How to measure crank length. Copyright © 2017 Damon Sprague
Have child stand close to, but not touching wall, shoes on, shoulder width apart. Use a clipboard or similar, place flat against wall with long edge in crotch high as possible before protest. Measure from floor to top edge of clipboard including fractions with tape measure. Convert fractions of an inch to decimals for equation, top number divided by bottom i.e 3/8 = .375. Example, 20.25 for a 20 1/4) inseam. Then divide total number by .173 for crank length in mm. So 20.375 divided by .173 = 117mm. Now if your child is a stomper, better to go with the longer size 120mm as you are unlikely to find a 117mm crank. If your child is a spinner, the next size down would be better such as 115mm. As a rule too long is worse than to short of a crank. Also note young beginners are not typically going to be "spinners". Don't count on feedback from young riders, careful observance to determine proper fit of all the components will result in safer, faster, better riding and more fun so scrutinize the fit. *notes: If using clipless (clip, clip in) pedals the shorter length will be better than long. Also try this on yourself to see if it is close to what you may be using on your adult bike. You can also find a decent chart at BMX ultra though it will render long sizes for little riders. DO NOT use Dans- way way outdated. Other calculators use the ambiguous term "inseam" which means what exactly? My method tells you exactly how to get the measurement to plug into the formula. My measurement method works well with my formula and I stand by it. If you try and mix my measurement method with other calculators you will not get solid results- those are not designed for my measurement. They are designed to use what ever your interpretation of "inseam" is since it is not clearly defined.
A discussion about Q factor. Copyright © 2017 Damon Sprague
For little riders, you do want your cranks as tight as you can get them. Even if you have to shave the inside of the crank a little for clearance. It is worth it. These little riders have an extremely limited power potential and you want to maximize it. Levering the bike side to side rather than pushing it forward is wasted power and not helping balance so bringing everything into the center as much as possible is a big deal. The width of the cranks is termed Q factor and tighter is better. A width of 130mm outside to outside on the crank arm is very good, smaller the rider the more important this is obviously as they will have narrower hips and shorter inseam causing them to look bowlegged on the bike. This is what we are trying to minimize. Watch out for cranks as that bow outward worsening the situation. Answer mini bow outward more as the length increases, in the shortest sizes, they are relatively as good as the straight arm cranks from LDC, tronix, ,turn3, round2, and VSR. Helium, snap and bombshell rockets are the tighter of the straight arm cranks. 2 piece cranks will be heavier and wider and I don't recommend them for the little guys, save that stuff for 100 pound and up riders. I always try and get cranks as tight as possible no matter what frame the're on. It's about efficient power transfer and improving fit and balance.
For frames I'd recommend redman, black crown, Ssquared, LDC or Staats at this point not because they are lighter but due to rear end design allow for you to run the tightest BB you can get. Helium Racing currently sells Staats and Black Crown frames. I've seen 52t front chaining with a 103mm BB with no interference on certain frames. Some other frames with wider chainstays will not allow certain BB/crank combos without a gear rubbing or crank hitting the chain stay. Redline/GT and other frames with wider chainstays require a longer spindle to clear longer cranks. Sometimes you can get by with a short BBs on these frames if you are using 120 or shorter cranks.
A low cost way to reduce Q factor is shortened pedal spindles. We machine spindles in house for certain popular pedals and can also sell you a pair ready to go. Customers are reporting 2 sec fast laps times, another reports 4-5 bike lengths faster from first use. These have won at the worlds and can help your little rider too.
If I was choosing between a tighter Q factor or a few grams lighter I would pick the tighter Q factor parts. It's simply more important than weight. We sell the lightest and the tightest cranks, bottom brackets and pedals available. Use all three together for a trifecta of speed.
Immediately he was faster out of the gate..
Just from changing the one part he was 2 seconds faster on his lap times!
My son is 4-5 bike lengths faster now just from that one change.