Cycling has made its habit of rejuvenating itself in its own image and in their efforts to find the best wheel size.
Exploring the many shades of gray between mountain bikes and road bikes threw up a trend from many of the best gravel bikes for offering the option of two wheelset sizes – 650b or 700c – for the same frameset at the point of purchase, as Pinarello Grevil 9 or Specialized Diverge. Bikes such as the Polygon Bend R5 or Merida Silex + 8000-E even come with the smaller size as the default spec.
The central principle behind this approach is that, without a significant effect, the road has a large frame geometry. So, for example, a 650b wheel with a 47mm tire would have a similar total diameter as a 700c with a 28mm tire. This massively broadens the range and, the manufacturers hope, appeal of the bike.
Where did 650b wheels come from?
Rather than a completely new innovation, the 650b is a blast from the past, having been traditionally known to have been known to have been completely damaged into obscurity by the 1980s.
And this is not the first time it has been resurrected. Over the past decade or so it has been introduced into mountain biking as it was before the standard 26-inch rims and the new-kid-on-the-block 29ers. As the road with mountain biking nomenclature, at least in the UK, the imperial measurement is used, it has become known as a 27.5-inch wheel.
So the first thing to realise is that 650b and 27.5-inch are the same thing and, for comparison, so are 29er and 700c. The ‘b’ in the name is also a little confusing as it denotes the width of the rim – but the 650b are wider rims than the 700c, hence their suitability for larger tires in both mountain bikes and gravel bikes.
So why choose 650b wheels?
The ability to put bigger tires, with bigger treads on 650b wheels will give riders more grip on unstable road surfaces and increased dampening to soak up the bumps lumps that come their way. A vital part of this is the ability to run the tires at a lower pressure, which helps to improve their traction with the ground.
The smaller rims will also give gravel riders the option to downsize. Choosing a turret gives a slightly smaller overall diameter and will give you a more responsive ride experience, especially on short, sharp rises in tight or tight cornering.
In this scenario, it has the effect of lowering the height of the bike so it can appeal to shorter riders and, in fact, more and more bikes – gravel included – now come with a 650b wheelset rightly recognizing that a one-wheel-size-fits-all approach is unsatisfactory.
Another argument for the smaller rims is the weight advantage. Simply smaller and using less material, the 650b wheelset will weigh less than the equivalent 700c wheelset. Many of the best gravel wheelsets, however, are now available in many sizes, allowing for riders to choose their own adventure.
Why choose 700c wheels?
With all those great reasons to choose 650b tires on gravel, why would you choose to stick with those tired old 700c roadies?
Well, unfortunately, it’s not quite as simple as all that.
The main counter-argument to 650b is that old chestnut of rolling resistance. If you’ve ever tried riding a mountain bike with lovely big tires for eating up the mud and grime on a smooth, flat tarmac for any amount of time you’ll know how much of a grind it can be. Fitting a tire with thick tread and running it at lower pressure will enlarge the contact area with the ground and increase rolling resistance, which is the energy lost from the compression and relaxation of the part of the tires that touches ground. Add in the mix that knocks out deep in the treadmill and it will slow you down.
And, for the same reason that 29ers became popular in mountain-biking, the bigger 700c rim will roll over better than the 650b if both are using the same size of tires.
There is an argument for aerodynamics as well, with the thicker tires creating more drag, a factor that comes into greater impact the faster you go. However the fact that the Pinarello Grevil 9 is available with 650b wheels does not fly in the face of that logic somewhat.
And while there is the potential for a large number of options, it is not possible to do so. . Downsizing too much on tire size will also start to affect the carefully considered geometries of the bike frame.
650b is here to stay?
In fact, there is already the indication that gravel bikes’ dalliance with 650b might be coming to an end.
When Cannondale released the dual-suspension Topstone Lefty in the summer of 2020, it was fitted with 650b rims and 47mm tires, following on from its predecessor the Slate. But the latest versions of the Topstone Lefty have 700c x 44 as standard, only losing out on 3mm of tire thickness. Frame designers are getting better at accommodating wider tires on 700c wheels and BMC’s 2022 version of the URS One has clearance for up to 45mm on 700c rims.
So while the bike-building world can not be foreseen, choosing a bike designed for 650b tires may not be the best option for future proofing – though, the vast majority have 700c. Given gravel bikes are almost exclusively fitted with disc brakes, there are 650b wheelset with gnarly tires for the rough stuff and a 700b wheelset with more tarmac-friendly treads.
In summary, which is better – 650b or 700c?
Still no clearer? Well, that’s because – like all things bike – there is no right or wrong answer. Essentially, deciding which type of riding is going to be influenced by what type of riding you will be doing with them.
Gravel bikes originated in the USA on – you guessed it – gravel roads, but for most riders it’s probably the only terrain you’ll be using it on.
The gravel tag is bikes across quite a wide spectrum, from slightly thicker-turret road bikes at one end to, arguably, drop-bar mountain bikes at the other. Their main draw is the versatility they offer, giving riders the ability to stray off the tarmac and deep into the countryside.
This picture will look very different to different cyclists – for many, the reality will be spending most of the time riding on tarmac, only diverting off on badly maintained roads every now and again. If the rolling resistance factor is heightened – on tarmac, 700c tires are going to be faster than 650b in most cases. Shimano, for example, says 700c is the wheel of choice for its top gravel racers focused on speed and efficiency.
But if you intend to push your gravel bike to its extremes, straying and staying on tracks and trails where the ground is constantly shifting beneath your wheels, you are going to benefit from increased grip and traction .
A key difference between gravel bikes and cyclocross bikes is that they are often less race orientated, geared towards adventure and exploration. So 650 is the potential for a nimbler, more playful ride.
On this note, comfort and control should not be overlooked. The increased contact with the ground that results from thicker tires is usually associated with rolling resistance – and the detriment to speed – but a lesser-known factor also affects the pneumatic trail, which impairs the stability and handling of the bike.
When steel-frame specialist Fairlight designed its gravel-oriented Secan and Faran models, pneumatic trail was a key consideration. Both bikes come with the option of 650b or 700c wheels and Fairlight uses the example that a 650 x 47mm tire would feel more stable than a 700 x 28mm tire if they were used on the same frame, each at their recommended pressure.
Interestingly, among the Faran’s a list of intended uses – the French touring discipline that first popularized the 650b size – perhaps proving the wheel has, in fact, come full circle after all.