I have a steel tandem with a rear seat tube that sticks out above the top tube, with an external clamp for the seatpost. In order to accommodate a shorter stoker, I'd like to cut about 1 cm from the seat tube. How can I make a good cut without ruining the frame? And do I have to lengthen the slot facing the rear of the bike? Or should I take the tandem to a framebuilder to do all this?
You can do it! You might try using a fork cutting guide to get a straight cut. Yes, you should lenghten the slit in the back to its original length. I think a hack-saw should do it just fine. Clean up the cut with a file and you're done. I've never done it, but I don't think you should be afraid to work on your bike. good luck.
homebuilt FWD recumbents, Genesis 20" folding bike, RANS Sequoia, Canondale Beast of the East, 70's Peugeot
How much length sticks up from the top tube? This may sound crazy to some, but if there is enough length there (for tool access), I would suggest that you get a pipe/tubing cutter and use that to make the cut. It will make a nice, clean, SQUARE cut on the tube.
Here's a couple of pics, sorry they're a little fuzzy. Looks like all I can take off is 0.5 cm off the seattube, otherwise the slot in the back would be lower than the welds for the seat stays. The crosshatched area is the part that the seat collar clamp would cover. So I have to ask myself is it worth the trouble for just 0.5 cm ....
You might try a seatpost with a lower head profile. This means that the distance from the maximun insetion level to the saddle rails is less. You might also try a saddle with less padding (SLR, etc,...) to help bring the effective saddle height lower.
I have seen frames where the seat tube slit extends below the attachment of the seatstays. I don't know if sliting below this level will copromise integrity.
orbea steel road bike, my very own track bike, penny farthing.
there are a couple other options. i have seen seat posts that were build for aluminum frames that were built without any sort of binding. the seat post worked like a quill stem. it may be hard to find and if you find it, it may not fit the seat tube.(i believe one of these frames was made by schwinn)
another option is to make a binding like strawberry cycles