Building a kids bike

I am building a bike for my daughter based on 20" wheels, with a steeper seat tube (74) and slacker head tube (70 or 71). Is there a trail (or a head tube angle and fork rack combo) that works well for new riders? Also, been trying to figure out why all the production kids bikes I've seen (and that we own) have such high bottom brackets (i.e. little drop)... am I missing something? Just watching my daughter ride her bike makes me think a frame with more drop would be appropriate and helpful. And thoughts/suggestions on some general design parameters would be appreciated. Thanks.

JC

Bb height is a function of crank length on small bikes. There is just so much you can drop the bb with 20 inch wheels.

My method for doing a one off for my kids is to do my final check in bike cad. I can visually tell if the design will be wonky or have some problems. I also design the bikes on the slightly big side so as the saddle goes up they grow into the frames and not directly out. Most importantly, I let the kid pick the color. As far as forks go I've always used off the shelf mtb forks for the mtb bikes so I worked with that rake.

One builder who has experience with different wheel sizes uses the 81* castor angle as a guide for steering geometry. This number comes from decades of their and another's experience. There are builders about who have a lot od 20" experience, any BMX or freestyle builder as example. Bike Friday uses 20" wheels on lots of their bikes too. Andy.

I measured our kids bikes seat tube angles when I started on a 24" for my son and they are all on the steep side. 75* & 76* for the 20" and 24" bikes we have. I went with 76* and 70* HTA. I'm interested in the 81* caster angle measure. I'll have to look at that and see where I landed. I'm done with the fork and front triangle so, it's locked at this point.

Bottom bracket drop is tricky. More seems good but, could be disastrous - or at least painful. For an adult, we know not to pedal through corners above a certain speed - kids don't know that and I've seen my son pedal through corners and clip a pedal so be careful there.

My 6yo is just about to grow out of a 16" wheel GT scamp that she's been riding (with no training wheels) since her 4th birthday.

Attachment 74950

One of the good features of this bike has been the slack seat tube angle which means that the reach increases more as the seat goes up.

I think that it also has shorter cranks and a lower bb than other similar bikes which helps to lower the stand over and center of gravity.

Duane nails the issue with BB drop; pedal clearance when cornering. Also, you didn't mention if you were building a geared bike, but if so, you'll have to lengthen he chain stays to achieve a reasonable chain line (longer wheel base is a bit slower, but the extra stability helps when they get going downhill). My daughter's 20" wheel road bike used BMX cranks and 1X9 Shimano drive train with a bar end shifter.

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Originally Posted by duanedr

I measured our kids bikes seat tube angles when I started on a 24" for my son and they are all on the steep side. 75* & 76* for the 20" and 24" bikes we have. I went with 76* and 70* HTA. I'm interested in the 81* caster angle measure. I'll have to look at that and see where I landed. I'm done with the fork and front triangle so, it's locked at this point.

Bottom bracket drop is tricky. More seems good but, could be disastrous - or at least painful. For an adult, we know not to pedal through corners above a certain speed - kids don't know that and I've seen my son pedal through corners and clip a pedal so be careful there.

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Andrew, thanks for the input. One question though - I am not familiar with castor angle in relation to bicycle steering geometry. I did some research but wasn't able to clarify what is meant by castor. Could you provide a bit further detailed explanation? Thanks.

I am aware of castor as it relates to cars, but when it comes to bikes, everything i have learned to date has centered around head tube angle, rake and resulting trail.

Jonathan Clay

Yes, my chainstays are 395mm long. I'll be going with an older 8sp xtr rear with a single chainring in front. I want to do disc brakes but am working on how to get the caliper inside the rear triangle. The seatstays are going be fairly light and I don't think they'll withstand the braking forces.

How would one measure the castor angle on a bike? The diagrams I find don't make it clear what the upper bisect is. I'm assuming, given standard fork dropout, the lower bisect is through the axle. Or is it as simple as adding the fork offset angle (for example, a 7* fork crown) to the HTA?

Thanks as always!

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Originally Posted by JuanGrande

Andrew, thanks for the input. One question though - I am not familiar with castor angle in relation to bicycle steering geometry. I did some research but wasn't able to clarify what is meant by castor. Could you provide a bit further detailed explanation? Thanks.

I am aware of castor as it relates to cars, but when it comes to bikes, everything i have learned to date has centered around head tube angle, rake and resulting trail.

Jonathan Clay

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As described by Bill Boston, castor angle is the angle WRT the ground with the steering axis/ground intersection and the axle. Remember with a rake the axle is off set from the steering axis. The formula is tire radius divided by trail.

I have found that most of the bikes I have ridden, measured geometry and wanted to replicate had a castor angle between 80 and 81 degrees. Andy.

I built a kind of road bike with straight handlebars with 20” wheels for my daughter when she was 6 about 20 years ago. Somewhere I found 125mm cranks . The formula I used to determine BB drop was based on a desired BB height of 260mm (265 if one is more careless about pedaling through corners) if using 170 length cranks. Subtracting 125 from 170 means I can lower the BB height 45mm to 215mm when using 125mm cranks. This 215mm I subtract from the wheel radius to be my BB drop. There are 2 sizes of 20” wheel sizes. I chose the bigger 451s. The tire circumference is 1545mm according to a Cateye computer chart for 20” X 1 1/8” tires. 1545 ÷ 3.141 = 492 ÷ 2 = 246. So subtracting the wheel radius of 246 from a desired BB height of 215 means my drop will be 31 or if one wants to keep it simple, 30mm.

I remember the design to be fairly challenging because I couldn’t set her small body up on my fitting bicycle. Eventually I made a step through frame with an effective top tube length around 15”. My head angle was 71º with about 25mm of rake (I don’t have my build sheet handy to know exactly what it was). The seat angle was 72º with a set back seat post (since her handlebar height was a bit above the saddle). It all worked great for her until she outgrew it. I also needed to find a smaller saddle and brake levers. It had rear derailleur gears only.

The quote I’ll always remember, “Daddy, why can I go so much faster than my friends?“

So, I found this on recumbent forum site at Ideal Caster Angle ? [Archive] - BentRider Online Forums

" If you extend the head tube angle down till it touches the ground, and then place a line from the point of intersection with the ground, back up through the hub center, the angle of this line is the caster angle. "

I'll have to measure when I get back out to the garage tomorrow.

I'm noticing most others are using shallower seat tube than my reference bike. My reference bike is a Redline Conquest 24" cross bike which was my older son's bike. It has a 76* STA. Because my DT miter was a bit sloppy at the top where I had to adjust for the ST/BB fillet, my DT/ST angle closed up a bit so the STA is a bit steeper yet. I need a better process for that part. Do most folks tack both ST and DT to the BB before doing the full fillet?

The HTA and fork rake are similar to what Doug mentions so, that's encouraging. I'm at 71* 30mm of rake for around 67mm of trail.

Thanks everyone, this helps a lot. Back to drawing and hopefully brazing soon.

Jonathan Clay

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Originally Posted by duanedr

" If you extend the head tube angle down till it touches the ground, and then place a line from the point of intersection with the ground, back up through the hub center, the angle of this line is the caster angle. "

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By that definition, castor angle = HTA + asin(trail / Rwheel) where Rwheel is the radius of the wheel, tyre included.

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Originally Posted by Mark Kelly

By that definition, castor angle = HTA + asin(trail / Rwheel) where Rwheel is the radius of the wheel, tyre included.

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Here is a very good video on a few different aspect of steering, enjoy!

Also with small wheels you run zero BB drop, and in many cases BB *rise*, and from memory 1.5" front axle offset is common for BMX forks - for real BMX your HT* will run much steeper then the ST*

- Garro.

https://www.youtube.com/watch?v=W8RZS85u6Sg

I wondered if castor angle was in bikecad, so I asked Brent, yep its there:
Arctan(Trail/Wheel Radius) | www.bikecad.ca
cheers
andy

Good Evening,

I built a few Kidsbikes with 20" wheels.

Attachment 75172

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Attachment 75174

I used the attached drawing and it worked pretty well:

Attachment 75175

Greetings,
Georg

I did this one a couple of years ago for my then 6YO. I can't tell if it rides really well or if she's a good rider, or whether kids even notice much related to ride quality, but it sure has worked out well for her. I wasn't sure what to shoot for so I essentially copied angles and rake from what I like and scaled it down (including tube diameter.) The BB doesn't really have any drop, and front crank was 175 shortened to around 120 or something. I also ground off the teeth of the big ring to make a rock ring and protect her legs from outer teeth (they can't use it anyway.) The only real problem my kids have had is having enough strength to shift gears. Levers seem easier to manage than twist shifters, but not by a lot.

It's really rewarding to put a kid on a quality bike, so good luck and enjoy!

-Ryan

Attachment 75179Attachment 75180

I build a lot of kids bikes. The 20" have no BB drop. 24" get about 40 mm. For the 24", I design around ~ 390 - 410 mm AC fork with ~45 mm offset. I like 70 degree head angle. The kids ride rigid on our local trails, and other XC. When we go to KT or highland for some lift-served downhilling, I put a 26" suspension fork on the front - slacks the front end out and adds the monkey motion to keep things under control.

Most important is to shorten cranks. Both Shimano and SRAM made a good cheap crankset that can be shortented down to 110mm if need be. I just do it on my lilttle mill, but I know that there is a vendor out there that shortens crancks - can't remember the name now though.

I have found that the WTB "Speed she" seat is deal for kids. Nice and short, not too wide with a decent amount of paddling and doesn't weigh a ton.

I have found that the kids can work the trigger shifters better than the twisters. The front shifting is the hardest. Part of it - with top pull deraileurs - is the angle of the cable if the housing stop is put on the ST. The ST is so short that there is an acute angle on the cable. Put the housing stop on a standoff, or use one of those pre-machined housing stops used for cantilever brakes to make that cable run as straight as possible. Or skip the front changer all together and just change it by hand. Granny for off road and middle ring for road.

Hydraulic disks are the best for little hands. Or sue the Avid rollamajig on rim brakes to multiply the cable pull so their fingers can reach the levers.

Nothing better than a real bike for a kid. They get better every single ride!

B

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Originally Posted by MDEnvEngr

I build <cut>

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Please add a first and last name to all posts here.
Thanks atmo.
http://www.velocipedesalon.com/forum...rum-36866.html

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Originally Posted by e-RICHIE

Please add a first and last name to all posts here.
Thanks atmo.
http://www.velocipedesalon.com/forum...rum-36866.html

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Bob Spooner
East Hampton CT

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

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.

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.

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.

Attachment 118944

chaz

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.

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

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Originally Posted by 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.

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??

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

@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.

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

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Originally Posted by 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

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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.

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)