I’m not flaming you CA, I’m joking. You are sensitive about your lovely creations aren’t you? 🙂

Iain have a look at my calcs – if they are wrong pease let me know.

boblo, I’m doing stuff that no one else is and it’s working, it just gets a bit tedious to get flamed for that alone, I don’t flame folk when they are being constructive.

ever ridden a tandem – that joint is under twist

Tandems not nearly as stiff, more weight (which is pedalling) on the back too.

Reinforce it sideways. Otherwise it’ll fold at the dropout.

howso?

Al, how round is the top tube in the donor frame ?
I managed to dig up some pics of Frankenbike.
I used a stem, bought specifically off here for the job. It was a Titec one IIRC, had to be pretty long but with a round shaft. I lopped off the fork clamp, then cut two thin slots lengthways along from the cut. This then slid over the cut-off top tube of the donor frame & clamped with a large bolt-up seat clamp from a Scott Ransom. I slid a length of steel bar inside the top-tube to stop it crushing. I cut the slots a bit long though, so planned on getting another stem.
The bar clamp went onto the seat-post with a shim.
Didnt get as far as finishing it off, but it rode fine. I had my 15st brother sat on a seat on the rear frame while we rode around on the contraption, much to the amusement of the wife. We didnt die.
Only area of concern was the b/b used for the drop-out interface. I’d modelled up a beefed up axle that I was planning to get made here at work. It was really just an idea that I spotted on the Xtracycle group on Flickr.

24032010303 by pten2106, on Flickr

21032010291 by pten2106, on Flickr

24032010304 by pten2106, on Flickr

24032010302 by pten2106, on Flickr

If I understand your calcs I think you’re just calculating the percentage of the total load that will be on each end of the bike based upon how close to each end it is. The point is that whatever load is at the rear wheel will exert roughly double the load at the GT dropout because the rear frame is pivoting around the seatpost connection and the distance from this to the rear axle is roughly double the distance to the GT dropout.

I think you could have a very simple seatpost connection because it is only in compression. Just file the top tube to fit neatly around the seatpost and use a couple of zip ties (jubilee clips might be safer but possibly unecessary).

Ta taki, TT is square section but there are some square stems around, getting teh size and angle right could be tricky.

Planning on snug washers on bb bolts. It’ll be solid I reckon.

EDIT ta Iain, that is what I am thinking. I have accounted for the leverage and doubling – 36kg to 72kg.

Taki, did you consider getting that welded up? Assuming both similar grades of ally and weldable, might be a neat(er) solution.

You said you rode it? It’s quite a short cockpit for the stoker, how’d he/she get on?

Yes a shim would be used but still not explaining myself very well.

Line up the rear bike over the front bikes seat tube, insert seatpost & shim. There would be some heavy jiggery pokerey, cutting and welding to get the dropouts and angles correct but pretty sure it could be made to work.

Whatever you end up doing all the best with it. 🙂

You said you rode it? It’s quite a short cockpit for the stoker, how’d he/she get on?

It was my brother, & it was just to see if it held together. Wasnt used with two chainsets, but I guess it could have.

Line up the rear bike over the front bikes seat tube, insert seatpost & shim. There would be some heavy jiggery pokerey, cutting and welding to get the dropouts and angles correct but pretty sure it could be made to work.

Thats exactly how it went together, I bolted the crank into the dropouts, then lined up the clamp & slid the seatpot through, then the shim.

The plan was to buil a frame for a long seat, like an xtracycle.
I’d designed a seat unit for my two lads, that would lock in to place. To be honest, it rode fine, but you underestimate how much room it takes to maouvre something that long.

iain, I did a simple analysis in Ansys, but cant find the file. It basically showed a shear load on the axle, & a heavy compression load on the seatpost joint. It also showed a heavy shear load on the seatpost, between the clamps. If you can imagine, the rear frame is pushing the seat post forwards, while the front frame is pushing the post rearwards but is further impacted by the rider weight causing a bending moment. I modelled in a length of steel tube slid inside the seat post & this beefed that up nicely. It showed the joint under compression from the front & rear.
I’ll try & dig the Ansys file up, but I’m more CFD based than Stress.

😯

Stem could go inside the top tube of the rear bike if it’s oversized – make a bung out of wood that fits snugly, drill hole for stem then bolt it all up. Or glue would be good too actually, I know you like it 🙂

Re loads – isn’t this going to be a load carrier? If so the load could easily be balanced over the rear wheel so loads on the middle would be reduced?

Although to be fair Al, this would be a lot easier if you learned to weld 🙂

molgrips, I am considering getting some wlding kit, it can’t be that difficult (I have actually tig’d ti before!)

Welding thin tubed bike frames can be hard tho…

Eureka! (oddly that’s teh name of the Jamis in question).

The 31.6 seatpin on the rear bike has an ID of ~26mm, I’ll saw a bit off, split it to go to 27.2 ID, cut a hole in the TT of the rear bike’s top tube, insert the tube adn glue/CF it in.

Pics to follow…