MTB Wheel Sizes Guide: 650 And 29 Explained3 January 2022
The current crop of MTB wheels sizes is displayed in the infogram below.
As you can see there are five apparent different sizes. In reality, though it's simpler; there are actually only three different wheel /rim sizes and on the 650 and 29" rims you can run normal, or + size tyres. Three wheel sizes- and bigger tyres for the two largest wheels. It's that simple. Now let's look at how they differ physically and how that converts into different handling and suitability for differing riding styles.
The Measurements, BSD and OD
BSD stands for Bead Seat Diameter. This is the measurement from one side of the rim to the opposite side, measured exactly from where the tyre bead sits on the rim. It's the effective width of the rim and is used pretty much universally to describe the rim size.
The other measurement is OD, Outside Diameter. This measurement is taken across the width of the tyre and wheel and is the effective standing height of the wheel with a fully inflated tyre attached.
26" The original (and Best?)
Originally MTB's came with only one wheel size, the humble 26". Bigger than a BMX and smaller than a road bike wheel the 26" wheel offers a compact wheel that can be built into a stiff structure without excessive spoke tension, it helps keep rotating mass down and allows for nimble and agile wheel that can quickly respond to steering input. Pros: small, light, stiff, agile.
650b (27.5") The Goldilocks size?
25mm wider at both the rim and tyre tread the 650b wheel increases circumference to give a slightly larger contact point for better grip and an improvement in roll-over that allows the wheel to roll over objects as opposed to being pushed back by them. Pros: Improved rollover and larger contact point.
650b+ (27.5+) Plus Size 650 wheels
There's no change in the rim size with 650+ wheels, when compared to normal 650b wheels, all that is changed in that you're now rolling on fatter rubber. Typical plus size tyres are considered as 2.8" and over, with 3" being the norm. The plus size tyre gives you a much larger contact point, more tyre volume that allows for lower pressures for extra grip and the ability to soak up bumps and lumps without needing as much suspension as a skinnier tyre. In terms of circumference the 650+ wheel and tyre combo with 3.0" tyre is actually as big as the 29" wheel with a 2.3" tyre. Pros: enhanced contact patch, better grip from lower pressure, improved rollover ability.
29" (29er) Wagon Wheels
A standard 29er rim has the same circumference as a road bike rim, it's the biggest MTB rim size and has a BSD of 622m. Add a 2.3" tyre onto this and you have a wheel that stands 740mm tall at the tread and rolls over trails obstacles more easily than any of the smaller wheels. You get a better contact patch than with the standard 650b wheel and the slightly larger wheel carries its momentum better. They're now pretty much the standard for skinny XC wheels where their roll over ability compensates for a lack of suspension travel on skinny XC bikes. Pros: big contact patch, excellent rollover, carries momentum. (The standard road tyre, 700c, shares the 622mm BSD measurement with the 29" of-road tyre- as if it isn't confusing enough already!)
29"+ (29er+) Wagon Wheels Go Fat
There's no change in the rim size with 29+ wheels, when compared to normal 629" wheels, all that is changed in that you're now rolling on fatter rubber. Like their 650b+ cousins typical plus size tyres are considered as 2.8" and over, with 3" being the norm. The plus size tyre gives you a much larger contact point, more tyre volume that allows for lower pressures for extra grip and the ability to soak up bumps and lumps without needing as much suspension as a skinnier tyre. In terms of circumference, the 29+ wheel and tyre combo with 3.0" tyre is actually 38mm taller than a standard 29" wheel with a 2.3" tyre. Pros: enhanced contact patch, better grip from lower pressure, improved rollover ability.
Of course, there's no free lunch in cycling. Bigger wheels weigh more, they have to use stiffer thicker spokes to built up in equally stiff wheels as their smaller counterparts. The tubes, weigh more, the tyres weigh more, and all that has to be accelerated and decelerated as you ride. Larger wheels have greater angular momentum so they are a little harder to steer, you gain 'flickability' but you get a bike that stands up better in the corners. Ying and Yang.
We like Plus size wheels, they offer lots of the fun and excitement of a dedicated fat bike, but do it in a much more versatile package. We're not sure how much the 29+ will catch on, that's a lotta weight to be hauling around on a bike. For 650+ is where the real sweet spot is. You retain the agility and nimble feel of a smaller wheel, you get most of the fun factor and added traction of a Fat Bike wheel but with less weight penalty and you get the rollover of a 29" wheel but with far greater versatility. At the end of the day, with any Plus size bike you can always swap back to the standard tyre size whenever you want. for us, that's where the magic lies, in being able to choose the right bike set-up for the ride ahead.
If Plus size wheels sound like they might be your thing check out the bikes below, there's more coming soon.