What problem are you trying to solve that stops you from just buying a pre-made rack?
this post was submitted on 20 Jul 2024
20 points (100.0% liked)
bike wrench
821 readers
1 users here now
A place to ask bicycle repair questions, and for bike shop monkeys to share advanced non commercial wrenching resources (no YouTube self promotion). This is only for repair related topics.
founded 1 year ago
MODERATORS
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)
to grue: its mainly because i have the tools and time and i can make it cheaper than buying it (buying things ready spoils the fun for me) also i cant find a frame mounted front rack unless I pay someone to make it custom
If you want it to be easily repairable (you mentioned this in another comment) I'd go with small diameter aluminum tube and a very very good radius bender if you can AC tig weld, or stainless steel (maybe 316?) if you can only do DC tig. The reason for this is because neither option "requires" paint, though the aluminum does benefit a little from paint. Paint makes everything harder to repair (assuming the repair involves a welding process) because you have to grind it off again if you break something or want to make a modification.
Also, I totally get wanting to make things. My hobby projects are normally zero percent about saving money and 100% about having fun. Most things I make in my shop would cost about 1/4 of the price if I just went to the store and bought them, especially factoring in time and consumables.
EDIT: I suppose this comment really gives away what region I live in lol
I suppose this comment really gives away what region I live in
Does it? I can't quite recognize which part of your comment is particular to a specific region.
FYI for folks considering stainless steel welding, copious ventilation is required as the chromium component gives off some awful gases. To be clear, all welding processes give off bad gases, but stainless steel's gases sounds particularly unpleasant.
I'm not aware of a welding process that is safe for humans without at least a dust/fume extractor. That and a respirator with the appropriate filters for the application that fits under a welding hood is basically standard equipment even for hobby welding in my opinion. Also hex chrome is a particulate, not a gas, and tig welding uses solid rods (not flux core) and a much smaller heat affected zone so it's already better in that area AFAICT, but I'm not an expert in this area.
As far as particulates when welding go you should watch how careless people are when they grind their tungsten electrodes containing all kinds of dangerous additives with their respirator around their neck because "I'm not welding so I don't need it".
So yes, this is good advice. We should all think more about the things we do, even when working on home improvement projects and mundane stuff, that could end up damaging our lungs.
in my experience going to the store almost always costs more (but then again I only make simple single material stuff that dont require crazy equipment) To give this rack as an example, steel front racks are going for 100eur++ (check those surlys lol) and for what I want (which needs someone that mades custom racks) will easily go above 200eur.. (the plan is to make it with at least half)
for me its not just the fun.. the reasons are mainly ideological. (making, repurposing, reusing over consuming)
This gave me an idea. Have you considered buying a used higher quality rack and modifying it? Might save some time and get you what you want with less new material, and sort out some of the harder parts to fabricate because they'd be done already, possibly only requiring a little bending to align with your eyelets. Plus you'd be saving something from someone else's scrap pile.
Around me there are a few community run used bike part shops. They typically have lots of racks and other parts kindof organized into piles or boxes by type. I can usually find something close to what I need if it's not too specific.
What do you mean by bike rack: a car roof or hitch rack? A stationary rack/stand? Rear rack on a bicycle?
I assume rear rack but then they're usually aluminium no?
alu is to weak and not a good alternative to cromoly (i had the pizza rack from specialized fail on me on heavy load)
Ah yeah well they have weight limits sure
Good luck on your build though!
edited the title. I meant any type of rack that attaches to a bike frame