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  1. #1
    Light Makes Right GV27's Avatar
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    Cassette for kid's MTB

    Hi all,

    I recently bought my son a new Trek MTB with 20" wheels. (MT60) It's got a cassette with a 28-tooth big cog. Does anyone know where to get one with a 32-tooth big cog? We live on a mountain and he needs as low a gear as he can get.

    Thanks,

    Chris

  2. #2
    GATC
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    Loose Screws is a good place for oddball pieces. Keep in mind that a 28t cog on a 20" wheeled bike is a lower gear than a 34t cog on a 26" wheeled bike.

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    It depends on the cassette. Some of them have the larger cogs "permanently" together. In quotes because you can drill out the rivet to separate them. The individual cog cost may not be worth it compared to a new cassette. I think this is an 8 speed, no? If not then likely a 7. I don't think Trek is selling any 6 speeds in their kid bikes, but I could be wrong. In any case, a new cassette might be the simpler route. Nashbar has some pretty inexpensive ones. You may need to lengthen the chain with the larger cog.

  4. #4
    WillFam-Clovis,CA
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    Also consider the possibility of changing the small front chain ring to a smaller size by a couple of teeth. That will make a huge difference and is not too expensive.

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    Quote Originally Posted by WillFam-Reno View Post
    Also consider the possibility of changing the small front chain ring to a smaller size by a couple of teeth. That will make a huge difference and is not too expensive.
    I think this is single chainring bike (most 20" bikes seem to be, but not all). Regardless, getting a smaller chainring on the front may indeed be easier. You'll just need to know the BCD (Bolt Circle Diameter) for the crankset.

  6. #6
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    This bike has a one piece crank-chainring assembly with 32T, it also has 2 positions for pedal attachment at 120mm and 140mm. This crank is also used on the Trek with 16" wheels, so I dont think there is one available with less than 32T.

  7. #7
    Light Makes Right GV27's Avatar
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    Quote Originally Posted by masiman View Post
    It depends on the cassette. Some of them have the larger cogs "permanently" together. In quotes because you can drill out the rivet to separate them. The individual cog cost may not be worth it compared to a new cassette. I think this is an 8 speed, no? If not then likely a 7. I don't think Trek is selling any 6 speeds in their kid bikes, but I could be wrong. In any case, a new cassette might be the simpler route. Nashbar has some pretty inexpensive ones. You may need to lengthen the chain with the larger cog.
    No, it's a 6-speed. SRAM shifter. We wouldn't be having this conversation if it was a 7- or 8-speed! I've got them laying around the garage. All the big brands - Trek, Specialized, Giant and Schwinn were selling essentially the same bike. Some had Shimano, others SRAM. According to Trek it's a Shimano cassette. Loose Screws did have one with a 32T big cog but with a 24 - 32 jump between the two biggies. Maybe the answer is to get that, drill it out and make my own.....

    Yeah, the crank's a one piece deal. I'd have to replace the whole thing with a new crank. I know a place that'll do a 140mm crank, but they're as much as the bike cost! THE reason I chose the Trek over the others was the 32T chainring - the other brands all had 36T. All non-swapable. What the heck a little kid is supposed to do with a 36-12 I have no idea - haulass down hill I suppose!
    Last edited by GV27; 03-07-09 at 01:56 PM.

  8. #8
    Light Makes Right GV27's Avatar
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    Quote Originally Posted by HardyWeinberg View Post
    Loose Screws is a good place for oddball pieces. Keep in mind that a 28t cog on a 20" wheeled bike is a lower gear than a 34t cog on a 26" wheeled bike.
    Thanks for a link - see comment above for what I found.

    As far as gearing, that's why gain ratio is such a good idea (http://www.sheldonbrown.com/gain.html). Haven't plugged in the numbers, but I bet the 3cm difference in crank length between his bike and mine turn the gearing around. 'Course the 100lb weight difference comes into play....

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    Derailleur

    You sound experienced wrenching on bikes, but a friendly reminder to check and make sure the rear derailleur can handle that large of a cassette.

    Our daughter primarily used the lowest 3 gears at that age, and we ride flat ground! She never came close to high gear (20"- 7 x 1).

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