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I am in the process of building a single speed 29" hardtail for my brother to putt around the BMX track w/ his 6 year old. I have been researching gear ratio's but still a little lost. I found a formula (linked) for gear inches so I can convert a 20" BMX gearing over to the 29" wheels.

https://surlybikes.com/info_hole/spew/spew_single-speed_gearing_101

I googled 20" BMX gearing and 44/16 kept popping up. Using the calc. it equates to 55 gear inches. I currently have a 30t front ring and could use a 16t cog? Does this sound like a good starting point?
 

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Snow Dog
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my only thought is that BMX gearing is for gaining large amounts of speed in a short amount of space...will he want that on the bigger 29" wheeled bike? Might reference what Surly has on their Lowside bike as well....
 

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Bikes in jeans
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I more or less had to do some trial and error based on the type of riding I do, so it may be tough to find the correct answer for the situation.

But, that gearing sounds fairly aggressive to me with so much larger wheels than a 20in, but I guess it depends on the rider as well. I run closer to 46 unless it's something that will never see climbing, and then closer to 50.
 

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Life's a Garden, dig it!
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Much depends on your terrain. Generally speaking, you want to start out around 2:1, so a 32:16, yes. Then adjust for terrain and wheel size.

With wheel size getting bigger (from the old 26" standard), adjust to more teeth in the rear to make up for it. I.e, a 29" wheel is roughly 10% bigger than a 26er, so add 1.6 teeth (rounded up to 2), for an 18 tooth cog as a starting point. If you're climbing more, go up an other tooth or two.
 
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