I am currently working on a 14" bike for my kids. I am heavily leaning on frame design from European manufactures with slight tweaks for my kids small stature.
The problem I am running into is that trail is so short due to the small wheels. I am struggling to decide where I want the trail number to end up.The most trail I can get with “normal” is 36MM with a 69 Degree head angle and 300MM offset.
My concern is that 36M of trail is on the most extreme low end for adult bikes. While I want low trail for this build how low is too low for a kids bike? For reference most companies 14 inch bikes have around 20MM of trail.
Something that I feel is untested, but makes sense in my brain, is the relationship to intended speed and trail numbers (the self centering ‘caster’ effect).
With the most extreme end of DH racing for high speed and high trail numbers (loads of bump inputs to push the front wheel off line, big need to recenter the steering) it also makes sense that a kid who is learning how to ride and what a bike is capable of wouldn’t (maybe shouldn’t?) reach the same peak velocities that a full grown adult would hit. A lower trail number makes sense to me.
Also, a more beginner rider might have a tendency to steer with the handlebars instead of leaning the bike, so a quicker handling front end might also make sense.
All of this is N=1, anyone else have a different methodology?
It is true that you will want a relatively low trail number because of the lower speeds most kids will reach; however, you have to remember that trail is dependent on wheel diameter as well so it is expected that it will be quite a bit different than a big with twice as large wheels.
There are many prominent brands that use a fork offset of zero on their 14" learn to ride bikes with head angles around 70. These bikes can be very squirrely at speed but are great for learning. If this is intended to be more of a ripper bike than I think you are definitely on the right track.
When designing kids bikes (and adult bikes), I find it helpful to compare geometry of different frame sizes through ratios. For instance, look at the ratio of trail to wheelbase as compared to your preferred geometry and size to get a better understanding of how the handling will compare.
I think using ratios is a good idea. That is how I am determine most other measurements.
They have a big name 12” strider bike with a lot of offset and 71 HTA. I have not calculated trail. That bike is so awful handling they have struggled to get used to it. The front flops so badly that the whole bike falls over with little steering input. It’s so bad that I had to make a rubber steering damper inside the head tube to make the bike rideable.
It always scared me to death when my seven year old went ripping down hills on the pavement at warp speed because her commercially available bike always got bad speed wobbles.
I went out of my way to get her a decent amount of trail on her custom 20 incher. I think I ended up around 65mm of trail. But it took a 66* head angle with 30mm of fork offset to get there. (Going from memory)
So far she hasn’t complained about the ride. Not that eight year olds give lots of constructive feedback. And the speed wobbles are under control.
I guess my largely unsubstantiated opinion is that trail is trail regardless of wheel size and there needs to be a certain amount of trail for stability. With that said, it’s important to look at the bike as a whole and not get hung up on one metric.