Sheldon style homebuilt tandem anybody?

[drubie]

OK, so there's two cro-moly mountain bike frames here, and I'm trying to follow the Sheldon instructions for building a tandem, concise version follows:

1. Two steel frames, one bigger than the other one.
2. Cut the seat stays off the front frame, leave the chain stays.
3. Cut off the front half of the head tube on the rear frame, mate up to front frame under seat.
4. Bend the front frame chain stays inwards to touch the down tube on the rear frame.
5. Align the frames.
6. Weld the rear frame head tube to the front seat tube.
7. Weld the dropouts on the front frame to the rear down tube.

I've tacked it together, but I'm thinking that the chain stays are going to pull off the front down tube and I need a bit more reinforcement there otherwise it'll collapse. The obvious solution (put a bolt through the front dropouts that hold a metal strap over the down tube of the rear bike) seems like the best way to solve it. Anybody else done this?

(I should have taken some pictures!)

[}SkOrPn--7]

Picture may well help to make further comments or give advice.......

[hartleymartin]

I've been thinking about trying this myself, but I don't have any welding equipment.

Another idea which has been floated was to cut off the chainstays on the front bicycle and use a 1" x 2" box-section piece of steel, suitable filed to shape at each end and welded to the two bottom brackets.

[mikedufty]

Forget the welder and use duct tape instead

Instructions here

http://www.instructables.com/id/Duck-ta ... n-bicycle/

[drubie]

The box section steel welded between the bottom brackets does sound like a better idea, but I'm wondering whether it'd lower the centre of the bike so much you would end up striking pedals. I think I'll end up doing something like that though.

Here are some pictures - hilariously pointing out my welding is awful is redundant. A big problem I had was trying to balance the weld penetration without blowing through the seat post metal - hard to do in a continuous bead so I have stitched it instead. Yeah, amateurs and stick welders are always a dangerous combo but at least I'm having fun Note how many blobs of metal are sticking either to one frame or the other

The whole frame:


Chain stays tacked onto the down tube - this is the weakest link I reckon:


Hilarious welding on seat tube. Yes, this will get cleaned up:

[ausdb]

Have you seriously thought about the implications of the quality of those joints? Not just for yourself but for the other poor soul you manage to talk into being your stoker. Grinding them back is not going to do more than show you how little is actually holding it together

Grind off the old bird droppings, separate the joints and clean them up to bare metal and and borrow an oxy acetylene welding set. Bronze brazing will do a plenty strong joint and you will have a better chance of making a reasonable joint with a little practice than trying to stick weld thin tubing with an AC buzz box welder. Sheldon also makes mention of heating the chain stays of the front bike and bending/moving them in the bottom bracket shell (the front bike looks like it is lugged construction?) and then attaching them further up the down tube of the second frame. You should really get rid of the dropouts from the chainstays attach the tubes directly to the downtube of the rear bike.

[AverageSpeed]

Drubie, I can offer no advice, just encouragement. As my father used to say, "It's a hell of a big hole that a pound of putty won't fill". Seems both bikes are well-committed (wedded even) to the experiment.......

Can't wait for future installments - solving the second chain-wheel, freewheel, handlebars for the rear bike, DON'T FORGET BRAKES! Need any help spoking up a sturdy set of wheels?

[WyvernRH]

The box section steel welded between the bottom brackets does sound like a better idea, but I'm wondering whether it'd lower the centre of the bike so much you would end up striking pedals. I think I'll end up doing something like that though.

Having built a few tandems in my life (from tubing, not two bikes ) I can only say that Hartley has the best suggestion structure-wise. Currently that is going to flex and then snap, probably as you suspect when there is so little holding the stay to the tube. before the photos I thought you meant that you ran the stay tubes either side of the down tube and then brazed/welded them to the tube giving at least some contact area. That sounded dodgy enough but what you have is just um.. much, much dodgier
I reckon you did quite well if you did that with a stick! I wouldn't go near tubing that thin without a TIG or an Oxy torch and if using Oxy I'd have brazed it up.
I reckon the idea of grinding it back, hiring a TIG, an Oxy set or maybe a good MIG setup and having another go sounds a good move. get yourself a a good 'boom tube' to connect the bottom brackets (It doesn't have to be level). Oh yeah, when you braze/weld the tube to the bottom bracket shellsput some old cups in there to prevent distortion which you will likely if you are welding or you feed too much heat in brazing.
Cheers
Richard

[mpa61]

I haven't built one myself, but I just bought one(see pic). It's currently located in Adelaide, so I haven't seen it in real life yet.



From what the owner said, it was 2 AZTEC MTBs TIG welded together. Not sure what he use to join the BBs...could be 1.5" water pipe for all I know. Probably should have asked first, but I figured for what I paid for it, the parts alone would cover the cost for some up coming tinkerings. I am also not sure if it is going to be strong enough...I may yet put another diagonal brace on it.

[drubie]

Have you seriously thought about the implications of the quality of those joints?

Yes. Yes I have and nobody has sat on it beside me to work out whether the thing was plausibly rideable. I'll mig it when I'm happy with the configuration and decided what to do with that bottom joint. Removing the lugs from the chain stay and bending the stays up as suggested would probably be weaker - those things can only be bent so far before they go past their elastic limit.

With the frame layed out, it's actually fairly easy to see how the rest will work: The back handlebars can sit in an aheadset style of stem attached to the front seat tube. The stoker gets both gear shifts, the captain gets the brakes. The outer chainrings will end up as the synchronising chain drive. The tech bits are far easier than the structure.

The only really long cable will be the rear brake, the rest is kind of a doddle once the structural stuff is sorted. Wheels - the two on the bike are steel rims (I'm thinking they'll be a touch stronger than alloy) but if we ended up riding it more than casually, I'd buy a proper tandem anyway.

mpa61's nice looking homebuilt is clearly far more professional, but I'm only in about $20 so far so we'll see how it turns out Notice how on his bike the synch chain is on the other side like a proper tandem? Nice!

[hartleymartin]

I think that Sheldon recommends brazing the two frames together, as apparently it doesn't affect the strength of the steel tubes like welding does. You COULD try drilling a hole in the downtube of the rear frame, and using a bolt through the two rear drop-outs.

Also, I'd be concerning myself with the bottom-bracket height. My "Twenty" has a 10" BB height, and with 170mm cranks there is 2.5" of air between my pedals and the ground, and I have to be careful with pedal strike.

[rkelsen]

hilariously pointing out my welding is awful is redundant.

No kiddin'! My dad would have a myocardial infarction if he saw these pics.

Apart from that, it actually looks pretty good. One question though: Are you sure you'd want the gear levers on the stokers' handlebar? What if you were out riding without a stoker one day?

One thing that I've always wondered about tandems is, what happens when the captain farts? Does the stoker automatically cop a mouthful of egg? There were a couple of tandems on the ATB ride on Sunday, and the short distance between the captain's bum and the stoker's face looks like it could be really bad for the stoker if the captain had pasta with broccoli for lunch.

[drubie]

One question though: Are you sure you'd want the gear levers on the stokers' handlebar? What if you were out riding without a stoker one day?

One thing that I've always wondered about tandems is, what happens when the captain farts? Does the stoker automatically cop a mouthful of egg?

Not too fussed about the shifting - it's something that can be fixed later if the frame works out OK. Putting it into low/low with a single rider would be fine.

As for the captain farting - at the moment I think the motion of giggling on the back would likely snap the Pequod in half.

(yes, I'm going to get some vinyl cut letters in Peugeot script and call it the Pequod, largely because the captain is crazy)

[il padrone]

Aaaaahh!! These things so often end in tears. A lot of fun for a while, but I've seen my mate's cobbled up tandem finish up in two pieces

Save the pennies for a real tandem is my recommendation. The simple value of having an ellyptical BB to set the timing chain tension is more than worth it.

[drubie]

but I've seen my mate's cobbled up tandem finish up in two pieces

Which bit broke il padrone? It seems to me the weakest link is the "boom" that should go between the bottom brackets, as in a big ladder like this it's really the only tube that is in tension rather than compression. To make it strong, I think the boom should really go around the brackets or the seat posts somehow so that when the tube pulls outwards under load, it isn't just pulling inwards on the welds.

the top joint seems to me like it's most likely to be affected by rotational forces as the frame twists, so a decent weld or braze will be enough. The problem is I find plenty of pictures of grinning idiots standing next to their duct tape/bolted together tandems but no pictures of broken ones to verify whether my guesses are close or not.

[lemmiwinks]

Would you like some 1.something mm (don't remember exactly but they are sub 2mm) rods mate? I've got heaps and they're really good for the thin stuff.

[drubie]

Would you like some 1.something mm (don't remember exactly but they are sub 2mm) rods mate? I've got heaps and they're really good for the thin stuff.

Might be better than the welding rods I'm using, but I suspect it's an operator fault

I'm wondering now whether I can't use the duct tape bike for a bit of inspiration, but instead of sticks and duct tape use some concreting reo rod. Loop it around the seat tube on the rear bike and join it along the chain stays toward the front, then use a couple of bits either side of the top tubes. It would save having to weld on the bottom brackets.

I'm starting to think Sheldon's instructions left out a fair bit since his method doesn't seem all that strong...

[lemmiwinks]

Might be better than the welding rods I'm using, but I suspect it's an operator fault

I'd offer to weld it up for you but I suspect I'll be a tad busy for the foreseeable future

I'm wondering now whether I can't use the duct tape bike for a bit of inspiration, but instead of sticks and duct tape use some concreting reo rod.

Use stainless steel radiator hose clamps instead of duct tape/cable ties.

[hartleymartin]

I'm wondering now whether I can't use the duct tape bike for a bit of inspiration, but instead of sticks and duct tape use some concreting reo rod.

Use stainless steel radiator hose clamps instead of duct tape/cable ties.

How strong are those hose clamps? I've only used them to hold bidons onto my frames - would they be strong enough to hold two frames together?

[lemmiwinks]

How strong are those hose clamps? I've only used them to hold bidons onto my frames - would they be strong enough to hold two frames together?

Very. The screw mechanism would fail before the material itself. They're light enough that you could double or quadruple up. Personally, while the duct tape bike is amusing, I'd only weld or braze myself.

[hartleymartin]

Thanks,

I MIGHT have a go at a Sheldon-style home-brew tandem if I can figure out a good way to put it together securely without welding or brazing. (Obviously with an view to braze it together should it work!)

[drubie]

Thanks,

I MIGHT have a go at a Sheldon-style home-brew tandem if I can figure out a good way to put it together securely without welding or brazing. (Obviously with an view to braze it together should it work!)

The hose clamps don't offer any rotational resistance- ok for stopping the pipes separating but not folding sideways. This isn't as easy as that darned interwebs says!

The duct tape bike uses sticks to stop the frame pivoting around the joint. It's hardly ideal but the problem is very real. Welding or brazing are the only way. BUT some way of solving the boom issue at the bottom of the bike is unfortunately unique to the frames you've got.

[hartleymartin]

I was hoping that the bolt through the front bike's chainstays and the rear bike's downtube might have been sufficient.

[drubie]

I was hoping that the bolt through the front bike's chainstays and the rear bike's downtube might have been sufficient.

I think that'd be an even worse idea than trying to tag the stays to the tube - drilling the tube will make it very weak and likely the bolt would just tear through the front of it. Better to use some kind of clamp - although that will only stop it pulling forward. It still needs some kind of rotational stiffness added somehow that a clamp probably wouldn't offer.

[Thoglette]

Save the pennies for a real tandem is my recommendation. The simple value of having an ellyptical BB to set the timing chain tension is more than worth it.

I bought a very cheap new tandem last year - $350 including shipping if I recall correctly. Branded "Omega"

Sometimes it is possible to pay too little. With this, I have come close

It does not have an elliptical BB - just another derailleur on the left side.

It did come complete, if pre-bent (1020 so easily fixed) and with dreadful build quality. (Not the vendor's fault).

Chrome and paint are paper thin - but the main derailleur is a decent Shimano one.

The frame appears to be spaced for 126mm wheels, but has a 135mm MTB hub. It has 26" rims and MTB brakes, but appears to have space for 27" or 700C with appropriate brakes

I've corrected the basic construction faults (bad cabling, stem bearings loose, chainline all wrong), swapped the knobblies for street tyres; added fenders and replaced the stamped-steel brakes with some Tektro units. Oh, and replaced the middle triple with an old biopace triple. Still some fettling to do (like rechroming or replacing the bars!)

It's fun, but if you know you want one, spring the $600 (and up) for a real one.