Building a kids bike need advice.

[BigMike_an_his_cheap_bike]

Hello ratrodbikes!

Ok... to start with.
I found a specialized rockhopper on craigslist, the ad said make an offer. So i emailed an offered $8, well he took the offer. The rockhopper needs a lot of work. Its an 18" or 19" frame. Almost everything was shot on the bike.
I'm building this for my 7yo daughter. I am going to use the original fork, with a bmx stem an bmx bars. I am also going with 165mm cranks.

I want to build 24" wheels. I want a geared rear hub with a brake, I'm thinking a 7 speed hub. Ok here's where i need help... I dont know what the tooth count on the front sprocket should be. Originally the front sprocket had three chain rings but I would like to build it with one front chainring.

I'm building this cause we do a lot of city riding. The extra gears will help her pedal at her own pace once she learns how to use them.

Thank you in advance for any an all help.

 

[LukeTheJoker]

Sounds like you got a good deal!

Are you sticking with a derailleur set up or going to an internal geared rear hub?
I'm not very knowledgeable on Internal hubs, but if you are sticking to the Derailleur set up, usually a front sprocket around the size of the current middle chainring will be close, but that will depend a lot on your daughter, her leg strength etc...

It might be best to set it up with no front derailleur and just manually swap between your current 3 chainrings until she decides which one she likes, then you know it will work.

I'm trying to imagine a 7 year old on a 24" MTB, and all I can say is that she must be tall for her age!

If you are going Internal rear, @Bicycle808 is knowledgeable on gear ratios, under/overdrives and the like...

Luke.

 

[LukeTheJoker]

Welcome to the forum too!

Luke.

 

[horsefarmer]

I like Luke's idea. What does she ride now? Use this to guide your selection. Maybe she could test ride a similar bike? Gonna be some trial and error before you get it perfect.

 

[BigMike_an_his_cheap_bike]

Hey Luke, Horsefarmer!
Originally the rockhopper bike had front an rear derailleurs.
I would like to put a rear 7 speed intenal geared hub w/ brake on it for her.
I would like to know what size front sprocket would go best with the rear igh.
Currently she has a small bmx bike with 145mm profile cranks and 25/9 gearing. She does well on that bike but she's ready for a bigger bike and more gears.

 

[deorman]

I'm not familiar with the various internal hubs currently available, but I can tell you that some of them are mostly overdrive with which you would want to use 40t or lower, and some are mostly underdrive or both over and under, with which you would probably want to go with mid 40s. I'd have to agree that Bicycle808 is a good source for info on that subject.

 

[horsefarmer]

Here is a very cool drivetrain analyzer program I found. http://www.soulbikes.com/gears/

Using the program and plugging the info you gave on your girls bike incl. 20" bmx wheel shown in first 2 pics.

3rd pic shows results of calculation.

You can change hubs, gears, wheel sizes and see their effects.

 

[LukeTheJoker]

Hey Luke, Horsefarmer!
Originally the rockhopper bike had front an rear derailleurs.
I would like to put a rear 7 speed intenal geared hub w/ brake on it for her.
I would like to know what size front sprocket would go best with the rear igh.
Currently she has a small bmx bike with 145mm profile cranks and 25/9 gearing. She does well on that bike but she's ready for a bigger bike and more gears.

You can get a rough idea for single speed with just a calculator and the specs of the old bike.

The current bike has a 25/9 ratio, or 2.8 to 1 on a 20".

2.8x20=56
56/24= 2.3

So to get the same distance per pedal rotation you need a 2.3 to 1 ratio on the 24", if you are running a 16 tooth rear you will need a 36 or 38 tooth front, an 18 would need a 40 or 42 tooth front.

That is if you only had 1 gear though, you need to know the gear ratios of your chosen hub (overdrive/underdrive) to work it out for multispeed applications.

Luke.