Kids Bike Gearing

Bicycles for Toddlers to Teens

Kids Bike Gearing

Postby pstanbis » Wed Apr 30, 2014 7:14 pm

Gday All,

Just got given a fantastic, near-new Avanti Spice 20" w/Nexus 3 speed, for my 6yr old, who has well and truly outgrown her 16" Trek Mystic. She is very tall for her age, and has no trouble riding and handling the bigger bike, however the region we live in is seldom flat, and she is struggling with the length of the stock gearing.

I have already swapped out the cranks for some shorter ones, which made a marked difference in comfort, but now im trying to figure out how low I can/should go on the gearing. It currently runs a 40t Chainring with a 20t sprocket on the rear hub, but it rarely (ie never) sees even 2nd gear, let alone 3rd. I can source one piece crank chainrings from the local BMX guys, so have a wide range to choose from, but I've no idea what might be apprpriate. Anybody got some experience in such matters ?

Here's my current options using the very handy gearing calculator....
http://www.gear-calculator.com/#KB=34,50&RZ=11,12,13,14,15,17,19,22,25,28&TF=85&UF=2099&SL=2

Cheers,
Phil
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by BNA » Wed Apr 30, 2014 8:56 pm

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Re: Kids Bike Gearing

Postby MattyK » Wed Apr 30, 2014 8:56 pm

How about a larger rear sprocket? Eg a 24t will drop your gearing 20% - almost one gear lower.
http://www.moruyabicycles.com.au/conten ... pline.html
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Re: Kids Bike Gearing

Postby silentbutdeadly » Thu May 01, 2014 10:07 am

If the chain ring can be swapped on the front and is something like 104 or even 80 something BCD then you can get rings down to 30 tooth from Wolftooth Components in the US. They are very very nicely made too. But MattyK's response is a good one.
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Re: Kids Bike Gearing

Postby pstanbis » Thu May 01, 2014 10:39 am

Thanks Matty, didn't realise the rear sprockets came so large ! Might be an easier swap than the chainring, although will probably require new/lengthend chain ? single speed, so not exactly a bank-breaking expense....

Silent - Unfortunately nothing so simple.... is a cheapy one-piece crank setup with the american style BB. I can get 25t all the way through to 36t rings to suit, although not with the nice little chainguard currently fitted...
Cheers,
Phil
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Re: Kids Bike Gearing

Postby KGB » Thu May 01, 2014 11:21 am

I'd put the longer cranks back on, in what way did the shorter ones "improve comfort"?
A longer crank is a bigger lever which should make it easier to get up the hills on a bigger gear. Pantani rode 180mm cranks in the mountains, despite his short stature.
Definitely swap to the 24t cog though as well. It might only require a couple of extra links. I'd be splicing in a bit of old/offcut. If you don't have any then the LBS should be able to help you out.
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Re: Kids Bike Gearing

Postby MattyK » Thu May 01, 2014 4:22 pm

Betting Pantani had longer legs than a 6 year old girl...

Most kids bikes have horribly disproportionately long cranks. eg a typical crank might be about 1/10th of your height. A 6 year old girl might be 120 cm, but not many kids bikes would have 120mm cranks.
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Re: Kids Bike Gearing

Postby MattyK » Thu May 01, 2014 4:31 pm

pstanbis wrote:Thanks Matty, didn't realise the rear sprockets came so large ! Might be an easier swap than the chainring, although will probably require new/lengthend chain ? single speed, so not exactly a bank-breaking expense....

Most of those 3-spline sprockets are 1/8" wide. You might require a new chain if you don't already have a 1/8" chain on the bike.
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Re: Kids Bike Gearing

Postby KGB » Thu May 01, 2014 6:09 pm

MattyK wrote:Betting Pantani had longer legs than a 6 year old girl...

Most kids bikes have horribly disproportionately long cranks. eg a typical crank might be about 1/10th of your height. A 6 year old girl might be 120 cm, but not many kids bikes would have 120mm cranks.


I'll bet you he wasn't anywhere near 180cm tall either!

Point taken about kids bikes though.
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