with bumpy terrain because it doesn't fall into holes between bumps as far as a smaller tire. This if 100% true. Notice I said "interact" previously instead of "roll". Once an object is rolling, there are laws of physics that come into play. Bike wheels have a certain weight to them. Rolling bike wheels have rotational inertia. When I built-up my 29'er, I built it with wheels that were roughly the same weight as my 26" wheels, and with tires that were actually lighter. (can't remember for sure but I think the 26" was a IRC Serac UST's at around 750gram and the 29'ers were Bontrager tubeless ready at 650g). Overall lighter wheels but still felt a LOT harder to spin up. I knew the physics going on here.
The formula for Rotational Inertia (or Moment of Inertia) = mass * R squared. R being the distance the mass is from the rotating axis. Because the R is squared, changes in this dimension have a big impact were as changes in mass do not.
Here are some Moment of Inertial calculations on 3 different tires (the rotational inertial of the rim and spokes is being ignored here since it is minimal)
The Kenda Nevegal is available in all 3 sizes: 26", 650b, and 29". I chose this tire because it comes in all 3 sizes, and it's a "real" tire(more on this later), and the exact same construction/tread across all 3 sizes.
The weight of these tires are approx 610grams, 650grams, and 833grams respectually.
The Moment of Intertia for these 3 tires is approx 66.5gm2, 76gm2, and 113gm2 (grams meters squared). Notice there is small difference between the 26' and 650b, but a huge difference going to the 29er. This is that R squared thing kicking in.
Back to the "real" tire comment I made above. I consider a "real" tire one that I can ride the local trails with, works well in most terrain wet or dry. I can rip the local bike park with it AND race on it. A do-it-all tire. I don't keep mutiple wheels sets around with different tires for different things. I need a do-it-all tire and ones that durable enough to handle it.
Notice that the Moment of Inertia is only slightly higher for the 650b tire than the 26" and it's the same great tire, same great tread pattern I like. Now look at the 29er". It's almost double the 26" tire.
Now of course this is for a real tire, sized up to 29". If 29er bikes had tires like this on them in the bike shop, it would feel so heavy during the "parking lot" test that nobody would buy them. That's why they have "29'er specific" tires. These are typically smaller (2.0 vs 2.1 or bigger), have lighter casings, and thinner rubber with a race tread pattern. The pro's you see winning races are on these, and the bikes in the bike shops come something like this. I wouldn't recommend doing downhill runs at your local ski resort with these.
Using the same Moment of Intertia(MOI) formula, we can find out exactly how light a 29'er tire would need
to be to have the same moment of interia as 26" tire. Using the MOI of the Kenda Nevegal 26*2.1 at 610grams as the target, a 29" tire would have to weight 490grams. Using the MOI of the Kenda Nevegal 650b*2.1 at 645grams, a 29" tire would have to weight 560grams. Both of which are pretty light tires probably recommended for racing or light XC stuff. No all-mountain stuff or DH runs at your local ski resort.
The type of terrain 29er's really suffered in was super twisty, rolling single track. The longer bike, the constantly changing speeds with the harder-to-turnover wheels, and the constantly changing direction with a front wheel that has a high MOI. If you spin a wheel with both hands on the axle and try to turn it, the wheel will try to resist this change and it will actually twist on the vertical plane. The higher the MOI of the wheel, the more resistance the wheel has to changing direction. This is were the 650b front wheel is able to maintain the nimble and flickable characteristics of the 26.
OK, so far we have a bike that maintains most of it's 26" dimensions/geometry or in my case, it's the same frame/fork. We have a slightly bigger wheel/tire that does roll over stuff better (maybe 1/3 better), and does it without the rotational inertia penalty, and you get to use a real tire that you like (provided it comes in a 650b version). You still have a short, nimble bike basically.
Another thing I discovered is that the jump from 26" to 650b wasn't enough to make the standard triple ring set (22/32/44) feel awkward. Standard triples don't really work with 29er's. Hence the reason they came up with the double 26/39, 24/36, 30/40 setups and so on. This is all dependent on the type of terrain you ride and/race on of course and all of this is only my personal experience.
