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Discussion Starter #1
I made my kids their bikes so far, but I basically know what to do when it comes to road geometry. I moved to the mountains and I'm going to crank out some MTB this winter. But I don't know the current orthodoxy for mountain geometry. I can see that mtb geometry has changed a lot in the last 20 years whereas road seems to have been pretty stable forever. Head tube angle, seat tube angle, handlebars height, trail etc. If there are bikes out there to copy I'm not above doing that at all, but a lot of 20" mtb seem pretty generic or based off bmx geometry.

The kids are between 5 and 9.

Unless you can recommend a light and non crappy suspension fork I will probably be building rigid. I'll probably be building with 406 rims and BMX tires. Other than that everything else is on the table but I'm thinking a 1x setup with megarange freewheel which has worked out pretty well on a tandem so far.
 

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I searched and researched before I bought a bike for my grandson for his 5th birthday. He started on a balance bike and I had him on some 16" BMX stuff waiting for him to get big enough for a good 20" bike.

The 2020 Commencal Ramones 20 that I got him has been fantastic in the four months he's been riding it. Although it's a bit big for him as far as feet down while seated, it's been no problem for him at all and he's got some room to grow into it. He's very capable on it and has really impressed me with what he can do and how far he'll go. The bike has great brakes, has good gear ratios, shifts well and the geo and handling seem spot on. It was $399 plus about $35 for shipping.

Here's a link to the 2021 version, looks like different colors is the only change.

https://www.commencalusa.com/ramones-20-black-white-2021-c2x31842183
 

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If you want to go down the rabbit hole of trying to make a lightweight 20" suspension fork;
https://forums.mtbr.com/families-riding-kids/suspension-fork-shortening-932945.html

As for the frame building, realize that kids are going to outgrow a 20" bike so fast that it will not get much use, maybe 2 years max from the time they can first ride it until it is too small and is time to move up to a 24". Ive always been able to find an abundant supply of decent quality, cheap used kids bikes to use as frame donors for my kids bike build projects, save the framebuilding project efforts for something that will get used longer.
 

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Discussion Starter #5
Good tip on the Commencal Ramones 20. I will probably just copy the geometry on the website for that one. Of course buying one would be the smart thing to do but I need a winter project anyway, and I think I can build one a little better.
 

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Discussion Starter #6
realize that kids are going to outgrow a 20" bike so fast that it will not get much use, maybe 2 years max from the time they can first ride it until it is too small and is time to move up to a 24". Ive always been able to find an abundant supply of decent quality, cheap used kids bikes to use as frame donors for my kids bike build projects, save the framebuilding project efforts for something that will get used longer.
This is true but I have a deep pipeline of kids...4 of them with my youngest is 8 months and my oldest is 8 years. The first road bike I built has been ridden to death by 3 kids already, so any MTB I build will probably stick around for a while.
 

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Good tip on the Commencal Ramones 20. I will probably just copy the geometry on the website for that one. Of course buying one would be the smart thing to do but I need a winter project anyway, and I think I can build one a little better.
LOL! Yeah, at that price it's quite a deal. However, finding the bike in stock was a huge problem for me so if you feel the need you can justify the build that way!

The geo is very good, I suppose you could go a degree or two slacker HTA for "more modern" numbers, but this works. The large volume tires at low pressure and the geo as is make for a bike that seems ideal.
 

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A 20" wheel is going to give you much less trail for a given head angle and rake combo, so you might even want to go slacker - to get 100mm trail you'd need a head angle around 61 degrees. Whether that much trail makes sense at such a short wheelbase is another story, but there's certainly room for experimenting...
 

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Discussion Starter #9
In that case I can build the fork with zero offset, which I've done before. That would help increase the trail more without increasing the wheelbase.
 

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Discussion Starter #10
So I started looking closer at this Commencal bike and had some questions.



First of all, why the long chainstays? I usually build with the tire slammed as close to the seat-tube as I can get. Why is there all this space (green drawing)? Could it be just for chainline? I think I can get around the clearance by using a yoke. Is there any geometric reason the SS should be this long?

Second, it's spec'd with 145mm cranks and I think a kid this size is probably better with 120. With 120mm cranks the BB could be lower, and there's more toe clearance. Any downside to dropping the BB by 10mm or so?

If I build this with a 0-offset fork, the resulting trail would be about 98mm...I don't see the trail spec'd on the website. I usually build 20" road bikes with 50-60mm of trail. I don't know if there is a downside to too much trail on a MTB.
 

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With small wheels and short chainstays, the chainline can get excessively angled when shifted into the highest and lowest gears. I think the long chainstays are more about preserving the shifting than providing ideal riding geometry.

Ive noticed that beginning rider kids definitely like to be able to sit on the saddle and reach their feet to the ground at the same time. A high bottom bracket design makes this hard to achieve. Shorter cranks and lower BB height will suite a beginning rider better.
 

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I think its just chainline geometry. Kiddo is a 40lb 5yo, and he learning to manual the front up a bit when he wants to, so I don't think it hurts

The cranks are very long, it's one thing I thought I would change, but he's used to it now. Still considering it though.

20201111_131524.jpg
 

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I think there's enough variety in kids bikes that you should be able to find something that suits your prejudices without commissioning a frame. Norco and Spawn are two brands who clearly spend time developing bikes to be ridden hard. Spec and Cannondale... make sidewalk bikes with pogo sticks.

Marin just brought out some enduro-styled kid bikes that seem very strange at first glance - the forks are the wrong wheel size with short travel. But in fact they seem to be sized for the Junit forks that would be longer travel at the right size for the same A-C. You can only speculate what happened, but it's like they planned to make a $1500 bike with all the trimmings and then decided it had to be a $600 bike to sell

 

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So I started looking closer at this Commencal bike and had some questions.

First of all, why the long chainstays? I usually build with the tire slammed as close to the seat-tube as I can get. Why is there all this space (green drawing)? Could it be just for chainline? I think I can get around the clearance by using a yoke. Is there any geometric reason the SS should be this long?
I say use a yoke and put on sliding dropouts -- that way you alwasy slide the tire back a bit for chainline if you need to.


Second, it's spec'd with 145mm cranks and I think a kid this size is probably better with 120. With 120mm cranks the BB could be lower, and there's more toe clearance. Any downside to dropping the BB by 10mm or so?
Everyone in our house has shorter than "normal" cranks and nothing bad has happened to any of us. I ride with 140mm cranks on a 29+ bike, my son has 102mm cranks on his 20" and 24".
 

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Unless you can recommend a light and non crappy suspension fork I will probably be building rigid. I'll probably be building with 406 rims and BMX tires. Other than that everything else is on the table but I'm thinking a 1x setup with megarange freewheel which has worked out pretty well on a tandem so far.
Manitou Junit is probably the king of kids suspension forks right now -- weight is 1574g for the 20". It uses a 1-1/8 to 1-1/2 tapered steerer and uses a boost through axle, which gives you room up to 20x2.8" tires:
https://hayesbicycle.com/collections/forks/products/my21machetejunit_z?variant=34635433836589

Trailcraft is slightly lighter at 1420g, but lower travel / not quite as nice.
It's also straight steerer and works with regular 100mm QR hubs:
https://www.trailcraftcycles.com/product/tc30-fork/
 

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Discussion Starter #16
Here's what it looks like with the original geometry, vs. with the CS shortened by like 20mm and the BB dropped by about 15mm, and 120mm cranks instead of 140mm.

I think I can build a shorter fork and a shorter headtube. That would drop the front, but I'm not sure if there would be an advantage.
 

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Discussion Starter #18
I'm going to wait until I get my rims and tires and measure the actual wheel circumference, then I might slam the front lower.
 

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Discussion Starter #19
So, I have the frame done but I need to decide where to run the cables. I have a rear derailleur and a rear disc brake.

Run both cables under the top tube?
Run both on TOP or sides of the top tube?
Should I do that thing where you run a bare shift cable down and under the BB with one of those plastic guide bits on the bottom of the BB?

Is it currently trendy to run full length housing or should I do cable stops?
 
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