Only just heard of them

I’ve had a spin on a cannondale slate and loved it. Seems to get the smooth ride of a big tyre but accelerates faster than a 700x40c would.

Would definitely consider putting it on my bike if I change the wheels.

I think you’ll find the touring and roughstuff crowd have been riding 1.5″ and wider tyres for years, but didn’t realise you need some marketing wonk to create and new sub-niche and give it a name to realise how good it was…

Thats not the point…

The idea is a 650b wheel with a 41c tyre is the same diameter as 700x23c so it makes it feel a lot sharper/racier than a massive 700 tyre but still comfortable. Best thing is you could have a frame that only takes up to a 28c and you could then fit a very big 650b tyre.

I’m a little with dovebiker on this – people like Jan Heine (https://janheine.wordpress.com/?s=650b) have been running 650b large volume tyres for a long time, but i suspect that’s stayed under the mass market radar as the focus has been on off-road.
Seems the new excitement over gravel bikes/racing has forced the market to look again at comfort and have re-discovered that large volume tyres are as fast as thin ones if the carcass is supple enough.

this might be of interest https://www.bikeman.com/bikeman-blogs/650blog/1771-650b-conversion-guidlines

It’s all surprisingly interchangeable.

Grand Bois Hetre tyres are a bit of an institution for gravel roads in 650b; around 42mm / 1.7″. They’re made by Panaracer in Japan and mine came from Acycles in France.
Apparently, they’ve been around for many years; have had them on the 26″ tandem for two summers, and find that they’re ‘sprightlier’ than the original 26×2.0 road slicks that it came with. The tandem is run with the original 26″ wheels in winter, slick on the back and a semi on the front. The outer diameter of the 42mm 650B is about the same as a large 26″ tyre or small 700c.