Mountain Biking - Middle School fork swap.

Not Old School , that would have been me mounting the first after market straight blade MTB fork on my Bianchi MTB in the 80's.

This project uses:

Mid 90's Thin Blue Line Ozone Pro.
http://img293.imageshack.us/img293/3006/sidev2ku3.jpg

The objective being to swap out the FS front end with a straight blade rigid fork.
I use this as a winter commuter now but I want to re-purpose it to include some fun CX (I'm old so I can use a MTB frame) and possibly some polo. I've got dirt drops and road bars that can be added once the right shorty stem is found. I've got the long travel road levers for V-brakes.

Here's where I need help. I do indeed love the geometry as it is. I've measured Axle-Crown and got 437mm. I start searching for reasonably priced straight bladed forks at that general length. In my research I'm reading about sag adjusting the measurement as well. If I do 10% I end up with 393mm.

THIS IS OVER an inch difference. My forks don't sag much as most of what I do is on the streets now and I've had em cranked for ages. Please if anyone has some experience in what the hell to do so I don't end up destroying the handling of a well loved beast..... I would be forever grateful.

Sag is based on the amount of travel the fork has, not on the total axle-crown measurement...10% is on the low side for sag, too.

Ok.... now if I don't generally engage the suspension by virtue of being on the street mostly. Should I simply look for rigid that matches the A-C measurement of the current fork as it is when extended?

That would all depend what you want. The frame was probably designed around a range of axle-crown lengths, so just depends how tall you want your front end to be to an extent, and how that fork you're using fit the frame in the first place. If you get great response from the frame with the current fork's ride height, I'd just go with an approximation of the axle-crown as sagged currently...and you'll be somewhat limited in any case by available fork axle-crown lengths.

coolio Thanks that gives me a range to work with. I think if anything I juuuust might lower the front a tiny bit. Not so terrified to not match the length exactly now.

Surly Instigator fork comes in at 440mm.