Trav1s

Ok, short guy here who recently discovered that 165mm cranks on my hybrid and vintage mountain bikes and have wondered about crank lengths for kids, especially as they transition for 24" to smaller 26" wheeled? bikes. Then I watched this video this morning and it has me thinking it is even more important than initially considered.


I'm working on a Trek 800 Sport 13" frame for my daughter and know the 170s that came on it will be too long for her when it initially fits.

Anyone out there played around with shorter cranks for kids riding smaller 26" wheeled bikes? If so, what did you use?

I realize this is a touchy subject so I post with caution...

Trakhak

Since you're thinking about crank arm length, you're knowledgeable about bike sizing. Just wanted to provide a link to a helpful site for anyone reading this thread who might be about to buy a bike that's too large for a child.

Here's a source for figuring out how to buy the correct size for a kid's bike that will be both safe and fun to ride.

With that out of the way:

Back when I worked in bike stores, one of my peeves was parents trying to put their kids on bikes for them to "grow into." Six-year-old kids on 24" bikes, for instance. (Never mind size: those kids often lacked the strength to use hand brakes or gear shifters.)

Most parents want to pick the bike that's best for the size of the kid, of course. But I also encountered plenty who would buy a bike that was absurdly big and then, when the kid had eventually grown enough to barely fit safely on that one, buy the next size up.

Sometimes, of course, they would listen when it was pointed out that, if they instead bought the correct size each time as the child outgrew the last bike, they'd buy at most one extra bike over the course of 10 years of buying kid's bikes. (Maybe not even one extra bike, since many kids are happy to stick with 20" BMX bikes until they're old enough to drive.) Sometimes not, though.

Trav1s

Size is important for sure and that's a great resource.

My goal is to take it one step further and ensure that when the size is right other things line up with the size.

I'll do some digging and share back with the group.

Trav1s

https://bikedynamics.co.uk/FitGuidecranks.htm#1

I'm going to play around with this info for my daughter and select a crank accordingly. Origin8 has reasonably priced options that give flexibility up upgrade in the future.

Trav1s

Origin8 ranks available on ebay but not found on their website - https://origin8.bike/

• 140 mm
• 150 mm
• 155 mm
• 160 mm
• 165 mm
• 170 mm
• 172.5 mm
• 175 mm

https://www.ebay.com/sch/i.html?_fro...crank&_sacat=0

t2p

just checked some notes on the kids bikes …

the small 20” bmx racers had 140 - 155 mm cranks

the 24” bikes had 160 mm cranks

the small 26” bikes they rode had 170 mm cranks … they quickly grew through this stage (and left me there lol)

Trav1s

t2p thanks for the scoop. I'll check the current 24s in the garage and add the info to the mix.


I scored the lightly used goody for a fraction of the new cost. https://www.ebay.com/itm/126239384152. One more pice is in the puzzle.

At this point, I am thinking 160 might be the best choice but that's still up for debate.

Trav1s

Info on her current Quick 24 -145 mm cranks

Frkl

As a 6'3" adult who rides 150-155mm cranks on my bikes, and has had really good experience (as with all crank length posts, this is my experience, yours may vary!), i have always found cranks on kids bikes way too long. Well, it is getting better, but still long.

In terms of stability alone, a kid pedalling a long crank is like marching in a marching band with knees moving way too far up and down.

On the 20" bikes i have installed 115mm cranks, and on the 26, i have 153mm. The 24 currently has 153mm too, but it isn't in use. Once it is again, i will be shortening these too.

Garthr

I have proportionately long legs for my height of 74" and I ride 150mm cranks also, with a more mid foot placement. With this I have optimal power transfer.

If you look at bmx websites you'll find lots of short cranks in various BCD's, bolt patterns and BB standards. The Origin8 cranks, often listed as the "Origin8 Alloy Crank Arm Set", I've ridden for some 5 years now. They "only" made do down to 140mm though. Sinz for example makes the "Expert" square taper 110 BCD single ring cranks that go down to 115mm.

I have a custom set of the Andel triples @150mm from Bikesmithdesign in Minnesota .https://www.bikesmithdesign.com/Short_Cranks/ . I think the shortest of all the shortened cranks he offers is a 90mm, wow ! He does custom work also but not many cranks can be safely shortened and he won't do just any crank you send him. Surely not anything hollow ! When I wanted another though, I really didn't want another one in black, so I kept looking at the many alternatives. Among what I found as that Andel themselves in Taiwan indeed makes some cranks in 150mm. The one I bought above, known as the RSC6 from Andel, sold as the Dimension Cross crank in the USA, is made in the factory down to 150mm, and in silver ! I bought 3 sets last year from them as if I'm going to pay for the air shipping, I may as well max out the box ANDEL ENTERPRISE CO.,LTD.

Yes, kids and adults alike can ride cranks too long and adapt, but it's not necessarily optimal.

Trav1s

I appreciate all of the responses. I'm thinking 150mm might be the best bet considering she is on 145mm. The Origin8 stuff makes that easly and I have been impressed with their other products that I own. Observation - As my daughter has grown, I have watched her rock less from side to side as she rides the Quick 24.

Frkl

I also ride with a more mid foot position. interesting, I wonder if that is common with short cranks.

The 115s on the 20-inchers are made by Lasco and I got them here: https://www.bike-components.de/de/3m...0-mm-32-Zaehne
The 155s are Sugino XD2s, and the 150s are Stronglight Impact Kid cranks, which are also actually Sugino XD2s. They are available down to 130mm
The 152s are Suntour XCT Jr., but these are "hot forged" and I don't think they are made any more. While probably plenty strong, I do think these are best reserved for light riders/kids.

Russ Roth

if you haven't ordered that yet, one more option that's easy to overlook and comes in 152mm.
https://www.srsuntour.us/collections...ucts/zeron-1-x
SR Suntour gets a bad rap because, let's face it, they make some crap low end forks that end up on the cheapest bike shop bikes. But this is a solid crank, the BB hasn't failed in 2 years of use so far, youngest hasn't grown into it yet. But no complaints. As I recall it came with the BB so while 100.00 might not seem cheap, add a bb, chainring, and bolts to the origin8 and you're not far off. I've run the origin 8 on their road bikes but this crank is purpose built if you're really mtbing.

Trav1s

Thanks, Russ! I keep the Suntour crank in mind. I picked up a like new Origin8 chainring on ebay and need a crank to go with it. Then again, I could use chainring on another project. Time will tell.