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I have this bike:

http://www.bikebarn.co.nz/2015-merida-big-nine-20d-white.html

I want to swap out the forks for some rigid forks.

Please tell me what specs I should be looking at to get the right fit.

Thanks!

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I have recently just done the same thing and switched from suspension to rigid forks, although with a much older bike. There are a few things that you need to know.

The first is your type of headset, it will either be threaded or threadless. If you have a stem that clamps on the steerer tube and then your handle bars clamp onto the stem, you have a threadless headset. I can tell by the picture you have linked too that you do have a threadless headset.

The second thing you need is the diameter of you headset, which is generally 1 1/8 inches for modern mountain bikes but if you can find a specs sheet for your headset it will specify. If you cant find the info for your headset, just measure the steerer tube with a set of calipers and look at this page (http://sheldonbrown.com/cribsheet-headsets.html) under "Threadless Headset / Steerers" and reference the measured dimension to identify the size of steerer tube.

Last, and probably most importantly, you will need to know the axle to crown measurement. This is basically the measurement from the middle of your front axle to the top of your forks. There is plenty of info on the internet on how to take this measurement but the important thing is sit on the bike and get someone else to measure or get a friend of similar size to sit on the bike while you do it, this will ensure the slack is taken out of the suspension. This measurement will help you to get forks that are a suitable replacement length for your current ones. This is important because your frame has been engineered to work with forks of a given length and altering that geometry by a significant amount will alter the handling characteristics of your bike.

One thing that can help make it easier to select forks is to buy models that have been specifically made as replacements for suspension forks with a given amount of travel. If you know the amount of travel of your current suspension you can buy some forks that are specifically made to compensate for that amount of travel (i.e surly instigator 1.0 forks are made to replace shocks with 100mm of travel). It is still worth checking that the axle to crown measurement is similar to your current setup as they can vary in size by manufacturer. For example my original forks (2002 Rock Shocks Judy C) originally only had 80mm of travel but they are relatively long and I needed the Surly instigators to get the correct length. (They work awesomely well, by the way).

When picking new forks also make sure that they have the correct brake mounting fixtures. You will often need to specify for disc, or cantilever, or both. When you get the forks, it is probably easiest to take them to your LBS and get them fitted as they will need to take the fork crown race off your current forks and move them onto the new ones. This is best done with specialist tools, you can do it yourself, but it is not worth risking the damage to components as the crown race is specific to your headset and you may be up for a new headset if you damage it.

Also make sure the new forks are made for your size of wheels, 26, 27.5 29 etc.

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If you are buying rigid mountain bike forks - you will need to buy the correct length for the suspension corrected geometry of your frame.

The bike in your link has a 100mm fork - the majority of forks with a specified travel will measure the same length (axle-to-crown) plus or minus 10mm. If memory serves me correctly - the length of rigid fork you would be looking for is about 440mm axle-to-crown.

420mm = 80mm travel fork 440mm = 100mm travel fork 470mm = 29er Rigid fork

The hardest part of fitting your forks will be the crown race removal and re-fitting from your old forks to new forks. Its possible to gently lever the crown race off using an old chisel to get under it and tap it out - going around it tapping evenly.

And to re-fit the crown race onto the new forks using the jaws of an old & large adjustable spanner - again - going around it tapping evenly to remount flushly onto the seat.

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