More rubber and air volume with no increase in overall wheel/tire diameter. That matters more in smaller frames as you surmise, but it can also effect stability, agility and handling in any bike. Once you get over 35mm wide tires I personally find that a 700c wheel/tire starts to feel a little ponderous. Some people appreciate the stability, rollover and momentum advantages that a larger overall diameter brings but I like a bike that's a little quicker to react to turning input but that still has a generous width and air volume for secure cornering, cushion and float in loose dirt, so 650bX42-52 suits me well. It really comes down to personal preference-it's nice to have options!

 

I'm probably going to throw a set of 27.5 wheels with 2.2" tires on my Checkpoint. For some rides (those that include singletrack), I'd like a little more tire than my 700x38. But I already have toe overlap, and that becomes more of an issue on singletrack, so I want to keep the diameter in check.

If not for the toe overlap, I'd just run bigger 700c tires.

 

If you want to run big tyres, and by big I mean around 47mm+ 27.5 makes more sense on a gravel/adventure/big tyre drop bar/whatever bike.

It's MUCH easier to design a frame to accept a wide 27.5 in the back. Most of the time gravel frames use 68mm BB shells and have clearance for a road compact crankset. This makes things complicated compared to MTBs when you want to cram a big tyre in there.

Another big one, much less toe overlap. It's super annoying. Unlike MTBs gravel bikes have steep head angles and shorter top tubes. Throw on a big 622 tyre, maybe fenders too and the toe overlap can be deadly even on larger frames.

Multi wheelsize compatibility. Generally speaking a 622 28-30mm tyre is about the same as a 650x47mm in terms of height. So if you want you can switch back and forth between a more offroad and a more road oriented setup without affecting BB height etc.

 

As I go further down the rabbit hole of gravel biking I see plenty of manufacturers are selling 27.5 inch specific gravel bikes/frames and I don't see why.

We all know why 27.5 is sometimes preferable for mountain bikes but none of those advantages appear to apply to gravel riding.

Unless you're a shorter rider, 700c/29 inch wheels seem like the only choice for covering the varying surfaces of gravel grinding. So what I am missing about 27.5 for gravel?

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I'm 6'3 and my custom frame with 700x35 tires has a touch of toe overlap. It's surprising how much toe overlap constrains the design. 650b (sorta) frees average sized riders from this limitation.

 

The reasons are better traction off-road, better handling in some conditions and a bit more plush of a ride.

650b wheels/tires are really fun to ride- Especially for a higher % off offroad.
I know I descend on dirt faster on them, I also corner and take switchbacks better- Especially uphill switchbacks.
650b does eliminate overlap for me.

But I will say that 650b is not faster in most conditions (like a race- where speed is paramount)-
Maybe there are exeptions depending on the rider and the course- But that is my take.

I have both sizes for my bike 650b and 700c.

 

It's easier to keep road bike dimensions with 650 and a high volume tyre. Fits in the same silhouette (plus width obviously) as a 700c with road tyres.

 

Interesting info. I'm surprised gravel riders wanted to run such wide tires but it make sense of you're mixing in singletrack and looser dirt roads.

The line between gravel and XC bike continues to blur. Lots of interesting ways to set up a gravel bike these days.

 

Interesting info. I'm surprised gravel riders wanted to run such wide tires but it make sense of you're mixing in singletrack and looser dirt roads.

The line between gravel and XC bike continues to blur. Lots of interesting ways to set up a gravel bike these days.

Might be just me but i'll call most drop bar offroad things gravel bikes.
To me a gravel bike is more like a versatile use it everywhere bike. Big tyres certainly help.

On Bombtracks website this thing is in the gravel bike category and they call it a "hyper-gravel bike".

 

Bombtrack is such a cool name for a bike company. Love it.

That rigs looks like my gravel bike which is a steel XC hardtail with drop bars and a carbon MTB fork. The ability to run real mountain bike size tires opens up a whole new level of riding.

 

Pictures? [emoji106]