UniChris

Something I've long wondered about: how are child-sized bicycles evaluated for handling quality?

A young man of my acquaintance has unfortunately had two crashes on a 20 inch five speed lately, where he seemed to get into a steering oscillation while riding down a hill at speed, once descending from a bridge over the highway on a paved rail trail, once on a steeper off-road path.

I know that directional stability of a two wheel platform critically depends on things like head angle and "trail"...

But how is it figured out it if this is correct on a frame which an adult can't ride to evaluate? Are particularly compact teens employed?

Presumably the position and size of the rider relative to the frame also matters - I once tried a cheap kick scooter where there was enough flex in the head tube that if your pushed (from memory) forward on the handlebar the front wheel would move back and create an alarmingly unstable regime.

Ultimately how can design or fit issues be distinguished from the bicycle equivalent of "pilot-induced oscillation" when its not possible for an expert rider of similar size to try the bike out?

VegasTriker

Have you checked things like the bearings and hubs in this kid's bike? It might be something as simple as the cones that hold the bearings in place not being properly adjusted. https://www.parktool.com/blog/repair...and-adjustment A quick way to find out if they are grossly maladjusted is to lift a wheel and rock it side to side. There shouldn't be any play in the wheel but it should turn freely if released with a light push. It's not wheel size either as many recumbents use 20" wheels and they have rock solid handling at speeds of at least 35-40, mph (for the quality bikes and trikes). Unfortunately, on some cheap bikes with really crappy hubs even careful adjusting is not going to make the bike handle at high speeds.

fietsbob

FWIW My Bike Friday has 20" wheels.. (disc brakes, Rohloff Hub)

in the Local bike shop they have 20" wheel Dahon Folding bikes & 20" wheel BMX bikes

if you got your kid a bike at Wally-world it may not even be properly assembled..

Ironfish653

Been setting up a lot of 20” kids’ bikes this summer, both for my kiddos and their friends.
Most of them have cup-and-cone headsets and hubs that can easily get out of adjustment, given the typical bumps and bangs that kids put their bikes through.

Also, I’ve noticed that most of them have short, high stems which with the narrow bars, are good for making them maneuverable at low speeds, especially with small, not-so-strong riders. That’s not exactly optimal for high-speed handling.

UniChris

Took a ride over there this afternoon and grabbed the front wheel of his bike with my legs and tried to exercise any play by turning the bars but could not.

With brake pads as reference the wheel runs truer than some I've ridden, and the spoke tension seems reasonable, so it didn't seem warranted to flip it over and commence a fork-referenced tensioning/truing.

It does however have an unpleasant rumble if you spin it. If I try really hard to put sideways pressure at the tire I can make it move a tiny bit, but not much for the size of system.

It's interesting that this thread was independently created at about the same time: https://www.bikeforums.net/general-c...le-shimmy.html

"Hey kid, if you feel like you're losing it squeeze the top tube with your knees"...not. More like "let's remember not to go too fast down hills"

The fork says "specialized" and the brake calipers are on the front of it, so while I'm told it was a used bargain it doesn't seem department store grade or misassembled - essentially imagine a past-generation step-up from the Riprock Coaster 20 on the same frame and rigid fork but with a 1x5 drivetrain, rather than the current step-up which has a 1x8 and front shock. Literally, it is the "Streetrock 20" which seems to no longer be an offering.

Viich

Is he steering by moving the handlebars instead of leaning? Common for kids to have not made that switch yet as they move from 'I can ride on two wheels' to 'I'm a daredevil', and will look like that, especially if they're nervous as they go fast.

UniChris

An interesting question, but in the case of neither incident would there appear to have been any desire to go anywhere other than straight ahead.

Though naturally that can require some correction.

AnthonyG

In my time I have ridden child's bikes VERY fast down hills. I also suspect headset adjustment as the culprit yet I don't think that you have tested it correctly.
What you do is, hold the front brake on, and then rock the bike back and forth to see if its at all loose. Also test to see if its too tight.
If it is well adjusted then its likely that the child needs needs to be taught to steer properly.