Expert: Dave - 7/31/2008

Question
Please excuse me if i am in the wrong site .  I need some help.  My 8 year old son is finally starting to ride.  He broke the same leg twice over the last two summers and has become a little shy about riding.  He is 48 inches tall. with short legs.  His older brother gave him his GT Fly. This has finally gotten my son to try.  Unfortunetly it appears the crank arms are to long.  I measured them at 22 ctm.  How do I determine the correct length?  Where do i find shorter cranks?  both crank arms can be removed from the front sproket.

Answer
Hello Marc. Crank arm length is measured in millimeters from the center of the crank bolt to the center of the pedal spindle or the hole it threads into. I am guessing what you measured was overall length. Your crank set I would guess is about 170-185mm. Most companies don't make anything less than 165mm. I have seen cranks as small as 155mm. There is a chart that we have at my shop that I looked up online. Here it is:

Leg Length .............. Suggested Crank Length
60-65cm ................. 150mm
66-70cm ................. 155mm
72-75cm ................. 160mm
75-78cm ................. 162.5mm
79-81cm ................. 165mm
82-83cm ................. 167.5mm
83-86cm ................. 170mm to 172.5mm
87-90cm ................. 175mm to 177.5mm
91-94cm ................. 180mm
94cm +  .................. 185mm

Anything in his general area can be suited for comfort. Also, you can consider what type of riding he wants to participate in. If he is doing riding where he is standing out of the saddle, then crank arm length is less important that to those who are commuting around. BMX bikes generally don't follow rules of knee bend or correct position.

To get the proper crank arms, you can call or see your local bicycle shop and ask them to order a proper crank set. Usually they will order your parts and will arrive within a week following a 10% down payment. Crank arms aren't usually kept in stock in smaller shops. If you arrive at the shop, you may want to bring your son along to let him try the different crank sizes on the bikes stocked at the shop.

If you need anymore help with your son's bicycle, please do not hesitate to ask again. I hope your son has a lot of fun on his new bicycle!
-Dave