What is 650b?

By What Mountain Bike | Thursday, August 9, 2012 9.00am

The 'inbetweener' 650b mountain bike wheel size has been the talk of the industry for a while now and was the hot trend at this year's Sea Otter Classic. But just what are the advantages, and is it a fad or the future? We delve a little deeper into the issue.

What is a 650b wheel?

The term 650b comes from French tyre sizing nomenclature - the number indicating the approximate diameter of the wheel, including tyre, and the letter corresponding to an approximate width.

A 650b wheel is one that's about 27.5 inches in diameter, when measured from tyre edge to tyre edge. This compares to 26in for a standard mountain bike wheel and 29in for so-called "29er" wheels. 

Bear in mind that because tyre sizes vary, this is only a rough measurement. A high-profile tyre fitted to a 26in wheel will usually be 27in or more in diameter. In fact, fitting high-profile tyres to 26in wheels will give you a good idea of how 650b wheels with normal tyres will feel.

What are the advantages of 650b wheels?

650b wheels offer most of the quick acceleration feel and nimbleness of 26in wheels, but with a nod towards the smoother-rolling feel, extra stability and enhanced traction of 29in wheels. They're also more suitable for smaller riders than 29ers. “650b is a good ‘best of both worlds’ bike,” says Steve Wingham of KHS, one of the first brands to pin their colours to the middle wheel size. "Especially for the sub 5ft 9in gang who often have a bit of ‘big brothers’ bike’ feel on a 29er."

Frames designed for 27.5in wheels can be built using well-proven 26in wheel geometry and usually end up looking neater than the often gangly looking 29ers, with slightly better clearances. 650b wheels are lighter and stronger than comparable 29er hoops, and the same is true for forks. Many bike companies are on board with the new wheel size, with the likes of KHS, Jamis and Seven already offering 650b bikes and new models in development from Intense, Norco and Scott.

Kirk Pacenti (www.bikelugs.com) has been promoting 650b wheels for years. “On a hardtail it’s a bit of a wash," he says. "But 650b is a rational choice for almost all full-suspension designs. When taken as a whole, a 650b full-suspension bike is a lighter, stiffer, more compact and agile machine that, in my opinion, is a better handling bike over a wider variety of terrain than a 29er is.”

The middle wheel size could prove particularly popular at the 'gravity' end of the riding spectrum, where the 26in wheel still rules supreme. As BikeRadar forum guru and What Mountain Bike columnist Supersonic says: "The lack of long-travel 29ers has been largely to do with the physical constraints of what you can cram into the frame." It's much easier to squeeze 27.5in wheels and six-inches-plus of suspension travel into a compact, 'chuckable' frame than 29in wheels.

The khs team rode 650b bikes in the dual slalom race at this year's sea otter classic:

KHS reckon the 'inbetween' wheel size is the future for gravity racing they used 27.5in wheels at the Sea Otter Classic dual slalom and are working on a 650b downhill bike

How about the disadvantages?

A new wheel size means new wheels, tyres, forks and frames. Choice of these components is limited at the moment, although lots of manufacturers are adding 650b options to their ranges for 2013. FOXRockShox and Magura will have 650b-ready forks available before 2013, and X-Fusion and White Brothers forks already accommodate 27.5in wheels.

DT Swiss, Reynolds, Stan's and SRAM are among many wheel manufacturers offering or developing 650b rims, and major tyre makers including Kenda have created moulds for 650b tyres, responding to requests from the industry and joining smaller manufacturers like Pacenti. This was an issue when 29ers started out as well, but within a year or so there were loads of great tyres, rims and forks to suit the bigger size.

Early 650b bikes will be expensive due to the limited parts choice and simple economies of scale, but prices should drop over time. The bigger wheels, and the longer frames sometimes needed to accommodate them, mean 650b bikes will also be heavier than equivalent 26in-wheeled bikes, though there won't be as big a weight difference as with 29ers.

Critics also argue that the 650b wheel is a jack of all trades but master of none – it doesn't smooth out trail undulations as well as a 29er wheel or provide as much traction, yet it's slower to accelerate than a 26in wheel and not as agile at slow speeds.

"We’ve only just reached the point where there’s enough supporting suspension fork, wheel and tyre product to service different 29er rider preferences," says BikeRadar test stalwart Guy Kesteven. "Adding another half-arsed inbetweener wheel size seems at best confusing and at worst like a cynical attempt to create another wheel size related sales spike. I may be proved wrong, but right now the Emperor’s new wheel size looks pretty naked to me."

We've been running 650b rims on a pacenti hardtail for a while now: we've been running 650b rims on a pacenti hardtail for a while now

We've been running 650b rims on a Pacenti hardtail for a while now but it's only now that the major mainstream bike manufacturers are getting involved

If 650b wheels have advantages, why have mountain bikes traditionally used 26in?

Regular mountain bike wheel size arrived at 26in as much by accident as by design. Many of the old steel-wheeled clunkers that gave birth to the original mountain bikes had 26in wheels, so early aluminium rims were simply made to replace them, although the long wheelbases and big tyre clearances of frames back then could easily have accommodated bigger wheels and tyres.

650b rims and tyres were readily available, as were the old 27in road rims, which effectively standardised into the 700c road rims of today. It’s those 700c road rims that have had their rim beds widened to take fatter 29er mountain bike tyres. By the time the early mountain bike bandwagon got properly rolling, 26in rims and tyre choices were becoming more plentiful than 650b or 27in. Inevitably most frames were being designed around 26in wheels too, so the habit stuck. 

Well, it stuck among most of the mainstream builders. A few custom builders were still meddling with big wheels on mountain bikes and it wasn’t long before Gary Fisher got people talking again by launching a range of 29ers. A few others followed and within a year or two the fork, rim and tyre manufacturers started creating more choices for the slowly growing base of big wheel fans.

Now that 29ers have earned a place in the mainstream, 650b has returned to the spotlight. It makes sense that wheels measuring half way between the established 26in and 29in standards should be considered.

This picture shows the difference in size of rigid forks designed for, from left to right, 26in, 27.5in and 29in wheels:

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This picture shows the difference in size of rigid forks designed for, from left to right, 26in, 27.5in (650b) and 29in wheels. While 29in wheels need a whole new fork chassis, some 26in forks can be adapted to fit 27.5in wheels by simply adding about 0.75in more room under the arch

So, will we all be riding 650b bikes in a couple of years?

With 29er wheels taking some 10 years to achieve anything like genuine acceptance, we don’t expect 650b to take over the world overnight. Nor do we expect 650b to supersede the other wheel sizes in time, rather to complement them and give bike designers the freedom to deploy the best size for the task, just as they already do with frame materials, suspension travel, transmission technologies and so on. What is certain is that 650b is coming. And sooner rather than later.

This feature is based on articles originally published in What Mountain Bike magazine, available on Apple Newsstand and Zinio.

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