Bike wheel sizes explained: 700c, 650b, 29 and 26

The evidence here is moderate: real tests, but not many of them. 10 sources, listed at the end.

What the tests say

What the numbers mean. ISO/ETRTO sizing uses bead seat diameter (BSD), where the tire bead sits on the rim:

• 622mm: road “700c”, hybrids and “29-inch” MTBs (“29er” is a marketing name for wide 622mm tires)
• 584mm: 650b, sold on MTBs as 27.5”
• 571mm: 650c, for triathlon/TT bikes and smaller road riders
• 559mm: most 26” MTBs¹

Outer diameter. Total diameter is roughly BSD + 2 × tire height². Using nominal width as a stand-in for height (calculated here; real tire heights run a little lower and vary):

So 650b x 47 matches a 700 x 28, not a 700 x 40. WTB, which launched the 650b x 47 Horizon “Road Plus” tire, describes it as having “the same overall wheel diameter as a 700 x 28c tire”⁶. To match a 700 x 40, a 650b tire would need to be about 59mm (2.3”) wide. Swapping a 700 x 40 for a 650b x 47 would lower the bottom bracket by roughly 12mm. For road racing, the UCI caps wheels at 700mm including the tire. By Elite Wheels’ figures that leaves about 38–39mm nominal on a 622 rim (38mm ≈ 697mm, 39mm ≈ 699mm), and a 40mm tire comes to about 701mm².

Stiffness and weight. Damon Rinard’s lateral stiffness tests found 650c wheels about 25% stiffer than 700c on equivalent hubs³. Wikipedia’s 29er article cites Mavic engineers saying 29” wheels were 39% less stiff and 10% heavier than a similar 26” wheel. That’s a secondhand manufacturer claim⁴.

Rollover vs acceleration. The usual trade-off is that larger wheels roll over obstacles more easily and carry momentum on rough ground, while being heavier, slower to accelerate and harder to fit on small frames⁴. Roues Artisanales measured how much energy it takes to accelerate wheels. An 820g wheelset needed 74 J and a 1,973g wheelset 141 J. For a 70 kg rider on a 4.1 kg bike, that difference is worth about 7 W over a 10-second sprint from 0 to 30 km/h⁵.

Mountain-bike trials: mixed. Ten Swiss national-team riders on a simulated cross-country course were faster on a 29” bike than a 26” (304s vs 311s, about 2%) at similar power⁷. In 55 cyclists on a 2,110m circuit, 29” was faster but transmitted more vibration and raised heart rate, with similar power⁹. Nine trained riders lapping a 3.48km course on 26”, 27.5” and 29” showed no performance differences⁸. In 49 highly trained riders on short rocky climbs and descents, times didn’t differ, but 26” gave more vibration at the coccyx uphill and 29” more at wrists and ankles downhill¹⁰. Nobody has published a study of 650b vs 700c on road or gravel, or of mullet setups.

Mixed wheels and rider height. “96ers” ran a 29” front with a 26” rear, and Trek sold a 29”/26” bike in 2007⁴. Today’s 29/27.5 “mullet” follows that idea. Toe overlap and standover are the usual concerns with 29ers for riders under about 5’5”⁴.

The numbers

• Bead seat diameters: 622mm (700c/29”), 584mm (650b/27.5”), 571mm (650c), 559mm (26”)¹
• Approximate outer diameter = BSD + 2 × tire height². So 650b x 47 ≈ 678mm ≈ 700 x 28, while 700 x 40 ≈ 702mm (calculated here)
• UCI road maximum: 700mm total, about 38–39mm nominal tire on 622 rims²
• 650c wheels were about 25% stiffer laterally than 700c (Rinard)³
• 29” wheels 39% less stiff and 10% heavier than 26” (Mavic claim, via Wikipedia)⁴
• About 7 W over a 10-second 0–30 km/h sprint, ultralight vs heavy wheelset⁵
• Swiss national team (n = 10): 304s on 29” vs 311s on 26” over a simulated XC course⁷

Myths and disagreements

• “650b x 47 = 700 x 40 in diameter.” By the standard formula it equals about 700 x 28. A 700 x 40 is about 24mm taller².
• “29ers are proven faster.” Two of four MTB studies (9 to 55 riders) found 29” faster on cross-country courses and two found no time difference. Two found more vibration on the 29” in some sections⁷⁸⁹¹⁰.
• “Rotating weight transforms performance.” Roues Artisanales’ data shows a few watts, and only during hard accelerations⁵.
• “27.5 is a different size from 650b.” They’re the same 584mm BSD¹.

Picking a wheel size

• To fit fatter gravel tires in the same frame, 650b x 47–50 can work. Expect a lower bottom bracket than with 700 x 40, and check the frame maker’s approved sizes².
• When you change wheel size or tire width, update your bike computer’s wheel circumference.
• For MTB, choose by frame fit and terrain, and demo if you can. The studies don’t settle which size is faster⁷⁸⁹¹⁰.
• Shorter riders: check toe overlap and standover before choosing 29” or 700c on small frames⁴.

Sources