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GTaholic
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Hi everyone.
Should I copy a BMX frame geometry when doing a MTB frame in same size using 20 inch wheels for a 6 year old girl?

I think all frames are too heavy for kids in their size, around 120-140cm long 4-6 year, they weigh almost the same as the bikes weigh.
Not so fun when in woods and playing with them.

What tubing are of decent weight, not the butted thin, not plumbing pipe type..
 

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BMX BB will be too high and probably lots of other things wrong..

I'd copy the Islabikes Beinn 20, but they don't give geo anymore on the website (probably because people copy it....). You can print off the photo and take scaled measurements (using wheel diameter to scale). Or my kid has one if you need any real measurements (bb height etc).

If you are in Europe, just buy Columbus Cromoly (Gara / Thron etc tubesets) from Ceeway in UK. You can get 16 / 19 / 22 / 25.4 / 28.6mm diameters in wall thickness around 0.8mm. Lengths to suit adult bike or can also buy 1.5m length. Much cheaper than aerospace tubes in Europe (e.g. £11 for 1.5m of 28.6). Link to the catalogue below:-

Spare Tubes
 

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Belltown Brazer
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Kid's bike design is fun. Put the BB as low as possible while allowing pedal/ground clearance when cornering. The pedals should be 2 - 3" off the ground. Most important is appropriately sized cranks. My 4 y/o is on 105mm cranks. Drill and tap an old set laying around. Use the cranks length to determine BB height. The seat will still be too low for efficient pedalling, because the little guys like to be able to touch the ground when seated (another reason to get that BB as low as possible). Measure the little guy and figure seat-bar distance. Use the shortest stem possible. Think about 0 reach stems or swept back handlebars. They'll start to need more reach pretty quickly. Chainstays don't have to be too short, but shorter than a typical BMX is good. If you are going to use a stock fork, mind the offset and figure the trail. Figure HTA to put the bike out on the high end of trail...they need all the help they can get going straight. Use decent brakes and little levers. Sinz makes some really small levers that are ideal. Use a grip shifter if you are going gears.

Oh yeah, use thin, small dia tubes. There doesn't need to be anything bigger than 1" besides the HT and BB. Use cut offs from your bigger bikes, or even 028 straight guage. 3/8 seat stays. Do not use a replaceable deraileur hanger...you will forever be straightening it out.

Route the cables under the top tube or anywhere other than the top of the TT. Keep that tube smooth.

Nothing better than building a bike for your kid. I've posted this before, but can't resist:
 

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Hi Blaster - I presume you mean 20" race BMXs? They are fantastic bikes (I sooo wanted to get one for my kids), but most I looked at had huge cranks and a correspondingly large bb height. Just looking at a few micros, they seem to have cranks around 130mm, minis 155mm. A guess at the bb heights would be 10". Rideable if you have a kid skilled enough for racing on a track and no need to sit down, but not ideal for messing about in the woods or seated climbing up long hills (where a reasonable saddle height would have them unable to touch the floor).

I'd go with everything said by MDEnvEngr for use on the trails. My 7yr old (short legs) is using 114mm cranks. For shortening cranks, you can use the smaller 1 piece bmx / kids bike pedal thread - the smaller hole means you have a wider choice of cranks that can be butchered (and the smaller LH thread tap is reasonably common as also used on classic motorbikes).

Gearing needs some thought. A 1:1 bottom gear ratio with 20" wheels means they can actually ride most stuff. Also worth thinking about hub gears - a friend has just set one up and it works far better than dragging a rear mech through the weeds (and a lot less learner gear shifting /chain loss hassle)
 

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Hi everyone.
Should I copy a BMX frame geometry when doing a MTB frame in same size using 20 inch wheels for a 6 year old girl?

I think all frames are too heavy for kids in their size, around 120-140cm long 4-6 year, they weigh almost the same as the bikes weigh.
Not so fun when in woods and playing with them.

What tubing are of decent weight, not the butted thin, not plumbing pipe type..
Hei Mattias,

I'm in the same situation with my 6 year old, and quite recently I copied the 20" Islabikes and did some tweaking to the geometry. Perhaps this will give you some inspiration?

Truls
 

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You are not going to save much weight by building the frame. The reason kids bikes are so heavy is because the crappy components. Its super hard to find light weight components for 20 and 24" bikes.

Great fun building for your kids though. I hope a can find some time to build frames for my kids this year.
 

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Not actually very hard to find lightweight 20" parts.

Even Islabikes etc come with steel cranks. Fairly easy and cheap to cut down aluminium cranks (this saves a heap of weight). Just need somebody with a miller to tap the pedal holes nice and square. The rest can be done with hacksaw and file.

Plenty of light 20" rims - some in 28 spoke.

Continental do reasonable lightweight knobblies and tubes.

Maybe BMX race forks (carbon - not sure what brake setup needed). Or I know a few people that run cut down Pace rigids (come with clamp on brake bosses and bottom dropout removed, cleaned out and re-bonded)..

Find some NOS 1990's lightweight flat bars (a scary 130 grams ish I seem to remember) - cut down these will weigh even less and be plenty strong enough for junior.

The hardest bit I've found is finding quality aftermarket ultra short reach brake levers. Anyone had luck with these?
 

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Not many 20" rims under 400g out there. The lights knobbies are 560g. Hubs, BB, stems, seat posts shifters etc will be the same weigt as on 26" bikes. It will be lighter than a regular kids bike but not much ligher than a 26" bike
 

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Brake levers?

Just curious - for those of you who have done this kind of project:
-At what age can kids manage hand brakes? Is there a brake lever that works especially well for small hands?
-Do kids need knobby tires? At what age can they actually ride on dirt at speeds that would make knobs necessary?

-Walt
 

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Most kids bikes are equipped with a brake lever from tektro with short reach. Some say a kid bike should have coaster brake. I would only recommend that if the child can't reach the break leaver. Most 5 year olds can use brake leavers safely. When a kid gets scared he/she will in many cases let go of the pedals. With break levers they still have a chanse to brake.

Havnt thought about knobbies just thought that they made sence in mud.
 

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My son got his BMX ( Norco mini)

at 5 years old,
And I was impressed at how well he manage the levers and the modulation. (his got front and rear brakes)
(But you need good brakes and good small levers...)

And I was way less scared seeing him down the hills with his pedals flat to the ground (no more cluster brakes) than before.
 

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Belltown Brazer
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Just curious - for those of you who have done this kind of project:
-At what age can kids manage hand brakes? Is there a brake lever that works especially well for small hands?
-Do kids need knobby tires? At what age can they actually ride on dirt at speeds that would make knobs necessary?

-Walt
My 4 y/o - average size hands - can handle hand brakes no problem. I did have to convert an old grip shift to put a brake on his pushbike when he was 3. He was following his big brother on the yard mtb trails with no brakes (other than feet) on the pushbike. This resulted in some crashing. Modifying the grip shift allowed him to actuate brakes with a twist.

Hydro disks would be ideal for little hands.

I put some small-spiky tires on the 4 y/o bike. A BMX race tire, and they work great on dirt. But no good on loose, mud or leafy trails.

I found that my older boy *needed* knobbies at 5.

I tried the BMX race tires on my 7 the y/o bike and they wouldn't stick with his speed. He did love the speed of the tires though, but didn't like washing out. He's running Maxxis Max Daddies, which are big and knobby, but roll smooth. The extra width is also very handy when he's riding skinnies too...it always seems like he's hanging half of the tire off the skinny.

B
 

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Dad
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Just curious - for those of you who have done this kind of project:
-At what age can kids manage hand brakes? Is there a brake lever that works especially well for small hands?
-Walt
My daughter at 5 was able to use hand brakes fine (aside from one time she panicked.) Use a thin grip and adjust the lever fairly close to the bars for a short reach. I found being light with small fingers, she doesn't need nearly as much finger clearance or cable pull to stop properly.

This was my attempt at a 20" mtb using adult parts cut down where I was able. It has worked well, but I think I'll slow the steering a bit on the next one slated for daughter #2.
 

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Long legged eldest could ride 20" wheels on tame off road from about age 4 1/2. Once it got slippy he needed knobs - they aren't putting a lot of weight on the ground so wheelspin on loose gravel, tree roots, front wheel washout etc. If just riding cyclepaths then probably doesen't matter so much.

Coaster brake works really well when they are tiny and hands are weak. They are a hinderance once a little older and go off road. As PeterE said - they sometimes panic at the worst possible moment. Also common for their feet to get bounced off the pedals - no feet = no stop). Coaster also gets in the way of restarting in tricky situations as they can't spin the cranks backwards to get some proper leverage for setting off.

400g x 20" rim is probably limited by other factors other than just rider weight - thickness of rim brake surface, ability to withstand spoke tension etc. I guess there is some scope for a lighter junior rim (esp if using disc brakes), but would just end up with adults seeing the weight and trying to use it for something crazy.... :)
 

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Light 20" parts are so easy to find. Taking about 3 minutes, I found several tires and rims way below what was mentioned above... 280 grams tires and 340 gram rims. And those were Maxxis DTH tires. Plenty for light trail riding. Of course the budget was pretty high, but we built a race bike for a 4 year old here, and fully built, with brakes and a 40-16t gear it weighed 7 pounds. Oh, it was a Steel SBC frame too. Light is possible, there are people making ridiculous stuff for their kids racing bmx... All of that can be used on a mtb.
A small kids frame can cut nearly ~5 pounds off of a 26" bike. Thats a huge jump in weight even if you cant find light components. The one on the right here weighs 1 lb 1 oz. use small diameter .028 wall and it will be over-built! Good luck with your build!
 

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From Russia with luv!
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I am now gathering information on a kids bike build. It's not for my kid yet, but it's for my friend's son.

What we don't have here in Russia are small Sinz cranks and levers. They don't even ship to Russia.
So I will need to mill and re-chase standard cranks.

I am deciding on the brakes too. I wanted to use hydraulic disc brakes. Wanting Hopes but they are very expensive over here.

For rims thinking of using front 20" trials rims.

Btw, the little guy is 5 y.o.



 
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