chucky

I'm thinking about modifying an aluminum frame I have...filing a 20mm deep notch into the end of a tube.

The thing that concerns me a little is the tube is closed off at the end. So I'd be effectively cutting off that closure (well not the whole thing, but cutting it in half).

Is there a significant difference in strength between a closed end tube and an open end tube? I suspect not or else seat posts and handlebars would be closed off at the end and they're not.

I weigh almost half of the frame's weight limit.

unterhausen

there is a difference between the strength of a tube when it is closed and when it isn't closed. It has to do with the tube wall being free to deflect at the ends when it is open.

chucky

Clearly, but the question is this difference significant? For example, there's a huge difference going from a flat strip of metal, to an L shaped channel, to a square tube. But closing off the end of the tube seems like it would have a minimal effect otherwise handlebars would be sold with closed off ends, no?

unterhausen

you really didn't tell us what you want to do. I'm not familiar with any bicycle frames that are loaded in a cantilever configuration like a handlebar is.

Falanx

Except it's impossible to butt a closed-end tube, and post-production welding it shut would warp the ends of the bars. But yes, you could sell the bar with little aluminium plugs in each end, and increase the cost of the whole unit. Little nylon plugs wouldn't have the same effect, but they're what the vast majority of the world uses to plud tube ends. They're sold 'unended', for want of a better word because it doesn't behove the manufacturer to add cost (for themselves).

chucky

650c wheels stock and the main tube is about 5mm too long to fit a 700c wheel in the dropouts (even if you file off the brake tab). What were they thinking?!

Or just machine a solid bar instead of a tube:


Not saying this isn't dumb, but I figured I'd post it for posterity.

chucky

Or maybe I should just make my own frame? 1 tube + chainstays + headtube + bottom bracket. But I'm concerned about getting the tube wall thickness right, especially the chain stays which might require some kind of reinforcement?

ftwelder

From my viewing angle, the way this frame is built it would be easy to cause a failure. There is no triangulation. That theoretical failure would occur just forward of the brake bosses at the end of the chain stay bends. I don't know what has been done in the way of reinforcement that I can't see but I would not make any modifications in the most vulnerable area of the frame.

unterhausen

he wants to cut a slot out of the boom tube between the join area between the boom tube and the chain stays. It would be a very bad idea. In this case, the closed end of the tube does contribute significantly to the stiffness of the bike. In addition, there would be no payoff to using marginally larger tires.

chucky

The main tube is actually somewhat triangular in shape and huge by conventional standards (2.5" in diameter). This is said to provide triangulation similar to smaller tubes arranged in a triangular fashion. Vertical loading is said to be somewhat unimportant as pedaling forces are all directed horizontally (so the frame need be vertically strong, but not necessarily stiff). Also, the stock frame is rated for nearly twice my weight.

I know, as frame builders, you guys have in mind the most efficient structure possible, but there must be some compromise for practicality. There's no point in having a frame twice as strong as I need it to be which can't accept the standard components I want to use.

Except that 650c (ISO571) is a dying standard with very little choice in the way of tires or rims. While I could use 26" (ISO559), again, tire and rim choices are somewhat limited for road use. Particularly, light weight puncture resistant tires are difficult to find for 559 wheels (it's either wide/puncture-resistant/heavy or skinny/light/puncture-prone) as are light rims for use with hub brakes. Mind you this is rotating weight in the worst possible place.

I also already have a set a 700c wheels I built to specification which I'm currently using on the steel version of this frame. Problem is the steel frame weighs 6.5 pounds whereas the aluminum frame weighs only 4.5 pounds.

I guess I should just look into getting a custom frame built then. Although there are few welds and I think I can make a good guess at specing the main tube, just as with modifying a stock frame, my main concern is getting the chainstays right.

NoReg

I think you could get away with it, the simplest version of it being to adding a web ahead of the slot out of epoxy and plywood, just wedge it in there, to support the vertical loading. Back when I couldn't weld, I built my trimaran, and it has 4 inch tubes that exit the floats, and that point was supposed to be supported with metal tubes welded in on the vertical axis. I couldn't do that, and I didn't like the reduction in tube strength welding would create. So I turned a bulkhead out of plywood and glued it in there. it works great. I think something expedient would get you your strength.

Second issue with any structural alteration is will a failure kill you. This doesn't sound like something where the downside is all that bad. Basically it would cause the tube to collapse, which you can obviate with the wedged wood, etc.. or the tube would split - can you get a band clap in there. Worst case the frame will crack at the rear stays, and probably give you some warning. It is significant that you will retain some of the plate on the end of the tube.

I am a little surprised at your overall reasons for the alteration, 650c seems increasingly popular. You don't have the choice you might in another, but is it necessary to get a new bike just for wider tire choice. depends on the use I guess. The same goes for 26, you're right about the choice, I use it for touring, so I know there are good options there.