Building a kids bike

[JuanGrande]

Just want to say thanks to everyone for chiming in on this thread.... It really helps and I greatly appreciate the advice. Time to start building. I will post some Friday Night Lights pics as I go. Thanks again!

Jonathan Clay

[duanedr]

One more lesson in frame building:

This frame is #2 and is a 24in MTB for my son. I obviously don't have my processes down perfectly yet. I attached ST to BB and then fully filleted the joint. Mentally, I think it just feel good to 'finish' something in my short bouts in the garage so, I try to finish a joint but, maybe....only tack the ST to BB shell. Then (days or weeks later) I added the DT - but since I'd built up a fillet around the ST, the joint was no longer mitered perfectly. I adjusted as best I could but, couldn't correct it all. As I finished up the front triangle, I noticed the DT/ST angle had closed up which ended up making the ST angle 2-3* steeper than designed. I thought that would be ok since I designed it with a zero offset seatpost. Then, last week I attached the rear triangle and finished up the braze ons etc.. As I was looking at it (and admiring my fantastic work!) I noticed the BB drop seemed more than designed. The steeper ST without changing the ST/CS angle has the effect of lower the BB!! AAUUGGHHH!!!!! I should have opened up the ST/CS angle!! grrrr... Maybe if I had been doing the layout over a full sized drawing, I would have noticed and changed this. I'll build the next one on a full sized drawing... sigh...

Anyway, there are other issues with the frame but, dang....there are some good details I really like about this one! He's still 6-9 months away from it fitting him so, I have time to get it right.

Lots of pictures on flickr page.

[false_aesthetic]

Bringing this thread back from the dead. Really curious about combining a short CS length (320-340mm). I have 9 speed components in the parts bin. With such a short chainstay length should I plan on limiting the RD to avoid the smallest and largest cogs?

TKS.

[hackdyne]

On a 20-inch bike shifting is a challenge if you want short chainstays.

I have one I built with a relatively slammed rearend (yoke required for 2.125 tires), but I built it with a custom 110mm hub and cranks. It uses a 6-speed freewheel with 1 cog removed and shifts across all 5 pretty well. I don't think a 9-speed would go on this bike because of the chainline. This was sort an an extremist attempt to build a light 20-inch MTB with usable gears and disk brakes. It comes in about 12 pounds. Total gear range is 14-32 of the freewheel.

The alternative, that we usually see on derailleur-geared 20-inch off-the-shelf bikes like the Commencal Ramones for one example, is artificially long chainstays. I have one I built this way too, with a commodity 135mm MTB hub and 7-speed freewheel. I think the chainstays are about 340 on that one and it shifts pretty well across all 7. The kids like it a lot but to be honest I mostly built it because I had an extra set of rims+tires, and you could build to the same rider fit on 24-inch wheels with slammed chainstays and gain from bigger wheels. So a 20-inch bike with full-range cassette, to me it's kind of a mutant with undersize wheels, not in an altogether bad way maybe, just a BMX-ish way.

You can see both bikes in the pictures, the smaller one on the right with just enough CS to clear fat tires using a yoke. On left, longer CS with standard drivetrain.

Short chainstays and q-factor are important to me in a MTB. Honestly for 20-inch wheels, if you are going to build it with short CS fitting of a 20-inch bike, I would think hard about going single-speed with a BMX hub and saving a lot of weight. I think BMX hubs are 120mm OLN which isn't far off from my 110mm hub and you can get some nice ones with small cogs. You can also get 120mm and maybe 110mm 3-speed hubs but you pick up a lot of weight.

Also you can find 3-speed derailleur hubs on ebay or aliexpress for folding bikes which are like 9t-24t or something like that, giving quite a lot of range with very limited space, but you're on your own finding a deraulleur and shifter. I have a couple in my bin but I haven't mapped the cog spacing to figure out what kind of shifter they would work with but it's probably 9spd-ish.

About the BB drop, I design BB drop based on crank length, based on avoiding pedal strikes. This depends just as much also on the pedals you will be using and the Q-factor of your cranks. With the custom cranks with Q-factor as low as 80mm, I can lower BB even further for a given crank length leading to lower standover and basically better fit for small kids. Crank length depends on the rider leg size. On a 20-inch bike cranks should be 100-115mm depending on the kid you are targeting. This depends entirely on the kid and I have built 20-inch and 26-inch bikes with identical crank length and identical BB height, because it was for the same kid...I usually build custom cranks at the sub-120mm size range but you can get BMX mini cranks down to 100mm and you can also pick up cranks this size from Aliexpress which aren't too bad, although I usually do put them on a custom BB because a 68mm BB gives just absurd q-factor. Always remember to give extra pedal room on fixed-gear bikes and tandems because you may be forced to pedal through corners. There is a CPSC spec that says the bike should be able to lean 30 degrees or something without the pedal hitting the ground, or something close to that and it's a good place to start.

PXL_20210401_013307073.PORTRAIT_3 (1).jpg

chaz

[thollandpe]

Chaz has clearly done his homework. Rock on, @hackdyne, you have not only figured some things out but also managed to explain them really well. As someone who has cobbled together a bunch of kids’ bikes, my hat is off to you.

[hackdyne]

Here is one of the hubs for 3-speed folding bikes. You can get them easily in disk version as well. I haven't build a bike with one but I would love to see someone figure out a matching derailleur. I was thinking about doing a geared road bike, but the kids will probably be grown before I get around to it.

https://www.ebay.com/itm/22416972769...YAAOSwm7JfbaXx

[e-RICHIE]

Bringing this thread back from the dead. Really curious about combining a short CS length (320-340mm). I have 9 speed components in the parts bin. With such a short chainstay length should I plan on limiting the RD to avoid the smallest and largest cogs?

TKS.

??

What wheel diameter are we talking about?
This thread seems a bit all over the place.

[false_aesthetic]

@e-R thanks for requesting clarification. 20x2.0 is what I'm aiming for.
@Chaz your post is great. it confirms a bunch of stuff I was thinking. Super helpful.

[hackdyne]

Here's another thing to think about when it comes to shifting with short chainstays. Microshift recently released their Acolyte groupset which is specifically targeted for kid's bikes. It is an 8-speed cassette, but it's 11-42 or 11-46 so it's high range. The derailleur also has a clutch although I'm not positive the clutch is very good after playing with it.

According to Microshift, 8-speed is better for kid's bikes because it runs better at terrible chainline typical of kid's bikes. I can't confirm this but I recently put an 11-42 Acolyte on a 26er with 340mm chainstays, and it runs great. I haven't really done a back-to-back of the Acolyte vs. similar 10speed or higher on the same bike, but I would like to see such a review. Anyway it's fairly cheap and if you are building a bike with marginal chainline, you have the Acolyte option now which could potentially run better than a 10speed with the same range, and still have a clutch derailleur. It's nice to be reminded that 8 speed chains are like $9 too.

chaz

[hackdyne]

Here is one of the hubs for 3-speed folding bikes. You can get them easily in disk version as well. I haven't build a bike with one but I would love to see someone figure out a matching derailleur. I was thinking about doing a geared road bike, but the kids will probably be grown before I get around to it.

https://www.ebay.com/itm/22416972769...YAAOSwm7JfbaXx

Replying to my own post since it's too late to edit...I tried to measure my 3-speed hub and seems the cog spacing is about 4.8mm. According to internet resources that corresponds to 8-speed, so 8-speed shifters and derailleurs should work. I'm now contemplating a build with this hub but didn't really want disc brakes.

[false_aesthetic]

That's great info, thanks.

I ended up going with Acolyte and am pretty stoked on it so far. Few shifting niggles but I think that will be solved with a chain keeper or limiting the RD. I'll post a pict when the bike is fully built up after paint. (This weekend)