Mountain Bike Reviews Forum banner
Cancel
Create thread New Forums

Crank length on kids bikes

Jump to Latest
1 - 20 of 45 Posts
F

Fargo1

· Registered
Joined
·
435 Posts
Discussion starter · #1 ·
Last night I came across this article talkinga about short crank lengths for kids. Determining Crank Length for Children

It made a lot of sense to me. So I decided to check on the crank length of the 24" Raleigh Mtn Scout I just picked up for my kid. I was astonished to find that it had a 165mm crank on it. Thats what I put on my wifes 29er. It seems way too long for 6-10 year old kid the bike was 'designed' for. Looking at specs on bikes from Trek and Specialized I see they run a shorter 152mm crank. That seems more reasonable to me. So why did Raleigh use such a long crank. (I guess,y because they stock them for other bikes)

Anyway, is the above article accurate? What are your experiences with crank lenghts on kids bikes? Should I be looking at getting a shorter crank for my kids bike? If so where do you find parts for kids 24" mountain bikes? This bike is a bit of a build project so I will be needing some other items as well.
 
#2 ·
I am a bit of a advocate for making sure crank length is right for kids. IMHO it makes a huge difference to their comfort on the bike, stamina and riding skill. Kids riding with cranks which are too long need to shift their body weight from side to side to reach the bottom and top of the pedal stroke effectively. This makes bike handling very hard. Look out for hips moving from side to side when standing up peddling. When sitting look at the knee angle when at the top of the peddle stroke. Compare to yourself or your wife. However if your child is super tall for their age then 165s might be ok. 10%of height is a decent starting point when selecting crank length but obviously some people have long or short legs for their height.

From what I have seen a lot of manufacturers (particularly in bmx) install cranks which would suit the tallest possible rider of that size bike. Makes no sense to me. From what I have observed your average 6 year old needs 125 to 130 mm cranks and your average 10 year old will ride 145 to 150mm cranks but once you get to age 10 their is a huge variation in height between kids. My tall for his age 5 year old has 120mm cranks on both his 20" bmx and 20" mountain bikes which works well for him.
 
Discussion starter · #3 ·
I've always wondered why my 6 year old could out ride my 8 year old. He has a lighter bike, but with shorter cranks. I always thought the longer crank would be an advantage to the older kid. Even if the bike was heavier. But now I think the lighter bike along with the shorter crank is actually a much better setup.

Anyway, my oldest was only about 51" tall last time I measured him. So at 10% of height that puts him at about 130mm cranks. A long ways from 165mm. So I think I need to either switch cranks or get a different bike. He just got the bike this weekend, so I hate to change bikes right away. But I could since we picked it up used.

The bike is pretty light because it has a rigid aluminum fork instead of suspension. So I'd kind of like to just build him a nice light bike. Where is a good place to find cranks or cranksets for his bike. This is the bike he has
Raleigh Bicycles Mtn Scout - 2011 Youth

I can't seem to find anything for a 24" bike at the bike places I look for my 29er. So any help in getting the proper crank is appreciated.
 
#4 ·
I've been going thru this, our Trek Superfly 20 came with 150 mm cranks which we took off right away and installed some 102mm ones we had which worked OK until the new cranks came in, which are 120mm.

I actually used the website you linked to. I'd rather my 5 y.o. not get knee injuries from too long cranks.
 
Discussion starter · #6 ·
XJaredX said:
I've been going thru this, our Trek Superfly 20 came with 150 mm cranks which we took off right away and installed some 102mm ones we had which worked OK until the new cranks came in, which are 120mm.

I actually used the website you linked to. I'd rather my 5 y.o. not get knee injuries from too long cranks.
Click to expand...
Yeah, I injured my knee 2 years ago biking and its still not healed. I don't want my kids to face that. I think I really need to do this.

I also came across this website that has addional links to pages I haven't read yet. Custom Cranks | Zinn Cycles website This website is advocating longer cranks. But I think they are talking more about taller than average riders, because they also talk about shorter cranks for shorter riders. The biggest thing I think is to get the proportions correct. The give some formulas there too. Using his formulas my kid should have a 120mm crank. Which seems a really short to me. So I have some questions if the formula is that accurate across the range from kids to tall adults. But even if its not super accurate, it still shows my kids should have a crank much smaller than 165mm.

This has really got me thinking. I'll have to do more research and calculating, but I think a crank somewhere around 130mm to a max of 152mm would seem reasonable.

So where can I find cranks of this length? Will I have to replace the whole crankset or just the arms. I'm pretty new to all this yet.

Thanks.
 
#7 ·
What type of bike is it? And how many teeth are on the front ring?

Most kids bikes are square taper cranks. So you can ask your LBS what they can get for you, it will be from the BMX world- or you can order online, whatever your preference.

In my case, I specifically wanted Sinz cranks for the pricepoint, and my LBS couldn't get Sinz, so they said order online, I got them from here:

BMX PARTS - Cranks - Crank Arms - SINZ Aluminum Square Crank - J&R BMX Superstore - BMX Bikes, Parts and Gear- Custom Bikes and Wheels

Now, I asked about how big the front ring was because my son's stock crank was a 32T- the smallest ring one can fit on a 5 bolt 110mm BCD crank is a 34T ring. A lot of kids bikes come with 32T crank, but I was unable to find any cranks aftermarket with 32T capability. My son will be ok with 34T though, his bike is a 7 speed.

Spawn Cycles told me that they hope to have 127mm cranks with 32T rings available by the end of this year, but nothing yet.
 
Discussion starter · #8 ·
XJaredX said:
What type of bike is it? And how many teeth are on the front ring?
...
Now, I asked about how big the front ring was because my son's stock crank was a 32T- the smallest ring one can fit on a 5 bolt 110mm BCD crank is a 34T ring. A lot of kids bikes come with 32T crank, but I was unable to find any cranks aftermarket with 32T capability. My son will be ok with 34T though, his bike is a 7 speed.

Spawn Cycles told me that they hope to have 127mm cranks with 32T rings available by the end of this year, but nothing yet.
Click to expand...
His bike is this Raleigh Mtn Scout. Raleigh Bicycles Mtn Scout - 2011 Youth I think Raleigh just threw on whatever parts they have available. His has a 165mm crank and an 80mm stem, in 2014 they had a 127mm crank and a 60mm stem. There is no consistancy on the bikes. Its crazy. They obviously didn't care what they put on the kids bikes.

Anyway, his crank appears to be a 4bolt setup with 3 rings. The specs call it a 'Pro Wheel 24/34/42t'. I don't know what you call the style of bottom bracket, but it looks like the bearings are internal and they screw in.

So I'm not sure if I will have to purchase a new crank or just arms. Honestly, if the price isn't too much I wouldn't mind converting him to a light 1x7 setup. He won't like giving up the extra gears, but it will be simpler for him.
 
#9 ·
Check this company out https://www.facebook.com/turn3racing

They offer 4 x 104bcd spiders as well as 5 x 94bcd spiders.

You will need to get a new bottom bracket. My guess is a 68x116 or a 68x118.

I have a set of these cranks on my sons bike with a RaceFace Narrow Wide 30T ring running a 68x116 bottom bracket and a 11x36 10spd rear cassette. The chain line is pretty good with this setup. Turn 3 also offers the Answer Ti Slider bottom bracket if you feel like spending the extra $$ I have been running 135mm cranks on all our 20" bikes for the last 3 years. I have found that is an ideal crank length for the kids riding this size of bike.
 
Discussion starter · #11 ·
Demo9 said:
Check this company out https://www.facebook.com/turn3racing
Click to expand...
Those look nice, but at $140 for the cranks plus having to purchase a new bottom bracket the price is too prohibitive. At that point it would be cheaper for me to buy a different bike with better specs and sell this one.

ladljon - did you drill out those cranks yourself? I thought about doing that, but I'm not sure I would get a strait enough hole.
 
#12 ·
This is a very interesting discussion. My almost 4 year old (98cm tall) still has 89mm cranks on his bike. I have some 102s ready to put on (thank you!) when he needs them.

I think I need to measure his inseam and look at some formulas to see if he has grown enough. I also desperately need to swap his 22tooth chainring for something bigger so would be handy to do both at the same time.
 
#13 ·
Fargo1 said:
His bike is this Raleigh Mtn Scout. Raleigh Bicycles Mtn Scout - 2011 Youth I think Raleigh just threw on whatever parts they have available. His has a 165mm crank and an 80mm stem, in 2014 they had a 127mm crank and a 60mm stem. There is no consistancy on the bikes. Its crazy. They obviously didn't care what they put on the kids bikes.

Anyway, his crank appears to be a 4bolt setup with 3 rings. The specs call it a 'Pro Wheel 24/34/42t'. I don't know what you call the style of bottom bracket, but it looks like the bearings are internal and they screw in.

So I'm not sure if I will have to purchase a new crank or just arms. Honestly, if the price isn't too much I wouldn't mind converting him to a light 1x7 setup. He won't like giving up the extra gears, but it will be simpler for him.
Click to expand...
The crank is one half of the arms- so you'll need to purchase a new crank. The Sinz I bought was $50. It is single ring though. Then I had to buy a ring which was like $20. And a chainguide which I assume is like $20, I haven't got the bill yet from my shop.
 
#14 ·
I had a machinist drill and tap the threads..and then I cut off the ends and machined them down....He charged me $100 for two pairs....I know that there are two peeps doing them for $90 for one pair. Just google short bicycle cranks. Check out Bikesmith Design and Fabrication....
Fargo1 said:
Those look nice, but at $140 for the cranks plus having to purchase a new bottom bracket the price is too prohibitive. At that point it would be cheaper for me to buy a different bike with better specs and sell this one.

ladljon - did you drill out those cranks yourself? I thought about doing that, but I'm not sure I would get a strait enough hole.
Click to expand...
 
#15 ·
XJaredX said:
I've been going thru this, our Trek Superfly 20 came with 150 mm cranks which we took off right away and installed some 102mm ones we had which worked OK until the new cranks came in, which are 120mm.

I actually used the website you linked to. I'd rather my 5 y.o. not get knee injuries from too long cranks.
Click to expand...
trek superfly 20 comes with 127mm crank. Either u measured incorrectly or someone put the wrong crank on it. Crankset length is measured center of hole for bb to center of whole for pedal spindle. Wanted to point this out since u said ordered a 120mm, chances are it'll be a hair shorter than the stock one.
 
#16 ·
tigris99 said:
trek superfly 20 comes with 127mm crank. Either u measured incorrectly or someone put the wrong crank on it. Crankset length is measured center of hole for bb to center of whole for pedal spindle. Wanted to point this out since u said ordered a 120mm, chances are it'll be a hair shorter than the stock one.
Click to expand...
No waayyyyy. The stock crank is giant. We measured it at my LBS and then for the heck of it I held the 120mm's I bought up to them.

I appreciate the input but I do know how to measure a crank- maybe you are in a different country where they spec them differently from in 'murica?
 
#17 ·
XJaredX said:
No waayyyyy. The stock crank is giant. We measured it at my LBS and then for the heck of it I held the 120mm's I bought up to them.

I appreciate the input but I do know how to measure a crank- maybe you are in a different country where they spec them differently from in 'murica?
Click to expand...
In complete agreement with XJaredX here. Trek superfly 20 bought in Australia - stock crank length = 150 mm. Way too long.
 
#20 ·
Ok well something went grossly wrong at the factory for the cranks then cause I measured my sons last night before u posted my reply to get exact number. They were supposed to be specced with 127mm crankset which is what my sons has. Oh and I live in Illinois lol. But could be possible my lbs caught that issue and had it corrected, hes good like that. 150mm in a 20" bike...had that happened to us I would have been all over Trek over that one. Owner of my Lbs used to be a rider for Trek and his brother works for Trek at their home office in Wisconsin. Been good for me as if I have had a complaint or warranty issue, its handled QUICK!!!.

All that being said, Trek listens, in my experience (yes I've dealt with them via email prior to learning connection my lbs has.) So all of you that have gotten hosed on that need to complain to Trek directly. At the very least it'll get changed, if not those that complain get a proper crankset. Put it this way, part number spec is precaliber 127mm crankset except with 6-8s stamped ring instead of ss ring.
Sent from my Nokia Stupid Phone using Tapatalk
 
1 - 20 of 45 Posts
About this Discussion
44 Replies
15 Participants
Last post:
jochribs
Mountain Bike Reviews Forum
posts
16M
members
534K
Since
1990
A forum community dedicated to Mountain Bike owners and enthusiasts. Come join the discussion about bike parts, components, deals, performance, modifications, classifieds, trails, troubleshooting, maintenance, and more!
Full Forum Listing
Explore Our Forums
General Discussion Downhill - Freeride California - Norcal Shocks and Suspension Passion