Thinking of giving it a go.
Popping a 29 inch wheel into a 445 a-c rigid fork on a sanderson solo.
What does it do to the ride?

I can *just* fit a 2.1 tyre such as a Bonty Jones into a 420mm a-c On-one steel fork, the heavy old white ones. There would be more clearance with other forks as the On-one has a very deep crown. That is in a frame built for 80mm travel (I think). It is a bit slack, which is no problem, but the BB is a bit high and that is a bit inconvenient, you really don't need a high BB on a rigid bike. It would be a bastard keeping the front down climbing with gears I reckon, but as a singlespeed it is fine, really easy to pop the front wheel over stuff and dig the back wheel in when you need grip. The big wheel does help a bit with grip and smoothing out the faster downhills, also that and the slacker geometry make steep downs and drop-ins less scary.

69er more marketing crap? or an idea by a bloke who had run out of matching wheels?

apparently a 425mm A2C will do it

I've made a 69er by fitting a 29inch wheel to a 26" On-one Scandal frame with 425mm axel to crown 26" exotic forks with discs. It works fine, geometry is great and there's plenty of clearence for a 29 inch wheel and tyre up front. Using a 425 A2C does not raise the front end, I think they are equal to a 100 mm travel fork which is what the frame is designed for.

I think 69ers are a great idea for rigid bikes. Acceleration from the smaller rear and ability of the front to roll over obsticles. Mine is great to ride.

It worked for me… now back as a standard 26" setup, but it was an interesting experiment to try (I used a specific Singular 69'er fork for my build). I did find the big wheel up front softened the blows a little bit, though the steel fork didn't help much!

IMAG0022 by ten_sim, on Flickr

IMAG0021 by ten_sim, on Flickr