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Old 08-18-14, 09:03 PM   #1
John 316
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Need for speed - Trek MT60 (youth 20")

First of all, I apologize for improper terminology that I will surely be using. I am a novice recreational cyclist and I'm looking for some advise on my daughter's new bike.

She is 6 years old and approximately 46" tall. She recently got a Trek MT60, her first multi-gear bicycle. It seems to be geared too low for her. What options, if any, are there for getting her some more speed?
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Old 08-18-14, 09:23 PM   #2
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Current configuration:
Front sprocket has 32 teeth, press fit on crank
6-speed freewheel gears are 14-28
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Old 08-18-14, 09:42 PM   #3
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The easiest (if possible) solution is a larger chainring. It may also call for a longer chain, but that's minor in the scheme of things. problem is that the crank may not have an interchangeable chainring, or if it does, there may not be much choice in sizes.

So, step one see if the ring is bolted on, and replaceable, then if so determine the bolt pattern, and shop rings.
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Old 08-18-14, 09:51 PM   #4
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Thanks FB but the chainring is not bolted on. Do you have any other suggestions? I have found freewheels with 13 teeth but am told that one tooth will not make a significant difference.
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Old 08-18-14, 09:56 PM   #5
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Thanks FB but the chainring is not bolted on. Do you have any other suggestions? I have found freewheels with 13 teeth but am told that one tooth will not make a significant difference.
going from 14t to 13t makes a difference of 1/14th, or about 7% or so. That may be enough, and freewheels aren't that pricy so it may be a reasonable approach.

Otherwise, it's a new crank, new chain, and maybe a new bottom bracket, so it's not a cheap proposition.
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Old 08-18-14, 10:18 PM   #6
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Any bike coops near you that might have used cranksets?

Otherwise - she'll develop a great spin!
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Old 08-18-14, 10:38 PM   #7
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Jyl - Is "bike coops" typo, or is it a term that I should know?
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Old 08-18-14, 10:43 PM   #8
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If used crank sets are available, how many tooth change should I make?
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Old 08-18-14, 11:26 PM   #9
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Jyl - Is "bike coops" typo, or is it a term that I should know?
Bicycle co-op.
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Old 08-18-14, 11:43 PM   #10
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Sorry, co-op. A bike shop that sells used bike parts and generally supports customers who want to save a buck and, often, learn to work on bikes. Used single chainring crankset might cost $30-50.
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Old 08-19-14, 07:43 AM   #11
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You can probably find other cranks with larger chainrings but the problem will be crank arm length. On children's bikes the crank arms are usually in the 140 to 155 mm range while adult cranks start at about 165 mm and your daughter is probably too small to use one comfortably. Also, on a small bike, the long crank arm is likely to let the pedal hit the ground on any reasonably tight turn. Go for the 13T freewheel.
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Old 08-19-14, 07:52 AM   #12
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You'll also need to change the chain ring guard to a larger size if you replace the crank. 38T seems to be a typical size for those types of bikes. I have one in my junk pile. A local shop might have a used set too.
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Old 08-19-14, 08:45 AM   #13
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Huh, where did my post go?
Plugging in the numbers, she ought to be able to get to about 13 MPH before turning the pedals becomes a chore.
Swapping in a 13T f/w might get her to 14.

Can't tell from the pic what might be done to the crank.

One (pricey) option is to rebuild the rear wheel around a Shimano Capreo hub, with a 9T smallest. That'd stretch the bike's range into 20 MPH.
Or an SRAM DualDrive.

Post some closeups of the crank and maybe something more can suggested.
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Old 08-19-14, 08:56 AM   #14
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A BMX crank like Amazon.com : Origin8 BMX Crankarm Set 140mm 110 BCD Forged Silver : Bike Cranksets And Accessories : Sports & Outdoors will accept 110 BCD chainrings and have 140 mm crank length. Might be hard to find one used, but at least used 110 BCD chainrings should be available.
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Old 08-19-14, 10:09 AM   #15
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There may be a low cost alternative depending on your (or a friend's) tinkering skills, and the specifics of the bike.

You could buy a chainring of the size you want, and having an ID small enough to overlap well on the existing one. A 3/32" BMX ring is a likely candidate. Then bolt the ring to the existing one. It has to be done right so the ring is concentric and doesn't tend to loosen and it also depends on adequate clearance on the frame (as would ANY larger chainring).

If the skill set is there, this might be a workable, but not beautiful solution for a bike that your daughter will probably outgrow soon enough.
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