Diverging from their road-going ancestors, gravel bikes offer a choice of wheel sizes: the standard 700c as seen on road bikes, or slightly smaller 650b – with many gravel frames designed to be able to accommodate both.
You can often choose between these gravel wheel sizes when ordering your new gravel bike setup, too.
What are the differences between these hoop sizes, and what are the pros and cons of each? Should you have a set of each with different tyres for tackling different terrain?
Here, we take an in-depth look to help you decide which are the best gravel wheels for your rig.
What is 650b?
What does 650b actually mean? The name for these wheels – which are essentially the same thing as 27.5in mountain bike wheels – is derived from a French sizing system, just like 700c (29in equivalent).
The number relates to the approximate diameter of the wheel, including the tyre, while the letter relates to the approximate width.
We use the word approximately here, because this measurement includes the tyre, which can vary greatly in size according to the tyre’s profile. To be more accurate, from rim bed to rim bed a 650b wheel measures 584mm in diameter, with a 700c rim measuring 622mm. Bit less catchy, isn’t it?
When we talk about 650b wheels for gravel riding though, we typically associate this smaller wheel size with larger-volume and width tyres, as there’s usually more tyre clearance in the frameset to accommodate this chunkier rubber compared to 700c wheels.
Therefore, when compared to a 700c wheel with a narrower tyre, the effect of the decreased diameter of the 650b wheel rim is lessened due to this increased tyre size. Effectively, the rolling diameter of the two different setups is usually more similar than you might expect.
Pros of running 650b wheels on a gravel bike
650b wheels are typically used in conjunction with wider tyres – in the region of 45mm up to 2.1in (53mm) – compared to 700c wheels.
This allows riders to run lower tyre pressures, yielding enhanced grip as well as improved comfort. A greater cushion of air can also reduce the risk of pinch punctures, especially when set up tubeless.
Greater grip comes in handy on the rougher trails, while increased comfort is especially useful for longer, multi-day bikepacking tours.
Just as some shorter riders tend to opt for 650b tyres on the road for better handling in proportion to the frame size and reduced toe overlap, this can also be the case when it comes to gravel riding.
Lastly, 650b wheels tend to be stronger than 700c, as the shorter spoke triangle leads to a more robust construction.
Cons of running 650b wheels on a gravel bike
As 650b wheels and tyres for gravel riding are relatively new, there’s a slightly more limited selection to choose from, although this is improving.
Most 650b tyres will be higher-volume options (45mm and above), and usually feature a greater level of tread, like the 47mm WTB Sendero, which is essentially a little cross-country tyre.
If you’ve already got a 700c wheelset, then you’ll need to fork out for a 650b alternative. Thankfully, there are a range of options on the market to suit most budgets, from entry-level builds to bespoke, lightweight and race-focused carbon wheelsets.
Remember you’ll also need inner tubes to match your new wheels – whether that’s for your standard setup or a spare in case of any tubeless setup issues. Check out our full guide on tubeless tyre repair before taking to the trails if this technology is new to you.
Pros of 700c wheels on a gravel bike
As 700c wheels have been the norm for a long time, there is good availability of gravel-specific wheelsets on the market.
Axle-standard incompatibilities aside, there’s nothing technically stopping you from using a road bike or mountain bike wheelset on a gravel bike.
However, remember that you’ll need to check the internal rim diameter before you buy because that largely determines a wheelset’s suitability for use with gravel tyres.
To be precise, ETRTO tyre standards (reviewed in 2020) recommend 21mm internal-diameter rims for 29 to 34mm tyres, 23mm internals for 35 to 46mm tyres, and 25mm for tyres measuring 47 to 57mm (up to 2.2in).
As well as good wheel availability, 700c tyre availability is also better, along with a whole raft of different widths and tread options
Many people claim that a larger-diameter 700c wheel will offer easier rollover, just like a 29er mountain bike setup. However, it’s more nuanced than that when it comes to gravel.
Of course, if both 700c and 650b wheelsets were set up with the same width of tyres – let’s say 40mm – then the 700c wheelset would have a greater external diameter, and therefore greater rollover.
However, most people opt for a wider, higher-volume tyre for 650b wheels, which significantly reduces this difference between the setups’ external diameter, and hence the impact on rollover.
The other primary benefit of 700c wheels that is often quoted is the reduced rolling resistance, which is why 700c is the most popular choice when it comes to gravel racing.
This is a bit of a broad generalisation and is dependent on both tread and tyre width chosen, as well as tyre pressure.
For a more in-depth look at rolling resistance and tyre choice, check out Simon von Bromley’s extensive guide.
Cons of using 700c wheels on a gravel bike
The downside of 700c wheels is that you may be significantly limited to the maximum tyre width you can fit. This may also extend to limits on mud clearance and space for traditional mudguards.
This is dependent on frame design, however, and many modern gravel bikes will have oodles of tyre clearance on 700c wheels.
650b or 700c, what’s right for you?
Just like all cycling gear choices, the best wheel size for your gravel bike will depend on what kind of gravel riding you enjoy and what you’re hoping to get out of your time off the tarmac.
Wanna get rowdy and hit up some MTB trails, or want to cross a mountain range with everything you’ll need for a week? Try 650b.
Looking to go fast over smoother hardpack and tarmac with no compromise on speed? 700c will probably be best for you.
There’s so much more to this question than simply wheel size though, because all these attributes are also dependent on your tyre choice and how this interacts with the wheel size.
It appears preferences are changing too, as James Heaton, WTB’s European marketing specialist explains. “Our wider 700c gravel tyres are growing in popularity. Frame designers are creating bikes with more and more tyre clearance and it’s allowing more riders to appreciate the benefits that come with larger-volume tyres.
“Previously, it was largely riders with 650b wheels that could enjoy the extra comfort and grip that comes with bigger tyres, but it looks like the world of floaty fun is slowly opening up to those with 700c wheels too.”
Ollie Gray, Hunt Bike Wheels’ road and gravel brand manager seems to agree. “It’s common to see clearance for 700 x 45 now, whereas a couple of years ago it was necessary to run a 650b wheel if you wanted a high-volume tyre. Frame design has caught up, so running bigger wheels with bigger tyres is easier than it used to be.”
According to Ollie, sales figures show that 700c looks to be crowding out 650b as the de facto drop-bar wheel size. “However, I don’t think 650b will disappear,” he continues, “and honestly, I really think a pair of 650b wheels can be a bit of a no-brainer when it comes to gravel bikes. It’s a really easy way of making one bike feel like two.”