rekall

so here's a line of thinking that i can't seem to "let go" since i've become increasingly aware of various folders' mechanism, for, well, folding and also looking at older designs (raleigh twenty, vintage pugeot, et al)...

how hard would it be to cut an existing non-folding frame in half, and weld on locking hinges (a hinge with some sort of clasp) or plain hinges and then drill holes for a locking pin... ??

i imagine this is perhaps how the original folders were prototyped... well, i want to do this to my bmx! the frame is already small enough to be quite portable, if it were only folded in twain! obviously i'll have to use TWO hinges for both the top and bottom bar, but no biggie, really. and i already have become acquainted with a neighborhood welding expert who's adept at modifying bikes, to boot (he's made some really crazy looking tandems, recumbents, cruisers, off-roaders all from bits of other bikes).

so... am i nuts? is this possible? anyone here done it or seen it done?

and more specifically, where do i source the kind of hinge i'll need?
i don't even know what i'm looking for when i visit hinge/clasp manufacturers' websites...
the kind of hinge that has two flat bits and comes together with an integrated locking mechanism/lever...
what's that even called?

EvilV

If I tried that, I'd only succeed in making it easier to take the two bits of bike to the dump.

Bacciagalupe

It's rather difficult to do properly, even for an experienced framebuilder.

For example, you can retrofit S&S couplers onto a steel frame. However, S&S won't sell direct to the public; I don't think they sell them to a framebuilder until they meet specific criteria (presumably including signing waivers and having big insurance policies). Most of it is that if you don't do the procedure properly, the frame will split under load and the rider will end up eating pavement / dirt.

This is especially important if you want to continue to use this bike for BMX purposes (e.g. off-road, grinding, tricks etc). Not many folding bikes are made with those kinds of tolerances in mind.

Also, what's the frame material? Steel is fairly easy to weld, but aluminum is much trickier. Even the S&S guys won't do aluminum.

The hinges are proprietary; you'd have to design and build your own, or jack them off of other bikes.

I think you'd be much better off picking up a used folding bike, and upgrading parts as needed.

simsles

Yes, you are nuts.
I doubt that you would achieve a satisfactory result for the work invloved.
Most BMX bikes already have quite a small frame and both seat and handlebars can be lowered/twisted significantly, making the package even smaller, and with 20" wheels would probably fit in medium sized car.
Also they have a high bottom bracket which is not good for riding any distance.
My advice is to give up before you start and get yourself a folder which has been designed as such. Even a poor folder is likely to be better than anything you might achieve by carving up a BMX bike.

nekohime

Well, if this is purely for experimental purposes, why not? But if you are actuay gonna use the bike I'd advise against it because of the abovesaid reasons.

MichaelW

I have a dahon cadenza which hinges on the top tube and seat tube. The alignment of the hinges, their position on the tubes and their orientation in relation to the whole bike is fairly critical. The biggest challenge will be building a jig that holds your bike so you can "cut and shut" with both hinges in alignment.
If you really want to do this you need to figure out the hinge position (suggest you use cardboard tubing/ductape scale model, then prototype the jig (using an junk frame) before taking a saw to your bike.
The hinge unit need not be super complex like the Lockjaw, check out some simple ones like the Raleight 20 shopper.

griftereck

a 230mm grinder might cut both tubes at the same time

EvilV

I think it would be best to take the frame you're thinking of doing this to, and throw it straight in the skip /dumpster, just so you save some time and money, not to mention raising your blood pressure through frustration.




Waiver - I do realise that some people are immensely more skilled at metalworking than I am. I know this because if it were down to me, we'd all be chipping away at flints and we aren't.

Rick@OCRR

If you've read all of the above and still want to have a go at it, why not? Even if it was a dismal failure all you will have lost is a BMX frame and some of your spare time.

So yes, you are nuts, but remember that's a social definition and it need not hinder you! If nothing else, you will learn a lot about mechanical problem-solving in a three dimensional universe (and that's always fun!).

By all means, keep us informed on your progress and present us with any specific problems that you may not find immediately solveable. We may be able to help. Or we may not. But if you already know how it's going to turn out, where's the adventure in that?

Rick / OCRR

rekall

wow i wasn't expecting any reponse no less some of the more technically-minded sound-offs - thanks all!

yeah hinge position of the top relative to bottom tube is critical and the two cuts would have to be made perfectly in line with each other and exactly perpendicular to flat ground... not exactly that easily done with two seperate cuts even with careful measuring and proper equipment (mitre saw & a LARGE workbench w/ clamps).

i kinda figured the hinge/latch combination was proprietary as i haven't seen Anything similar from plain-vanilla hinge manufacturers' sites... it's probably an in-house custom job with a butt hinge machined onto or welded onto a clasp/latch to form the unified mechanism... if you look Really closely at the design of the pugeot folding bike's hinge it seems to be this case.

the s&s coupler is precisely in line with another line of thinking i've had on this topic, namely -- does it need to be a hinge? the other way i was thinking i could achieve the same result would be to cut the frame, find steel tubing that's smaller in diameter than my bike frame tubing, fit sections inside my top and bottom tube at the cut points, drill holes through both sets, and use locking pins, ubolts, or ulocks to join them whole again... it appears the s&s achieves the same result, albeit, in a far more elegant manner.

ironically the s&s site has a link to one of my oft-frequently local shops as one of their official framebuilders/partners - https://www.sandsmachine.com/bp_bil.htm -- though the place has been renamed 'bicycle stable', and montana norvell is Much older and more experienced now

the s&s doesn't seem to account for locking/security... which is kind of an issue... at least my idea of using a ubolt to join either the top or bottom tube would solve this problem.

as an alternative to a u-lock, a minimum of two hitch receiver locks could be used... these are straight pins meant to be used to secure a car/truck hitch:


https://www.masterlockcycle.com/product_receiver.shtml

there are many brands/diameters/material of these.

major upshot of ditching the notion of a hinge? NO WELDING.

faffer

What about avoiding interrupting the middle of a frame tube? You could create something like a rear triangle that bolts onto the main triangle, and it would be way easier. For example there are designs for recumbents and tandems that involve sticking two small frames together. You could have a folding tandem (not exactly solving your original problem, I know) if you stuck the two halves together non-permanently.

rekall

it's a possibility, but the main top and bottom tube are WAY long so you'd have a really short back section and a really lengthy front section rather than a bike nicely lobbed in half... also the chainring would be sticking out anyways, so you don't gain much by considering a 'cut point' further back on the frame.

rekall

chopping up and trying this on a used frame that Isn't in use by my current bike is a good idea... i'm keeping tabs on a few ebay auctions for some decent-condition gt, diamondback, redline frames that would all be suitable replacements to my current frame...

edit @ 5:54am - on a whim after being up late (insomia) i checked ebay buyitnow's for 'bmx frame fork' and found a Totally Sweet simple chrome unstickered solid-condition mongoose frame and fork, even including cranks, for < $100 (< $70 actually With shipping). i will NOT feel bad about experimenting on this one to work out a hypothesis.

griftereck

you'd have been better with a painted frame as it would be easier to refinish.

rekall

*****-and-a-half finding a source of metal pipe that has inner/outer diameters that would serve as either an inner or outer 'sleeve' to the top and bottom tubes of my newly acquired mongoose frame, but at last i was able to find suitable diameters in short-order (don't need to buy in bulk) from SMALLPARTS.COM (all caps because their site is Totally useful for project builders/prototype people).

going with a two-sleeve scheme where i'm slipping the original halves of the frame tubing between short segments of pipe with a bolt through them & secured with really nice hand knobs from JW Winco ... https://www.jwwinco.com/products/sect...ska/index.html


1 1/4" outer diameter [outside bike frame tube]
https://www.smallparts.com/dp/B000FMU...F&pf_rd_t=2901

1" outer diameter [inside bike frame tube]
https://www.smallparts.com/dp/B00137P...F&pf_rd_t=2901

the slight discrepency in diameters will be met with 1 1/2" diameter heat shrink tubing, also providing the bonus effect of keeping metal from rubbing on metal
https://www.smallparts.com/dp/B0013HR...F&pf_rd_t=2901

already sawed my mongoose frame in half with a hacksaw... which was effort, but not as much as i might have guessed.

pictures to follow once things start to take shape.

colinlewis

I have done this with a BMX frame. I cut the top tube and down tube. I put the bike back together using two sleeves. Each sleeve was a piece of tubing that just fit over the frame tubes, and had a slit at each end
I used hose clamps (aka "jubilee clips") to tighten the sleeves onto the frame. It worked, but the hose clamps had to be very tight to keep things from slipping.

So I ended up with a heavy, single speed bike that could be disassembled/reassembled in about 10 minutes time.

It was an experiment to see if it could be done. With a little bit of welding I could have made proper clamps rather than using the hose clamps.

One thing I learned is that you can not get by just attaching the down tube and leaving the top tube free - the two halves of the bike quickly rotate in opposite directions around the clamp.

I then stuck a wooden dowel in the top tube to keep that from happening. It worked, but I found that during riding the top tube is not only compressed (front and back pushed together) but also tensioned (opposite motion).

So, i wouldn't say you were any crazier than I am.

rekall

glad i overengineered and using four bolts and two sections of tube...

as i'd mentioned previously, i was going to Really overengineer this and do both an inner and outer 'sleeve' but as it turns out that's easier said than done... you'd have to come up with some way of retracting the inner sleeve into the frame completely while you line up your bike halves, then slip them into position inside the frame -- some kind of 'slit' cut into the frame and a little 'catch' sticking out of slit bolted or welded onto the sleeve would be needed to provide you with a sort of 'sliding door' device...

so i'll continue as planned with just the outer sleeves and i'll reevaluate after a few test rides to see how it holds up; if it's strong/stable etc.

as for locking considerations i decided to just invest in a longer chain to use with my new york fugghetaboutit mini u-lock... i can think of a bunch of times i wish i had a longer chain than my current 3'3" length, so this upgrade was on its way anyway... i got the trimax t-hex 5' which is a bit longer than the kryptonite noose (4'-something) which will allow me to cinch a loop around the back tire & rear brake frame, around to the front tire, onto a bike rack or street post, and completing the loop with the u-lock on the vertical frame tube beneath the seat post... this way even if someone takes the time to undo all four of my bolts and removes the tube sleeves, they're still looking at two bike halves very much attached to each other and the rack/sign/post/tree... which is good enough protection for only the times i'm too lazy to disassemble the bike and lock the two halves to each other doubled-over.

at some point in the future, however, i may upgrade to these:
https://www.urbanbiketech.com/product-p/m10dual.htm

intended for 10mm rohloff axles, but there's nothing stopping me from substituting two of my 3/8" bolts for 10mm bolts and attach them with these special locking nuts -- then there'd be no removing them without the special 'pit' and bolt turning tool.

i'd like to add that cutting metal, especially stainless steel, with a miter saw and metal blade is FRIGGEN FUN!! sparks galore, man-with-power-tools-grunt-tastic.

rekall

wow dude, i think you lite a fire under me arse, cuz here i am now with a wonderfully re-assembled bmx frame... they say a picture's a worth a thousand words, and here's four!









i had to mark the two tube sections with 'B' and 'T', and directional arrows, on account of eyeballing the drilling of the holes -- there's only ONE way this baby comes back together, but at least it does, and in quite a Sturdy fashion, i might add.

rekall

anyone wondering about the black 'something' towards the front of the frame, that's heat-shrink that i applied up there so that i won't scuff up the frame with my chain lock as i did on my other bike (i like to wrap it around there & lock it instead of wearing it or throwing it in a bag)... i figured as long as i had access to slip 2" heat shrink on there, i might as well

rench123

I've been thinking a bit about this mod. the open tubes are obviously not as strong now that they have been chopped - aside from the compression and tension that they get, they will also be subjected to crush forces exerted by the sleeves. Maybe some wooden dowels inside the open tubes will help?

At any rate - Please update us on what happens with this experiment.

Cheers!

rekall

your point is valid but my educated guess is that lateral forces (sideways i.e. turning) aren't as much of a concern as distal (up/down, vertical) on account of sidewalk bumps and potholes... in either case it won't be clear if i need to upgrade to a different/stronger metal (314/304 stainless or hardened brass) until i've ridden it and seen the result. just holding the finished frame now, though, i'm pretty confident i won't see significant stressing for a while.

rekall

here we go! all built (it was a LONG day yesterday)

in two halves for travel...


let's go for a ride!


just took it on a test spin and i didn't notice any chatter/clatter/etc.
good to go!

somnatash

Wow, you really did it you got determination - and it looks good too
I keep my fingers cross that is does not break down under you.
(only time I did something similar, I ended up on the ocean with a surfboard in two parts which was not intended)

rench123

Hey, cool setup! I see two cables toward the rear triangle - so I guess you are running an internal gear hub?

rekall

you have a good eye - i sure am! sturmey archer SRF3 (freewheel 3spd).
it isn't super-amazing in terms of performance gain but 1st gear makes hills a Breeze,
which is all i wanted of it; i'm quite pleased.

my coupler section doesn't rattle during rides but it does require that i tighten it before the next ride... not that huge a hassle, really. just add it to the list before you go riding... brakes? check; headset? check; seat height? check; coupler? check!