I am planning to build a steel bike myself and want to pair it with a custom stem. I came across someone selling 3d printed stems in my area. So I decided to design myself a stem that is compatible with ACR. I know how to 3d model, but what kind of loads should I apply to the stem in CAD? I'm only 50kg so not too worried about it not being super strong.
You’d probably have to measure in real life with a load cell to get good numbers.
I think the greatest forces occur when one is sprinting out of the saddle and pushing and pulling on the handlebars. It’s similar to the deadlift exercise where even amateurs can lift twice their body weight. The ugly thing about this usecase is that one hand is usually pulling up while the other is pushing down, so you get a lot of torque on the stem and load cycles with changing direction.
Hitting potholes obviously creates big forces too, but even with your arms bent you usually don’t break your nose on the handlebar, so I’d say it’s similar to bench press or dips where most people can do slightly more than their body weight.
So I’d expect/design for at least the rider’s body weight pushing and/or pulling on either end of the handlebar at the same time. This means your stem has to take twice the rider’s body weight in the up or down direction as well as a torque depending on your handlebar width.
Another data point: Riding with a person sitting on the handlebars usually doesn’t break the stem.
Stems are generally really beefy with thick walls (much thicker than your normal frame tubes).
Edit: Just in case it’s not obvious: A catastrophic failure of the stem is extremely bad and is going to make you crash. I’d never really trust a home-made stem.