Why 650b on gravel bikes?

It'll come, I bet.  Partly marketing of course but also the same way 29 has come to dominate xc

The 650b was/is to fit bigger tyres in a conventional-looking frame but I'm sure 47/29 is faster than 47/27

Bigger tyre so it can have more volume.

More volume can make for a more comfortable ride offroad with no suspension.

Tyre clearance without stupidly long back ends whilst allowing the use of bigger chain rings and front mechs.

Cos it makes the gravel come alive, obvs.

I  have an Open UP, which i think is probably classified as being at the roadie end of the niche.  I have a set of road wheels and also a set of 650b gravel wheels to try and properly differentiate the two, rather than them simply be a case of road wheels with knobbly tyres, and I am glad I did that.

Geometry, full 29er gravel bikes feel quite slow and difficult. So 700x40something or 650x50something are a compromise.

Weight, small wheels are lighter and stronger for the same crossection (so can be lighter still).

Terrain, you just don't need that extra rollover ability when your designing a bike for smooth-ish trails. Gravel bikes can be pretty capable, but I'm not smashing my front wheel up foot high steps.

I have a 650b GB that can also use 700c up to a certain diameter. Certainly better damped with the wider and taller 650b tyres. Can be run at lower pressure too without risking damage.

I guess it keeps the geometry closer to the roadie roots as well? 29er wheels with MTB tyres would need longer bikes and slacker forks or higher front ends. They are available but 700c/650b is far more popular.

All good points above on geometry compromises. Basically add wheel OD for bump amplitude, tyre volume and pressure for bump frequency. There's crossover but it works as a basic way to look at it. A 29er MTB benefits from both but a gravel bike tends to be more about frequency, small bumps at speed.

When I bought my Ribble TCR I opted for 700c wheels as most of the gravel roads around Munich are pretty smooth and could be driven with a lowered sports car.

Having ridden lots of gravel in Spain I'm tempted to look for a set of 27,5 wheels. The tracks there are much rougher and the extra volume of the tyres would definitely make the ride more comfortable and reduce the risk of punctures.

I think it was all about clearance to begin with. My bike fits 700x50 so I see no reason to go 650b and works well with road wheels and gravel wheels

Bigger tyre so it can have more volume.

More volume can make for a more comfortable ride offroad with no suspension.

But if my XC bike is rigid with 29x2.35 tyres, why get a gravel bike with 27.5x2.0?

If a gravel bike had 29x1.5 or 29x2.0 it'll be faster.

we chose 650b x 49 for Mrs BS’s gravel wheels as it’s pretty similar rolling radius to the 700c x 32 road tyres she runs on another set of wheels, that way the bike feels the same no matter which she has on.

Many earlier models of gravel bike were optimised for 700x40, so in order to fit bigger tyres, so fitting 650B rims meant you could squeeze in tyres up to 50mm. More recent models from some brands have pushed the wheel size envelope to accommodate 700x50 and more. When riding chunky stuff, give me the bigger wheel size every time and TBH it doesn’t make much difference on mixed surface rides.

I think I'd prefer something that can take up to 29x2.00. If I needed wider tyres I'd use the XC bike.

As others have said, it’s about having higher volume tyres whilst maintaining similar geometry.

Although to be honest, I’ve found having super high volume tyres on a gravel bike to be a poor substitute for front suspension on a mountain bike.

Also, what with 28 inch wheels?

Sorry, meant why do some come with 28 inch wheels and not 27.5 or 29?

why do some come with 28 inch wheels and not 27.5 or 29?

Wait until you find out that 28” wheels are the same size (rims) as 29er 😳

Because the only decent mud tyres bitd where senderos, only available in 27.5. Thank fully 700c has caught up with the likes of tundras and cinturato S.

650 and the associated bigger volume is good for bike packing over MTB terrain too

Although to be honest, I’ve found having super high volume tyres on a gravel bike to be a poor substitute for front suspension on a mountain bike.

That's because they are very different bikes. Comparing the two and hoping for the same outcome is a hiding to nothing

Wait until you find out that 28” wheels are the same size (rims) as 29er 😳

Oh the joys of cycling. It's like needing multiple languages to buy one thing.

For me it’s the 2 sets of wheels thing

650b with 47mm tyres is the same diameter as 700c with 38mm tyres

But I’m sure 700c with 47mm tyres would be better if you were building a purely off road bike

Because, at around the time that 29er riders realised that you could go 27.5 and have a bigger tyre #plusbikes, 'gravel' bikes became a thing and riders realised they could do something similar for a bit not gnarabilityness #roadplus. And to make their bikes less similar to CX bikes.

It's an easy way to get a different bike too, by having a second wheel set. I've done it on my 29er hardtail and my hipster hybrid - it's fun. Which is all good.

I agree on a gravel / do it all bike on less twisty and gnarly terrain, a faster rolling 700c/29 wheel is surely better?

Next bike I want 45mm 700c with full touring mudguards capability.

Initially I was think road bike but a gravel bike with 700x30 and 700x whatever CX bikes use might be best for me for long exploring rides or winter forest rides.

I reckon 29er wheels (and by that I mean 700c with big chunky tyres) and a gravel bike with road friendlyish geometry might give you toe overlap issues. A bike designed around long reach and a short stem as well as a slack head tube would alleviate that, but then that's not all that for when on the road.

Gravels are all about compromise and for me a bike with 2 pairs (or 3 🤭) of wheels - 700x38ish and 650b X 2.1 or maybe just 47mm covers things well.

Another way to look at is a road bike capable of taking wider (maybe 35mm) tyres. I'd be using it mostly on the road.

They're called "allroad" bikes I think.

That new Vitus looks like a good option for road & light gravel:
https://www.chainreactioncycles.com/p/vitus-venon-evo-gr-rival-axs-gravel-bike

While I get the advantage of 650b allowing more volume in approximately the same rolling radius, I'm actually happy enough with 700x40c to not really want to experiment and kind of feel like if really want more cushioning then the obvious choice is then a 29er MTB.

Having said that I do think Gravel bikes might benefit more from riders experimenting with different width and/or pattern tyres front and rear (much like MTBers have for a long while now). Even roadies are starting to recognise that front and rear tyres do slightly different jobs and hence have started playing with the width and pressures to taylor the ride, that has real value. Lots of graveilsts that I meet (and me too currently TBF) seem to just run a matched pair without really considering it...

If Paul Brodie says 650B.....

-

-

https://www.youtube.com/watch?v=-1N3dHq6nbc
- https://www.youtube.com/watch?v=-1N3dHq6nbc -

I agree on a gravel / do it all bike on less twisty and gnarly terrain, a faster rolling 700c/29 wheel is surely better?

That's the Q isn't it, is the 700c actually faster rolling? I'd say there's a crossover point where a (eg) 700x42 and 650x50 are pretty much equivalent on basic gravel tracks. One may feel more responsive on road, the other cushier over roots or rocks.

If the tyres are equivalent in theory the larger 650x50 should roll more easily on easy gravel tracks as it's able to be run at a lower pressure. I'd agree with that from my own riding, but also that a 700x50 is easier rolling still. So it's really down to the geometry and the compromises between road and off-road feel / handling.

My gravel riding tends to become a bit road linked MTB-lite, so I find higher volume 650b has more net positives than medium volume 700c.  But that’s just an opinion.

I always assumed gravel bikes would move more and more towards 700c as geometry is refined in the same way as mtb has done. Some people like Sonder are already designing in good 700c clearance

But if my XC bike is rigid with 29×2.35 tyres, why get a gravel bike with 27.5×2.0?

If a gravel bike had 29×1.5 or 29×2.0 it’ll be faster.

It's all relative:

A downhill bike with 29x2.6 tyres is faster than a road bike with 700x28c.

And a road bike with 700x28c is faster than a downhill bike with 29x2.6 tyres.

Just depends on the surface.

TBH like most bike things if you have to actively talk yourself into buying it then it's just marketing and you probably don't need it. I have a gravel bike, but don't have an xc bike because round here the XC doesn't really warrent anything more.

XC is not really a useful term for the next level up from the tougher end of gravel. Did you see the XC at the weekend? There are a shit ton of levels between the two in 2023. Mince-core, mince and mince-lite are three (non PC) levels in there somewhere!

What I do agree with though....once shod with even fast rolling 2.1 rubber, no matter what the size of the hoop, your intended percentage of tarmac probably needs to be minimal. That's how I ride mine when it's all estate road at the easy/fast end with a good few sections where I'd ideally be on a mountain bike but with chunky rubber (and a dropper) on the gravel bike I can fudge it and still have fun.

Lots of graveilsts that I meet (and me too currently TBF) seem to just run a matched pair without really considering it…

Been running cinturato M up front and H out back for the "summer" comedy rear drifting in the wet though

<p style="text-align: left;">This would be for mostly tarmac and a little off road. Maybe 30 miles on tarmac and 5 to 10 off road per ride.</p>