Folding Bikes - DIY folder???

I've been thinking for a while of trying to make a suitcase size bike for taking abroad. IT has to be 26 or 700 for touring from foreign jump off points, so it will need to be a pretty big suitcase, but bear with me.
After a lot of thought I took a hacksaw to an old steel frame I had lying around - no value, so nothing lost.
I cut it near to the seat tube on both crossbar and downtube.
Conveniently, a modern stem will fit both tubes neatly. I have a couple that have a longish span (though I would prefer one that had longer), which I slide onto the cut tubes. I can reassemble the frame, slide the stems over the cut, tighten them fully, and hey presto, the frame is solid (from a stretching and pulling point of view, anyway.
I need to put the wheels on and actually try it out, but I'm hoping it'll hold together without any flex.
I'm thinking of putting a length of steel tubing into the downtube (luckily, again, I find that I have some which will fit - this should give extra rigidity and that crucial join, which is where it would come apart under normal weight should the step clamp fail). Having this only on one tube will also mean the frame will actually come apart, as if it were in both disassemble/assembly would be impossible.

I'll try it out on monday and let you know how it rides.

Make sure those stems are on nice and tight!

You are a brave person!

That's not a folding bike. That's not even a coupled bike. That's just an accident waiting to happen.

A sleeve in the downtube that extends three inches in and three inches out to join the two pieces. A sleeve extending three inches into the top tube each way sliced at the cut to reinforce the clamping area. This would add weight, but also add some strength.

Anyway, I am no engineer, just a shade tree mechanic.

Well, it strikes me that bikes by there very nature are accidents waiting to happen, in that there are so many parts held together by single bolts or combinations of bolts.
Consider a pedal in a crank arm; the diameter of aluminium around that axle is not that great. Consider the forward force on a wheel when it is pushed with all your weight. The actual contact point of that bolt to the frame is miniscule.
Handlebars themselves, on which you put all your weight are held by those two small bolts on your stem. The tollerances are miniscule, and yet we trust our lives to them.
Consider how much metal actually holds a caliper brake in place, and think how much you trust it.

icithecat, that's what I figure. Only, the top tubes needs some slot and bolt affair so the sleave can be slid into the tube and back. If you imagine, as you pull the tube apart at the bottom bracket, the top tube moves up as well as back, meaning effectively it can;t move.
I klind of like the idea of maybe an even longer, sturdier tube in the down tube and none in the top - gives greater strength, but allows for uncoupling.

Here's an image, btw, kindly forwarded to me by Velonomad on the framebuilding forum. It's of a 40 year old design that I based mine on, except I use parts readily available (stem). Notice the sleeve at the bottom with the quick release levers, and also, notice on the top tube teh insert. Again, effectively what I am doing. It still needs refining, and I might never actually use it, as any baggage for airline use might still be too big (limited to wheel size). But I like a challenge, and it keeps me off the streets. I'll let you know how I get on.

http://www.vintagebicyclepress.com/images/Hersedemontable.jpg

Of course, I have to say, I'm in no way advocating anyone do this. I'm just trying it for myself becasue I have an inquisitive nature. perhaps if humans didn;t ahve that nature, we'd all still be walking everywhere.

The safety question is if it would fail without any warning. Try riding it without fixing the quick release. If the bike just rattles then you will have time to stop and tighten the QR, if not it might be trouble.

I think the forces on the bike would tend to push the crossbar on top together, and pull the downtube apart.

You might drill through the tube walls and stems and insert a 4 pins as a safety. The drill holes will weaken the tube. I wonder if drilling vertically or horizontally would be better?

I think it is kind of clever. My guess is you will need to adjust the QRs often.

I once thought about using plumbing couplings.

This would be the proper way to couple a frame:

http://www.sandsmachine.com/smovie.gif

I do apprecate the input, That Forum Guy, but you can't buy the S&S couplings, and the cheapest frame with them I've seen is about £800. I got a Private message from another chap doing the same thing as me, (though he's using a homemade clamp), so it's not just me.

Geor8ge, looking at the image, I think you're right. I think that's why M. Herse's design is cut like it is on the down tube (do you think fate's trying to tell me something with regards to M. Herse's name?)
Good idea with the unfixed test. I'll do that.

I think the naysayers are being a little pessimistic. I remember the early howls that S&S couplers were obviously unsafe and would come loose while riding. Ritchey's system just uses one tiny clamp and the seatpost and still seems to work.

The angle cut on the Herse downtube allows the top tube to telescope (it isn't a straight butt fit). The downtube has a long external sliding sleeve. It sounds like your system could do with a little more contact area between frame halves.

Apart from the Herse demountable, there was a similar retrofit system in the late 1970s or so (Pingle or Pengle, from vague memory) that didn't seem to have too many problems (noted in Bicycling! magazine). The sleeves may have had pins in addition to clamps. I wouldn't fit it to a thin gauge frame though.

I'm all for experimenting, I just don't think it's a good idea to go travel with a bike that may suffer a catastrophic failure. Have you tried posting in Alt bikes about hinges? Some of the guys there are pretty clever.

I'll try that, That Forum Guy, thanks for the heads up.
LWaB. I know what you mean about the contact area. I'll see how secure it feels.

Tell you what, though. I like the look of some of those 26" wheel Dahons. I read that they ride well and are light. I'm not sure how suitable they would be for touring, though - racks and all. I also like a steel frame (though I notice some have suspension posts). Anyway, we'll see.

I've just come across this, though, which has got me thinking again.

http://www.chwhite.btinternet.co.uk/assets/images/dahonallegrotoplug.jpg

Look at that cut. Seat post. Genius.
I've got a steel mountainbike frame with quite low stays. I could probably get away with a cut like that.

Catastophic failure is not necessarily that inevitable:

Is amounts to knowing about what forces a joint like the proposed OP is subjected to, and making the parts that make up the joint stronger than those forces. eg, the downtube suffers tension from rider weight. Knowing what that tension amounts to, and how much force it would take to pull apart a friction joint, is a knowable exercise.

A friction joint is found in virtually every single bike on the road in the seat post and the steering assembly. Once the clamp is done up properly, those joints are enormously strong. It is more likely that the tube would fail than the joint.

While I am no expert, I am in agreement with Jur on this one. A coupler type connection has some obvious advantages, but I don't see why this couldn't work,
Juan

Well, I took the bike out today, and it feels fine to me. No more flex than I would expect (and that with me putting my feet on the down tube, one hand on the seat post and one on the stem and pushing and pulling like mad.
I even rode it with the bolts loosened, just to see.
I decided to add frame clamps either side of the main clamps (the kind that hold brake cables onto frames - but I will try and go for something more sturdy - I have some old parts from a triple crown suspension fork that fit perfectly - ahh, standardization). I was also riding without an insert, which I will add for peace of mind to the downtude.
All in all, It rode well.
I have to strip the frame and build it up again with beter parts to get a true feel, but so far, I'm happy with it.

I was wondering whether it was a redundant exercise, as my tourer is double butted 531, and probably wouldnt have the strength in the tubes when cut to survive, but then I weighed the cut bike, and it was actually lighter than my 531 tourer, and by about 6kg. Bizarre. Maybe its becasue it's a smaller frame and had no rack, but then again, it has, at present all steel everything - wheels, chainring, stem, so depending on the ride, I may just stick with this.

All I'd need is a bag that would take it now.

Congrats!

How about some pictures?

Let me clean it up a bit; don't want to scare you with too many rough edges.