How to Change the Freehub in a Trek Mountain Bike
If your cassette cluster (rear cogs) are getting loose or stiff, you may need to change the freehub. Note that this does not mean having to replace the entire hub. That would mean you would have to rebuild the wheel. Instead, you can perform this simple task with little more than an allen wrench. As noted on Sheldon Brown's encyclopedic website, mountain and road freehubs are interchangeable, even though the length of the hubs themselves are different. If your aim in switching the freehub is to increase or decrease the cog count, this may or may not work, depending on the make.
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Instructions
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1
Remove the rear wheel from your Trek mountain bike.
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2
Lace a chain whip around one of the larger cogs in such a way that the handle starts at one point and the chain circles counterclockwise around the cog from that point.
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3
Insert a lock ring tool into the lock ring notch (inside the smallest cog).
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4
Secure the hexagonal lock ring tool with a common crescent wrench and squeeze the wrench and chain whip in the opposite direction to screw off the lock ring.
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5
Pull off the cassette (block of cogs).
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6
Remove the axle's locknut on the left side of the wheel with a cone wrench and a crescent wrench. Fit the cone wrench onto the slender hexagonal bolt and the crescent wrench onto the wider locknut further outside.
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7
Pull out the axle and bearings.
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8
Place a 10 mm allen wrench into a vise, with the handle vised in place and the short part of the wrench sticking up.
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9
Lay the wheel onto the wrench so the wrench goes into the axle on the cog side. Turn the wheel counterclockwise. This removes the freehub.
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10
Put on a new freehub, tightening it with the 10 mm allen wrench. Insert the bearings, reinsert the axle and tighten the bolt on the bearings tightly, and then back it off a little. The bearing must move freely, of course. Then hold that bolt in place with the cone wrench and tighten on the locknut. Replace the cassette and screw on the outer cog. Note that you don't need the chain whip because your pedaling during a ride when in the highest gear will tighten it.
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