General Cycling Discussion - What do I need to know before I buy new forks?

A question from a relative noob. I want to replace the front suspension on my commuter / utility bike with a rigid fork. What do I need to know before I purchase the new forks (probably online / used)? IE: what are the variable sizes / measurements I need to be concerned with. The bike is an old Kona with Marzocchi front suspension.

My guess is that there is a hub size that is different between road bikes and mtb's as well as a head tube size variable. Am I right? Anything else I need to consider? I'd like to keep costs to a minimum and reuse existing brakes, wheels, etc.

Any info is appreciated
.

is it for 26" or 700c wheels?
does it take cantilever or disc brakes?

is the steerer 1" or 1-1/8"?
is the steerer threaded or threadless?
how long does the steerer have to be?
threadless can be shortened by however much is necessary.
threaded implies buying a fork with just a little longer steerer.
neither type can be made longer.

is the offset or rake about the same?
don't replace a 38mm rake suspension fork with a 50mm rake rigid fork.

is the axle to crown distance about the same?
shorter forks will quicken the handling some and taller forks will slow it down.

The hub size will most likely be the same.

I'd say go into the LBS and say you want a basic, suspension corrected Tange fork for the bike. Unless it's Tange Infinity or Tange Prestige, it'll be relatively inexpensive. You need special tools to remove the crown race and install it on the new fork anyway. The tools for cutting the steerer aren't nearly so specialized or expensive but they aren't worth buying to use just once either.

In any case, with few exceptions, I don't buy stuff online unless I'm doing the work myself. My LBS doesn't give me any grief if I find NOS parts on closeout though. If I buy the fork from them, they'll swap the crown race for free. If I find a $300 fork $20 cheaper online and ask them to do it, I'll get charged $10 for the swap and the shipping cancels whatever I thought I might be saving.

You got me at "special tool". Thanks for the info.

Does adding a suspension fork make the bike geometry slack though, if the frame is built before suspension forks became the norm?

I second taking it to a LBS... lots of variables to consider (cable routing, brake compatibility, wheel compatibility, and including perhaps a new headset), and needing to cut it to the right length is a big thing.

Typical Axle to Crown height for an 80mm fork is about 450mm. Feel free to correct me if I'm wrong but if I recall correctly an old rigid mountain fork was about 390mm(?). Every 20mm takes a degree off of the head tube angle so your old school 70.5º bomber suddenly ends up with a 67.5º head angle and it raises the bottom bracket and lengthens the wheelbase by around 18mm each as well. I've never done it and it's probably still rideable but it isn't anything I would do.

Finding a used 63mm fork and pairing it with a steeper 71º head angle will keep things at a more reasonable 69º and 12mm. I have a old bomber hardtail that currently runs 69º and 8mm higher than stock and it's kinda fun to ride around guys on XC full suspension. It doesn't do sharp switchbacks well at all but they can't believe I can ride downhill type stuff on an 80mm hardtail. It's actually the bike's geometry that gets it done, not the travel.

Typical Axle to Crown height for an 80mm fork is about 450mm. Feel free to correct me if I'm wrong but if I recall correctly an old rigid mountain fork was about 390mm(?). Every 20mm takes a degree off of the head tube angle so your old school 70.5º bomber suddenly ends up with a 67.5º head angle and it raises the bottom bracket and lengthens the wheelbase by around 18mm each as well. I've never done it and it's probably still rideable but it isn't anything I would do.

Finding a used 63mm fork and pairing it with a steeper 71º head angle will keep things at a more reasonable 69º and 12mm. I have a old bomber hardtail that currently runs 69º and 8mm higher than stock and it's kinda fun to ride around guys on XC full suspension. It doesn't do sharp switchbacks well at all but they can't believe I can ride downhill type stuff on an 80mm hardtail. It's actually the bike's geometry that gets it done, not the travel.

Ahh, I've got so much to learn....

BTW guys, it's presently got front suspension and I'm going to rigid forks. Not sure if that was clear in my original post....

Thanks for the info though. I'm going to go the bike shop....