crank advice?

[53 willys]

so i'm thinking about going with some longer cranks for my sons bike and have a few questions about cranks and bottom brackets...

i'm looking at 150mm expert cranks from rhythm...if we go this route the bike would need the BOB style bottom bracket. this set up from the weights I have gathered would be almost 5.0ozs heavier then if we keep his TI sinz BB and go with the 150mm "answer" cranks.....

so is the bigger outboard bearing BB that claim to spins easier worth the extra weight and cost?? or would the square taper answer cranks and sinz TI BB be better?
this style with TI BB
http://www.answerbmx.com/AnswerMiniCranks.html

or this style with BOB
http://www.crupibmxracing.com/crank-expert.html

thanks for any advice or tips you can offer!!

[joker75]

For someone running 15omm cranks, I would think that you should be fine with the lighter cranks. Once you start putting on some weight, and putting down a lot more power, is when the outboard bottom brackets, and stronger crank arms are going to show their worth. The BOB may spin slightly easier, but probably not enough to make much of a difference.

In my opinion, if you want to give the little guy some help, you would be better to save a few bucks on the bottom bracket, and upgrade the hubs/wheelset, or even just the bearings in his current hubs. If you think about it, the rear wheel is turning around 2.5 times for every rotation of the cranks, and that's when he's actually pedaling. Then factor in your hubs bearings are turning at a much faster rate for the entire track, when you cranks are only turning for the short stretches that you are pedaling and you should be able to see why the crank bearings aren't near ar critical as a good hub bearing.

[53 willys]

thanks for the reply.....he already has rhythm wheels and hubs so we are good on that side of it..I was not really wanting the BOB or cranks for speed...just was measuring his inseam and according to the crank chart on JR bikes he should be running 150mm-155mm cranks for his 22" inseam vs his current 145mm cranks.....he's 7yr and 4'3"tall.
is the crank chart on JR bike accurate? should we be running 150mm-155mm cranks?

[JAlvey]

The crank length charts are just a guestimate. The real answer is that you should be running the longest cranks that your kid can spin. If the cranks are too long, you will see the rider start to really chop instead of spin. If you're seeing a lot of side to side movement when pedalling fast, then the cranks are too long. You will find that the longer cranks will help out of the gate pretty quickly.

Based on the age and height you are at, the 145's are too short. 150, probably 155 would be better. You can stay with standard square taper BB and cranks for now until he's closer to 80-90 lbs, then you will need to change over either to an ISIS style crank/BB or A 2-peice BOB style.

SS makes an adjustable set of cranks that you could use for years. Another option would be to try and track down a used set of cheap sinz type cranks. There are a lot of other parents just like you who have growing kids. You can often find old sinz cranks used for $10. Sometimes checking with parents with kids 1-2 years older than yours is a good idea. Buy some in 150, 155, 160 and try them all, once you find the size that works, buy a more expensive set. AC makes a mini crank that is almost as light as the answer and you can find for $38.

[53 willys]

thanks for taking the time to post up guys.....getting some cheap ones sounds like a really good idea.... ...I will try that and see what works best for him...then upgrade to something light weight if we like that size.
thanks again!