jitterbug_will

So I'm at about a beginner level when it comes to fixing my mtn bike, and right now I'm in the process of changing the freewheel. I've got a 13/28T freewheel made by some obscure company. Originally and atm I'm planning on buying the Shimano 7-speed Tourney freewheel MF-HG37 (13-28T). I know this will replace my current freewheel, but I'm wondering if I can get a freewheel with more teeth, and if I can, would I need to make changes to my chain or anything? Thanks.

Bill Kapaun

Shimano makes some with a 34T largest cog. Look up Mega Range.
If your chain is correctly sized for your existing FW, it'll be too short for any cog larger than 28T.

Cross Creek

Unless you very recently replaced your chain, you shouldn't reuse the old chain with a new freewheel.

SkyDog75

You'll need a longer chain, like Bill said. Use a chain that's too short and bad things can happen when you try to shift onto that big cog and the chain won't reach. You'll also want to use a new chain like CC said, so that you don't quickly wear the teeth on your new freewheel by using a worn (elongated) chain.

In addition, you may want to confirm that your rear derailleur can handle the size of your large cog and has sufficient wrap capacity if you go to a freewheel with a much larger big cog. The max cog size and wrap capacity should be easy enough to look up for a Shimano derailleur, but figuring out the wrap capacity you need will require a little math.

"Wrap capacity" is the derailleur's ability to take up the slack in the chain when you shift into smaller chainrings and cogs. As mentioned above, your chain needs to be long enough to handle the big chainring + big cog combination. Imagine that same chain length wrapped around your small+small combo. You'd have a lot of slack left over and the chain would droop if it wasn't for the spring-loaded pulley cage on your rear derailleur. The longer that cage, the more slack the derailleur is able to take up, and thus the bigger difference it can handle between the small+small and big+big combo. The amount of slack the derailleur is able to take up is measured as its wrap capacity. You can calculate the wrap capacity you need using the formula below:

Chain Wrap = (large chainring – small chainring) + (large rear cog – small rear cog)

For example, let's say I have a mountain bike with a 13-28 cassette and 22/32/42 chainrings on its crankset and a Shimano Acera M360 rear derailleur. Using the formula, I calculate (42-22) + (28-13) to get a needed wrap capacity of 35. The derailleur's wrap capacity is 43, so it can handle my 35-tooth difference in chain wrap. And the derailleur can handle a 34-tooth large cog, so my 28-tooth big cog won't be a problem.