Is a bolt together frame a bad idea?

I'm considering building a frame that is bolted together instead of welded. Imagine something along the lines of Tinker Toys. The junctions would be aluminum blocks connected by square aluminum tubes. The head tube block could be CNC milled to accept the bearing cups for the headset. The junction for the seat tube/down tube could be CNC milled to accept a bottom bracket. The rear suspension triangle would be built in similar fashion. This is very much in a high level conceptual idea stage. The advantage I see is that the geometry can be easily changed by using shorter or longer tubes, and obviously no welding is invovled. Is this a bad idea? Assuming I used an adequate sized tube with adequate wall thickness, will the weak point be the bolts?

I don't see why not considering people make bicycle frames from wood or bamboo, and you can't weld those The downside is it would likely end up being heavier or weaker than an equivalent welded frame.

"Bolt together" bike from 1935.
On display at Velo Museum in Nijmegen, Netherlands.
Museum is worth a trip.

Punx0r said:

I don't see why not considering people make bicycle frames from wood or bamboo, and you can't weld those The downside is it would likely end up being heavier or weaker than an equivalent welded frame.

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Well, we haven't seen anybody build a wood or bamboo electric bike with 20 lbs of batteries and 6Kw of power. That would definitely put my concerns to rest.

It is the Tri-cruise 2000 that is a bolt together kit. I had one for years and made tons of modifications. It has a rear suspension from a cheap steel bike attached. Worked great. It is also known as the Americruiser. Here is the link.

http://www.americruiser.com/

otherDoc

YES it is spend your time and money else where's

Why not, if it is made to cruise. On a erformance bike there are enough bolts and screws on components already, I don't see the need to add some to make the frame, but to make it heavy and weak.

StinkyGoalieGuy said:

Punx0r said:

I don't see why not considering people make bicycle frames from wood or bamboo, and you can't weld those The downside is it would likely end up being heavier or weaker than an equivalent welded frame.

Click to expand...

Well, we haven't seen anybody build a wood or bamboo electric bike with 20 lbs of batteries and 6Kw of power. That would definitely put my concerns to rest.

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Yes we have, it was bamboo bike with green cromotor and battery box.

http://endless-sphere.com/forums/viewtopic.php?f=3&t=41321

Box tubes fitted into square junctions will put all the stress in localized spots, unless it is a press fit. You could not adjust the frame geometry without having slop in the fitment. A better approach would be twin vertical plates with cross braces. It could be modular to allow for adjustment of various geometries, without relying on sloppy fitments to get there.

Bolting flat pieces together to create frame tubes will be heavy, as noted above.
However, motorcycle frames sometimes include an aluminum back bone section of frame, bolted to a steel section of frame.
You may be onto something.

Put me down firmly in the "bad idea" camp, and here's why: A "bolt together" design means that you have multiple bolts and bolt mounting points that can be points of critical failure. This in and of itself is not necessarily a bad thing, provided you engineer your design so that all anticipated loads can be dealt with without deformation or failure. The problem is that with a bolt together design there is a much smaller window of partial failure, as compared to a welded design.

What do I mean by this? Here's an example; imagine riding along at 30+mph on a downhill, and you hit a pothole, and suddenly you've exceeded what your frame can handle. In the case of a welded design, you may end up cracking a weld or partially breaking a weld, but it is very unlikely that you would have a weld totally fail in one moment. A bolted joint, on the other hand, is far more likely to catastrophically fail, meaning one moment you have a working frame, the next you don't; all while at speed.

I don't know about you, but personally I like to have as many cards stacked in my favor as possible when I'm designing something that I'm going to put my butt on. There's a reason so many bicycles out there are designed around welded tubes. It's because it is a very lightweight, robust and forgiving way to design a bicycle frame.

No doubt a welded frame will be lighter and stronger, but I don't have access to a welder and getting someone to do a one off job is hard to find, especially for the type of frame I want to build. They just don't have jigs for it. I also want the ability to change geometry easily.

Also, I'm not thinking of a traditional bike frame, where this is 1 main tube going to each junction. I'm thinking more along the lines of a Ducati style frame, with multiple smaller tubes run in parallel, creating a space frame (I think i'm using the right terminology). So in that sense, if 1 tube breaks, it's not necessarily catastrophic.

You could bolt it together and reinforce everything with thread lock and then JB weld it if you don't plan on taking it apart ever. You could use some of that epoxy that Doctor Bas uses.

Proper adhesive joint design can be tricky. You need to ensure the loads are taken in tension, compression or shear and avoid peeling or cleaving.

Just as an aside, Sid Gowdy, the developer of the Americruiser bolt together alloy trike had formerly built ultralight aircraft with similar "bolt together" technology. It seems to work for those types of aircraft.
otherDoc

The real problem comes from a non precision fit. The bolts will slam the holes overtime, making them bigger and eventually failing. Such designs rely on friction joints to prevent movement. You don't have to weld, consider brazing (steel).
Learning:
Wisdom- 1st hand knowledge, the wise old man.
Intelligence- Learning from mistakes of other's, from a book.
Teaching- Students pose great questions you tend to overlook.

It will be a learning exp, but that's not necessarily a bad thing. Why re-invent the wheel, spend that time learning electronics, fewer ppl understand that. Maybe even trade your comprehension w/ a welder who wants an Ebike as well.

Forget bolting stuff together. I want to see a 1 pc 3d printed frame! Any takers?

Ask and you shall receive.
http://endless-sphere.com/forums/viewtopic.php?f=3&t=50795

They guy has a website and is selling the frames now, cant remember what it is tho.