this post was submitted on 20 Jul 2024
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I had a long answer about why cromoly versus aluminum, but recognizing the XY problem, I think @grue is right that we need to step back and address the requirements: is there something that pre-made racks can't solve for you?
In another comment, you mentioned a Specialized Pizza Rack failing under heavy load. Can you describe what sort of load you had on it? The manufacturer rated it for 15 kg (33 lbs), and presumably that's with even pressure, not concentrated on the weaker arms jutting out.
I have something like this rear rack, which attaches to my bike frame to bosses near the rear axle and near the top of the seat tube.
Although this is rated for 60 lbs (27 kg), I've had static loads on this rack approaching 100 lbs, as my bike doubles as a handy sawhorse in my garage. While riding, 60 lbs would already be causing its own stability problems, whether the load is atop the rack or at the bottom of panniers on both sides. Having mounts close to the axles makes better use of the aluminum's strength with straight-down forces
If your issue was the arms of the Specialized Pizza Rack bending due to a wide, heavy load, one possible solution is to use a conventional rear rack without the jutting arm, then mount a sheet of wood as the wider top surface. Wood is also very forgiving for cantilevers, since it bends a lot well before breaking or folding over, so you get advanced warning before something gives.
alu is a no go as it cant be repaired easily. (and its also weaker than steel in my experience)
in the end i want to easily put 30kgs on a frame mounted rack that is made from me.
Can you clarify why aluminum is not easily repairable compared to steel? While it's true that steel tubing will resist dents and bending better than equivalent-spec'd aluminum tubing, steel is subject to damage from rust. As the other commenter said, this means repairing a steel structure requires sanding off the paint layer, which isn't required at all if the natural color of aluminum stock is acceptable. And both can be welded using readily-available processes, so repairs should be substantially similar to initial construction.
As for strength, steel is indeed stronger in tensile and yield strength. But for tube segments in compression, that quality of steel isn't being exercised, since you can always use aluminum tubing with thicker walls to support your expected payload, and still be lighter than the equivalent steel tube section. 30 kg is well within what commercially-available aluminum round tubing can support, for the dimensions of the average bike rack.
What can be a possible consideration for steel vs aluminum is whether you have significant sideways forces. A payload centered above the rear axle simply follows where the front wheel is pointed. But if the payload is heavy and behind the rear axle, it swings to the outside of a curve, causing flexing in the rack.
Aluminum tubing should be designed so that sideways forces do not "collapse" the structure by allowing the tubes to be kicked out from underneath. This applies to all tubing, but steel's high stiffness compared to aluminum means steel segments can be much longer before running into these same considerations.
But I think we're still getting ahead of things. Can you describe what your current bike is, or what existing mount points it has for racks? And also perhaps a description or sketch of how the rack would be loaded? The engineering is necessarily different for a 30 kg child versus two 15 kg panniers.
https://hembrow.eu/dscf5165.jpg im thinking something like that (thanks for the info and the links, lots of reading to do)