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Discussion Starter · #1 ·
This may be a bad idea, but I can't find a better solution.

9-year-old son has a true 24" inseam. His bike has 165 mm Sora triple cranks. After seeing him ride his bike and doing some research (http://www.myra-simon.com/bike/cranks.html), it seems like 150 mm cranks would be better him until his inseam gets to 26."

[This is actually conservative. His legs are about 70 percent the length of mine. Arguably, if he were to run cranks proportional to the 175s I run, he should run 125 mm cranks.]

So, I figured the cheapest and simplest soloution would be to just drill and tap the existing cranks. I realize that the holes would have to be perfectly parallel to the exising holes. If I screw up, these are only $70 cranksets.

Any downside to my plan? Any alternative solutions that aren't expensive or require significant changes in the drivetrain? I know that I can get TA "chainsets" from the UK in that size for about $200, but this is overkill moneywise.
 

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Discussion Starter · #2 ·
Or make my own crankset?

Here's an alternate solution. Probably more elegant.

I can get 150 mm Bulletproof BMX crankarms for $28. I can buy 48 and 34 chainrings from nashbar for a total of about $40. After postage and the cost of bolts, I can create a custom compact crank for less than $100. I can then get longer crankarms as my kid grows. And yes....I'm assuming that my kid's not fickle and won't abandon cycling.

Since the rings won't come with the pins and ramps, will this noticeably affect shifting?
 

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things to worry about

NoMSG said:
Here's an alternate solution. Probably more elegant.

I can get 150 mm Bulletproof BMX crankarms for $28. I can buy 48 and 34 chainrings from nashbar for a total of about $40. After postage and the cost of bolts, I can create a custom compact crank for less than $100. I can then get longer crankarms as my kid grows. And yes....I'm assuming that my kid's not fickle and won't abandon cycling.

Since the rings won't come with the pins and ramps, will this noticeably affect shifting?
You might need a new BB as well. I did have an experience with a BMX BB that appeared to be the wrong taper angle for road cranks - the crank cracked at the BB axle. Shifting will not be as good without the pins and ramps, but it might be good enough. Actually, I think your first plan is fine. I did this for both my kids when they were little and riding on the back of a tandem. Even if the hole are a little off from the crank center line, it's not a big deal. The key thing is to get the holes perpendicular to the crank. I did it in a drill press with no problem. Kids are never going to be able to put enough force on a crank to worry about there being an extra hole in the middle (after you move the pedals back out to the original hole).
 

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Discussion Starter · #4 ·
Thanks Kerry.

Kerry Irons said:
You might need a new BB as well. I did have an experience with a BMX BB that appeared to be the wrong taper angle for road cranks - the crank cracked at the BB axle. Shifting will not be as good without the pins and ramps, but it might be good enough. Actually, I think your first plan is fine. I did this for both my kids when they were little and riding on the back of a tandem. Even if the hole are a little off from the crank center line, it's not a big deal. The key thing is to get the holes perpendicular to the crank. I did it in a drill press with no problem. Kids are never going to be able to put enough force on a crank to worry about there being an extra hole in the middle (after you move the pedals back out to the original hole).
I'm gonna tap the holes.
 
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