Building a kids bike

[JuanGrande]

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

[Jonathan]

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.

[Andrew R Stewart]

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.

[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.

[hugh]

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.

gt_scamp_girls_bmx_bike_16_19427601.jpg

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.

[GSmith]

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.

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.

[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

[duanedr]

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!

[Andrew R Stewart]

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

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.

[Doug Fattic]

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?“

[duanedr]

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.

[JuanGrande]

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

Jonathan Clay

[Mark Kelly]

" 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. "

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

[steve garro]

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

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.

[afwalker]

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

[GEBLA]

Good Evening,

I built a few Kidsbikes with 20" wheels.

Microgebla52.jpg

GEBLA-KinderMTB20-09.jpg

GEBLA-MTB-20Zoll01.jpg

I used the attached drawing and it worked pretty well:

radgeoKidsBikeHT.pdf

Greetings,
Georg

[Ryan Christbaum]

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

9055671426_758a7c7f1e_k.jpg9053444457_17a6ff132c_k.jpg

[MDEnvEngr]

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

[e-RICHIE]

I build <cut>

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

[MDEnvEngr]

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

Bob Spooner
East Hampton CT