I’ve just set my FS up 26 rear 29 front.

First ride tomorrow night.

I’ve done 26r/29f as a winter bike.

The donor bike was an on-one 456 with 26r rigid fork and 29r front wheel.

For that particular combo it gave me a rigid bike with similar geometry to the 456 with a sagged 140mm fork but with better mud clearance and lighter weight. The bigger wheel out front gave the usual 29r thing of rolling over bumps easily and a slightly bigger contact patch.

As a rigid combination it worked well but I think mainly because it was the best way of achieving the right overall geometry from the frame rigid (it felt “twitchy as” with that fork and a 26″ wheel).

Liteville have been selling bikes like that for years…

From their website….

“Scaled Sizing
The correct frame size of a bike influences the performance capability of an entire bike. Yet it is not only the frame size that is to fit the rider, but also the wheel size. This is why our frames, depending on the frame size and the body height of the rider, can be built with different wheel sizes (24”, 26”, 27.5” and 29”).

Different wheel sizes? Because it makes sense!

Front: rolling smoothly and safely.
When you hit the dirt, your front wheel has the toughest job. That is why it should be as big as possible without having disadvantages regarding the chassis design.
Passing obstacles becomes a lot easier. On top of this, the tracking as well as the braking performance of the front wheel are improved. And there is more to it! The increased gyroscopic force additionally improves the stability of the bike where it is needed most. The rougher the trail is, the more you will have your front wheel support your riding.

Rear: Light, stiff, highly agile
A moderate wheel size in the back makes possible kinematics that allow for a short rear end of the bike. At the same time, passing obstacles with the bigger front wheel is a lot easier, no matter how big the obstacle is. In comparison to a conventional mountain bike with two equally sized wheels, the inertia is reduced and thereby leads to easier acceleration and more control. It goes without saying that a bigger wheel in the back, even for small riders, would allow for better rolling in flat sections, yet in challenging downhill sections, equally sized wheels provide less stability and thus less control. It is not what you expect from a bike that is designed for tough terrain!”

I tried it on my Geometron but I’m not usually a fan of 29ers.  It made it feel like a 29er.  I changed back to 27.5 and the trails regained their life 🙂

I’ve done it on my 2019 Specialized Stumpjumper 29 with a 27.5×2.3 on the rear and a 29×2.5 WT on the front. It’s meh…
I’ve also tried 27.5 back and front on that bike. That’s better.
I would say i’m not good enough to tell the difference but that wouldn’t quite be truthful. I’m not good…but I can tell the difference.
On steep stuff it’s great. i’m 5′ 7″ and I don’t scrape my arse on the back tyre with that set-up (to be expected) but on everything else it does feel a bit weird – too slack imo.
27.5″ back and front on a bike designed for 29″ is actually ok and is definitely more nimble but after all my experimenting I have reverted to 29″. I guess I’m just more used to it.
Does that help?

69er Inbred. Rigid. SS. Makes a big difference (in a good way) to running it 26 front & rear.

Tried that a few years ago with the NS Eccentric. Never happened again, make of that what you will 😉

Ah, me was lying! I’ve done B+/29+ on the Cookie:

I test rode a couple of geometrins. I found the same as caldog above. It was a little bit easier to ride in really tight and slow trails, but worse everywhere else compared to the 29 front and back. I found I was going quicker into corners on 29, it was smoother and handled better on the fast rough tracks.

it was only a shirt ride on both though, and there may still be done merit in it.